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. 1894 Excerpt: ...or housings made at the joints in cutting the f1bres reduce the effective sections considerably, consequently large pieces of timber have to be used to do the required work: this adds great weight to be carried by the supports, and interrupts the passage of light in open roofs. Wrought iron ties would be oftener employed if it were not for the trouble and delay caused in getting small quantities executed and difficulty of fixing ceiling joists where such are required. Suspending the ceiling joists by iron straps to the ties is a somewhat unsatisfactory recourse sometimes adopted. Objection.--Composite roofs of wood and iron are subject to this objection, that the timber principals do not expand or contract appreciably with the changes of temperature, while the iron members do so considerably. This can easily be imagined to have the effect of lengthening the tie until it ceases to act, or until the feet of the principal rafters spread; or, again, the contraction of the tie rod may have the effect of bending wood principals or overstraining the tie rod. F Tie-beams.--The trusses that have timber tie-beams are suitable for supporting ceilings. Principal Rafters.--The principal rafters in composite roofs usually are of timber, this being convenient for fixing the purlins. Fig. 285. Typical Trusses.--The following are typical composite trusses:--For spans from 20 feet to 30 feet. Figure 285 shows a wrought iron king bolt and cast iron head, the joint at foot of struts being made by intro ducing a straining piece. Figure 286 illustrates a truss for a 20 feet span, the tensional members being of wrought iron, while the struts and principal rafters are of timber. The German truss may be used, introducing a king bolt and tie rods, those being the members in tension,...
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