Cyclopedia of architecture, carpentry, and building : a general reference work . action we take moments about the right end, and tofind the right reaction we take moments about the left end. Thus,if Rj and R, denote the left- and right-reactions respectively, thentaking moments about the right end, (R^X 24) - (1800 X 15) - (600 X 15) = 0, or 24R,= 27,000 + 9,000 =: 36,000; hence -^1 = —^f]— ~ 1,500 pounds. Taking moment about the left end, - R,X 24 + 1,800 X 9 + GOO X 9-- 0,or 24R, = 16,200 + 5,400 ^ 21,600; hence ^i— 900 pounds. 190 STATICS 57 As a check, add the reactions to see if the siun
Cyclopedia of architecture, carpentry, and building : a general reference work . action we take moments about the right end, and tofind the right reaction we take moments about the left end. Thus,if Rj and R, denote the left- and right-reactions respectively, thentaking moments about the right end, (R^X 24) - (1800 X 15) - (600 X 15) = 0, or 24R,= 27,000 + 9,000 =: 36,000; hence -^1 = —^f]— ~ 1,500 pounds. Taking moment about the left end, - R,X 24 + 1,800 X 9 + GOO X 9-- 0,or 24R, = 16,200 + 5,400 ^ 21,600; hence ^i— 900 pounds. 190 STATICS 57 As a check, add the reactions to see if the siun equals that ofthe loads as should be the case. (It will be noticed that reactionson trusses and beams under vertical loads are determined in thesame manner.) 2. It is required to determine the reactions on the truss rep-resented in Fig. 28 due to the wind pressures shown (2,700, 5,400and 2,700 pounds), the truss being fastened to both its supports. The resultant of the three loads is evidently a single force of10,800 pounds, acting as shown in Fig. 34. The reactions are. Fig. 34. parallel to this resultant; let Kj and Itg denote the left and rightreactions respectively. The line aho is drawn through the point 7 and perpendicularto the direction of the wind pressure; hence with respect to theright support the arms of R^ and resultant wind pressure are acand 5c, and with respect to the left support, the arms of H^ and theresultant wind pressure are ac and ah. These different arms canbe measured from a scale drawing of the truss or be computed asfollows: The angle 11a equals the angle 417, and 417 was shownto be 30 degrees in Example 3, Page 26. Hence al> = 14 cos 30^, Ic = 28 cos 30=, ac = 42 cos 30°. 191 58 STATICS Since the algebraic sums of the moments of all the forces actingon the truss aboiit the right and left supports equal zero, Kj X 42 cos 30^ = 10,800 X 28 cos 30^and E2 X 42 cos 30^ = 10,800 X 14 cos 30°. From the first equation, 10,800 X 28 ^ .^^Kj = -^—2
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