The method of calculation is the same as for wooden beams, except that, since the strength of I-beams is not readily computable except by reference to tables in the handbooks published by the manufacturers, such tables will be utilized. The tables always give the safe load which may be carried on an I-beam of given dimensions on any one of a series of spans varying by single feet. If we call W the total load (or upward pressure) to be resisted by a single cantilever beam, this will be one-fourth of the load which can safely be carried by a beam of the same size and on a span equal to the offset.

187. Example

Solve the previous example on the basis of using steel I-beams.

The offset is necessarily 5 feet 3 inches; at 1,000 pounds per square foot, the pressure to be carried by the beams is 5,250 pounds for each foot of length of the wall. By reference to the tables and interpolating, an 8-inch I-beam weighing 17.75 pounds per linear foot will carry about 28,880 pounds on a 5 foot 3 inch span. One-fourth of this (or 7,220 pounds) is the load carried by a cantilever of that length. Therefore, 7,220 5,250 = 1.375 feet =16.5 inches, is the required spacing of such beams. When comparing the cost of this method with the cost of others, the cost of the masonry concrete filling must not be overlooked.