Foundation Bearing Capacity (pad and strip base:ACI 318)
Check for Pad Base Bearing Capacity
Bearing capacity calculations are done using service (soil) -combinations.
Total base reaction:
| T |
|
Fswt + Fsoil + Fdl,sur + Fll,sur - P |
Moment about X axis:
| M x,c |
|
Mx,sup - P * ey - tftg *F y,sup |
Moment about Y axis:
| M y,c |
|
M y,sup + P * ex + tftg *F x,sup |
Where:
| Lx |
|
Length of foundation in X-direction |
| Ly |
|
Length of foundation in Y-direction |
| Af |
|
L x * L y = Foundation area |
| tftg |
|
Depth of foundation |
| Ds |
|
Depth of soil above the foundation |
| lx |
|
Length of column/wall in X-direction |
| ly |
|
Length of column/wall in Y-direction |
| Ac |
|
cross section of the column/wall segment |
| ex |
|
eccentricity in X direction |
| ey |
|
eccentricity in Y direction |
| ρc |
|
density of concrete |
| ρs |
|
density of soil |
| Fswt |
|
Af * tftg * ρc = foundation self-weight |
| Fsoil |
|
(Af - Ac)*Ds* ρs = soil self-weight |
| Fdl,sur |
|
(Af - Ac)*scdl = Dead load from surcharge |
| Fll,sur |
|
(Af - Ac)*scll = Live load from surcharge |
| scdl |
|
Surcharge in dead loadcase |
| scdl |
|
Surcharge in live loadcase |
| P |
|
axial load acting on support in service combinations |
| Mx,sup |
|
Moment acting on support around X-axis in service comb. |
| My,sup |
|
Moment acting on support around Y-axis in service comb. |
| A c |
|
cross section of the column/wall |
| F x,sup |
|
Horizontal force acting on support X-direction in service comb. |
| F y,sup |
|
Horizontal force acting on support Y-direction in service comb. |
Eccentricity of base reaction in X-direction:
|
eTx |
|
My,c / T |
Eccentricity of base reaction in Y-direction:
| eTy |
|
Mx,c / T |
If abs(eTx) / Lx + abs(eTy) / Ly ≤ 0.167
Then base reaction acts within kern distance - no loss of contact in X-direction, and:
Pad base pressures:
| q1 |
|
T/Af – 6* My,c / (Lx*Af) + 6* Mx,c / (Ly*Af) |
| q2 |
|
T/Af – 6* My,c / (Lx*Af) - 6* Mx,c / (Ly*Af) |
| q3 |
|
T/Af + 6* My,c / (Lx*Af + 6* Mx,c / (Ly*Af) |
| q4 |
|
T/Af + 6* My,c / (Lx*Af - 6* Mx,c / (Ly*Af) |
Max base pressure:
| qmax |
|
max (q1, q2, q3, q4) |
Else base reaction acts outside kern distance - 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
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.
If abs(eTx) / Lx ≤ 0.167
Then - no loss of contact, and:
max base pressures for segment:
| qmax |
|
T/Af + max[- 6* My,c / (Lx*Af) , 6*My,c / (Lx*Af)] |
Else - loss of contact and
max base pressures for segment:
| qmax |
|
2*T/[3* Ly* (Lx /2 - abs(eTx))] |
where
Ly = segment width