A course in structural drafting; . t is desired that a ten-sion member shall-resist a small- amountof compression also, the form of themember must usually be changed. When the loads on a simple trussare downwards, as is nearly always thecase, the upper chord is always in com-pression and the lower chord always intension. In the web system the strutsand ties alternate. through bridge is represented in thefigure. The. essential difference between aHowe truss and a Pratt truss is that inthe former all vertical members are ties(tension members), and all diagonalmembers are struts (compression mem-
A course in structural drafting; . t is desired that a ten-sion member shall-resist a small- amountof compression also, the form of themember must usually be changed. When the loads on a simple trussare downwards, as is nearly always thecase, the upper chord is always in com-pression and the lower chord always intension. In the web system the strutsand ties alternate. through bridge is represented in thefigure. The. essential difference between aHowe truss and a Pratt truss is that inthe former all vertical members are ties(tension members), and all diagonalmembers are struts (compression mem-bers) while in the latter the opposite isthe case. The method of constructinga diagram of .stresses ,is practically thesame for both trusses. The duties of diagonal and verticalmembers in the Howe truss are the re-verse of what they are in the Pratttruss. The maximum stress in anyvertical web member of a Howe truss,and. in the diagonal meeting it at theupper chord, occur ? when the joint atthe foot of the vertical member and all. Fig. 47. Diagram of Howe Truss ? The favorite style of truss now usedfor moderate spans is what is commonlyknown as the Pratt truss which waspatented in 1844 by Thos. W. and CalebPratt. As a metal structure it possessesadvantages over all other forms oftrusses THE HOWE TRUSS. The Howe truss was devised by Wil-liam Howe, in 1840. It is an excellentform and is much used in this countryin localities where timber is cheap. Fortrusses constructed entirely of metal,however, it is not as economical as thePratt truss. In the modern types of the Howetruss the lower chord is usually con- structed of metal. Such a truss for athe joints at the right are loaded, theothers being unloaded. PRATT TRUSS. The vertical web members of thePratt truss are struts while the diagonalmembers are lies. In a Pratt truss, the maximum stresswill occur in the diagonal web memberin any panel when all joints at the rightof the panel are fully loaded and thejoints at the left of it are
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