Spread Footings

The preceding results will be applicable to built footing courses only when the pressure above the course is less than the safe crushing strength of the mortar (see § 106 and § 255). The proper projection for rubble masonry lies somewhere between the values for stone and for concrete. If the rubble consists of large stones well bedded in good strong portland-cement mortar, then the values for this class of masonry will be but little less than those given for stone in Table 61; but if the rubble consists of small irregular stones laid with portland-cement mortar, the projection should not much exceed that given for concrete. Footing courses should not be laid of small stones in either natural-cement or lime mortar.

Brick Footings. The off-sets in Table 61 for brick-work are the combined off-sets of one or more projections in terms of the total thickness of the one or more projections.

Plain Concrete Footings. A concrete footing should be built as a monolith for its full depth, since the deeper the beam the greater its strength; but the outer upper coiner may be stepped to save concrete, provided the combined projection in any case does not exceed that given by Table 61 or a similar computation.

After the safe length of the off-set of the footing has been determined, it should be examined to see if it is safe against failure by shearing. Footings are subjected to heavy loads and conse quently to great shearing stress, which should be carefully provided for. For the shearing resistance of stone, see § 20; for brick, see § 82; and for concrete, see § 408.

Eccentric Footing. It is frequently desired to place the outer face of the wall upon the exterior boundary of the lot; and in such cases, if it is impossible to secure permission of the adjacent owner to extend the footings into the adjoining property, it becomes necessary to support the wall upon an eccentric footing somewhat as shown in Fig. 83. Of course, with this arrangement the pressure upon the soil will not be uniform along the line AB. The pressure at A may be computed by equation 1, page 354, and that at B by equation 2, page 354, by noticing that M = W e. W is to be regarded as the weight of a unit length, say 1 foot, of the wall and the footing; and S in the above equations is the area of a unit of length of the footing, which in this case = 1 x AB = 1.

The value of e is shown in Fig. 83.

Since the pressure at A is considerably more than the average upon the footing (see !: 692), this method can not be employed for a heavy building upon a soft soil; and it should never be used except as a last resort, since better results are likely to be obtained when the pres sure upon the soil is uniform under the footing.

Cantilever Footing.

A modification or rather an improve ment upon the eccentric footing described in the preceding section was developed at Chicago. The improvement was devised especially • for tall steel-skeleton buildings, and is usually employed to support columns rather than walls. The lower end of an exterior column, which supports the heavy exterior walls, rests upon the short arm of a gigantic cantilever beam which in turn is supported upon a founda tion built at a considerable distance inside of the lot line, a lighter loaded interior column •resting upon the long arm of the canti lever. This method has been frequently employed in Chicago and elsewhere. Obviously it could be used to support a masonry wall by building the wall upon a longitudinal beam or girder and supporting the latter at intervals upon a Series of transverse cantilevers.

Reinforced Concrete Footing.

In designing a reinforced concrete footing for a wall, the first step is to adopt a ratio of the working stress of the steel to that of the concrete (§ 470-71 and 474-75); and the next is to assume a depth for the beam and compute the per cent of steel, p, by equation 10, page 228, and also the position of the neutral axis, k, by equation 3, page 227, and then compute the resisting moment by both equation 5 and 6, page 227. If the safe resisting moment of the cantilever does not agree with the moment of the pressure on the soil, which in the nomen clature of ; 696, is 4 P then a new depth must be assumed and the computations as above must be repeated.

After the off-set has been adjusted to resist the bending moment, its shearing strength should be tested (see 4 458 and 476-77). The bond stress should also be tested (see 4 455-57 and 472-73).

The upper outer edge of the footing may be stepped to save concrete; but as each reinforcing bar is likely to extend the full width of the footing, each off-set need be tested only for shear.

The above relates to the footing under a wall; and if a square footing for a column is to be designed, notice that the corners will have a projection of 1.4 times that of the sides, and that there fore the thickness of the footing should be equal to approximately 1.4 times that for a wall giving the same average pressure on the soil, and in a diagonal unit section there should be approximately as much steel as in a section perpendicular to the side of the footing.