1 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 column B, given by
Let (b1 x b1) and (b2 x b2) be the size of column A and B respectively. Taking moments of footing areas about the centre of column B, we get
From Eqs. 3.14 and 3.15, the unknowns L1 and L2 can be known in terms og any suitable value of B. [Alternatively, the widths B1 and B2 of each footing can be kept different, and lengths L1 and L2 may be kept equal (=L) and suitable equation on the line indicated above can be formulated to determine B1 and B3]
This pressure intensity will be uniform for both the individual footings. The slab of each individual footing is
designed as cantilever slab, having sagging B.M. in each of the cantilever portion. The strap beam transfers a part of load of footing A to footing B, in such a way that C.G. of the two loads coincides with the C.G. of the footing areas. In doing so, it is subjected to bending moment and shear force all along its length.
FIG. 3.25 GRILLAGE STRAP FOOTING.