/smash/get/diva2:633742/FULLTEXT01.pdf

/smash/get/diva2:633742/FULLTEXT01.pdf
Ferrocement Super-Insulated Shell House
Design and Construction
JAN LUGOWSKI
Master’s Degree Project in
Energy Technology
Stockholm, Sweden 2013
EGI-2013-046MSC
Master of Science Thesis EGI-2013-046MSC
Ferrocement Super-Insulated Shell
House Design and Construction
Jan Lugowski
Approved Date
Examiner
Jaime Arias
Supervisor
Peter Kjareboe
Commissioner
Contact person
Abstract
The purpose of this paper is to explore the ferrocement building technique for sustainable housing. Ferrocement involves the use of conventional cement with fine aggregate and
several layers of steel, with the advantage of higher strength than conventional reinforced
concrete, limited formwork and thinner sections. It is particularly suitable for thin shell
structures, where geometry minimizes bending loads. Architectural flexibility is one of the
main priorities considered in sustainable housing, along with energy efficiency, occupant
comfort, resistance to seismic and tornado events, affordability and durability. Ferrocement’s historical and present applications are covered, along with other building techniques,
in order to establish best practices and possible improvements. Reducing construction labor
is a particular focus, which has limited ferrocement development in recent years. Computer
modeling of shell form finding is described, with three case studies created. A structural
analysis method is described and applied to each case study to verify general building code
safety. Energy modeling is performed in two climates for each case study in the United
States and compared to key PassivHaus energy demand limits. Net zero energy use is possible with on-site solar photovoltaic generation.
Keywords: shell structures, concrete, ferrocement, green buildings, energy modeling
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Acknowledgements
Thank you to my examiner, Jaime Arias, and advisor Peter Kjareboe for their support during
the project and allowing me full flexibility in exploring a topic which I deeply care about. The
project allowed me to combine my previous working experience with material learned during
the SEE program. I’m grateful for the holistic approach that my mentors embraced.
Jan Lugowski
Stockholm 2013
ii
Contents
1 INTRODUCTION
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 FERROCEMENT CONSTRUCTION
2.1 Overview . . . . . . . . . . . . . . . . .
2.2 Competing building systems . . . . . . .
2.3 Opportunities for improving ferrocement
2.4 Proposed building techniques . . . . . .
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3 MODELING AND DESIGN OF CASE STUDIES
3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Functionality . . . . . . . . . . . . . . . . . . . . . .
3.3 Software implementation . . . . . . . . . . . . . . . .
3.4 Design guidelines . . . . . . . . . . . . . . . . . . . .
3.5 Case study 1: Shell Retrofit . . . . . . . . . . . . . .
3.6 Case study 2: Calabaza . . . . . . . . . . . . . . . .
3.7 Case study 3: Ballena . . . . . . . . . . . . . . . . .
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4 STRUCTURAL ANALYSIS
4.1 Overview . . . . . . . . . .
4.2 Methodology . . . . . . . .
4.3 Material properties . . . . .
4.4 Loads . . . . . . . . . . . .
4.5 Evaluation strategy . . . . .
4.6 Case study 1: Shell Retrofit
4.7 Case study 2: Calabaza . .
4.8 Case study 3: Ballena . . .
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5 ENERGY SYSTEM DESIGN & ANALYSIS
5.1 Overview . . . . . . . . . . . . . . . . . . . . .
5.2 Methodology . . . . . . . . . . . . . . . . . . .
5.3 Design considerations . . . . . . . . . . . . . .
5.4 Case study 1: Shell Retrofit . . . . . . . . . . .
5.5 Case study 2: Calabaza . . . . . . . . . . . . .
5.6 Case study 3: Ballena . . . . . . . . . . . . . .
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6 CONCLUSIONS
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BIBLIOGRAPHY
60
iii
A EXAMPLES OF FERROCEMENT CONSTRUCTION
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B DESIGN AND CONSTRUCTION DETAILS
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B.1 Seismic loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
B.2 Wind loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
B.3 Energy model details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
C SOFTWARE
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C.1 Modeling Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
C.2 FEA Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
C.3 Energy Analysis Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
iv
Chapter 1
INTRODUCTION
1.1
Motivation
The purpose of this thesis project is to explore and improve the ferrocement building technique
as applied to sustainable housing. Ferrocement construction refers to a specific style of steel and
concrete construction. It’s a method that involves the use of much more steel reinforcement
layers in the structure and a concrete mix that includes sand rather than coarse aggregate.
The resulting structure can be thinner than traditional reinforced concrete construction, while
retaining superior strength. Material costs are typically competitive with conventional building
techniques, but there is considerable manual labor involved in setting up the steel reinforcement
in the field. This is the reason why today ferrocement is used mostly in countries with cheap
labor.
A major appeal of ferrocement is that buildings can become very interesting in shape.
Famous examples include the works of Antoni Gaudı́, Felix Candela and even ship hulls. Taking
advantage of shell shapes that naturally place the concrete in compression is a key part of
designing with ferrocement. Shapes such as domes and catenary beams are especially wellsuited. However, more complex shells are very difficult to analyze by hand and therefore a
computer tool such as Finite Element Analysis (FEA) can enable complex designs.
The term “sustainable housing” has many aspects. For clarity, it is defined in this thesis as
incorporating the following features:
Aesthetics A beautiful building will be cherished and maintained for a long time, reducing
the need for new construction and increasing occupant comfort.
Strength The structure’s strength to withstand natural disasters such as hurricanes and earthquakes is key in many regions and avoids costly reconstruction.
Construction Minimal ecological footprint during construction, including the use of locally
sourced materials, renewable materials, minimizing energy use, minimizing waste, minimizing disturbances to neighbors and avoiding the use of ecologically-sensitive land.
Fire resistance Use of materials that give occupants time to evacuate. Preferably the structure should not be flammable nor weaken considerably during a fire.
Energy efficiency Minimize the energy required to heat, cool and electrify the building. Includes strategies such as passive solar heating, insulation, air heat recovery, heat pumps,
efficient appliances and smaller living spaces.
Energy production To minimize the impact of external power plants, the building should
generate as much of its energy needs as possible. Strategies include solar water heating,
solar photovoltaics, biomass combustion and small wind turbines.
1
CHAPTER 1. INTRODUCTION
Thermal comfort Stability of the interior temperature can be increased with thermal mass
to balance changes in the outdoor climate. Rounded building profiles reduce the surface
area for a given volume, which reduces thermal losses of the building envelope.
Air quality Fresh air should be ventilated constantly and preferably filtered to minimize CO2 ,
dust and other pollutants. Ventilation rates are compromised by maximizing energy efficiency. Air quality also means minimizing materials which give off Volatile Organic
Compounds (VOCs) and other harmful gases to the occupants.
Natural light Sunlight in the building increases occupant comfort, although maximizing glazing comes in conflict with energy efficiency.
Noise External noise should be attenuated in the structure. Internal noise from occupants and
equipment should also be minimized.
Affordability The building technique should be as economically accessible as possible.
Building codes and practices typically cover only the bare minimum of sustainable building
ideology. For this reason, voluntary accreditation was created to help fill this gap. Examples
include the American LEED [63], British BREEAM [12] and Swedish Miljöbyggnad [56]. Each
takes a different approach, where LEED focuses on lowering energy costs and site selection,
BREEAM focuses on minimizing greenhouse gas emissions in construction and operation and
Miljöbyggnad focuses on occupant comfort.
The target is to reach net-zero energy use for a typical family in a given year for any climate
in the continental United States, while allowing for architectural flexibility and strength that’s
not available in conventional house building methods.
1.2
Objectives
The thesis is comprised of three main objectives:
Improved field construction Evaluate ferrocement construction of today and of the past.
Evaluate other building methods to find improvements that can be carried over. Attempt
to reduce field labor and the number of steel reinforcement layers. Explore novel concrete
techniques, such as textile forms, fiber reinforcement, prefabrication and others. Incorporate thick insulation, which historically hasn’t been a priority in ferrocement. The goal is
to present a viable building technique that incorporates the lessons learned. This work is
presented in Chapter 2.
Structural analysis Create geometry modeling and FEA framework for efficiently analyzing
the structure as exposed to loads, including deadload, wind and earthquake loads. The tool
should provide automated stress and deflection reports on the entire building. The goal
is to optimize the structure’s strength and provide support for building code compliance.
The FEA tool will be used on three house case studies. Modeling and design work is
presented in Chapter 3. FEA work is presented in Chapter 4.
Energy analysis Evaluate the annual energy needs of three house case studies. Find the
appropriate energy system technologies and perform rough system sizing. This work is
presented in Chapter 5.
The project’s scope and focus is visualized in a more general context in Fig. 1.1.
2
CHAPTER 1. INTRODUCTION
Figure 1.1: Scope and points of focus for the thesis. Remodeling and demolition are not in the
scope.
1.3
Methodology
A strategy for achieving the objectives above is described here, specifying the owner profile,
building purpose and applicable building codes.
1.3.1
The owner-builder
The entire thesis is meant to be viewed from the perspective of an owner-builder who would like
to design a unique house and build it efficiently. Efficiently here means getting the best value of
time, labor and money. Do-it-yourself (DIY) building techniques are preferable, as they allow
the owner-builder to better control the balance of labor costs and time.
1.3.2
House case studies
The focus of the thesis is on single family houses built on their own lot and without specific
size limits. A family of four is the intended occupant. The house should at minimum contain a
kitchen, three bedrooms, living/dining area, two bathrooms and a utility room.
Three case studies are designed and analyzed with the tools presented in this thesis. Each
house is designed to the above constraints, but with low, medium and high budgets. This affects
how large and elaborate the house is as well as which building techniques can be used. Two climates are considered: the hot and humid Miami, Florida and the mixed cold/hot Chicago, Illinois. Hurricane and tornado wind loads are assumed to be the maximum specified by the
building code, specifically Southern Florida. Earthquake loads are assumed to be the largest in
the country found around the San Andreas fault in California and the New Madrid fault in the
Mid-West. Flooding is not covered because it is best addressed by not building in flood planes
or raising the foundation. The intent of considering extreme conditions is to enable building a
safe structure anywhere in the country. The house designs and specific loads are described in
Chapters 3 and 4.
3
CHAPTER 1. INTRODUCTION
1.3.3
Retrofit case study
New construction isn’t always practical or possible. For this reason, one of the case studies is the
retrofit of an existing wooden frame house with a Disaster-Resistant Shell (DRS). The shell is
concrete and its purpose is to transform the building into a storm shelter and resist earthquake
damage. In the United States, houses in tornado or hurricane-prone areas may incorporate a
separate concrete storm shelter in the basement or buried just outside. The purpose of these
shelters is only to save the occupants, while the house itself would still be destroyed by a storm.
The retrofit of Swedish multi-residential buildings was also considered. These buildings are
conventional multi-story apartment tower blocks, typically constructed out of concrete. These
apartments were built in a government project called Miljonprogrammet (Million Program) as
part of a construction boom in the 1960s and 1970s in Sweden corresponding to a growth in
population. These buildings have been the focus of energy efficiency improvements in recent
years as part of a goal to reduce energy use in buildings by 50% by 2050 [55]. However, there is
relatively little potential in leveraging ferrocement in these retrofits. The structure is already
strong and tornado & earthquake loading isn’t a major concern in Sweden. At most, cosmetic
enhancements could be implemented to the facades, windows and entryways. These buildings
are much more simply and economically retrofitted with external rigid insulation and improved
windows.
1.3.4
Building codes
Incorporating building codes is an important part of this thesis such that techniques and designs
proposed can be built in practice. The country of focus is the United States, where the International Residential Code (IRC) [29] is predominantly used. The IRC applies to single family
houses, duplexes and townhouses limited to three stories. Anything bigger is covered by the
International Building Code (IBC) [30]. The IRC and IBC replaced regional legacy codes that
were in use up till the end of the 1990s, namely the Uniform Building Code, National Building
Code and Standard Building Code. States and municipalities may add extra requirements, such
as minimum foundation depths to avoid frost heaving. Voluntary LEED accreditation is not
directly covered in the thesis.
The IRC itself is very limited in its design flexibility for concrete walls and roofs. Walls need
to be at least 100 mm thick [29, §R611], which is too much for ferrocement. The designer is given
the option to use American Concrete Institute’s Building Code Requirements for Structural
Concrete, ACI-318 [5]. ACI-318-11 is therefore the code that drives most of the design in this
thesis. Chapter 19 of the code specifically addresses shell structures and allows for computer
analysis to justify the design. The minimum concrete cover is 13 mm for shell reinforcement
not exposed to weather. There is also flexibility in the type of reinforcement used, as long as it
meets tensile strength requirements.
ACI provides ferrocement-specific guides in the form of ACI Committee 549 Guide for Design, Construction and Repair of Ferrocement [8] as well as State-of-the-Art Report on Ferrocement [7]. Both of these documents are intended to supplement the main instructions in ACI 318,
providing guidance on material selection (sand, cement, mesh, admixtures), construction, repair
and maintenance.
When load values are to be selected, ACI-318 refers to ASCE 7: Minimum Design Loads for
Buildings and Other Structures [9]. This code provides details for seismic, wind, dead and live
loads.
A code specifically for ferrocement exists in the form of International Ferrocement Society’s
IFS 10-01 [31]. This code is a quasi-legal document which provides building recommendations
and techniques and is much more detailed than ACI 549. It follows ACI codes as closely as
possible, since by itself it cannot be used for certification.
4
CHAPTER 1. INTRODUCTION
Supporting building codes are also part of the design process and are introduced in the
relevant sections of the report.
1.3.5
PassivHaus and net-zero
The PassivHaus concept was created at the PassivHaus Institute in Germany. It’s a building
specification that results in extreme savings in space heating, to the point that conventional
heating systems aren’t necessary [37, 22]. To achieve heating requirements of no more than
15 kWh/m2 -yr, windows are required to have U-values under 0.8 W/m2 -K, heat recovery units
must be over 75% efficient, sub-soil pre-heating may be used and passive solar gains are leveraged. Wall insulation is very thick to reach a U-value of 0.1–0.15 W/m2 -K, airtightness is
emphasized and thermal bridges are avoided around penetrations. Ground source heat pumps
are used to meet the remaining demand for space heating and cooling, as well as hot water
heating. Energy-efficient appliances are used. Solar water heating and photovoltaics offset (and
sometimes completely cover) the energy provided from external sources. Net zero CO2 emissions
are also required, made possible by on-site renewable energy.
Net-zero buildings are a more general concept, where energy consumption and CO2 emissions
are zero summed over the year [15]. Several international initiatives exist to promote, codify
and subsidize net-zero buildings, but in general it is up to the owners to decide on the most
effective solutions.
Requirements of PassivHaus are stringent and designed for the climate of Central Europe.
In this thesis, it’s assumed that it makes much more economical sense to focus on the net-zero
target and only incorporate PassivHaus solutions where justified. More specifically, net-zero
energy use is the goal of each case study and emissions from external sources are not considered.
5
Chapter 2
FERROCEMENT
CONSTRUCTION
2.1
Overview
Ferrocement is a type of steel-reinforced concrete. The basic mixture is Portland cement and
sand (three parts sand to one part cement by volume) and no larger aggregate stones are
added [19]. Water is carefully controlled and the mixture has a plaster-like consistency. A
mixture that’s too wet is difficult to apply and forms weaker bonds with the steel. Chemical admixtures may be added to change workability, accelerate or retard set time, reduce the
water required (plasticizers), increase strength, increase adhesion, add color and enhance water resistance. For example, acrylic is useful plasticizer to increase strength, water resistance
and improve adhesion to existing concrete (such as in cold joints). Even the basic cementsand mixture exhibits less cracking, higher strength, better adhesion and smoother surface than
traditional concrete mixtures.
Reinforcement includes more steel layers in the form of reinforcing bars, welded wire and
mesh, together known as the armature. The layers are close to each other, allowing for thin
shells that carry loads uniformly. The concrete mixture is worked into the armature by hand
like plaster. Significant labor is required to setup the armature skeleton, attach the mesh layers
and pack the concrete. There is more steel and less concrete used as compared to traditional
reinforced concrete. Steel can be replaced by natural fibers such as bamboo and hemp. Glass
and fiber reinforcement is explored later in this chapter.
