Cyclopedia of architecture, carpentry, and building : a general reference work . ermine these, construct the polygon for the four , AB ( inches long with arrowhead pointing to the left)represents the 7,800-pound force and BC ( inches long witharrowheads pointing down) represents the 2,580-pound force. Nextdraw from A and C two lines parallel to the unknown forces andmark their intersection D; then the force polygon is ABCDA andthe arrowhead on CD and DA must point upw^ard and to the rightrespectively. CD ( inches = 2,580 pounds) represents the 164 STATICS 31 force exerted
Cyclopedia of architecture, carpentry, and building : a general reference work . ermine these, construct the polygon for the four , AB ( inches long with arrowhead pointing to the left)represents the 7,800-pound force and BC ( inches long witharrowheads pointing down) represents the 2,580-pound force. Nextdraw from A and C two lines parallel to the unknown forces andmark their intersection D; then the force polygon is ABCDA andthe arrowhead on CD and DA must point upw^ard and to the rightrespectively. CD ( inches = 2,580 pounds) represents the 164 STATICS 31 force exerted on the joint by member 84; since the force actsaway from the joint it is a pull and the member is in ( inches == 5,220 pounds) represents the force exertedupon the joint by the member 36; since the force acts away fromthe joint, it is a pull and the member is in tension. We have now determined the amount and kind of stress inmembers 12, 13, 23, 24, 34 and 36. It is evident that the stressin each of the inembers on the right-hand side is the same as the ISoolbs. stress in the corresponding one on the left-hand side; hence furtheranalysis is unnecessary. 2. It is required to analyze the truss represented in (ff), the truss being supported at the ends and sustaining twoloads, 1,800 and 600 pounds, as shown. (For simplicity we as-sumed values of the load; the lower one might be a load due to asuspended body. We shall solve algebraically.) The right and left reactions equal 900 and 1,500 pounds asis shown in Example 1, Page 56. At joint (1) there are threeforces, namely, the reaction 1,500 pounds and the forces exertedby members 13 and 14, which we will denote by F^ and Fg respect 165 32 STATICS ively. The three forces are represented in Fig. 20 [I) as far as they are known. These three forces being in equilibrium, their horizontal and their vertical components balance. Since there are but two horizontal components and two vertical components it follows that (
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