132-1
THEORY AND PRACTICE OF ENGINEERING.
Book II.
resist the strain to which they are subjected: the feet of the main timbers are securedby iron straps, and from the outer walls are projecting corbels, to afford a firm rest to thejoists and braces which are framed into them.
The obliquity given to a rafter lessens theeffect of the weight which it bears, precisely inthe same ratio that its length diminishes itsstrength: for in all beams, when lengthened,there is an additional weight to be supported,which diminishes their strength, and in calcu-lating that of rafters, we must remember that theslight flexure produced by the transverse straindiminishes their strengthby resisting a longitu-
dinal force. /\
m -d a tarn ni d
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Fig. 2118.
In framing the feet of the principals into the tie-beam, care should be taken to placethem at some little distance from its extremities, at the same time not allowing them to beprojected a long way within the walls, which might occasion the bending of the tie-beam, anevil which often occurs when the slant of the roof is very flat; in many of our churchescovered with lead, this portion of the construction is very faulty. Every roof, howeverwell put together, must undergo some change from its original form, from the shrinking ofthe timbers, but a scientific knowledge of this fact will sometimes prevent any great evilfrom arising ; to know how much a piece of timber will yield when compressed will enableus to give it such dimensions that its form will be retained, and its efficiency continued :when any part of a roof assists in suspending the floors below, it becomes important toconsider the nature of this new strain, and to provide properly for it. In the presentexample, the upper floor forms a portion of a trussed arch, which assists in carrying theprincipal rafter at the point where the suspension rods are attached, and as long as thefeet of the rafters are maintained in their place, there is perfect security, provided also thatthe tie-beams are not drawn asunder, which hold in the wall plates and corbel timbersbeneath them. A truss of this description will bear great weight, as it forms but onepiece of framing, the timbers of which cannot pull out, each being acted upon in the mannerof a strut, and if of a dimension to resist bending, there is no danger of the whole retainingits strength and usefulness.
Iron rods are now often substituted for king-posts, and all other situations where beamsperform the office of ties, and it is advisable to introduce iron sockets whenever it is possible,to receive the ends of all braces and struts, so that the principal timbers may not be weak-ened by cutting mortises or notches to receive them. The principal rafters may be set inan iron shoe, which may' be bolted down upon the tie-beam, and every purpose of a mortiseanswered without the labour of cutting it or destroying the strength of tie-beam. Althoughiron is twelve times heavier than fir timber, it is more than twelve times stronger, therefore