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 |
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Moment about X axis: | ||
M x,c | = | Mx,sup - P * ey - tftg *F y,sup |
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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. |
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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: |
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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: |
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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: |
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qmax | = | T/Af + max[- 6* My,c / (Lx*Af) , 6*My,c / (Lx*Af)] |
Else - loss of contact and max base pressures for segment: |
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qmax | = | 2*T/[3* Ly* (Lx /2 - abs(eTx))] |
where |
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Ly | = |
segment width |