Strap Footings

When the distance between the two columns to be supported on a combined footing becomes large, cost increases rapidly, and a strap footing, or cantilever-type footing, may be more economical. This type of footing, in effect, consists of two footings, one under each column, connected by a strap beam (Fig. 9.47). This beam distributes the column loads to each footing to make the net soil pressure with unfactored  loads uniform and equal at each footing, and with factored loads uniform but not necessarily equal. The center of gravity of the actual column loads should coincide with the centroid of the combined footing areas.

The strap beam is usually designed and constructed so that it does not bear on the soil (Fig. 9.47b). The concrete for the beam is cast on compressible material.
If the concrete for the strap beam were placed on compacted soil, the resulting soil pressure would have to be considered in design of the footing.
The strap beam, in effect, cantilevers over the exterior column footing, and the bending will cause tension at the top. The beam therefore requires top flexural reinforcement throughout its entire length. Nominal flexural reinforcement should be provided in the bottom of the beam to provide for any tension that could result from differential settlement.
The top bars at the exterior column must have sufficient embedment length to develop their full yield strength. If the distance between the interior face of the exterior column and the property-line end of the horizontal portion of the top bar is less than the required straight bar tension development length, the top bars should have standard end hooks to provide proper anchorage (Art. 9.49.5).
Strength-design shear reinforcement [Eq. (9.40a)] will be required when Vu > Vc, and requirements for minimum shear reinforcement [Eq. (9.81)] must be observed when Vu > Vc /2, where Vc = shear carried by the concrete (Art. 9.47).
For the strap footing shown in Fig. 9.47, the exterior column footing can be designed as a wall spread footing, and the interior column footing as an individualcolumn  spread footing (Arts. 9.75 and 9.76).

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