STRAP FOOTING OR CANTILEVER FOOTING.

A strap footing comprises of two or more footings of individual columns, connected by a beam, called a strap. When a column is near or right next to a property limit, a square or rectangular footing concentrically located under the column would extend into the adjoining property, which may not be permissible. In that case, a trapezoidal combined footing may be an alternative. However, if the distance between this column and the adjoining column is large, the combined trapezoidal footing will be quite narrow, with high bending moments. In that case, strap footing may be provided. The strap beam, connecting the spread footings of the two columns, does not remain in contact with soil, and thus does not transfer any pressure to the soil. The strap, assumed to be infinitely rigid, serves to transfer the column loads on to the soil with equal and uniform soil pressure under both footings. The individual footing areas are so arranged that the C.G. of the combined loads of the two columns pass through the combined C.G. of the two footing areas. Once this criterion is achieved, the pressure distribution below each individual footing will be uniform.

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.