The most common causes of failure of ferrocement (and reinforced concrete in general) are
steel corrosion and incomplete mortar penetration. As reinforcement rusts, it expands and
cracks the concrete. Cracked concrete allows more air and water to reach the steel, leading to a
destructive circle. The most common causes of corrosion are internal air pockets and exposed
steel. Air pockets are solved by vibrating the concrete right after pouring and/or manually
pushing the mixture as is done with troweling. Concrete cover is always required for any steel
such that it’s not exposed to the atmosphere. In ferrocement, since the outer-most mesh is close
to the surface, it’s best to use galvanized steel.
The failure mode of ferrocement is different from standard reinforced concrete. Given a
large impact or point load, a thin shell will deform (sometimes buckle) and result in cracks over
a wide area. The structure will however remain intact. In contrast, reinforced concrete will
tend to break, crumble and delaminate, causing the structure to fail.
One of the first documented uses was in 1847, when Frenchman Joseph-Luis Lambot filed a
patent for a wire-reinforced boat. It wasn’t until the 1950s that ferrocement was used in large
public structures as well as houses. For example, there was a community of builders around
Santa Barbara, CA at this time who found the low cost as well as earthquake and fire resistance
6
CHAPTER 2. FERROCEMENT CONSTRUCTION
of ferrocement houses to be perfect for the region’s needs. More examples are described in the
following sections. Today, ferrocement is used mostly in countries with low labor costs, usually
for water tanks and houses.
2.1.1
Water tanks
Ferrocement water tanks have long been recognized for their longevity, low maintenance and
inexpensive materials of construction. As an example, a ferrocement tank manual describes the
construction procedure for a 60 m3 water tank [20]. The armature cutaway and finished wall
are shown in Fig. 2.1. The floor, cylindrical wall and dome roof consist of:
1. Reinforcing bars spaced in 40–50 cm squares. Rebar thickness is #3 (9.5 mm) or #4 (12.5 mm).
2. Welded wire attached on both sides of rebar. The wire is spaced in 15 cm squares and
5 mm thick.
3. Steel lath, or thin gauge expanded steel, is attached with hog rings to the armature’s inner
surface. Two layers of chicken wire can also be used.
4. Plaster sand mixed with cement and worked by hand and trowels into the armature
simultaneously from the inside and outside.
5. Coloring and waterproofing paint.
Figure 2.1: Wall section for ferrocement tanks and finished wall armature [18]
There are nuances to be aware of when creating the floor-wall and wall-roof joints, temporary
supports, penetrations, access hatch, etc. but in general construction requires little skilled labor.
It is a technique still used in countries where low material costs are important and labor is
inexpensive. Fiberglass Reinforced Plastic (FRP) has gained in popularity for small tanks and
steel plate for large tanks due to lower labor costs. Where the water is corrosive, an alternate
strategy is to cover a plastic tank with a thin ferrocement cover to protect from the sun and
extend lifetime considerably.
2.1.2
Boats
Boats were one of the original applications of ferrocement, extending into the 20th century. During both World Wars, ship builders were encouraged to use concrete to save steel. If constructed
properly, ferrocement boats are strong, low maintenance and of similar weight to composite and
wooden ones [27]. An amateur rush to DIY boat building in the 1970s and 80s, pushed by the
promise of cheap projects, led to many poorly-built boats and ferrocement’s poor reputation.
7
CHAPTER 2. FERROCEMENT CONSTRUCTION
This is one of the reasons why the technique is rarely in use today. Experienced builders and
quality inspectors have not been replaced over time and even a well-built boat is more difficult
to sell. Another perceived drawback is the uniqueness of each boat, as mechanized production
hasn’t advanced as compared to wood and composites. Nevertheless, it remains the cheapest
building technique for amateurs for boats longer than 8 m [26]. A few companies like Hartley
Boat Plans still sell DIY manuals and full-size patterns, advocating ferrocement.
The building technique is interesting to study because it is perhaps the strongest and highestquality ferrocement shell possible. It’s the most labor-intensive and contains the most reinforcement layers. A documented example of an 8.5 m boat built in South Carolina in the 1970s that
is still in service is presented here [1]:
1. Rebar rod trusses are laid out, welded and covered in mesh. Truss skeleton hangs from
ceiling of a building
2. Trusses are connected with spring steel wire, 50 mm spacing top to bottom, 100 mm side
to side. Welded at front and back of boat, otherwise wire tied. This creates a fairly dense
mesh
3. Two layers of chicken wire tied on top of spring steel mesh
4. Two layers of cross-welded wire mesh on top of chicken wire
5. Mortar with polyester resin is pushed through armature and troweled smooth
6. Finish coats of concrete applied and ground smooth
7. Paint and waterproofing
Figure 2.2: Ferrocement boat construction in the 1970s [1]
There are problems which arise with building boats this way. There can be eight or more
layers of mesh and the intent is to overlap layers such that largest hole is no bigger than a pencil
to improve strength and impact resistance. This dense armature is difficult to pack the mortar
into and air voids remain. Dry packing can also occur, where the mortar is dewatered through
pushing and doesn’t cure properly. It’s therefore not surprising that amateur boats tended to
be of poor quality.
An alternate technique called Laminated Ferrocement (LFC) was developed and patented in
the 1960s by Martin Iorns. Rather than the mortar packed into the armature, the idea behind
LFC is that the armature is packed into the mortar. First, a thin (3 mm) layer of mortar is
sprayed onto a mold of the hull. Expanded metal lath is placed on top and lightly pushed in.
Another thin mortar layer is sprayed on and the next layer of mesh set, rotated 90 degrees
8
CHAPTER 2. FERROCEMENT CONSTRUCTION
against the first. The process that Iorns used had five layers in total and a hull thickness of
19 mm. Air pockets and dry packing were eliminated. Labor was significantly reduced by not
building a skeleton frame or tying mesh together. This system underwent extensive testing for
commercial sea-worthiness certification by the U.S. Coast Guard in 1976, including tension,
compression, bending, shear, impact and fatigue [58].
LFC was the industry’s best chance to build with concrete. However, boatyards have moved
on to fiberglass and other materials, pushed by easier quality control, mechanization of production, public perception and insurance rates.
2.1.3
Houses
There are many inspiring ferrocement houses in existence. Architects like Antoni Gaudı́ have
left their mark with imaginative and playful creations. Ferrocement’s plasticity and strength
has been used for roofs covering large open areas without columns, furniture built into the
structure and sculptures, among many others that are difficult with other building methods.
Illustrated examples of what is possible are presented in Appendix A.
Contemporary ferrocement house builders are rare in the U.S. One example is Steve Kornher,
who builds mostly in Central Mexico and documents his projects under his company Flying
Concrete [35]. Steve’s architectural creativity is exactly the aim of this thesis. He prefers to
use light-weight concrete, where locally-available pumice is mixed with cement in a 8:1 ratio
by volume for walls and 5:1 for roofs. He offers a 400 year warranty on roofs and prefers a
vault design for strength. The typical roof section is shown in Fig. 2.3. The other advantages
of vaulted roofs are little formwork needed, the inherent compression-only load distribution,
simple geometry and pleasant interior space.
Figure 2.3: Roof section used by Flying Concrete (adapted from [35])
An interesting point is that thick rigid insulation can be incorporated into this design,
although it is not typically used in Mexican houses due to the mild climate. Another point
is that reinforcing bars are not used at all for smaller vaults. The armature is temporarily
supported by wood and shaped welded wire. The supports are re-used between vaults. There
are some nuances to consider when tying into the wall and incorporating a second floor, such as
casting an edge beam and nailing the mesh into the wall. The thickness of the strong concrete
increases to 100 mm if light-weight concrete is used. The bottom-most thin shell is placed first
so that the concrete above doesn’t sag the mesh under its weight. The advantage of light-weight
concrete is that it’s easier to work with and can even be nailed into.
Walls are typically constructed out of concrete blocks and reinforced. Arches over doors and
windows are also done with blocks. If a more elaborate window shape is needed, a ferrocement
mesh is used. Windows and doors are often custom-made; it’s important to deter theft with
9
CHAPTER 2. FERROCEMENT CONSTRUCTION
steel bars so it’s an additional challenge to create appealing designs that don’t look like jail bars
and fit into the architecture. The climate allows for single-pane windows and doors. Examples
of construction by Steve’s crew are shown in Fig. 2.4.
Figure 2.4: Examples of ferrocement construction from Flying Concrete [35]
Javier Senosiain of Arquitectura Orgánica also builds beautiful houses out of ferrocement (see
Appendix A for examples). Other builders have variations of these methods, by for example
building walls in ferrocement, not using light-weight concrete and installing standard windows.
However, the basic approach remains similar and is considered the “base” procedure to build
upon later in this chapter.
2.1.4
Large shells
Large shell structures can be built with the minimum of materials if the shape redistributes
loads into the membrane. In other words, the critical design concept in very large thin shells is
to eliminate bending moments. Where this isn’t possible, arches and beams must be built in to
take the bending loads. Catenary curves are used as much as possible in arches and shells to
distribute the weight in pure compression.
Italian Pier Luigi Nervi designed and built large enclosures with rib-reinforced shells, including aircraft hangars, the exhibition hall in Turin, Italy (Fig. 2.5a) and many sports arenas [47].
He understood that concrete properties vary outside the control of the builder, even if the
mixture is consistent. The geometry of his designs was simple and aesthetically pleasing; seemingly intricate designs were accomplished simple patterns. Prefabrication was used extensively,
including the rib trusses and ferrocement panels.
Félix Candela was a Spanish architect, engineer and builder who worked in Mexico. Like
Nervi, he was also a proponent of simplicity in shape and extensively used hyperbolic paraboloids
(see L’Oceanografic in Valencia, Spain in Fig. 2.5b). These doubly-curved shapes were very
strong, allowing for minimal use of concrete. One famous example, the restaurant Los Manatiales in Xochimilco built in 1958, has a shell span of 45 m and concrete thickness of only
4 cm [47]. The curved shapes were possible to define completely by straight lines, so standard
wooden boards could be used for formwork. Catenary vaulted roofs were also used extensively
in his work. Candela had access to cheap Mexican labor and limited mechanization on his
projects.
Swiss engineer Hans Isler is widely known for the thin shells he designed using fabric models.
The basic design was done by suspending a piece of fabric by the edges, letting the shape form
under its own weight and trimming the buckles. Isler noted that hanging revered shapes are
nature’s best solutions to a given statics problem and constraints [45]. Physical models were
precisely measured and laid out the construction, without the need for complex mathematics
or computers.
10
CHAPTER 2. FERROCEMENT CONSTRUCTION
(a) Nervi’s exhibition hall (1949)
(b) Candela’s L’Oceanografic (2002)
Figure 2.5: Large shell projects [47]
There are many other notable engineers of the 20th century. What makes large shell structures so interesting to study is each project’s unique approach and the improvements in builders’
techniques. Lessons such as strength in double curvature, prefabricated patterns and ribbed
shells carry over to ferrocement on a house scale.
2.1.5
Sculpture
Sculptures have been a popular application of ferrocement and perhaps the most experimental in
terms of techniques. Commercial examples range from large donuts to two story cowboy boots
and other such gimmicks to attract clients. Backyard artists create garden bridges, statues,
gazebos, benches, fake wood and many others. Since structural strength is not as critical as
buildings or boats, sculptures are the natural choice for exploring new ways of using ferrocement.
Notable examples include:
ˆ Fiberglass, hemp and other flexible meshes soaked in mortar and built up in small sheets
like paper maché
ˆ Complements to mesh reinforcement for difficult curves such as wire weaving and fibers
ˆ Foam cores carved by hand or hot wires
ˆ Foamed concrete used as a lightweight core, reinforced on surface
ˆ Various molding techniques such as painted silicone (for copying), earth and sand casting
as well as textiles
ˆ Coloring with acrylic or latex paint, acid staining and pigments
Many of these techniques carry over to structural work and are explored in more detail later
in the report.
2.2
Competing building systems
The purpose of investigating other building methods is to gain more insights in how ferrocement
may become simpler and more economical. Several different systems are described here, chosen
either for their prevalence or ingenuity.
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CHAPTER 2. FERROCEMENT CONSTRUCTION
2.2.1
Wooden frame building
Wooden frame building, also referred to as stick building, is the most common type of house
construction in the U.S. The reason is that fast-growth wood is cheaply available, easy to work
with in terms of material softness and low weight, design simplicity and room for remodeling.
Code compliance is easy to achieve with extensive worker and regulatory experience as well as
established, pre-engineered design details. Light-weight construction does well in earthquakes.
Standard wood beam dimensions reduce costs of materials. Tools for construction are relatively
few and inexpensive. Prefabrication is possible and generally frame construction is quick.
Briefly described, poured concrete or blocks are used for the foundation and basement. Walls
are constructed of wooden frame boards (studs) which are usually 2 x 4 inches (51 x 102 mm)
and spaced at 16 inches (406 mm). Walls are assembled as a box on the ground and stood up.
Exterior walls are covered with a structural wood panel such as plywood to increase rigidity. A
vapor barrier covers the plywood and siding is attached. On the inside, insulation fills the gaps
between the studs and gypsum drywall is the inside surface. Roofs are sloped, covered with
plywood and asphalt shingles. Triangular A-frames are the most common roof structure, also
assembled on a flat surface. The floor may be a concrete slab or wooden joists covered with
plywood. An example frame and details are shown in Fig. 2.6.
Figure 2.6: Wooden frame cutaway and example structure [10]
Unfortunately, stick building has many serious disadvantages that motivate a different approach. The most significant is the low strength against high winds. Every year the hurricane
and tornado seasons destroy houses in the U.S. despite building code provisions, sometimes even
entire towns [11]. Many more are heavily damaged by flying debris and fallen trees. Another
disadvantage is vulnerability to moisture and significant damage from flooding as well as the
danger of mold growth. Insects such as termites are a nuisance in some areas and increase the
cost of maintenance. Fire resistance is relatively poor even with fire retardants impregnated
into the wood, requiring expensive sprinkler systems in public buildings and more expensive
insurance. Noise from footsteps and floor creaking is common.
Although not necessarily inherent to wooden houses, thermal performance of the average
house is poor. Air tightness relies on a vapor barrier liner underneath the exterior siding and
can easily be installed incorrectly or damaged. None of the other wall components provide air
tightness unless there is a brick or plaster facade. Insulation is typically fiberglass or mineral
wool in-between studs. By design this space is usually limited to 100 mm and thermal bridging
of the studs is not accounted for. External rigid insulation can correct the problem but is
relatively rare. Insulation of the roof is usually done by actually insulating the top-most ceiling
12
CHAPTER 2. FERROCEMENT CONSTRUCTION
and leaving the attic space uninsulated. This can also be corrected by also adding insulation
to the roof framing.
2.2.2
Structural Insulated Panels
Structural Insulated Panels (SIP) is a concrete-foam building system, where rigid foam serves
as insulation and shotcrete “formwork”. Foam is typically expanded polystyrene (EPS) or
extruded polystyrene (XPS) molded into panels by a manufacturer. This core is 100–300 mm
thick. Plastic or metal tie rods penetrate the foam and hold steel mesh on both sides. Depending
on whether the composite sandwich will be required to handle large bending loads, the ties can
be diagonal such as the MetRock Panels [23]. Steel mesh can vary and can be 25 x 25 mm and
1.6 mm thick [23]. Service conduits and outlets are inserted into the foam where needed. Once
the foam panels are assembled into a building structure, 40–50 mm of shotcrete is applied to
both sides of the foam followed by a surface finish. The final assembly is shown in Fig. 2.7.
Reinforcing bars and thicker concrete layers are used only selectively in critical parts of the
structure.
Figure 2.7: SIP system [53]
This proven building technique enjoys commercial success in the U.S., especially for stores,
restaurants and similar applications. The foam structure is quickly erected and when combined
with an efficient shotcrete contractor the building is quickly completed. Advantages include fire
resistance, strength against earthquakes and strong winds, energy efficiency through insulation
and envelope tightness, low maintenance, flexibility in interior/exterior surface finish, noise
absorption and favorable total lifetime cost. All parts of the structure use the same system floors, wall, roofs, columns and arches. There is flexibility in decorative features as the foam
can be cut to any (flat) shape easily. Building code conformity is long-established. Interior
concrete serves as thermal mass.
This system can be used by the owner-builder without even the need to purchase specialty
panels. Rigid foam boards and steel mesh are readily accessible. Foam can be cut to the desired
design. Specialty mesh ties and spacers are available (and even patented) from companies like
SIPcrete, but these also be made by hand from plastic or metal rods. Finally, shotcrete does
not require specialty contractors. One can spray a concrete mix with a tool like the Stucco
Sprayer (see Fig. 2.8) offered by MortarSprayer.com [43] connected to an air compressor. It’s
possible to apply up to 5 m3 /hr of concrete this way.
