Foundation Bearing Capacity (pad and strip base:EC2)

Tekla Structural Designer
Modified: 7 Jun 2023
2024
Tekla Structural Designer

Foundation Bearing Capacity (pad and strip base:EC2)

Annex A of EC7 allows bearing capacity to be checked using two sets of partial factors: A1 and A2.

In Tekla Structural Designer the bearing capacity check is performed on STR load combinations using set A1 and on GEO load combinations using set A2.

Alternatively, an option is also provided to check a Presumed Bearing Resistance in accordance with EN1997-1cl.6.5.2.4).

Check for Pad Base Bearing Capacity

Total vertical force:

F dz = γ G *(F swt + F soil + F sur,G ) + γ Q * F sur,Q - F z,sup
                                 

Moment about X axis:

M x,c = M x,sup +F z,sup * y 1 + h*F y,sup
                                 

Moment about Y axis:

M y,c = M y,sup +F z,sup * x 1 + h*F x,sup

Where:

L x = Length of foundation in X-direction
L y = Length of foundation in Y-direction
A = L x * L y = Foundation area
h = Depth of foundation
h soil = Depth of soil above the foundation
l x = Length of column/wall in X-direction
l y = Length of column/wall in Y-direction
x 1 = Offset in X-axis. (Distance between centre of the pad to the centre of the support in X-direction)
y 1 = Offset in Y-axis. (Distance between centre of the pad to the centre of the support in Y-direction)
γ G = 1.35 = Permanent partial factor - unfavourable action when Set A1 used
= 1.0 = Permanent partial factor - unfavourable action when Set A2 used
γ Q = 1.5 = Variable partial factor - unfavourable action when Set A1 used
= 1.3 = Variable partial factor - unfavourable action when Set A2 used
Fswt = A * h * γ conc = foundation self-weight
Fsoil = (A - A c )*h soilsoil = Unfactored load from soil
γsoil = Density of soil - user input
Fsur,G = (A - A c )*sc G = Unfactored load from surcharge for permanent loadcase
Fsur,Q = (A - A c )*sc Q = Unfactored load from surcharge for variable loadcase
scG = Surcharge in permanent loadcase - user input
scQ = Surcharge in variable loadcase - user input
Ac = cross section of the column/wall
Fz,sup = Vertical load acting on support in STR/GEO limit states- (from analysis)
Mx,sup = Moment acting on support around X-axis in STR/GEO limit states- from analysis
My,sup = Moment acting on support around Y-axis in STR/GEO limit states - from analysis
Fx,sup = Horizontal force acting on support X-direction in STR/GEO limit states - from analysis
Fy,sup = Horizontal force acting on support Y-direction in STR/GEO limit states - from analysis
   
                                 

Eccentricity in X-direction:

ex = My,c / F dz
                                 

Eccentricity in Y-direction:

ey = Mx,c / F dz

Uniform rectangular stress distribution method

Effective length in X-direction:

L'x = Lx - 2ex when ex > 0
L'x = Lx + 2ex when ex < 0
          

Effective length in Y-direction:

L'y = Ly - 2ey when ey > 0
L'y = Ly + 2ey when ey < 0
   
          

Design bearing pressure:

fdz = Fdz / (L'x * L'y )

Presumed bearing capacity method

If

   
abs(ex) / Lx + abs(ey) / Ly 0.167
   
                                           

Then Base reaction acts within middle third - no loss of contact and:

Pad base pressures:

q 1 = F dz /A - 6* M y,c / (L x *A) + 6* M x,c / (L y *A)
q 2 = F dz /A - 6* M y,c / (L x *A) - 6* M x,c / (L y *A)
q 3 = F dz /A + 6* M y,c / (L x *A) + 6* M x,c / (L y *A)
q 4 = F dz /A + 6* M y,c / (L x *A) - 6* M x,c / (L y *A)
   
                                           

Max base pressure:

q max = max (q 1 , q 2 , q 3 , q 4 )
                                           

Else base reaction acts outside middle third - loss of contact.

In this case the pressure calculations are more complex - in Tekla Structural Designer these are done using sets of equations presented in an article by Kenneth E. Wilson published in the Journal of Bridge Engineering in 1997.

Note:

Seismic combinations: The presumed bearing capacity method uses SLS combinations in the bearing checks - however as there is no clear Eurocode guidance on service factors for seismic combinations, in Tekla Structural Designer they are not currently assigned. If using the presumed bearing capacity method, to avoid the check being performed for zero loading you are advised to consider which service factors might be appropriate and update the seismic combinations manually.

Check for Strip Base Bearing Capacity

The principles used in the strip base bearing capacity calculations are similar to those for pad foundations. Only the direction X is checked (around Y-axis) using segment widths.

Design bearing pressure:

f dz = F dz / (L' x * L y )
   
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