Foundation Bearing Capacity (pad and strip base:ACI 318)

Tekla Structural Designer
Modified: 2 Jun 2023
2024
Tekla Structural Designer

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

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