Beams may be fixed, loose, or continuous. When fixed they are built into the structure; if loose they are supported only; and are continuous when they have more than two points of support in their length.
For practical purposes, a continuous beam may be considered one-half stronger than when supported at two points only.
The strength of a solid rectangular beam varies directly as its breadth, the square of its depth, and inversely as its length.
The formula for finding the breaking weight (B.W.) of a solid rectangular beam supported at each end and loaded at its centre is as follows:
B.W. = (bd2)/l × c
where b = breadth in inches;
d2 = depth multiplied by itself in inches;
l = length of beam between supports in feet;
c = the constant found by experiment on the timber in use.
Example.—Find the B.W. of a solid rectangular beam of Northern fir, 9 in. by 9 in., and 10 feet between supports, loaded at its centre.
The constant for Northern fir (Table I.) is 448.
B.W. = (9 × 92)/10 × 448
= (9 × 9 x 9 x 448)/10
= 14 tons 11 cwt. 2 qrs. 111⁄5 lbs.
One-fifth of this must be taken as a safe load, say 2 tons 18 cwt.