The function of the strap beam is to transfer the load of heavily loaded outer column to the inner one. In doing so,the strap beam is subjected to bending moment and shear force and ti should be suitably designed to withstand these.
Fig. 3.23 shows varity of ways in which straps may be arranged, and their choice depends upon the physical conditions of each specific case.
FIG. 3.23 COMMON ARRANGEMENTS OF STRAP FOOTINGS.
Proportioning of strap footing. Fig. 3.24 shows two columns A and B, transmitting axial loads W1 and W2 and area spaced l apart, centre to centre. Let W' be the total weight of both the individual footings. If A1 an A2 are the individual footing areas, an qs is the safe bearing capacity of the soil, we have where B is the common width of each footing an L1 and L2 are the individual lengths of the footing. The length L2 is arranged centrally under column B. The C.G. of resultant load W = W1 + W2 falls at x from the centre of...
Grillage strap footings. Strap footings are commonly constructed in reinforced cement concrete. However, for steel stanchions, grillage strap footings may be used, specially for the circumstance where the depth of footing has to be shallow and where soil has low bearing capacity. Fig. 3.25 shows typical details for such a footing.