The key disadvantage of this building system is that non-flat surfaces require special foam
molding. Architectural freedom is therefore limited without changing the structure of the
insulation. Such changes are explored later in this chapter.
Concrete isn’t the only type of skin commercially available for SIPs. Oriented strand
boards (OSB) are made of fast-growing trees and replace concrete on both sides of the foam.
13
CHAPTER 2. FERROCEMENT CONSTRUCTION
Figure 2.8: Stucco Sprayer for DIY shotcrete [43]
They are purchased as a complete panel. The advantage of this type of SIP is less labor: after
erecting the panels and sealing the joints, external siding can be directly attached and the interior surface finished. Companies like Thermocore [57] offer custom-cut panels with window,
door and conduit provisions. They claim that a house shell can be erected in 2 days. The
wood’s softness is an advantage when attaching to it. The disadvantages of using wood are
reduced fire resistance, reduced strength and vulnerability to moisture and insects.
2.2.3
Insulated Concrete Forms
Insulated Concrete Forms (ICF) are SIPs in reverse: hollow foam blocks are stacked to form walls
and concrete is poured inside the cavity as shown in Fig. 2.9. It is effectively a quicker method
of building with conventional reinforced concrete, as the labor-intensive wooden formwork is
unnecessary. The thickness of the foam varies based on the desired insulation value, but is a
minimum of around 50 mm. It is common to see commercial systems with thicker outside foam,
for example 100 mm. Steel or plastic ties hold the foam blocks together as well as support
reinforcing bars. A concrete pump truck is ordered once each story of the building is assembled
from foam blocks. A mesh needs to be attached to the foam before plaster or stucco is applied.
Figure 2.9: ICF system wall and slab [48]
ICFs are most appropriate for basement walls, where the outside foam can be covered in
plastic and the system saves on formwork. It is also apparently a better system with respect to
sound attenuation inside the building than SIP.
14
CHAPTER 2. FERROCEMENT CONSTRUCTION
A disadvantage of this system is the cost of ordering a concrete pump mixer and pump trucks
multiple times during construction. Great care must be taken to avoid weaknesses in the foam
blocks because it’s possible to blow out a wall entirely during a pour. Another disadvantage
when compared to SIPs is that the concrete isn’t as effective as a thermal mass. Also, there
is more work to finish the foam surfaces as compared to the SIP concrete surfaces ready for
plaster or stucco. Construction in general is more complex than SIP.
The roof cannot use this system, unless it is a flat roof. Architectural details also require
another technique such as SIP. Overall, it is difficult to recommend the ICF system over SIP.
2.2.4
Steel-framed ferrocement
The “ferrocement” construction technique that’s commercialized by companies such as Amcor [3] is similar to wooden stick building, except that the framing is prefabricated using lightgauge steel. A layer of exterior mesh is attached such that a layer of concrete may be applied to
form a thin, monocoque shell as shown in Fig. 2.10. The erection of the steel frame in the field
is very quick and requires little labor to assemble. Window openings and conduit provisions
are part of each panel. The same system is used for floors, walls, arches, columns and roofs.
The builder is responsible for selecting insulation, such as fiberglass that’s conveniently pre-cut
for wooden frames. The finished building has the advantages of being highly fire-resistant,
light-weight and resistant to flying debris. Am-cor claims that the system is competitive in
price with wooden framing and can be assembled quicker, providing a number of residential
and commercial built examples. This system is not strictly ferrocement because only one layer
of steel mesh is used. However, it is indeed a simple, clever and economical solution for an
owner-builder.
Figure 2.10: Am-cor panel system [3]
It’s noteworthy that Am-cor’s chief architect, Angus W. Macdonald, has experimented with
a wide variety of meshes and types of construction in general. He notes that expanded metal
laths have the most favorable cement interlocking and provide a large surface area for the cement
to interact with. This is critical for a one layer system, but this experience is also useful for
multi-layer systems where the type of mesh often isn’t seen as important.
Disadvantages of this system include thermal bridging of the steel frame, limited customization and low internal thermal mass. Insulation in-between wall beams must be supplemented
by, for example, rigid insulation on the inside to completely cover the steel’s conduction path.
Thermal mass inside must be done separately, adding cost. The lack of architectural freedom is
difficult to overcome, as the prefabricated shapes are not intended to be curved or customized
15
CHAPTER 2. FERROCEMENT CONSTRUCTION
to a great extent by the manufacturer. The structural strength of the structure appears to be
engineered into the steel beams to conform to IRC and other codes.
2.2.5
Earthbags
The earthbag system involves building walls and sometimes roof using polypropylene bags or
tubes filled with local soil [34]. Bags and tubes with misprints are purchased inexpensively. The
bags are stacked like bricks on top of a concrete or stone foundation. Each bag is tampered
after being placed. Barbed wire strands are placed horizontally in-between each course to bond
the bags together and add tensile strength. Vertical reinforcing bars are spaced every meter.
A reinforced concrete ringbeam is poured at the top of the wall if a wooden framed roof is
installed. Otherwise, the bags are shaped into a dome. Wooden frames are used for doors
and windows. The interior and exterior are plastered with a mix of straw, clay and sand and
painted with a lime mixture to resist water. Examples of houses under construction are shown
in Fig. 2.11.
Figure 2.11: Earthbag houses under construction [25]
The entire process relies on few external materials and very inexpensive, ecologically-friendly
buildings can be constructed this way. Since rounded shapes are more stable, earthbag buildings
heavily incorporate them. In some cases, building code compliance is achieved by incorporating
conventional wooden/concrete posts and beams. Thermal mass is very high, which makes for
comfortable buildings in hot climates. The earthbag system is very similar to traditional adobe.
In climates where insulation is necessary, an additional external layer of bags can be incorporated filled with insulation. Several projects have used insulation fill directly, such as volcanic
rock, which solves the problem with one layer of bags. Since material weight is reduced, the
building process is also quicker. The projects built this way have been in areas not regulated by
building codes, however. The concept of insulation-filled bags is explored later in this chapter.
The disadvantage is that manual labor is extensive with earthbags, with relatively few
opportunities for automation. Moisture must be carefully controlled by roof overhangs and
good drainage.
2.2.6
Earthships
Earthship is a building system popularized by Michael Reynolds that strives for completely
off-grid houses [51]. The defining characteristic is the use of automotive tires recovered from
landfills to build the walls. Tires are stacked like bricks and each tire is filled with soil from
the site and compacted. Non load-bearing walls are often built with bottles and soil mortar,
allowing for curved and colorful designs. The walls are backfilled with soil from the north,
making the roof easily accessible. Strength of the walls is enormous, as is the thermal mass.
16
CHAPTER 2. FERROCEMENT CONSTRUCTION
Walls are plastered and painted on the interior. Flooring is placed directly on compacted soil
after good drainage is ensured. Roofing consists of tree log beams and boards, plastered on the
outside. In desert climates of the South-Western U.S. where Earthships are most commonly
found, insulation is not even necessary as thermal mass evens out the hot days and cold nights.
The earth’s naturally stable temperature is also conducted through the thick walls.
More than just an ecological structure, the house as a complete system is covered by the
Earthship system as shown in Fig. 2.12. Building floorplans are based on passive solar concepts.
The buildings stretch East-West, are covered completely in the North and have all windows
facing the South. This allows for copious amounts of natural light and heating. The roof and
window shutters are designed to control the amount of sunshine reaching the interior, based
on the local climate. An internal greenhouse is right behind the large windows, resulting in a
source of food and cooling from the plants. Usually a windowed wall separates the greenhouse
corridor space from the interior to mitigate solar heating and serve as a thermal barrier.
Figure 2.12: Earthship system [21]
Rainwater is collected into a cistern and filtered for consumption. Water is recycled several
times: gray water from sinks and showers feeds botanical cells and finds re-use in toilets. Black
water is run through another set of botanical cells. The result is that the greenhouse and outside
plants are nourished, while eliminating the need for a sewage connection.
Electricity is generated using solar PV panels facing the south and stored in batteries.
Additional generators such as wind turbines are sometimes added in remote areas. Direct
current appliances reduce the total electricity use. HVAC energy needs are low.
Earthships have been built in a variety of climates. Wet regions require additional drainage
provisions, more roof cover and water-resistant plaster. Cold climates incorporate insulation
behind the tire wall. Architectural flexibility is fairly extensive due to the use of soil and/or
concrete shaped by hand. Built projects exhibit curved walls, colored glass embedded in walls,
arches and other creative features.
Manual labor required to construct the structure is extensive, although relatively unskilled.
Filling tires with soil and compacting with sledge hammers is perhaps the single most timeconsuming part of construction. The structure is most often built by the owners and the hired
17
CHAPTER 2. FERROCEMENT CONSTRUCTION
help of 5–10 laborers.
2.2.7
Inflatable domes
Monolithic concrete domes are commercially available in many sizes and configurations. Aside
from the high strength of an ideal dome shape, other attractive features include fire safety,
energy efficiency from incorporated insulation, fast erection and low price. Commercial solutions
generally follow the same construction procedure:
1. A reinforced concrete ringbeam foundation is built. The ringbeam transfers loads to the
ground, provides stiffness and holds the airform during construction.
2. The airform membrane is attached to the ringbeam, sealed and inflated with fans.
3. Polyurethane foam insulation is sprayed onto the membrane from inside the airform. The
insulation has the added function of stiffening and stabilizing the airform. It is 50–100 mm
thick.
4. Reinforcement bars are attached to the inside of the insulated shell. Windows and door
forms are attached to the polyurethane shell to block off the shotcrete.
5. Shotcrete is sprayed from the bottom up in rings inside the structure in layers, achieving
approximately 75 mm thickness.
6. Air pressure is removed once shotcrete cures. Windows and doors are cut out. Overhangs
are attached to the outside.
The finished structure is shown in Fig. 2.13. Hiring specialty contractors to spray foam
and shotcrete is the most expensive part of the construction process, but the contractors can
be efficient due to the simple geometry. The owner-builder can complete all other tasks with
unskilled labor. Various cost estimates place concrete domes as comparable to wooden stickbuilt construction or a bit more expensive. The total cost is considered much lower due to low
maintenance, long lifetime, energy efficiency and resistance to natural disasters.
Figure 2.13: Dome structure cutaway [42]
Disadvantages of concrete domes built this way include architectural limitations, echo propagation inside and sub-optimal use of interior volume. A large dome will have a two story
18
CHAPTER 2. FERROCEMENT CONSTRUCTION
arrangement of rooms around the perimeter and the tall center of the dome can be tricky to
utilize.
There are several lessons learned which may be applied to ferrocement. The concept of
inflatable formwork needn’t be restricted to dome shapes. The membrane can be made cheaply
to custom shapes. The option of creating smaller inflatable forms can be considered, where the
outside of the form is used instead. Rigid insulation that’s sprayed on will be revisited later in
Section 2.3.1. The shotcrete shell’s strength, durability and versatility are proven by decades
of dome building, even without the benefit of mesh reinforcement. Dome architecture has been
made more interesting in projects where large sections are cut out, blurring the transition with
outdoor space and a tarp-like roof effect.
2.2.8
Molded foam
International Dome House Co. of Japan [33] produces a system where molded EPS blocks
are assembled directly into dome houses. Although limited to one story, the buildings can
be assembled very quickly with minimal labor and by design provide great insulation. The
interlocking foam blocks are light-weight (40–65 kg apiece) and perfect for earthquake-prone
Japan. The system adheres to local building codes despite not using any structural beams. The
175 mm thick foam material includes fire retardants. The company claims very low costs of
transportation and construction, although exact numbers were difficult to find.
The number of foam shapes available is low, but many configurations are possible due to
the modular design as shown in Fig. 2.14. Buildings can even be linked together using the basic
foam shapes, allowing for star-like house layouts. The attractive price and ease of painting the
exterior has also led to adoption by small stores and kiosks.
Figure 2.14: International Dome House system [33]
Attempts to contact the company to find out if the system is available outside of Japan
were unsuccessful. This highlights a key disadvantage of dependence on the manufacturer. It’s
likely that the system would need to be structurally reinforced to meet IRC, which is why
it isn’t available in North America or Europe. However, there is an exciting opportunity if
a concrete shell is added. The foam’s inherent weaknesses in terms of debris protection and
fire resistance can be corrected. By applying a layer of shotcrete on the interior, this system
effectively becomes similar to monolithic domes described in §2.2.7 with the key advantage of
no inflation equipment or foam spraying contractors. Shotcrete application would also become
simpler, as the foam is much more rigid. A stucco or thin concrete shell exterior would further
protect against debris in strong winds.
2.2.9
Concrete printing
Contour crafting is a concrete printing technique being developed at Loughborough University in
the UK. The concept of additive printing is the same as a 3D plastic printer: a thick, fast-setting
concrete mix flows out of a computer-controlled nozzle (see Fig. 2.15a). Fiber reinforcement is
incorporated into the concrete mix. The structure is printed in layers and complex shapes can
be created. The ultimate goal of this research is to produce a gantry crane that could be placed
19
CHAPTER 2. FERROCEMENT CONSTRUCTION
on site and an entire building printed as shown in Fig. 2.15b. A high level of automation is the
goal.
(a) Wall printing
(b) Gantry printing house
Figure 2.15: Contour crafting concrete printing [40]
The technology is still in its infancy, but has already been used to produce intricate sculptures and building wall concepts. It is not yet commercialized and the first printed building is
yet to be completed. The high level of automation is attractive, but the technique is not suitable
for ferrocement. Objects can be embedded in the wall, though a thin shell with multiple layers
of steel does not appear to be feasible to print. Reinforcement will need to be improved to
provide safe housing and allow for thinner concrete sections. Using this technology to prefabricate complex wall and roof panels may well become an attractive alternative to on-site gantry
printing.
2.2.10
Concrete cloth
Concrete Cloth is a product made by British company Concrete Canvas that is a fabric impregnated with a dry cement mixture. The fabric consists of a sandwich of three layers, where
the cement powder is constrained by tightly-knit upper and lower faces (see Fig. 2.16a). The
cloth is engineered such that it can be rolled out like a carpet and soaked in water. The fabric
is available in thicknesses of 5, 8 and 13 mm in 1 m rolls and has a rated 10 day compression
strength of 40 MPa [17]. It was originally developed for applications like ditch lining, slope
protection and tank lining, where only a minimum of labor is required to cover areas quickly.
There is no danger of over-watering the cloth and 80 percent of strength is reached after only
24 hours. Cost is in the range of $70-120 per m2 , depending on thickness and amount purchased.
(a) Fabric structure
(b) Inflated shelter
Figure 2.16: Concrete cloth [16]
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CHAPTER 2. FERROCEMENT CONSTRUCTION
Concrete cloth has been used on housing projects, mainly as a skin on an existing building
for the purpose of hurricane protection. It protects the building very effectively against flying
debris. Another housing application has been emergency shelters and military outposts, where
the cloth is sewn at the factory to be an inflatable form. Installation on-site only requires two
hours, where the fabric is rolled out, door installed, fabric anchored, inflated and soaked with
water. It becomes a usable shelter after one day (see Fig. 2.16b). The 54 m2 version of the
shelter costs about $35,000.
The clear advantage of this fabric is how quickly it can be deployed and adapted to any shape.
In ferrocement, it could replace the outer-most steel mesh layers and also serve as formwork. For
example, a shell could be built by attaching the fabric to both sides of a reinforcing bar skeleton
and fiber-reinforced concrete used as the middle binder. Another attractive application is for
smaller details and joints where bending steel mesh is arduous, such as column-roof junctions,
furniture and artistic features.
The material is fairly new to market, having been patented in 2010. Direct competition
is non-existent, which is reflected in the price. There is considerable engineering involved in
its manufacturing, including fabric fiber spacing, binding of the three layers and selection of
cement additives. Not surprisingly, it hasn’t been reproduced by amateurs at this point.
2.3
Opportunities for improving ferrocement
2.3.1
Insulation
There are several insulation strategies explored here to achieve performance similar to PassivHaus. Two thicknesses are calculated, depending on the U-value desired, and presented in
Tab. 2.1. Fiberglass and cellulose lose a significant amount of insulating value when damp, so
they are only listed here for reference.
