Essentials in the theory of framed structures . Normal pressures in pounds per square foot when pressure on vertical surface is P = 20 lb. per square foot 4° 46 I in. to I ft. 3-3 5 0 3-S 9° 28 2 in. to I ft. 10° 0 14° 2 3 in. to I ft. iS° ° 18° 26 4 in. to I ft. •5 20° 0 22° 37 5 in. to I ft. 13-4 25° 0 26° 34 6 in. to I ft. 30° 0 30° 15 7 in. to I ft. 33° 41 8 in. to I ft. 35° 0 17-3 40° 0 45° 90 Sec. II ROOF TRUSSES Sec. II. Stress Analysis for Wind S 76. Wall Bearing Trusses.—The stresses in a roof truss areusually determined by graph
Essentials in the theory of framed structures . Normal pressures in pounds per square foot when pressure on vertical surface is P = 20 lb. per square foot 4° 46 I in. to I ft. 3-3 5 0 3-S 9° 28 2 in. to I ft. 10° 0 14° 2 3 in. to I ft. iS° ° 18° 26 4 in. to I ft. •5 20° 0 22° 37 5 in. to I ft. 13-4 25° 0 26° 34 6 in. to I ft. 30° 0 30° 15 7 in. to I ft. 33° 41 8 in. to I ft. 35° 0 17-3 40° 0 45° 90 Sec. II ROOF TRUSSES Sec. II. Stress Analysis for Wind S 76. Wall Bearing Trusses.—The stresses in a roof truss areusually determined by graphic methods. Theoretically, twostress diagrams are required—one for vertical loads, and onefor the wind loads acting normal to the roof. In practice,however, a separate stress diagram for wind is seldom drawn,except for trusses supported on columns; and then only forlocalities having high winds and no snow. The reason for this. Fig. 83. will now be shown in connection with the Fink truss in The following data will be used: The truss is supportedon walls; span, 40 ft.; rise, 10 ft.; length of bay, 15 ft.; windpressure, 20 lb. per square foot on a vertical surface. Thenormal wind pressure, taken from the table in Article 75, lb. per square foot of roof surface. The length of the topchord is ft.; hence, the area subject to wind pressurewhich each truss supports is X 15 = 334-8 sq. ft.; and thetotal wind load is X = S,ooo lb.; or 1,250 lb. perpanel. A half-panel load is supported at the eaves and peak,and a full panel load at each of the three other panel points. Il6 THEORY OF FRAMED STRUCTURES Chap. Ill The horizontal and vertical components of the total loadare 2,236 and 4,472 lb. respectively. Let Vi, Hi, V2 and H2represent, respectively, the vertical and horizontal componentsof the left and right reaction (see Article 52). F. = 5°°° ^ ^ = 1,39840 Vi = 4,472 - 1,398 =
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