Strap Footing Design
A strap footing consists of two heads and one strap connecting the two heads together. The heads are designed as isolated footings. The maximum bearing pressure is calculated for each head, and applied as a uniform pressure on the bottom surface. The bottoms of the columns are assumed fixed.
The figure below illustrates the uniform bearing pressure applied to the strap footing.
The strap is designed as a beam. It is assumed that no bearing pressure and self-weight act on the strap. The moment and shear are from the bearing pressures acting on the heads adjacent to the strap. To calculate the moment and shear in the strap, Substructure assumes a trapezoidal distribution of the bearing pressure on the bottom of the heads. This distribution creates an eccentricity between the resultant of the trapezoidal bearing pressure and the centroid of the head. In turn, this eccentricity produces two moments at the left and right end of the strap. Only the larger of the two moments is used in the flexure designSubstructure of the strap. The figure below shows the model used for strap design.
The bearing pressure applied on each side of the head is averaged from the two corners on that side. For example, as shown above, bearing pressure q1 is the average value of two corners on the left side of head 1.
The shear in the strap is calculated using the larger of the two results, R1 and R2. The strap footing is considered to act as a cantilever in the global Z-direction.