side of cross-section and f the ultimate resistance to compression, in pounds per square inch. To obtain the safe resistance, divide the ultimate resistance by 6. Approximate values of from 75 to 80 per cent. of those given for Georgia Pine may be assumed for White Oak posts. Girders. The condition of stress sustained by a girder is that of a beam supported at both ends and carrying a load evenly distributed, or concentrated at one or more points throughout its length. To compute the dimensions of an evenly loaded wooden girder use the following formulas: Breadth in inches = 3 LX W where B breadth in inches. What breadth of pine girder 12" deep will be required to carry a distributed load of 4,000 pounds when the span is 18 ft. and the fibre stress is not to be more than 800 lb. per sq. in.? Therefore, a girder 6′ × 12′ would be required. When the girder has its load concentrated at the center as is the case when it carries at its center a column or post, it will carry but half the load it is capable of sustaining when the load is evenly distributed. The formulas for girders when the load is concentrated at the center are The When the girder has a concentrated load at some point not at the center, it will carry more than if loaded at the center. strength of the beam is then expressed in the following ratio; mn: L\2 W ::: W when m = the distance of the concentrated 2 load from one end of the beam. n = the distance of the concentrated load from the other end. W = the safe load at the point indicated, and the other letters have the same values as before. The formulas for girders when the load is concentrated at some point away from the center are Safe Loads, in Tons, Uniformly Distributed for Wood Beams. In accordance with the Building Laws of New York, Chicago and Boston. F= Maximum Fibre Stress. L = Distance between supports, in inches. Safe Loads Uniformly Distributed for Rectangular Spruce or White Pine Beams, One Inch Thick. The following table has been calculated for a maximum fibre stress of 750 lbs. per sq. in. : taking a safety factor of 4. To obtain the safe load for any thickness: Multiply values for 1 inch by thickness of beam. To obtain the required thickness for any load: Divide by safe load for 1 inch. For oak, increase values in table by . For Georgia pine, increase values in table by %. 6" 5678 Span in 999 Feet. Depth of Beam. 7" 8" 9" 10" 11" 12" 13" 14" 15" 16" 600 820 1070 1350 1670 2020 2400 2820 3270 3750 4270 9 12 13 18 19 20 21 940 1090 1250 1420 920 1090 1280 1490 1710 1940 |