Table 2.1: Insulation thicknesses
Type
Sprayed polyurethane
Expanded polystyrene (EPS)
Extruded polystyrene (XPS)
Polyisocyanurate
Fiberglass batt, high density
Concrete with EPS beads
Cellulose
Papercrete
Perlite
Vermiculite
Thickness (cm) needed for U-value:
0.10 W/m2 K
0.20 W/m2 K
23
36
29
20
36
85-144
41
48–72
53
68
12
18
14
10
18
42-72
21
24–36
27
34
Source
[14]
[14]
[14]
[14]
[61]
[61, 49]
[14]
[39]
[28]
[14]
Rigid foam
Closed-cell rigid foam is the most effective type of insulation. It is readily available in 1.2 x 2.4 m
boards and various thicknesses. Polyisocyanurate is more expensive than EPS or XPS, but its
performance and lower required thickness can justify the extra cost. The problems in using
these boards with ferrocement include the limited flexibility of the boards to adjust to curved
surfaces, gluing consecutive layers to the concrete and other foam before the final lath can be
21
CHAPTER 2. FERROCEMENT CONSTRUCTION
attached as well as difficult penetration of lath ties through the foam. All of these challenges
can be solved, but the installation process becomes a labor-intensive combination of cutting,
gluing, hand-fitting and tape sealing each layer. The performance may in the end be similar to
simply using loose fill insulation.
Sprayed foam
Sprayed polyurethane and icynene have excellent insulating performance, adhere well to concrete, are airtight, are quickly applied and form arbitrary shapes. However, it is the most
expensive solution. DIY kits such as Foam It Green cost $445 per m3 of finished foam [24].
With a desired thickness of 20 cm, a modest family house would cost $20–30,000 to cover.
Contractors cost more for insulating existing homes, but may be cheaper in spraying one thick
continuous shell. For comparison, XPS boards cost $250–350 per m3 but require more labor to
install in this case.
Light-weight concrete
Light-weight aggregates can be mixed with cement to provide insulating concrete. Internal
thermal bridging guarantees that the U-value will never match pure insulation and must be
thicker. However, locally available materials such as volcanic rock (scoria), perlite, pumice,
vermiculite or recycled EPS make this a potentially inexpensive option. In ferrocement, it
could be used as an external shell with metal lath attached to the structure in stages to provide
formwork. Rastra’s EPS blocks, for example, are made of 85 percent recycled EPS [49].
Papercrete is composed of waste paper shredded and mixed with Portland cement. A mixture with paper and cement in equal weight shrinks by 5 percent [39]. Cement can be avoided
altogether by substituting clay soil and lime, but the increased shrinkage and drying time makes
it difficult to work with. There is a range of materials that can be mixed with, including ground
polystyrene. The technique is attractive because of its very low cost, including DIY mixers that
are slowly towed trailers with a lawnmower blade powered by the wheels. Papercrete is not
recognized as a structural material in the IRC despite its wood-like strength. A thin, sealed
protective concrete shell is needed to protect from moisture.
Aircrete or cellular concrete is another type of light-weight concrete that’s mixed with foaming agents. As it cures, the foam creates small air pockets that are 50–85 percent by volume
in the finished product. The mixture is very fluid before it cures and requires enclosed formwork. Commercially available aircrete comes in factory-cast blocks, as spray foam installed by
contractors or can be made on-site by the builder. Contractors cost significantly more than
spray foam. Foam agents can be purchased to use on-site but require special foam generator
equipment. If the volume of cast parts is high on a project, the expense may be justified because
casts are easy to handle and do not develop shrinkage cracks.
Bagged insulation
Earthbag builders have experimented the most with plastic bags filled with materials such as
shredded EPS, perlite and scoria. The bags (or tubes) can be stacked like bricks and lightly
tampered, forming a thick shell next to the ferrocement structure. String ties attached to the
ferrocement can hold a metal lath on the outside of these bags for a final thin concrete layer.
The advantage of this approach is that recycled or low-cost local materials can be used and the
bags easily follow the curves of the structure. Labor in filling and stacking the bags is quick. Air
voids need to be avoided, especially between bags. It may be necessary to use a small amount
of household spray foam.
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CHAPTER 2. FERROCEMENT CONSTRUCTION
It should be expected that a material like shredded EPS will not perform as well as a rigid
board. However, with tight stacking, there’s no reason why it shouldn’t perform well. The
advantages in cost and simplicity make this a very attractive solution.
2.3.2
Textile forming
Textile forming refers to the replacement of rigid forms by fabric materials. The fabric deflects as
concrete is poured or sprayed on, leading to a variety of tension geometry. There is considerable
freedom in the shapes possible in stitching the fabric but also in applying local tension or
constraints. Columns are made with closed forms which are stitched before pouring and only
require temporary supports. Wall panels are cast by horizontally stretching a fabric in a frame
and plastering.
One of the few research groups active in this area today is C.A.S.T.1 at the University
of Manitoba. They use tough, inexpensive and re-usable polypropylene geotextiles originally
intended for soil stabilization [64]. The fabric is slightly porous, which allows excess water
to escape the form during curing. An inner liner of stretch fabric, such as Spandex, can be
used for artistic effects. The geotextile’s open seam is tied in a variety of ways when columns
are constructed. When the fabric is removed after curing, the surface finish is of very high
quality and does not require additional plastering. Incorporating fiberglass mesh and fiber
reinforcement allows for casting thin shells without having to form steel or corrosion protection.
Molds for repetitive casting can be made by using a fabric which is smooth plastic on one side
and fuzzy carpet-like on the other that concrete adheres to.
The group has made curtain-like wall panels, double curvature beams, catenary barrel vaults
and many types of columns. Several examples are shown in Fig. 2.17. Unlike Hans Isler’s large
shell designs, the buckles of the fabric are left intact. Most of the columns are not reinforced
with more than fiber, although it would be possible to wrap a steel armature in fabric. The
fabric’s porosity would allow for using a wetter concrete mixture and thorough vibration to
ensure full penetration into the armature. Cutouts can be incorporated in the geotextile for
local bulges, where only the inner stretch liner resists wet concrete pressure.
Figure 2.17: Fabric forming at C.A.S.T. [64]
Fabric forming lends itself to prefabrication as well as laminating with several layers of
non-metallic mesh. Forms for roofs should be cast upside-down so that they naturally take
on a catenary shape. Field casting would be most suitable for tree-like columns. An entire
building could be cast in sections, although it becomes apparent that matching edge geometry
on complex curves is difficult. The only reasonable solution would be to design with straight
joints in mind that are consistent for the entire shell. It would not be practical to create more
1
Centre for Architectural Structures and Technology
23
CHAPTER 2. FERROCEMENT CONSTRUCTION
than a few curved beams for the cast’s frame. Accurate modeling of the final shape is tricky,
especially local buckling. These difficulties are worth solving, however, because of the low cost
of this type of formwork.
2.3.3
Non-metallic reinforcement
The armature is a critical component of ferrocement but requires tedious labor to attach and tie.
Non-metallic reinforcement has the advantage of corrosion resistance and cloth-like flexibility,
allowing for quicker setting and using less concrete. Fiber Reinforced Polymer (FRP) refers to
glass, carbon or aramid fibers embedded in a polymer resin and cast as bars, meshes and small
fibers [4]. A recently developed material is basalt, which is volcanic rock extruded into fibers
at 1400°C and mixed with resin to form rebar rods, thin ropes and mesh [54]. Basalt has a
number of relevant advantages over steel:
ˆ Twice the tensile strength of steel
ˆ Five times lighter than steel
ˆ Fire-resistant - does not lose strength until 980°C
ˆ Corrosion resistant to acids
ˆ Chemical adhesion to concrete in addition to mechanical
ˆ Low thermal conductivity
ˆ Meshes and thin rebars are very flexible
ˆ Cheap raw material should lead to lower cost with more mass production2
ˆ Immune to UV radiation (unlike fiberglass)
Basalt production is still low-volume, but the material holds advantages over other FRP
types for its resistance to alkalinity and lower cost. Research has been limited, although basalt
beams were found to perform favorably according to ACI 440 [46]. ACI 440 is a companion
code to ACI 318 which deals with FRP reinforcement [6].
Thin basalt rebar is as flexible as a rope, which creates intriguing possibilities such as
overlaying on top of an inflated form. Laminating complex curves in a mold also becomes much
easier with the mesh. Since concrete cover for corrosion protection isn’t necessary, shells can be
built with the LFC technique to be very thin and still include the necessary reinforcement to
comply with ACI 318 and ACI 440.
2.3.4
Fiber reinforcement
Concrete durability through resistance to cracks can be achieved by adding fibers to the mortar
mix. Fibers can be made of steel, plastics (polypropylene, nylon, polyester), glass or even natural
materials. Fibers are usually 10–75 mm long, less than a millimeter thick and come in a variety
of shapes [4]. They are inexpensive and have the additional benefit of adding mechanical bonds
between cold joints. Crack widths are reduced and shear and impact resistance improved. Many
ferrocement builders incorporate them, especially in sculptural projects where reinforcement
isn’t close to the surface. If a synthetic mesh (such as fiberglass or basalt) is used close to
the surface, there is less benefit. Fibers aren’t counted on for strength, however, and do not
contribute to the reinforcement strength in design calculations.
2
Currently, the cost is substantially more than plain steel rebar but cheaper than stainless steel
24
CHAPTER 2. FERROCEMENT CONSTRUCTION
2.3.5
Custom inflated forms
Inflated forms can be fabricated in any shape, anchored to the foundation, pressurized and frozen
with spray-on insulation. One example project is the Mexican Whale House by Arquitectura
Orgánica (see Fig. 2.18). Unlike the monolithic domes presented in §2.2.7, this project sprayed
on the outside of the balloon, which allowed for re-using the cloth and removed the need for
overhead spraying. There are numerous examples of DIY inflated domes made with nylonreinforced vinyl skin and joined with vinyl adhesive. Modern CAD software makes it easy to
get fabric dimensions by flattening a structure’s shape.
Figure 2.18: Deflated and inflated form of the Mexican Whale House. Yellow coat on inflated
form is the start of polyurethane foam [52]
Structures designed with inflation in mind automatically take on strength in compression.
The fabric may be attached to ribs and columns to force a local shape and provide attachment
to supports. If one goes to the cost and trouble of creating an inflating form, the armature and
concreting procedure should take advantage of the formwork.
One solution is to adopt the monolithic dome approach and ignore ferrocement. After
inflating the fabric, foam would be sprayed on the outside to about 75 mm thickness and foam
cutouts attached inside. A rebar skeleton would be bent into shape inside the foam, offset by
13 mm from the foam and fiber-reinforced mortar sprayed on. The resulting shell would need to
be at least 50 mm thick per ACI 318 to provide adequate concrete cover. A fiberglass mesh may
be set into the interior surface to reduce cracking. The outside of the foam would be sprayed
to final thickness, re-using foam from cutouts. The exterior would best be done using LFC,
spraying mortar and setting three layers of mesh. This way, the external shell would be strong
enough to support any additional architectural features.
Another solution would be to take full advantage of LFC. After the initial foam layer is set
and fabric removed, the foam which is beyond vertical would be cut into panels and laid on the
ground. These panels would be covered by mortar and metal lath to a thickness of 19 mm.
Removing the panels is necessary because attaching mortar to the foam overhead would be
difficult without the risk of delamination. Mesh on the edges would be left exposed so that
cold joints can be completed when the panels are re-connected. This approach could even lead
to prefabrication to a certain extent. Beams could also be made with LFC with foam cores.
Non-metallic meshes like basalt are more flexible and laying them into a curved form would be
easier. The code approval would be more difficult than the technique above.
Incorporating Concrete Cloth on one or both sides of the foam is tempting because it’s very
flexible and sets quickly. It would be adequate as the exterior surface if quick construction is the
priority. Its strength probably shouldn’t be relied on entirely for structural support and would
need to be reinforced. Its expense could be justified since spray-on foam wouldn’t be necessary.
The cheapest and thinnest Concrete Cloth would be used as the inflated fabric, since one side
25
CHAPTER 2. FERROCEMENT CONSTRUCTION
is air-tight. Once cured, laminate layers would be built up on the outside of this layer. Bagged
insulation would be placed before a final exterior LFC shell is formed. This approach reduces
costs from spray foam, although it complicates fabricating the tough fabric to be inflatable and
incorporate penetrations without cutting the cured layer.
A technique being developed by Monolithic Constructors is the uninsulated EcoShell [42],
which is built by inflating an air form and spraying shotcrete directly on the outside. A thin
layer is set and basalt ropes are wrapped around the structure at 200 mm spacing. Once a
reinforcing grid is finished, another layer of shotcrete is sprayed. The final thickness is about
38 mm. This provides the same advantages as using Concrete Cloth at a lower cost. The single
grid of ropes can be complemented by more mesh layers to create a laminate.
2.3.6
Prefabrication and automation
Conventional ferrocement offers little opportunity for prefabrication and automation. The skeleton bar may be accurately bent in a shop and power tools used in the field to assemble the
armature. Shotcrete saves on hand packing. The tedious work of attaching each mesh layer to
avoid overlaps as well as tying layers together is not easily automated.
Laminating lends itself much more to prefabrication, although it requires a mold. The mold
can be made of foam as described above to achieve a composite sandwich similar to SIP. Textile
forming is convenient for shapes for consistent edge shapes, which isn’t the case here.
2.4
Proposed building techniques
The purpose of this section is to challenge each step of conventional ferrocement house construction and incorporate ideas for improvements. Two very different shell building systems are
described here: armature-based and mold-based. Other aspects, such as foundation, columns,
sculptures, sub-floors and internal walls are the same for both.
2.4.1
Foundation
The only type of foundation considered here is the on-grade insulated slab with spread footings
under the walls. If not insulated, the footings are required by code to be under the local soil frost
line to avoid frost heave damage. Starter bars are embedded in the footings to attach the wall
armature and concrete later. The footings are reinforced with bars and the slab is reinforced
with welded wire. All utilities, such as water pipes and conduits must be placed beforehand.
There are several alternatives to concrete footings, such as using stones or earthbags. These
options are not explored here since the pouring of concrete for the slab is still required.
Slab foundations are very common in house construction. The only challenge lies in insulation: typically rigid insulation is placed under the slab but the footings create a significant
thermal bridge. A suitable commercial solution is found in Irish company’s KORE system,
which claims to have the lowest losses on the market and has been approved by the PassivHaus
Institute [2]. The system includes EPS blocks under the entire foundation as shown in Fig. 2.19.
Floor heating tubes can be set into the slab.
The slab can be floating type, or monolithic. For a double shell wall, double footing at
external walls is necessary to completely break the thermal bridge. EPS-100 and EPS-300
denote the load bearing capacity of the insulation. According to an engineering report done for
the foundations of PassivHaus using this system, the bearing capacity of EPS-100 is 48 kPa3 and
EPS-300 is 120 kPa [32]. Standard EPS boards rated according to ASTM C578 are available
3
The maximum bearing load is based on 1 percent compression of the insulation. ASTM-C578 ratings are
based on 10 percent.
26
CHAPTER 2. FERROCEMENT CONSTRUCTION
Figure 2.19: Slab insulation detail with molded EPS footing forms
in the American market, with ratings of 35–276 kPa. Loads on the insulation must be checked
during design.
KORE literature claims a U-value of 0.08 W/m2 -K for this floor system. They typically
install a Radon barrier below the concrete. EPS does not transmit moisture, so there’s no need
for an additional external barrier.
This design qualifies as frost-protected shallow foundation, which allows building above
the frost line in northern climates. That reduces the amount of concrete that needs to be
used. The insulation is also used as integrated formwork, which reduces labor and waste as
compared to a wooden framework. The labor in attaching insulation later is also eliminated.
Shallow foundations are commonplace in Scandinavia [44]. They are addressed specifically in
IRC §R403.3, which deals completely with frost protection rather than energy efficiency.
2.4.2
Columns
Columns follow the slab in construction sequence. A hybrid column construction technique
is considered here that merges a steel armature with textile forming. A steel rebar skeleton
is erected in accordance to standard details and connected to the starter bars in the slab
foundation. Standard here refers to vertical bars with circumferential rings around them. Layers
of mesh are wrapped around and tied to the skeleton to finish the armature.
Given a tight seam, an inner liner of stretch fabric shouldn’t be necessary. Two sheets of
polypropylene fabric are cut oversize to match the shape of the armature. Clamps are used
to tightly close the form on both sides. The pour is best done in three stages to control the
pressure: a minimal initial pour is done to minimize leaking at the bottom, then the form is
clamped and poured to half way up and finally the top half finished. The fabric needs to be
supported vertically from the armature to prevent horizontal creases. Local tension can be
applied to, for example, create a root-like pattern at the bottom of the column. The form needs
to be vibrated from the outside right after each pour. Some of the mesh will not be covered by
the pour, so plastering with concrete is necessary after the fabric is removed.
