Depending on the footing type selected, Substructure uses either the calculated maximum bearing pressure or maximum calculated pile reaction for the footing design. For flexure, one or two-way shear designs, Substructure allows the eccentricity between the footing centerline and column centerline. However, the eccentricity must be given in the global X-direction. For spread footing, uniform maximum pressure is applied at the bottom surface of the footing. Negative bearing pressure is not considered for spread footing design. For pile cap footing, the maximum pile reaction is applied at each pile location. Negative pile force is not considered for pile cap footing design.

The effect of self-weight and surcharge is not considered for the footing design, namely flexure and shear (one and two-way) designs. This means that the bearing pressure or pile reaction used in the footing design is without the contribution from the self-weight and surcharge.

When calculating either bearing pressure or pile reactions, the forces and moments at bottom of the column (top of footing) are used.

The figure below illustrates the auto design layout of footing reinforcement where Bar Dist (X-direction) is the distance from bottom of footing to centerline of reinforcement in X-direction (i.e., concrete clear cover + 1/2 reinforcement diameter) and Bar Dist (Z-direction) is the distance from bottom of footing in Z-direction (i.e., clear clover + diameter of X reinforcement + 1/2 reinforcement diameter in Z-direction).

Schematic Diagram Showing Auto Design Layout of Footing Reinforcement

Bearing Pressure Calculation for Footing

For isolated spread footings, forces and moments obtained from each load combination, at the top of the footing, are applied at the centroid of the footing. The footing is treated as a member with infinite stiffness. The bearing pressure at each corner is calculated, as follows:

where:

The self-weight of the footing and surcharge are added to the bearing pressure under service and factored load combinations, in addition to the pressure due to column forces and moments.

In addition, spread footing design based on the reduced effective footing dimensions and the related uniform distributed soil pressure is also supported in LEAP Bridge Concrete. See LRFD Art. 10.6.1.3 and 10.6.1.4 for more details of the theory. The option of Effective footing dimensions considered needs to be checked under Analysis Tab -> A/D Parameters -> Footing.

Nomenclature
Ag	Area of footing.
Ix	Moment of inertia with respect to global X-direction.
Iz	Moment of inertia with respect to global Z-direction.
d	Distance from extreme compression fiber to centroid of tension reinforcement.
dv	Effective shear depth used in LRFD shear calculations.
Mmax	Factored moment at critical section/checkpoints.
Asb-prv	Provided steel area at bottom.
Asb-eff	Effective steel area at bottom, after considering development length.
Asb-req	Required steel area at bottom.
Ast-prv	Provided steel area at top.
Ast-eff	Effective steel area at top, after considering development length.
Ast-req	Required steel area at top.
fs-t	Cracking/fatigue stress at top.
fs-b	Cracking/fatigue stress at bottom.
Ratio fs-t	Ratio between cracking/fatigue stress at top and allowable stress.
Ratio fs-b	Ratio between cracking/fatigue stress at bottom and allowable stress.
Vu	Factored shear force at section.
phi*Vc	Shear strength provided by concrete, having factored considered.
Bo	Perimeter of critical section for two-way shear.
Ao	Area within perimeter of critical section for two-way shear.
Avg.d	Average distance from extreme compression fiber to centroid of tension reinforcement.
Avg.dv	Average effective shear depth.
*	Warning flag if code requirement is not satisfied.