A Treatise on the Strength of Bridges and Roofs; With Practical Applications and Examples, for the Use of Engineers and Students (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1877 Excerpt: ...of inclined trusses or rafters, joined together at the apex and to the walls by one chord only. Let I = the horizontal distance, BC, between the walls, I' = the length of each rafter, h = the height of the apex, A, above the walls, w=the whole weight, uniformly distributed over the two rafters. The rafter to the right of A is kept in equilibrium by the vertical reaction of the wall, --, the weight on the rafter, --, and the inward reaction of the wall; hence, taking moments around A, Ave have for the moment of the latter, the difference between the moments of the other forces, or, 2 2 2 4 H=, (229) is the horizontal reaction of either wall. Taking moments around D, in the line of the tops of the walls, it will be found that the horizontal thrust at A is the same. iso.--Had the rafters been joined by both chords at A, it would be impossible to determine the proportion of strain to which each chord would then become subject. In such a case one rafter could not bear an equal or certain fixed proportion of strain to the other unless the mechanical construction possessed a theoretical precision almost unattainable in practice; and even were this done, the different expansions and contractions of the two chords would immediately affect the amount of strain upon each. All ambiguity of strains in trusses must, where practicable, be avoided, and we shall therefore consider the rafters as joined together by one chord only. 151. The proportion between the Horizontal and Vertical Reactions of the Wall.--To find the proportion between the horizontal and vertical reactions of the wall, we have, wl w., I--: -:: h: --8/i 2 4 Draw, in Fig. 58, the vertical line BE = //, and tlie horizontal line EF =--. If BE represent the amount of vertical reaction, or--, then EF will re...