An alternate solution is to apply concrete directly to the armature without fabric using a
drier mix. In this case, it may be best to wrap the mesh around the skeleton in stages so that
the column can be filled quicker from the top and troweled at the mesh. Adding fibers to the
concrete is a good idea in either case to limit surface cracking.
2.4.3
Steel armature
The use of steel reinforcing bars is an effective method of achieving an arbitrary shape of
reinforcement. They’re widely accepted by building codes, easily accessible, inexpensive and
serve as a stiff skeleton to attach all other layers to. Avoiding rebar in favor of thick welded
wire makes the structure flimsy and a welded wire grid is more difficult to shape by itself as
27
CHAPTER 2. FERROCEMENT CONSTRUCTION
opposed to forcing it to an existing rebar skeleton. Using only mesh is more difficult to justify
to building codes. To make field erection efficient, the following needs to be done:
Design Computer design contains every bar’s shape, location and overlapping bars.
Shop preparation Bending to be done in a shop - best by an automated bender and cutter.
Otherwise, a template system like an overhead projector can be used, projecting on a
floor and bars shaped with power tools. Projected image on ground is actual scale, so the
projector effectively makes custom templates and saves considerable work in measuring.
Each rebar piece is labeled. Bar overlaps are also marked.
Beam welding Beams of rebar are welded in the shop. Arches, columns and other small pieces
can be wire-tied in the shop as well.
Organization Bent rebar is stacked in groups so in the field they’re sequentially placed. For
example, vertical bars are attached to the slab from left to right in a given room and
horizontal bars from the bottom up.
Rebar attachment A quick method of attaching bars together is needed. Automatic wire
tying tools are available which take only one second to wrap and twist wire at a joint.
Welding Critical joints and beams need to be welded to further stiffen and strengthen the
structure once the entire skeleton is assembled. A small MIG welder is sufficient and can
run on a normal generator on-site.
2.4.4
Concrete composition
There are many options when selecting the concrete composition to build with. Based on Martin
Iorns’ mixture used for boat building and guidelines in IFS 10-01, a recommended mixture is
provided in Tab. 2.2. The minimum of water should be used to reach hydration for maximum
strength and minimum shrinkage. The addition of pozzolans such as fly ash or blast furnace
slag improves the mixture’s flow without adding water because of its small, spherical particles.
Acrylic aids in bonding cold joints, but is otherwise too expensive to use. The minimum 28-day
compressive strength required by ACI 318 is 21 MPa, although Iorns’ similar mixture was tested
by the Coast Guard to be above 60 MPa [58].
Table 2.2: Proposed concrete composition
Ingredient
Amount
Notes
Cement
-
Sand
1:2 cement–sand by weight
Water
0.4 water–cement by weight
Pozzolan
0.15 pozzolan–cement by weight
Acrylic admix
9% by solids ratio
Portland cement, typically
Type I per ASTM C 150
Clean (free of salt and organic compounds)
passes 1 mm sieve
Potable, salt-free. Adjust amount
based on sand water content
Improves impermeability
improves workability for spraying
Cold joints only. Example product is Acryl 60.
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CHAPTER 2. FERROCEMENT CONSTRUCTION
2.4.5
Sculptural details
Decorative concrete is easiest done by casting in molds. A wide variety of silicone molds is
available in the market for features such as false stonework, arch overlays, scrolls, wildlife
patterns, figure sculptures. These molds are best cast starting with a very thin layer of concrete
without fibers, followed by a mixture of mesh and fiber-reinforced mix as the shape allows.
Much of the shape can be left hollow, but a layer of mesh or rods need to be left exposed
to allow attaching to the structure. Other decorations that are less intricate can be plastered
directly into ferrocement features as they’re shaped.
29
Chapter 3
MODELING AND DESIGN OF
CASE STUDIES
3.1
Overview
This chapter presents the methodology and implementation of structural shell modeling. The
three house case studies are built using this system and also presented. The goal of the system is
to make the design and structural analysis as quick and easy as possible. To this extend, various
software solutions are explored. It’s unavoidable that some complexity exists, for example in
scripting, but the overhead of learning and using the tools should be as small as possible. The
designer should be able to convert hand sketches, rough dimensions and ideas into a functional
model with a few hours worth of work.
3.2
Functionality
The scope of features desired in the modeling system is:
3D geometry The entire house shell must be modeled as a solid, with discrete reinforcing
bars. Other reinforcement, such as welded wire and meshes are best modeled as shells
offset from the rebars. Ease of use of the CAD software is important, as is affordability
to the designer/builder.
Change management Changes in 3D geometry need to be easy to incorporate without manually changing surface points. This includes enlarging spaces, reconfiguring rooms, reconfiguring penetrations, changing shell shape and others. This is critical to successful
exploration in the design phase and optimizing the structure for loads.
2D drawings The 3D model must be able to export 2D drawings of any desired parts of the
geometry. This includes cross sections with dimensions. Every rebar’s dimensions and
shape can be exported separately to aid in shop bending.
FEA link The relevant geometry must be cleanly exported to the FEA software such that
additional tweaking isn’t necessary. This includes assemblies and material types. Relevant
geometry is the structural framework and doesn’t need to include non-structural details.
Scripting and automation The entire design and analysis process must ultimately be represented in macros and scripts. This is critical for automating routine tasks in creating
geometry and performing the structural analysis. There needs to be a template for any
30
CHAPTER 3. MODELING AND DESIGN OF CASE STUDIES
project, where the user concentrates only on the structure’s features and has the remaining workflow automated. Common text files should include all inputs such as material
properties and loads. Text files should also be used where possible for transferring information between different software packages and debugging. The scripting language should
be easy for the user to learn.
Post-processing All tasks such as generating standard views, key drawings, analysis results
and reports need to be automatically done for the user. Material quantity estimates for
steel, concrete and insulation need to be available.
It’s also important to clarify the limits to the scope. Building Information Modeling (BIM),
which involves all building elements is not the focus of this project even if the 3D model is
a significant contribution in that direction. Likewise, utilities such as electrical conduits and
wiring, water pipes, HVAC and lighting are not considered1 .
3.3
Software implementation
The software workflow is shown in Fig. 3.1. It is an iterative process, where the geometry is
changed based on structural analysis results. Automation through scripting is used heavily in
each program to allow for experimenting with various shapes and concrete composites. The
details of the software selection process are in Appendix C, including the plugins that are
required.
Figure 3.1: Modeling flow
3.3.1
Geometry modeling
The software used in designing the building shape is Rhino3D 5.0. It is a 3D NURBS2 modeler,
which is well-suited for curved geometry. The strength and flexibility of this software is in its
graphical programming environment called Grasshopper. Geometry can be entirely scripted
and re-generated when inputs change. This allows for quickly evaluating design changes. A
physics engine plugin allows a mesh to be inflated and in general simulate fabric. The final
mesh is exported to ANSYS with a custom script, that re-creates each surface with commands
native to ANSYS.
1
Utility locations and layouts could be added to the 3D model later on. This is important for construction
drawings and cost estimation, although it may be easier to use specialized software for this purpose.
2
Non-Uniform Rational B-Spline, a method of representing geometry with smooth curves.
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CHAPTER 3. MODELING AND DESIGN OF CASE STUDIES
3.3.2
Structural analysis
ANSYS 13.0 is used in the structural analysis. ANSYS is a general purpose FEA package
for pre-processing, solution and post-processing of linear or non-linear, structural and thermal
problems. A set of macros written for this project automate the re-generation of Rhino3D
geometry, apply loads and boundary conditions, solve load components, combine results into
building code requirements, and create tables and plot of results.
3.4
Design guidelines
In designing each case study, several design guidelines are taken into account. The design intent,
structure strength and constructability are sometimes in contradiction to each other. The latter
two are critical and take precedence.
3.4.1
Catenary and inflated shapes
A catenary curve is naturally created by, for example, a cable hanging under its own weight.
The cable’s shape puts all links in pure tension. This shape, when inverted, results in an arch
that’s in pure compression. This basic concept has been used in architecture for centuries to
create stone structures without reinforcement. It is the most efficient shape to bear its own
weight, also when the curve is revolved into a roof.
The equations describing the catenary curve are [50]:
L
S = 2a sinh
2a
L
−1
d = a cosh
2a
where d is the dip of the suspended cable from points at the same level, L is the span, S is
the cable length and a is the horizontal portion of the tension divided by the cable’s weight per
unit length (see Fig. 3.2). The equations are solved by trial and error.
Figure 3.2: Catenary shape
Circular (Roman) arches are the strongest shape to resist a uniform pressure load. They
are also the best choice where supporting a flat surface and a non-uniform vertical load which
increases down the arch, such as a bridge or aqueduct. A semicircular dome roof has the
advantage of no thrust forces when loaded by a uniform pressure, unlike a catenary roof. It is
the first choice in design of a vaulted dome ceiling that will support another story.
Pointed (Gothic) arches are slender and provide the greatest strength resisting a point load
in the middle, up to the extreme case of becoming triangular.
32
CHAPTER 3. MODELING AND DESIGN OF CASE STUDIES
3.4.2
Form finding
Form finding is a design exercise that involves finding a shell shape that connects a closed
boundary and exhibits uniform stress in every direction. The best case is where the shell
assumes a shape that eliminates shear forces and moments. The exercise involves simulating a
fabric sheet that reacts to imposed loads and observing the deflection. Most often, gravity and
external pressure are the loads to optimize for. It is important to incorporate double curvature
into shell structures to stiffen them and protect against buckling.
Rhino3D’s physics engine plugin is used for this purpose. A simple example is shown in
Fig. 3.3. A mesh is converted into a series of point masses and connecting springs, which react
to applied loads through Newton’s and Hooke’s laws. The mesh is constrained at the outer
edges and allowed to deflect to a shape where only membrane tension resists the load. When
the load is reversed, the shell acts only in compression. The mesh is updated to the deformed
shape and can be manipulated further, such as creating doors and windows.
(a) Internal pressure
(b) Fabric weight
(c) Point load and fabric weight
Figure 3.3: Form finding of a mesh
3.4.3
Ribs
Ribs are beams connected to the shell which serve to increase load-carrying capacity. They
are especially needed to reinforce large penetrations and resist concentrated loads, where shells
are at their weakest. Concentrated loads include columns, upper story connections, external
attachments and others. The addition of rib beams is one way to strengthen the structure
without changing its shape.
3.4.4
Shear walls
Shear walls are interior walls that are designed to resist horizontal loads on the structure, such as
seismic and wind loads. The IRC requires shear walls for conventional wooden houses. Concrete
buildings require them based on shear force calculations in ACI 318. In this project, adding
shear walls is either impossible (first case study) or not necessary (cases 2 and 3). Therefore,
shear wall design is not considered. Interior walls in general are only included in the energy
analysis.
3.4.5
Shape optimizing
Where a complex design detail is used as a structural support, such as a tree-like internal wall,
it is convenient to use a shape optimization tool. Such a tool takes as input a generic grid with
loads applied as they are in reality. The program determines which elements in the grid are the
most critical to stiffness, such that the “dead” material can be cut out. The amount of material
to remove is specified by the user. ANSYS includes the Topology Optimization tool which does
33
CHAPTER 3. MODELING AND DESIGN OF CASE STUDIES
exactly this, as shown by an example in Fig. 3.4 of a 2D wall profile. Macros were created to
automate the analysis for exploring such designs.
(a) Elements and loads
(b) Resulting optimization
Figure 3.4: Topological optimization of a plane. Circle holes are unmeshed to force their shape.
Seven loads are on top edge and five supports on bottom. The red shape is the optimized shape
for 50 percent material reduction.
3.5
Case study 1: Shell Retrofit
The purpose of a retrofit is to build a protective shell on an existing wooden house, especially
in high-wind regions of the country. The advantage of this approach is that it’s relatively easy
to implement with minimal disturbance to the occupants. A typical one story, three bedroom
house is assumed, with the wall shell having dimensions of 8 x 15.5 x 2.5 m, a roof angle of
30 degrees and an overhang of 0.3 m. Standard window and door sizes are assumed. The
geometry is shown in Fig. 3.5. The construction technique and selection of concrete composite
are discussed in the structural analysis chapter. The thickness of insulation is found in the
energy analysis chapter. This case study is intended to be the simplest and require the lowest
budget for an existing home owner.
Figure 3.5: Case study 1 geometry
3.6
Case study 2: Calabaza
This case study is designed to take advantage of ferrocement’s shape forming and requires a
high budget. The pumpkin in Fig. 3.6 is a two-story house, 14 m in base diameter and 6.5 m at
34
CHAPTER 3. MODELING AND DESIGN OF CASE STUDIES
its highest point. A large, tapering center column is necessary to support the roof. There are
12 sections. Windows are 1.6 m in diameter. The “eyes” are 2.5 m at their largest dimension.
The shape is created by defining a rib frame and inflating the surface. Continuity of curvature
is very important between the shell and column to minimize stress concentrations, which is why
inflation is done on the entire structure. The bulging shell is not an ideal shape in terms of
distributing dead load in that bending moments are created along the wall. For this reason,
stiffening ribs are added inside the “creases” in the shell. The 0.3 m ribs run continuously from
the wall, to the roof and down the column.
The column is hollow so that it may be used to channel rain water from the roof, either for
storage or discharge through a pipe. It’s also a convenient structure to wrap a spiral staircase
around.
Internal walls and the second story floor are not modeled even though they connect directly
to the outer shell. The reason is to ensure that the outer shell is structurally sound and can
withstand all loads on its own.
Figure 3.6: Case study 2 geometry
3.7
Case study 3: Ballena
The whale shape of this case study is inspired by Arquitectura Orgánica’s Mexican Ballena (see
Appendix Fig. A.1), that was built using an inflated membrane as shown in Fig. 2.18. The
geometry here is similar, but adds more windows as shown in Fig. 3.7. Total floor surface area
is 195 m2 . Ceilings are 3–4 m tall at their peaks and the entrance door hallway is 2.5 m tall.
Only the external shell is modeled for structural analysis. Internal walls do not directly connect
to the outer shell to avoid thermal bridging.
The shape is created by weighing down a flexible membrane with point masses distributed
on its mesh. Weights are adjusted throughout the structure to obtain the height desired.
Some internal pressure is added to reduce wall slope at the base. Double curvature is present
everywhere.
To create the inflatable membrane, a coarse-mesh is created and each surface flattened as
shown in Fig. 3.8. There are 315 panels in this example with an area of almost 300 m2 . Rhino3D
can be programmed further to improve mesh quality, label the panels, mark the connections
and increase size for overlapping. An overhead projector can be setup to project each panel in
life-size so they can be cut without measuring.
35
CHAPTER 3. MODELING AND DESIGN OF CASE STUDIES
Figure 3.7: Case study 3 geometry
Figure 3.8: Coarse mesh and example flattened panel pieces
36
Chapter 4
STRUCTURAL ANALYSIS
4.1
Overview
The intent of the structural analysis presented here is to generally establish a building’s safety.
The analysis methodology and the process of selecting software are presented. Several building
codes are referred to for establishing loads and load combinations. Simplified acceptance criteria
are presented for tension, compression and bending. Each of the three case studies is analyzed
for its ability to resist dead and live loads.
4.2
Methodology
It is not practical nor accurate to calculate each member of a shell structure according to
code formulas which assume rectilinear geometry. Fortunately, an elastic analysis of the entire
structure is allowed in ACI 318. Shell theory may be used, which simplifies the analysis geometry
and reduces calculation time as compared to a fully solid model. The model’s concrete structure
is assumed to be uncracked, linearly elastic, homogeneous and isotropic. In effect, only concrete
is analyzed and the reinforcement is designed for the structure based on tensile forces that have
to be supported.
The codes (ACI 318 and ASCE 7) require designing for service and ultimate strengths.
Service loads are what the building is expected to experience in its lifetime. As such, the building
is expected to be repairable after loading events (i.e. no plastic deformation in reinforcement,
limited crack widths). Ultimate loads are extreme cases, where only protection against collapse
is to be designed for. In this project, the evaluation is based on ultimate strength only.
Elastic, linear analysis is computationally efficient in that a given load can be solved for only
once and combined with others in post-processing. For example, dead load can be solved with
a factor of 1.0, as can each of the wind and seismic directions. In post-processing, the result
components (stresses and displacements) are scaled and added for any desired combination.
