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  =  F_{swt} + F_{soil} + F_{dl,sur} + F_{ll,sur}  P 


Moment about X axis:  
M _{x,c}  =  M_{x,sup}  P * e_{y}  t_{ftg} *F _{y,sup} 


Moment about Y axis:  
M _{y,c}  =  M _{y,sup} + P * e_{x} + t_{ftg} *F _{x,sup} 
Where:  
L_{x}  =  Length of foundation in Xdirection 
L_{y}  =  Length of foundation in Ydirection 
A_{f}  =  L _{x} * L _{y} = Foundation area 
t_{ftg}  =  Depth of foundation 
D_{s}  =  Depth of soil above the foundation 
l_{x}  =  Length of column/wall in Xdirection 
l_{y}  =  Length of column/wall in Ydirection 
A_{c}  =  cross section of the column/wall segment 
e_{x}  =  eccentricity in X direction 
e_{y}  =  eccentricity in Y direction 
ρ_{c}  =  density of concrete 
ρ_{s}  =  density of soil 
F_{swt}  =  A_{f} * t_{ftg} * ρ_{c} = foundation selfweight 
F_{soil}  =  (A_{f}  A_{c})*D_{s}* ρ_{s} = soil selfweight 
F_{dl,sur}  =  (A_{f}  A_{c})*sc_{dl} = Dead load from surcharge 
F_{ll,sur}  =  (A_{f}  A_{c})*sc_{ll} = Live load from surcharge 
sc_{dl}  =  Surcharge in dead loadcase 
sc_{dl}  =  Surcharge in live loadcase 
P  =  axial load acting on support in service combinations 
M_{x,sup}  =  Moment acting on support around Xaxis in service comb. 
M_{y,sup}  =  Moment acting on support around Yaxis in service comb. 
A _{c}  =  cross section of the column/wall 
F _{x,sup}  =  Horizontal force acting on support Xdirection in service comb. 
F _{y,sup}  =  Horizontal force acting on support Ydirection in service comb. 


Eccentricity of base reaction in Xdirection:  
e_{Tx} 
= 
M_{y,c} / T 
Eccentricity of base reaction in Ydirection:  
e_{Ty}  =  M_{x,c} / T 
If abs(e_{Tx}) / L_{x} + abs(e_{Ty}) / L_{y} ≤ 0.167 Then base reaction acts within kern distance  no loss of contact in Xdirection, and: 



Pad base pressures:  
q1  =  T/A_{f} – 6* M_{y,c} / (L_{x}*A_{f}) + 6* M_{x,c} / (L_{y}*A_{f}) 
q2  =  T/A_{f} – 6* M_{y,c} / (L_{x}*A_{f})  6* M_{x,c} / (L_{y}*A_{f}) 
q3  =  T/A_{f} + 6* M_{y,c} / (L_{x}*A_{f} + 6* M_{x,c} / (L_{y}*A_{f}) 
q4  =  T/A_{f} + 6* M_{y,c} / (L_{x}*A_{f}  6* M_{x,c} / (L_{y}*A_{f}) 
Max base pressure: 

q_{max}  =  max (q_{1}, q_{2}, q_{3}, q_{4}) 
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 Yaxis) using segment widths.
If abs(e_{Tx}) / L_{x} ≤ 0.167 Then  no loss of contact, and: max base pressures for segment: 

q_{max}  =  T/A_{f} + max[ 6* M_{y,c} / (Lx*A_{f}) , 6*M_{y,c} / (L_{x}*A_{f})] 
Else  loss of contact and max base pressures for segment: 

q_{max}  =  2*T/[3* L_{y}* (Lx /2  abs(e_{Tx}))] 
where 


L_{y}  = 
segment width 