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Steel Portal Frame EC3 RUNET software
The dimensions of the fundament Bx width lengthwise, By width in transverse direction, Bh the height of the fundament, and the size of the reinforcing bars of the fundament. The program may change the footing dimensions chosen by the user to fulfil the design criteria for the foundation design. If you don’t want a footing dimension to be changed then check the box next to the dimension.
Soil properties, soil bearing capacity qu [N/mm
2
], soil unit weight γ KN/m
3
, and soil angle of internal friction φ, can be selected by click .
Foundation depth hf is the depth of the bottom of the fundament. This depth is used for the computation of the passive earth resistance.
The high horizontal forces acting at the base are acting outwards as a result of bending in the columns due to vertical loading on the roof.
This is resisted in two ways.
Steel tie at column base A tie cast into the floor slab connected to the base of the columns. This should be considered more safe method to resist the horizontal forces at the base of the columns
Passive earth pressure on the side of the foundation.
In this case the earth filling and compacting on the side of the foundation must be performed carefully, so that the passive earth pressure is not reduced. The fundament transverse width By and the height Bh are used to compute the active area for passive earth pressure.
It is advisable to check the desired fundament height Bh on the side and let the dimensions Bx and By to be adjusted by the program. Bx and By are adjusted so the fundament weight has enough weight to resist uplift, (the foundation is also an important factor). By is adjusted also for adequate passive earth force to resist the horizontal base force outwards.
13.7.1 Foundation bearing resistance
The basis for the design of foundations is the bearing resistance of the soil.
The design bearing resistance may be calculated using analytical or semi-empirical methods.
Annex D of Eurocode 7 EN1997:2004 describes a method of obtaining the design bearing strength of the soil.
The methods of Annex D for drained and undrained conditions are implemented in the program.
The Design bearing strength of the soil is estimated for EQU, STR and GEO conditions.
The computation of design bearing strength is for drained and undrained soil conditions.
For drain soil conditions the important soil property is the angle of shearing resistance φ and the cohesion intercept c[kPA]. For undrained soil conditions the important soil property is the undrained strength c u
[kPa]. k
[°]
For the computation of design bearing strength other parameters are the dimensions and foundation depth of the footing, as well as the loading and the load eccentricities.
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RUNET Software www.runet-software.com 24
Steel Portal Frame EC3 RUNET software
In the foundation design of the program for the soil strength we use the soil bearing pressure q uk
(N/mm
2
)
. This is corresponding soil strength to the soil allowable pressure.
In the foundation design we use as design bearing soil pressure
q
ud
=q
uk
/γ
qu
, where
γ
qu
is the partial factor for unconfined strength. (Eurocode 7, Annex A). So to be consistent in order to convert the design strength estimated from Annex D of Eurocode7 to the soil bearing pressure used in the program the design value have to be multiplied by
γ
qu
,
γ qu
=1.40 for EQU and 1.00 and 1.4 for STR-GEO.
q
uk
= q
ud
. γ
qu
.
Click in the design of fundaments, and you get into a calculation window for design bearing resistance.
There you have an estimate of the soil bearing resistance q uk
which you may use in the program, from the soil and fundament parameters.
If there you check to include the calculations in the report, then the design bearing resistance will be set to the minimum estimated and the calculations will be included in the report of the footing design (remember that if you alter the dimensions or loading you have to reevaluate q uk).
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RUNET Software www.runet-software.com 25
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