ANSYS does the result manipulation for each node on a component basis, so that derived
results such as principal stresses are calculated only at the end. For example, a given node’s
stress in the x direction for load combination 1.2D +Evertical +Ehorizontal (dead load and seismic
factors) would be:
σx = (1.2 + Evertical ) (σx |D=1.0 ) + Ehorizontal (σx |E=1.0 )
The element type used in the analysis is SHELL281, which is an 8-node structural shell.
Each node has six degrees of freedom: translations and rotations in x, y and z directions.
ANSYS also provides a solid element, SOLID65, specifically for concrete. This element is
non-linear in that it is compression-only. Cracking planes can be visualized for each element
37
CHAPTER 4. STRUCTURAL ANALYSIS
if tension occurs. This approach was not used here because it is much more computationally
intensive and requires more detailed modeling.
To evaluate results, traditionally the structure is divided into sections and reinforcement
is designed based on stress results. The approach taken here is to evaluate each element of
the structure to ensure that a given ferrocement technique is adequate. Any areas that need
additional material can also be identified, namely, where tension forces are beyond the reinforcement’s strength or where concrete crushing occurs.
4.3
Material properties
Material properties used in the analysis are listed in Tab. 4.1.
Table 4.1: Material properties
Material
Property
Value
Source
Concrete
Young’s Modulus, E
Poisson’s Ratio, ν
Compressive Strength, fc
Density, ρc
20 GPa
0
25 MPa
2400 kg/m3
[5, §8.5.2]
[5, §19.2.1]
[31, §2.1.6]
-
Steel
welded wire
or rebar
Young’s Modulus, E
Poisson’s Ratio, ν
Yield Strength, fy
Density, ρs
200 GPa
0.3
400 MPa
7800 kg/m3
[31, Tab. 3-1]
[5, §19.3]
-
The amount of steel in the total composite is described by the volume ratio, Vr :
Vr =
Vreinf orcement
Vtotal
IFS 10-01 recommends a minimum Vr of 1.8% to obtain ferrocement characteristics. For a
welded wire mesh of N layers, wire thickness dw , concrete thickness h and wire spacing D, Vr
becomes [31]:
πN d2w
2hD
The Young’s Modulus and material density used in the analysis are the combination of steel
and concrete:
Vr =
Ecombined = (1 − VR )Econcrete + Vr Esteel
ρcombined = (1 − VR )ρconcrete + Vr ρsteel
4.4
4.4.1
Loads
Load combinations
ACI 318 requires the following load case combinations to be evaluated:
U = 1.4D
U = 1.2D + 1.6L + 0.5(Lr or S or R)
38
CHAPTER 4. STRUCTURAL ANALYSIS
U = 1.2D + 1.6(Lr or S or R) + (1.0L or 0.5W )
U = 1.2D + 1.0W + 1.0L + 0.5(Lr or S or R)
U = 1.2D + 1.0E + 1.0L + 0.2S
U = 0.9D + 1.0W
U = 0.9D + 1.0E
where:
U is the required strength
D is dead load
L is live load
Lr is the roof live load
R and S are the rain and snow load, respectively
E is earthquake load
W is wind load
These combinations need to be evaluated for each direction in loads such as seismic and wind.
Four directions are assumed for each (North, South, East and West). To simplify the analysis,
it’s assumed that roof and live loads can be ignored because they’re relatively insignificant for
small buildings as compared to wind and seismic. The two combinations with 0.9D are meant
to include the effect of dead load reducing the severity of seismic and wind. After simplifying,
the following 17 load combinations are explored:
U = 1.4D
U = 1.2D + 1.0WN,S,E,W
U = 1.2D + 1.0EN,S,E,W
U = 0.9D + 1.0WN,S,E,W
U = 0.9D + 1.0EN,S,E,W
4.4.2
Seismic loads
Seismic loads are applied as horizontal and vertical accelerations per Chapters 11 and 12 of
ASCE 7-05. The bottom of the wall has fixed displacements in each direction. The maximum
Seismic Design Category of D is used for the United States. The detailed calculations of the
accelerations are provided in Appendix B.1. The earthquake loading is comprised of horizontal
and vertical components:
E = Eh ± Ev
Eh = 0.385D
Ev = 0.28D
The vertical component is added in load combinations with 1.2D and subtracted in load
combinations with 0.9D.
39
CHAPTER 4. STRUCTURAL ANALYSIS
4.4.3
Wind loads
Wind loads are applied as pressure acting on the outside of the building. The pressure may
be positive or negative at any given point, depending on the wind’s direction and building’s
shape. The wind speed considered is 275 km/h, which corresponds to the maximum value for
the United States in ASCE 7.
Buildings with planar walls and roofs are easiest to analyze. Indeed, ASCE 7 provides
coefficients for common building shapes which are used for the retrofit case study. However,
more complex shapes require other sources for pressure profiles, such as those used on large
cylindrical storage tanks and spheres. The detailed calculations of the pressure profiles for each
case study are provided in Appendix B.2.
Wind loading for the Ballena case study is not calculated. This complex double curvature
structure is difficult to estimate with the limited tools available in codes. The most appropriate
solution for this building would be to find the shape coefficients from a Computational Fluid
Dynamics (CFD) analysis. This shape is excellent to resist wind loading and it’s expected that
seismic loading would govern the design.
4.5
Evaluation strategy
For each of the 17 load combinations, the following results are extracted for each node in the
model:
ˆ Displacements in global Cartesian coordinates X, Y and Z.
ˆ First principal stress, σ1 , for shell inner and outer surface. The maximum is reported.
ˆ Third principal stress, σ3 , for shell inner and outer surface. The minimum is reported.
ˆ Principal and component stresses of the shell’s middle surface.
Crushing is checked by comparing the largest compressive stress, σ1 , against the concrete compressive strength. It’s conservatively assumed that only concrete resists compression.
IFS 10-01 recommends that compression be limited to 0.45fc . Since each node is reported in a
generated table, it’s straightforward to find where crushing occurs and how large the region is.
The acceptance criterion is therefore:
σ1−surf ace−max < 0.45fc
Reinforcement adequacy is checked in two ways. First, the middle of the shell’s largest (negative) tensile stress, σ3 , is compared against the steel tensile strength. This is a check against
a mostly uniform tensile load and would not catch failure due to large moments. Second, the
shell surface largest σ3 is compared against a fraction of the steel’s tensile strength. A factor of
α of the steel is assumed to resist surface tension. ASCE 7 recommends a steel allowable of 0.6
of yield strength. The acceptance criteria are therefore:
σ3−mid
< 0.6fy
Vr
σ3−surf ace−min
< α0.6fy
Vr
This failure criteria approach is much simpler than the guidelines in IFS 10-01 or ACI 318
and is meant to only provide a general overview of the structure’s performance. A detailed design
would require extracting forces and moments, then explicitly checking resistance to moments,
40
CHAPTER 4. STRUCTURAL ANALYSIS
shear and tension using various adjustment factors. Crack widths would also be calculated.
The analysis done here provides a good estimate of stresses and displacements as well as regions
that would need additional strengthening in a detailed design.
4.6
4.6.1
Case study 1: Shell Retrofit
Analysis setup
This case is unusual in that the planar shapes of the walls and roof are not particularly suitable
to ferrocement’s strengths. The initial design of a 12 mm thick concrete shell with three layers
of 50 x 50 x 2 mm welded wire mesh proved to be inadequate. The roof geometry is prone
to sagging and the displacements and bending were too high. Sagging should be minimized
because it transfers weight to the wooden structure, which isn’t designed for the load. After
several design iterations, the conclusion was reached that a much thicker shell was needed. The
result is much closer to traditional reinforced concrete. Roof sagging is a much smaller problem
when internal walls provide support, which isn’t the case here.
The concrete composite consists of a total thickness of 75 mm with two layers of 150 x 150 x 8 mm
welded wire mesh. The galvanized mesh is placed 5 mm from the inner and outer surfaces to
provide maximum resistance to bending. The resulting volume ratio of reinforcement, Vr , is
1.8%. An additional mesh layer should be placed on the inside of the roof where bending is the
highest. It’s assumed that half of the reinforcement resists surface tension, so that α is 0.5.
4.6.2
Construction notes
The existing house makes this the easiest case study to build. Planar surfaces are perfect for
off-the-shelf rigid foam boards, which are attached directly to plywood after the siding and
roof shingles are removed. The first layer of mesh is attached to the foam and offset by 5 mm
using chairs. Bolsters are attached to offset the second layer of mesh. A finer square mesh, for
example 2 mm thick, is added around all windows and doors. To avoid formwork and maintain
flatness, expanded metal mesh is attached to the outer welded wire mesh in steps, plastered
and concrete is poured in. Alternatively, shotcrete can be applied in layers to only the welded
wire armature. The addition of fibers in the concrete mix reduces cracking and reduces slump.
The mass of the structure consists of 50.0 tons of concrete and 3.4 tons of steel.
4.6.3
Results
Representative displacement plots for two load cases are shown in Fig. 4.1, showing a roof sag
in the 1.4D load combination and a wall bulge in the 1.2D + Enorth combination. Specific notes
for each load combination are discussed here:
1.4D Roof sags by 5 mm. Tension due to bending is highest on the roof’s inside surface and
requires another layer of mesh reinforcement.
Seismic The walls bulge out a little due to horizontal acceleration. The maximum of 2 mm
occurs on the southern wall during the 1.2D + Enorth load combination. Generally the
1.2D combinations yield higher displacements and stresses than the 0.9D combinations.
Roof sag increases to 6 mm for Ewest and Eeast . The largest tension occurs on the roof
ridge, in the middle of each roof section and in the corners of window and door openings.
No crushing issues.
Wind Wind loads stress the structure less than seismic. Maximum wall bulge is 4 mm on
the north and south walls. The north and south walls exhibit more bulging than the
41
CHAPTER 4. STRUCTURAL ANALYSIS
east and west walls, even in cross winds. Roof uplift is maximum for 0.9D + Weast/west
combinations, but is under 2 mm. No crushing issues.
(a) Vertical displacement for 1.4D
(b) Horizontal Y displacement for 1.2D + Enorth
Figure 4.1: Case 1 sample displacement results
One strategy to improve the building’s performance would be to introduce curvature to especially the roof. This can be accomplished by changing the flat surface to a mini barrel vault that
rests on ribs. Another strategy would be to curve the entire roof into a half-cylinder. Both of
these options would reduce the concrete required and resulting loads. However, constructability
would be much more difficult in creating the armature and especially placing concrete from only
one side.
4.7
4.7.1
Case study 2: Calabaza
Analysis setup
The initial design of the shell was for a 25 mm concrete composite. It was found that the
displacements were unacceptable at over 25 mm under dead load and bending caused excessive
stresses. Ribs were added in-between shell bulges that spanned the wall. These still proved
insufficient at resisting bending, so they now run continuously from the walls, to the roof and
only get smaller down the center column. The “face” windows also needed to be redesigned so
that the center rib would be uninterrupted. The shell thickness also needed to be increased to
40 mm.
The volume ratio of reinforcement, Vr , of the composite described below is 3%. It’s assumed
that a third of the reinforcement resists surface tension, so that α is 31 .
4.7.2
Construction notes
The proposed construction technique is similar to classic ferrocement. A rebar armature of the
whole structure is erected. Expanded metal lath is attached on the interior of the shell. Two
layers of 150 x 150 x 5 mm mesh are stacked on the exterior. Concreting begins by plastering
the lath, which becomes formwork for shotcrete on the other side. The composite is 40 mm
thick after including 5 mm cover. There is no finer mesh on the shotcrete side as is commonly
found in classic ferrocement to speed up construction.
42
CHAPTER 4. STRUCTURAL ANALYSIS
Creating the armature and concreting the lower half of the shell first, including the second
story floor, would reduce scaffolding for completing the upper half.
Insulation is attached to the outside of the shell. Spray foam is the most convenient solution,
but also expensive. It would be more economical to use foam as only a glue that holds EPS
blocks together and seals gaps.
The exterior of the insulation may be smoothed and painted as is done with Monolithic’s
domes. Otherwise, a thin concrete shell can be built with either the laminated technique.
The mass of the shell consists of 51.8 tons of concrete and 4.5 tons of steel.
4.7.3
Results
Displacement under dead load is less than 2 mm for the structure. For seismic load cases,
the horizontal acceleration causes one side to lift slightly and the other to slump, as shown in
Fig. 4.2a. The maximum uplift for the 0.9D + E cases is 3 mm.
The highest stresses occur in the ribs, as they resist the shell’s tendency is to further push
out the walls. There are still surface stresses in the shell, especially at the top near the column
as shown in Fig. 4.2b. Thickening the ribs would reduce this tension. Window areas, especially
at the “face”, would also benefit from additional minor reinforcement. These openings are
generally not problematic because of the ribs.
Seismic load combinations govern in general. Wind loads only affect the large windows,
where the net force from pressure is resisted by the shell. Most of the windows are centered on
bulge sections to most efficiently distribute these loads.
(a) Vertical displacement for 1.2D + Eeast
(b) Surface σ3 for 1.4D
Figure 4.2: Case 2 sample results
In general, a structure like this which is designed with aesthetics as the priority is relatively
expensive. There is considerable effort devoted to studying weak areas, designing reinforcement
and ultimately more material is needed as opposed to designing with loads as the priority. This
is in stark contrast to the Ballena case study, which has a shape optimized for resisting loads
and performs accordingly.
43
CHAPTER 4. STRUCTURAL ANALYSIS
4.8
4.8.1
Case study 3: Ballena
Analysis setup
Several concrete thicknesses were explored and 25 mm was found to be the best compromise
between strength, material use and constructability. The entire shell has the same composite.
The volume ratio of reinforcement, Vr , is 2.6%. It’s assumed that half of the reinforcement
resists surface tension, so that α is 0.5. As previously noted, wind loads are not included.
4.8.2
Construction notes
The laminated ferrocement technique is used together with an inflated membrane. The membrane is inflated and foam sprayed from inside the membrane. The membrane is removed after
the foam has hardened. The concrete is built up in layers on the exterior’s surface in the same
way that boats were built by Martin Iorns. No skeletal steel, ties or formwork is necessary.
The concrete composite is 25 mm thick overall, with four layers of 25 x 25 x 1.6 mm
galvanized welded wire mesh. The reinforcement is placed close to the exterior surfaces. A
3 mm coat of concrete is troweled onto the foam’s outside surface (the foam may need to be
roughened for better adhesion). The first layer of mesh is pushed into the soft mortar. A very
thin layer of mortar covers the mesh. The second layer of mesh is pushed into the mortar, along
with small steel chairs that protrude 13 mm. The middle layer of mortar is added to the top
of the chairs. The last two layers of mesh are pushed on and a final cover of 3 mm applied.
The concrete mixture described in §2.4.4 is used to maximize strength and water-tightness,
including the use of acrylic at cold joints. The laminate is built up in sections from the bottom.
Scaffolding that extends over the building is needed.
The steel welded mesh may be replaced by a more flexible material such as basalt or fiberglass. Those meshes are much more expensive but are easier to handle and form curves with.
Corrosion is also not a concern.
Window and door overhangs are not needed for strength but for controlling rain run-off.
They are built with thin rebar before concreting starts. The armature is embedded in the
middle layer of the laminate, with mesh left exposed at the junction. Mesh and mortar is added
after the main shell is finished.
There are many options for finishing the interior. A single layer of expanded lath can be
attached to the foam and plastered to create a thin shell. Walls can be rigid foam panels, also
plastered on both sides. The foam is easy to shape into curved walls and adds sound isolation.
Ferrocement furniture and sculpture can be integrated into the walls.
The mass of the shell consists of 16.3 tons of concrete and 1.4 tons of steel.
4.8.3
Results
This structure is very efficient at distributing loads and minimizing bending stresses. The
catenary door opening and circular windows also minimize stress concentrations. There is under
1 mm deflection under any load combination in any direction. The maximum displacement sum
is shown in Fig. 4.3a, where the shell is more flexible around the biggest windows. There are
no regions that need additional reinforcement. Crushing is also not a concern.
An example of stress distribution for a seismic load combination is shown in Fig. 4.3b.
Tensile stresses are around the openings but the rest of the structure remains very uniformly
stressed.
44
CHAPTER 4. STRUCTURAL ANALYSIS
(a) Displacement sum for 1.2D + Ewest
(b) Surface σ3 for 0.9D + Enorth
Figure 4.3: Case 3 sample results
45
Chapter 5
ENERGY SYSTEM DESIGN &
ANALYSIS
5.1
Overview
This chapter presents energy modeling of each of the case studies. The methodology and
software are focused on estimating the annual energy demand in two climates: Chicago, Illinois
and Miami, Florida. Energy demand is split into electricity, heating and cooling. Various design
options are explored, such as insulation thickness, thermal mass and openings, to minimize
energy demand.
