This section is from the book "Building Construction And Superintendence", by F. E. Kidder. Also available from Amazon: Building Construction And Superintendence.

The safe loads for rectangular wooden beams supported at both ends and uniformly loaded over the entire span, are computed by the formula:

Safe load = 2 X breadth X square of depth X A / Span in feet.

the breadth and depth being taken in inches.

Values for A are given in the fifth column of Table I. When the load is applied at the centre but one-half of the load obtained by the above formula should be allowed.

. For a canti-lever beam fixed at one end and uniformly loaded, take one-fourth of the value given by the formula, and if loaded at outer end take one-eighth.

Thus, if a beam of 12 feet clear span will support 1,000 pounds distributed load, it will support only 500 pounds at the centre, while the same beam fixed at one end and projecting 12 feet will support a distributed load of 250 pounds, or a load at the outer end of 125 pounds. To find the safe load for a cylindrical beam divide the load for a corresponding square beam by 1.7. For other methods of loading the reader is referred to the formulae in the Architects' and Builders' Pocket Book.

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