Strength of Timbers - Carpenter's and Builder's Math, Plans, and Specifications - page 101

fiber-stress is known. For a beam with a loading that is evenly dis-
tributed along its span, this is the equation to use:

d 2

f × b ×

W =

L
where the following is true:
W

9 ×

= allowable evenly distributed loading, in pounds

f = allowable fiber stress in pounds per square inch

b = width of the beam, in inches

d = depth of the beam, in inches

L = length of the span, in feet

Figure 4-11 defines the dimensions of a beam— b , width; d , depth;
and L , length.

WIDTH

DEPTH b

LENGTH

d

L

Figure 4-11 A beam resting on knife-edge supports illustrates
the terms length, width, and depth.

Example What will be the maximum allowable load on a beam
whose nominal size is 6 inches × 12 inches and whose actual size is

5 1

/ 2 inches, with an 18-foot span? The timber is to
be 1500f stress-rated. This is the calculation:

/ 2 inches

× 11 1

1500 × 5 . 5 × 11 . 5 × 11 . 5

= 6735 pounds

9 × 18

If the loading is to be concentrated at the center of the span, use
one-half the load as calculated by the formula given. Therefore, for
the same timber as calculated, with the same span, the allowable
concentrated load will be
6735
2

= 3367 pounds