The goal is a net-zero house, referring to a net annual electricity use of zero. The house is
assumed to be connected to the grid and is able to sell surplus electricity from on-site generation
and likewise buy from the grid to satisfy shortages. The focus of this analysis is only on reducing
energy demand of the building. The electricity demand is provided for later design of on-site
renewable energy sources such as solar photovoltaics (PV) or small wind turbines. Heating and
cooling systems for each case study are conceptual and sized according to need. An estimate of
a solar PV installation is done for each case study, since it is usually the easiest to implement
in an urban environment.
Another goal of the energy system is to eliminate the direct use of fossil fuels. Natural
gas is widely used for heating in the U.S., currently being an economical solution. To this
extent, ground source heat pumps (GSHP) are incorporated into each case study to cover
space heating, space cooling and hot water. Although higher in capital costs, GSHPs allow for
complete electrification of the energy system, use of on-site generation and long-term savings.
District water heating and cooling are not considered because they’re generally not available
for single family houses in the U.S.
According to the Energy Information Agency (EIA), the annual average residential electricity
use was 9,240 kWh in Illinois and 13,572 kWh in Florida in 2009 [62]. Gas for heating is not
included. These values are a useful baseline for comparing proposed designs.
Finally, the PassivHaus certification requirements for specific annual energy demand are
≤15 kWh/m2 for heating and ≤15 kWh/m2 for cooling [37]. Heat loads are to be ≤10 W/m2 .
These are useful benchmarks of excellent performance, even if PassivHaus certification isn’t
specifically the design focus.
5.2
Methodology
EnergyPlus 7.0 is the simulation framework used in this project. This is an open-source, mature code developed by the U.S. Department of Energy. It calculates electricity, heating and
46
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
cooling demand based on a building’s temperature setpoints, shape, local climate, solar heating
gains, light requirements, indoor equipment heat gains and others. A transient heat balance is
calculated in hourly time steps for an entire year.
DesignBuilder 3.0 is the application used to provide a graphical interface to EnergyPlus.
DesignBuilder is where the geometry is modeled, zones defined with activity levels, material
properties assigned to the geometry and the simulation solved. Due to the program’s limitations
in importing CAD geometry and simple modeling tools, there is considerable simplification of
the curved surfaces of the Ballena and Calabaza case studies. Simple blocks are modeled to
reasonably approximate the true volume, surface area, glazing and building shape.
Each case study geometry is re-created in DesignBuilder, complete with internal room partitions. Ferrocement thickness is the same as presented in Chapter 4, while the insulation
thickness is varied to reach PassivHaus performance. Other components are assumed to be energy efficient, such as triple-pane windows and insulated core doors. In the case of the retrofit,
a base energy demand is also established by assuming conventional wooden construction.
Solar PV sizing is done using PVSYST 5.5. A fixed tilt angle is assumed, optimized for
annual yield in each city. Electrical demand drives the system’s size, as PVSYST calculates
the expected hourly irradiation for Chicago and Miami for a sample year. The goal of the PV
analysis is to estimate annual yield per unit area.
5.3
5.3.1
Design considerations
Thermal mass
Thermal insulation R-value ratings are based on steady-state testing per American Society
of Material Testing (ASTM) procedures. There are cases where thermal mass of a structure
contributes to a higher effective insulation value than would be expected from the R-value alone.
This is often called the “mass effect”. The heat capacity of dense materials such as concrete
creates a delay in heat transfer. This delay can be especially beneficial in climates with large
daily temperature swings around the desired indoor setpoint. For example, desert and elevated
altitude climates in Arizona, Colorado and New Mexico may have hot days over 30°C and cool
nights of 10°C. In these climates, the heat transfer reverses direction throughout the day. Heat
is absorbed during the day and released at night. Traditional adobe housing takes advantage
of this effect, holding a stable and comfortable temperature with little or no insulation.
The mass effect is beneficial even in constantly hot or cold climates. For example, delayed
heat transfer of peak temperatures can save on cooling costs because air conditioning is more efficient at night in cooler outside temperatures and electricity rates are lower. Cooling equipment
can be smaller since it is no longer designed for peak temperatures.
In the past, codes like the Swedish Byggnadsstadgan from 1950 provided different wall
conductivity values depending on the type of concrete used [38, Tab. 25]. However, there is
currently no standardized method to account for mass effects. Efforts have been made at the Oak
Ridge National Laboratory to dynamically test materials under various simulated climates [36],
but haven’t been adopted in building codes. Some manufacturers have their own claims to
favorably market their products.
Thermal mass effects are highly dependent on the local climate and are best handled caseby-case. In this project, the focus is to make efficient use of building materials. Therefore,
additional thermal mass can be added separately to the interior. The energy models do not
alter structural concrete thickness presented in Chapter 4.
47
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
5.3.2
Infiltration
Infiltration refers to the air tightness of a building, usually expressed as air changes per hour at a
pressure of 50 Pascals (ACH50 ). To determine infiltration on an existing building, a special door
is installed that contains a fan. All other openings in the house are closed. ACH50 is calculated
based on interior volume and how much air escapes through gaps in the building once 50 Pascals
is achieved. These uncontrolled losses lead to higher heating and cooling demand.
Natural infiltration for the energy model can be obtained from ACH50 . Conversion factors
1
vary by region. In this analysis, a value of 20
is used, which is a common approximation.
Therefore, the infiltration in the energy model is:
ACH50
20
An existing home may be assumed to have an ACH50 of 5, which is a requirement for an
EnergyStar 2.0 rating. This value is used in the energy model of the retrofit before the outer
shell is built. PassivHaus requires ACH50 of 0.6. The energy model uses this value for all three
ferrocement structures.
ACHnat =
5.3.3
Heat pumps
All heating, cooling and hot water are assumed to be provided by a reversible heat pump unit,
except for the retrofit baseline where natural gas and conventional air conditioning is used. The
heat pump type assumed is a closed loop GSHP. There is a wide variety of performance that
can be expected based on local climate, whether soil or ground water is in contact with the loop
and other variables. The U.S. EnergyStar program provides for minimum requirements [59],
which are the values used in the energy model.
EnergyStar requires a minimum average Coefficient Of Performance (COP) of 3.3. The
COP refers to heating performance and is defined as:
QH
W
where QH is the heat discharged into the building and W is the electricity needed to run
the heat pump. Heat pumps work more efficiently for lower temperatures, which is why radiant
floor heating is the best choice for the Chicago climate rather than radiators.
EnergyStar requires a minimum average Energy Efficiency Ratio (EER) of 14.1. The EER
is similar to COP in heat pump cooling and is defined as:
COP =
EER =
QC [BTU]
W [Wh]
where QC is the heat removed from the building. Cooling COP is more convenient to work
with:
COPcooling =
[BTU] 1055 [J/BTU]
QC
= EER
·
= EER · 0.293
W
[Wh]
3600 [J/Wh]
The heat pump COP is therefore 4.13. The retrofit baseline is assumed to have the reversible
heat pump upgrade in-place. The conventional air conditioner modeled in the original Case 1
house before retrofit has a COP of 3.
48
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
5.3.4
Water heating
Domestic hot water (DHW) calculations are based on U.S. Department of Energy residential
requirements [60]. Household consumption is estimated at 240 Liters of hot water per day at
57°C. An EnergyStar heat pump water heater is required to have a COP of 2.0. Annual energy
demand is 2,195 kWh. This value is assumed constant for all case studies. A typical gas heater
and needs 4,721 kWh to cover the same demand. In the baseline retrofit, it’s assumed that the
water heater has already been upgraded to a heat pump.
Solar water heating can be an economical addition in sunny climates like Florida. Drain water heat recovery may also be an economical addition. One example unit raises the temperature
of incoming water by 14°C [41].
5.3.5
Heating and cooling setpoints
The heating system is modeled to operate at an inside air temperature of 20°C and under. The
cooling system operates above 25°C. In-between these setpoints, outside air is used directly.
Natural ventilation is not used so that all air goes through a filter. This has the benefit of better
air quality (less dust, allergens) but in mild weather results in higher energy use. Mechanical
ventilation is assumed to be on at all times, with an air flow of 0.4 ACH of fresh air.
5.3.6
Heat recovery
Heat recovery ventilation is included to reduce loads on the heat pump. A thermal wheel is
assumed, with negligible energy needs. The assumed recovery is 70% for sensible heat and 65%
for latent heat.
5.3.7
Occupant activity
Each zone in the house includes activities by the occupants in the form of heat gains and fresh
air needs. A schedule of activity accounts for the house being empty on working days and
occupied otherwise. The schedule varies during the day and is adjusted for each room type.
The average metabolic rate is assumed to be 90 W per person. Other zone details are listed in
Appendix B.3.
5.3.8
Openings
Openings are a major area of thermal losses and need to be energy-efficient for the whole
building to perform well. Windows are assumed to be triple-pane, low emissivity, with a Uvalue of 0.99 W/m2 -K. External doors are assumed to have a U-value of 1.5 W/m2 -K. It’s
important to purchase windows and doors which provide an accurate installed U-value, which
includes thermal bridging at the frame.
Shading is important for hot climates to limit solar gains and avoid over-heating. Outside
shades are assumed in the model, which are closed once the window receives over 200 W/m2 of
solar irradiation. Overhangs may also fulfill this function.
The exterior doors present a thermal loss that can be reduced with a small room dedicated to
the entrance. This way, air exchange with the outside is more limited. An entrance is modeled
in the Calabaza case study.
5.3.9
Lighting
LED lighting is assumed in the building, with an energy intensity of 3.3 W/m2 -lux. Target
luminance for each zone is listed in Appendix B.3. Exterior night-time lighting is also included,
for a total of 50 W.
49
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
5.3.10
Solar PV
To illustrate the expected monthly output of a PV system, an example 20 m2 , 3.6 kW nominal
system is setup in PVSYST. The monthly electricity output for Chicago and Miami is shown
in Fig. 5.1.
Figure 5.1: Solar PV monthly electrical output for 3.6 kW system
Annual net electrical yield EP V can be estimated with:
EP V = G · A · P R · η
where G is the solar irradiation on the plane, A is the collector area, P R is the performance
ratio which incorporates thermal, inverter and other losses and η is the module conversion
efficiency.
It’s assumed that the modules are arranged in a fixed tilt, optimized for annual yield. The
tilt is 24° for Miami and 31° for Chicago. The modules selected are 300 W nominal, with 18.5%
efficiency. PR is 80% for Chicago and 75% for Miami. The difference in PR is due to the larger
thermal losses in hot climates. Snow cover is assumed for three winter months in Chicago,
removing all output.
For estimation purposes, it’s assumed that the system scales linearly with area. Based on
the sample system here, a system could be expected to produce 193 kWh/m2 -yr in Chicago and
282 kWh/m2 -yr in Miami.
5.4
5.4.1
Case study 1: Shell Retrofit
Description
The DesignBuilder energy model, including floorplan zones, is shown in Fig. 5.2. The wall and
roof ferrocement composite is variable XPS rigid insulation (see results) and 75 mm of outside
concrete. The ground floor is a covered but uninsulated 100 mm concrete slab with a U-value of
0.350 W/m2 -K. The original slab is not replaced for the retrofit. It’s assumed that the retrofit
has only had the HVAC and DHW components upgraded to heat pumps. The conditioned floor
area is 128 m2 .
Prior to the retrofit, the walls are assumed to be wooden studs with plywood sheathing and
fiberglass insulation, for a U-value of 0.432 W/m2 -K. The roof is also wooden studs, plywood
and asphalt shingles for a U-value of 0.254 W/m2 -K. Windows are double-pane with a Uvalue of 2.71 W/m2 -K. The existing building is left intact after placing the ferrocement shell,
50
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
Figure 5.2: Case 1 energy model
which means that less XPS needs to be added. Thermal bridging of the soft wooden studs
and fiberglass batt insulation is not directly supported by EnergyPlus. In this case, bridging is
ignored because it would only increase the effective U-value by less than 10%.
The retrofit baseline also includes lighting and appliance upgrades that reduce electricity
use and heat gains. The original building is also analyzed, where no upgrades have been done
at all. A summary of the retrofit stages is in Tab. 5.1.
Table 5.1: Case 1 retrofit stages
Name
Original stick-built
Baseline
Ferrocement shell
5.4.2
Insulation
Heat pump
Activity
Lighting
Fiberglass
Fiberglass
Fiberglass & XPS
No
Yes
Yes
+50%
Per App. B.3
Per App. B.3
+50%
Per §5.3.9
Per §5.3.9
Analysis results
A summary of the energy analysis results is in Tab. 5.2. Annual energy is broken down to
lighting and equipment electricity, heating and cooling. Heating and cooling are the electrical
load with the COP and EER incorporated. Heating includes only space heating and DHW
is added to the total electrical calculation. The U-value listed is based on the ferrocement
composite.
Table 5.2: Energy model results for Case 1
U-value
(W/m2 -K)
Elec.
Annual energy (kWh)
Heat
Cool
Total elec.
Climate
Construction
Chicago
Original stick-built
Baseline
75 mm XPS
150 mm XPS
0.432
0.432
0.239
0.156
5,067
2,871
2,863
2,862
7,191 gas
1,850
1,160
1,052
811
572
521
516
5,875+11,912 gas
7,488
6,739
6,625
Miami
Original stick-built
Baseline
75 mm XPS
150 mm XPS
0.432
0.432
0.239
0.156
5,069
2,918
2,903
2,900
0
0
0
0
3,292
2,003
1,502
1,473
8,361+4,721 gas
7,118
6,600
6,568
51
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
In terms of value for investment in energy savings, heat pumps and efficient appliances
provide a bigger benefit than additional insulation. Since the existing fiberglass is left in place,
the newly installed insulation can be thinner than the other case studies. It’s enough to install
75 mm to achieve PassivHaus’s goals of 15 kWh/m2 -yr for both heating and cooling.
In the best-case scenario, a solar PV would need to be 35 m2 in Chicago and 24 m2 in
Miami to cover electrical demand under a net-zero scheme. If solar water heating could cover
all demand, it would save 12 m2 in Chicago and 8 m2 in Miami. Solar DHW savings are the
same for all case studies.
5.5
5.5.1
Case study 2: Calabaza
Description
The DesignBuilder energy model, including floorplan zones, is shown in Fig. 5.3. The wall and
roof ferrocement composite is 40 mm of steel and concrete inside, variable EPS insulation (see
results) and 10 mm of outside concrete. The ground floor is a 100 mm concrete slab on top of
200 mm of heavyweight EPS for a U-value of 0.167 W/m2 -K. The same slab is also used for the
next case study.
This is the largest of the case studies, with a conditioned floor area of 440 m2 between the
two stories. Rooms are very large and could be reconfigured to include more bedrooms, an
office or workshop.
Figure 5.3: Case 2 energy model, 1st and 2nd story zones
5.5.2
Analysis results
A summary of the energy analysis results is in Tab. 5.3. Annual energy is broken down to
lighting and equipment electricity, heating and cooling. Heating and cooling are the electrical
load with the COP and EER incorporated. Heating includes only space heating and DHW
is added to the total electrical calculation. The U-value listed is based on the ferrocement
composite.
The annual heat balance and electrical loads for the Miami location with 300 EPS insulation
are shown in Fig. 5.4. Cooling is by far the largest electrical load, even when shading is
maximized to limit solar gains. The importance of heat recovery is especially apparent.
The Chicago results show a 73% reduction in heat demand when increasing EPS thickness
from 50 to 300 mm. Specific heating demand is under the PassivHaus limit of 15 kWh/m2 -yr for
the thicker EPS. The cooling is a bigger issue in Miami’s climate. A combination of steady high
temperature, large glazing solar gains and interior equipment gains result in specific cooling
demand. It can be seen that for a two story structure, where the surface area is smaller relative
52
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
Table 5.3: Energy model results for Case 2
U-value
(W/m2 -K)
Annual energy (kWh)
Elec. Heat Cool Total elec.
