Wednesday, November 14, 2012

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 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.


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