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 soil *γ soil = 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 ) |