Climate
Construction
Chicago
50 mm EPS
150 mm EPS
300 mm EPS
0.687
0.253
0.130
5,962
5,946
5,947
8,342
4,636
2,954
3,048
2,910
2,880
19,547
15,688
13,976
Miami
50 mm EPS
150 mm EPS
300 mm EPS
300 mm, 0.3 ACH
0.687
0.253
0.130
0.130
6,366
6,274
6,245
6,131
22
0
0
0
9,297
8,112
7,847
6,259
17,880
16,581
16,287
14,585
Figure 5.4: Key results for Miami 300 EPS
to volume as compared to the other cases, improving heat recovery is more important than
insulation. Investment should go towards window shading, more efficient heat recovery and
more efficient appliances.
In the best-case scenario, a solar PV would need to be 73 m2 in Chicago and 52 m2 in Miami
to cover electrical demand under a net-zero scheme.
53
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
5.6
Case study 3: Ballena
5.6.1
Description
The DesignBuilder energy model, including floorplan zones, is shown in Fig. 5.5. The wall and
roof ferrocement composite is 10 mm of steel and concrete inside, variable polyurethane foam
insulation (see results) and 25 mm of outside concrete. The model conditioned floor area is
195 m2 .
Figure 5.5: Case 3 energy model
5.6.2
Analysis results
A summary of the energy analysis results is in Tab. 5.4. Annual energy is broken down to
lighting and equipment electricity, heating and cooling. Heating and cooling are the electrical
load with the COP and EER incorporated. Heating includes only space heating and DHW
is added to the total electrical calculation. The U-value listed is based on the ferrocement
composite.
Table 5.4: Energy model results for Case 3
Climate
Construction
U-value
(W/m2 -K)
Annual energy (kWh)
Elec. Heat Cool Total elec.
Chicago
50 mm foam
100 mm foam
200 mm foam
0.506
0.266
0.136
4,177
4,159
4,150
2,125
1,336
819
722
715
712
9,219
8,406
7,877
Miami
50 mm foam
100 mm foam
100 mm, 0.3 ACH
0.506
0.266
0.266
4,238
4,243
4,119
0
0
0
2,886
2,875
2,173
9,319
9,312
8,487
In Chicago, the extra insulation results in significant reductions in heat demand, although
the difference in total electricity is reduced once DHW and equipment loads are considered.
Specific heating demand is under the PassivHaus limit of 15 kWh/m2 -yr.
In the hot Miami climate, increasing insulation shows much smaller gains. Instead, cooling
demand can be reduced by increasing window shading and decreasing the ventilation rate from
0.4 ACH to 0.3 ACH. Specific cooling demand is about 15 kWh/m2 -yr. Reducing internal gains
in the form of equipment and lighting would also be effective in saving on cooling, as these
gains dominate once window shading is in place. Current gains corresponding to lighting and
equipment are an average of 500 W for the household.
54
CHAPTER 5. ENERGY SYSTEM DESIGN & ANALYSIS
In the best-case scenario, a solar PV would need to be 41 m2 in Chicago and 31 m2 in Miami
to cover electrical demand under a net-zero scheme.
55
Chapter 6
CONCLUSIONS
Ferrocement construction is an exciting alternative to conventional wooden and masonry methods. In the context of sustainable housing, aspects such as strength, durability, architectural
freedom, occupant comfort, affordability and energy efficiency align well with ferrocement’s
qualities. This project explores its rich history and structure types, including water tanks,
houses, boats and large public areas. Various options are explored to reduce construction labor
costs, which is the biggest reason for its low adoption in today’s buildings. There are many
modern improvements possible, including new materials, innovative fabric forming methods,
computer modeling and analysis, among others.
Traditional ferrocement’s large labor requirements can be reduced. Computer-generated,
projected layouts allow for shop fabrication of rebar armature, which is only assembled on-site.
Inflated fabric, spray foam insulation and the laminated ferrocement technique can drastically
reduce armature work. The use of flexible, non-metallic materials such as basalt increases
durability and makes the mesh easier to handle and shape. Mortar composition can be improved
with fly ash for quicker shotcrete application.
Modern CAD tools allow for quick exploration of ideas and sophisticated form finding. The
combination of Rhino3D with the Kangaroo physics plugin is one of a few powerful options
available to a designer today. These tools allow an amateur to create shapes that in the past
required complex physical models. The high strength of a mixed funicular and inflated shape
is verified in the Ballena case study.
Structural analysis is a key part of preliminary design, especially with non-funicular shapes
like the Shell Retrofit and Calabaza case studies. High stress regions can be identified and
either reinforced or reshaped. All case studies in the project are analyzed with ANSYS, based
in part on building code requirements, to generally establish safety. Several design and analysis
iterations are done for each case study to find a solution that satisfies the architectural and
strength goals.
Energy efficiency is verified with DesignBuilder energy modeling. Many ideas are borrowed
from the strict PassivHaus specification, including the use of heat pumps, heat recovery, thick
insulation, energy-efficient lighting and appliances as well as window shading. Each case study
is able to meet PassivHaus specific energy demand limits for heating and cooling. On-site solar
PV system size recommendations would allow the houses to achieve net zero electricity use on
an annual basis.
56
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60
Appendix A
EXAMPLES OF FERROCEMENT
CONSTRUCTION
The purpose of this Appendix is to illustrate several notable ferrocement projects that have
been inspiring to the author. The examples are meant to show what is possible to build. The
descriptions are brief, with some extra sources included for further reading.
Figure A.1: Two ferrocement houses built in Mexico by Arquitectura Orgánica
(Above) The Nautilus House
(Below) The Mexican Whale House, done with single inflatable form and hardened with foam
More of their projects at http://www.arquitecturaorganica.com
61
APPENDIX A. EXAMPLES OF FERROCEMENT CONSTRUCTION
Figure A.2: Monolithic domes are usually unimaginative and almost awkward as a house shape.
However, with large cutouts, the building takes on new character.
(Left) The large dome house in Pensacola Beach, Florida survived a direct hit by Hurricane
Ivan in 2004 and Hurricane Dennis in 2005.
(Right) The Garlock house in the mountains of Colorado also benefits aesthetically from large
cutouts in the dome.
Figure A.3: Vietnam’s Crazy House, designed by Dang Viet Nga as a personal project to create
a tree-like fairy tale house. It now includes 10 guestrooms and tours are offered to recoup the
money of construction. Nga is the daughter of a past Vietnamese president, which likely helped
with the building permits.
The official website is: http://www.crazyhouse.vn/
62
APPENDIX A. EXAMPLES OF FERROCEMENT CONSTRUCTION
Figure A.4: Candela’s hyperbolic paraboloid Los Manatiales restaurant in Xochimilco under
construction and the incredible open space inside. The formwork was straight wooden boards,
with wire reinforcement laboriously laid down on top before the concrete pour. The concrete
was only 4 cm thick.
63
Appendix B
DESIGN AND CONSTRUCTION
DETAILS
B.1
Seismic loads
The following calculations are per ASCE 7-05. The purpose is to find the lateral acceleration
to apply in the structural analysis.
B.1.1
Inputs
Seismic Design Category: D (maximum, California)
F = 1.1 for two-story buildings (§12.14.8.1)
Fa = 1.4 for soil sites (§12.14.8.1)
SS = 1.5, maximum short term acceleration per §12.14.8.1
R = 4, response modification coefficient per Table 12.14.1
B.1.2
Calculations
The calculation procedure is per Chapter 12 of ASCE 7-05. Equation numbers correspond to
the code.
The design spectral acceleration at short periods, SDS , from Equation 12.14.11 is:
2
2
SDS = Fa Ss = · 1.4 · 1.5 = 1.4
3
3
Equations 12.4.3 and 12.4.4 establish the earthquake loading, E, in horizontal and vertical
components:
E = Eh ± Ev
The vertical component is added in load combinations with 1.2D and subtracted in load
combinations with 0.9D. Per Equation 12.14.6:
Ev = 0.2SDS D = 0.28D
The base shear component per Equation 12.14.11:
Eh = V =
F SDS
1.1 · 1.4
D=
D = 0.385D
R
4
64
APPENDIX B. DESIGN AND CONSTRUCTION DETAILS
B.2
B.2.1
Wind loads
Case study 1: Shell Retrofit
Based on ASCE 7-05, Section 6.5, Method 2: Analytical Method. Although not intended for
direct hits from tornadoes, the procedure is convenient for estimating “normal” winds at any
speed. Assume open building, with no internal pressure to counter external pressure.
Inputs
V = 76 m/s, design wind speed of 275 km/h
I = 1.0, Table 6-1, importance factor from Occupancy Category II
Kzt , Kz = 1, wind directionality and topography factors
Kd = 0.85, Table 6-4, wind directionality factor
G = 0.85, gust effect factor per §6.5.8.1
Roof angle = 30°
L/B = 8/15.5 = 0.52, floorplan width to length ratio
h/L = 3.65/4 = 0.91, mid-roof height to width ratio
Calculations
The velocity pressure, qz , is calculated from Equation 6-15:
qz = 0.613Kz Kzt Kd V 2 I = 0.613 · 1 · 1 · 0.85 · 762 · 1 = 3 kPa
Pressure on a given surface is determined from Equation 6-17:
p = qGCp − qi GCpi
where the internal pressure terms of qi drop out. The external pressure coefficient, Cp , is
based on surface type and wind direction. Positive values indicate wind pushing the surface and
negative values indicate suction. The relevant values are calculated from Table 6-6 and listed
in Table B.1. For wind parallel to the roof ridge, a constant value of -0.7 is used for Cp in the
analysis.
Table B.1: Pressure distribution for flat surfaces
B.2.2
Surface
Wind direction
Cp
Pressure [kPa]
Walls
Windward
Side
Leeward
+0.8
-0.7
-0.5
2.0
-1.8
-1.3
Roof
(Perpendicular to ridge)
Windward
Leeward
+0.2
-0.6
0.5
-1.5
Roof
(Parallel to ridge)
0–2 m from edge
Over 2 m from edge
-1.3
-0.7
-3.3
-1.8
Case study 2: Calabaza
The pumpkin shape is estimated here to be a cylinder with a dome roof. ASCE 7 only provides
Cp values for curved roofs, which in this case are approximated per Tab. B.2. Cylindrical
65
APPENDIX B. DESIGN AND CONSTRUCTION DETAILS
building shape coefficients can be found in British Standard 6399-2:1997 [13] and are shown in
Tab. B.3. The calculation of q is the same as above and the gust factor, G, is also 0.85.
In the FEA model, the pressures are interpolated for each element’s centroid location. The
transition from wall to roof is assumed to be beyond 45° from the horizontal.
Table B.2: Pressure distribution for dome roof [9, Tab. 6-7]
Location on roof
Cp
Pressure [kPa]
Windward edge
Middle
Leeward edge
-1.5
-0.9
-0.5
-3.8
-2.3
-1.3
Table B.3: Pressure distribution for cylindrical walls [13, Tab. 7]
B.2.3
Angle θ from
incoming wind
Cp
Pressure [kPa]
0°
10°
20°
30°
40°
50°
60°
70°
80°
90°
100°
120°
140°
160°
180°
+1.0
+0.9
+0.7
+0.4
0
-0.4
-0.8
-1.1
-1.05
-0.85
-0.65
-0.35
-0.3
-0.3
-0.3
2.6
2.3
1.8
1.0
0
-1.0
-2.0
-2.8
-2.7
-2.2
-1.7
-0.9
-0.8
-0.8
-0.8
Case study 3: Ballena
Wind loading for this complex double curvature structure is difficult to estimate with the limited
tools available in codes. The most appropriate solution for this building would be to find the
coefficients from a CFD analysis.
B.3
Energy model details
Zone activities in DesignBuilder are shown in Tab. B.4. The occupancy rate (people/m2 ) and
gains (W/m2 ) are adjusted for each room in DesignBuilder based on the area.
The weekday schedule is shown in Fig. B.1. The weekend schedule includes more time at
home. The total occupancy in the house is approximately four during the evening, night and
morning. The occupancy is zero during workdays and the family is present on the weekends.
66
APPENDIX B. DESIGN AND CONSTRUCTION DETAILS
Table B.4: Zone activities
Rooms
Bedroom 1
Bedrooms 2 & 3
Bathrooms
Living
Kitchen
Utility
Entrance
Occupancy
(people)
Lighting
(lux)
Gains
(W)
2
1
0.5
2
2
0
0.05
100
100
100
150
300
0
0
50
50
100
100
200
100
0
Figure B.1: Weekday occupancy schedule
67
Appendix C
SOFTWARE
C.1
C.1.1
Modeling Software
Selection
The modeling software that was considered for the project is listed in Tab. C.1. Polygon modelers allow for quick manipulation of vertices, edges and surfaces. People doing 3D sculptures
use this almost exclusively as conventional parametric CAD tools are “stiff” in comparison. In
this comparison, only SketchUp and Wings3D are polygon modelers. NURBS modelers such
as Rhino3D, however, can handle complex, smooth surfaces with greater ease. Scripting is
a vital feature not only because it extends the program’s functions but also since a scripted
model includes the entire history of commands to generate it. Variations are quick and easy to
re-generate. Scripting is a critical feature for automating large projects.
ANSYS is capable of creating simple geometry. However, it is much slower and more difficult
to implement an idea than with a dedicated CAD tool. Several key functions that exist as
Rhino3D plugins, like inflating a shell, would need to be scripted.
There’s a desire to use free software, especially open-source, for affordability as well as avoiding vendor lock-ins and future compatibility. This is the reason why FreeCAD, Wings3D and
OpenSCAD were strongly considered. Unfortunately, the maturity, feature set and graphical
programming of Rhino3D can’t be matched. SketchUp, despite its ease of use and large number
of plugins, isn’t suitable for fabric forming.
C.1.2
Rhino3D Plugins
The following plugins are essential to providing full functionality of Rhino3D. They are free to
install.
Grasshopper Graphical programming environment.
Kangaroo Physics engine that adds mass points to a mesh, which can be inflated, hung under
its weight or pulled like a fabric.
Weaverbird Mesh smoothing tools.
GhPython Adds Python scripting to Grasshopper. Used for ANSYS export and some geometry building.
68
APPENDIX C. SOFTWARE
Table C.1: Modeling software
Name
Notes
Scripting
Drawing
Price
SketchUp 8
Easy to use
Groups & layers
Import DXF1
Model library
Sunlight studies
Large user community
Yes, Ruby
Yes
Free (Prop2 )
FreeCAD
Classic CAD features
Excellent scripting features
Import DXF
Not as flexible as polygon modelers
Yes, Python
Yes
Free (OSS3 )
OpenSCAD
Easy scripting language
Import DXF
Many 3D printer users
Not flexible enough for arch.
Yes, OpenSCAD
No
Free (OSS3 )
Wings3D
Many 3D shaping tools
Limited 2D tools
Quirky interface
No
No
Free (OSS3 )
Rhino3D
NURBS-based shaping
Great scripting range
Professional support
Yes, multiple4
Yes
$250 student
C.2
FEA Software
ANSYS is used in this project because of its powerful features, mature code base and the
author’s previous work experience with it. The commercial license is expensive, which would
motivate finding free alternatives given more time. Free alternatives include Elmer and Z88,
both open-source. These alternatives are vastly limited compared to ANSYS, but would be
adequate for stress and buckling analysis.
ANSYS is a collection of several applications which cover model generation, meshing, various
solvers and post-processing. ANSYS Classic is the product developed since the 1980s, which is
the most powerful in terms of features but has a legacy, user-unfriendly interface. Workbench
is the company’s fairly recent product which improves on the user interface. Unfortunately,
Workbench’s improved user interface does not translate into higher productivity because scripting is much more complex than ANSYS Classic. Therefore, all FEA scripting is done in APDL
in ANSYS Classic.
C.3
Energy Analysis Software
The analysis engine that is used by DesignBuilder, EnergyPlus, is accessible from other software.
For example, Google SketchUp and Autodesk Revit both have the ability to export to this
1
DXF is AutoCAD’s native 2D format and used widely by many others
Proprietary code base
3
Open Source Software
4
RhinoScript, Python and graphical scripting with Grasshopper
2
69
APPENDIX C. SOFTWARE
format. A free alternative is OpenStudio, developed by the U.S. Department of Energy.
Revit is a capable BIM package, with a large library of real-world building components. In
this project, the difficulty in importing a fine mesh played a key role in deciding to rebuild the
model from scratch. Since the model needed to be rebuilt, DesignBuilder offered the advantage
of having a built-in interface with EnergyPlus. Revit can import simple Rhino surfaces and
afterwards zones, HVAC details and activity gains can be incorporated. Autodesk offers a
cloud-based energy analysis solution in the form of Green Building Studio. DesignBuilder can
import 3D models in the form of gbXML, but cannot generate these files directly from CAD
geometry.
70
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