Читать книгу Strength Of Beams, Floor And Roofs - Including Directions For Designing And Detailing Roof Trusses, With Criticism Of Various Forms Of Timber Construction - Frank E. Kidder - Страница 11

To determine the maximum safe load for a beam of known dimensions, loaded and supported as in Fig. 8 or Fig. 9.*

Rule 11.—To find safe load W, multiply the breadth of the beam by the square of the depth, both in inches, and this product by the value of A, and divide by 4 × L, in feet.

Example VIII.—What is the safe concentrated load for a spruce beam, 6 × 8 inches, fixed at one end, the point of application of the load being 6 feet from the support?

Answer.—Safe load equals 6 × 8 × 8 × 70 divided by 4 × 6 = 26,880 ÷ 24 = 1120 pounds.

Fig. 8.—Cantilever Beam Supporting Load at Its Outer End.

Fig. 9.—Another Form of Cantilever.

The beam will have the same strength whether loaded and supported as in Fig. 8 or as in Fig. 9.

To determine the SIZE OF BEAM to support a given load applied at a fixed point from the support, as in Fig. 8 or Fig. 9.

Rule 12.—First assume the depth. To find the breadth, multiply four times the load, in pounds, by the distance L, in feet, and divide by the square of the depth multiplied by the value for A.

Example IX.—What size of spruce beam will be required to support a load of 1120 pounds, applied 6 feet from the support?

Answer.—Assume 8 inches for the depth of the beam. Then the breadth will be equal to 4 × 1120 × 6 divided by 8 × 8 × 70 = 26,880 ÷ 4480 = 6 inches.