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Designing the Footing

beam, center, beams, feet and footings


The term footing is usually understood as meaning the bottom course or courses of concrete, timber, iron, or masonry employed to increase the area of the base of the walls, piers, etc. What ever the character of the soil, footings should extend beyond the fall of the wall (1) to add to the stability of the structure and lessen the danger of its being thrown out of plumb, and (2) to distribute the weight of the structure over a larger area and thus decrease the settlement due to compression of the ground.

Offsets of Footings. The area of the foundation having been determined and its center having been located with reference to the axis of the load, the next step is to deter mine how much narrower each footing course may be than the one next below it.

The proper offset for each course will depend upon the vertical pressure, the transverse strength of the material, and the thickness of the course. Each footing may be regarded as a beam fixed at one end and uniformly loaded. The part of the footing course that projects beyond the one above it, is a cantilever beam uniformly loaded. From the formulas for such beams, the safe projection may be cal culated.

Stone Footings. Table 10 gives the safe offset for masonry footing courses, in terms of the thickness of the course, computed for a factor of safety of 10.

To illustrate the method of using the preceding table, assume that it is desired to determine the offset for a limestone footing course when the pressure on the bed of the foundation is 1 ton per square foot, using a factor of safety of 10. On the table, opposite limestone, in next to the last column, we find the quantity 1.9. This shows that under the conditions stated, the offset may be 1.9 times the thickness

of the course.

Timber Footing. The rise of the transverse timbers (Fig. 17) may be calculated by the following formula: p' Breadth in inches X X A X s in which w = the bearing power in lb. per sq. ft.; p = the projection of the beam in feet; s = the distance between centers of beams in feet; D = the assumed depth of the beam in inches; A = the constant for strength which is taken for Georgia pine at 90, oak 65, Norway pine 60, white pine or spruce 55.

Steel Footings. The dimensions of the I-beams, Fig. 18, can be calculated by the usual formulas, by means of the strain to which the part of the beam in cantilever is submitted. The safe load per running foot is given by the expression S I 1 — — -- 6 m X in which W = load in pounds per running foot; S = 16,000 lb. per sq. in., extreme fibre strain of beams; distance from center of gravity of sections to top or bottom; I = moment of inertia of section, neutral axis through center of gravity; z = span in feet.

A ready method of determining the size of the beams is by com puting the required coefficient of strength, and finding in the tables furnished by the manufacturers of steel beams the size of the beam which has a coefficient equal to, or next above, the value obtained by the formula. C, the coefficient, is found by the following expression: C=4XwX? X in which w = bearing power in pounds per sq. ft.; p = the projection of the beam in feet; s = the spacing of the beam, center to center, in feet. Table 11 gives the safe projection of steel I-beams spaced on I foot centers and for loads varying from 1 to 5 tons per sq. ft.