The architect's and builder's pocket-book; A handbook for architects, structural engineers, builders, and draughtsmen

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. 1913 edition. Excerpt: ...from wind pressure, so that for ordinary construction and for spans not exceeding 40 feet it will be perfectly safe to omit counter braces. Manner of Supporting the Common Rajters--Purlins.--Before describing other types of trusses, it may be well to consider the manner of supporting the common rafters by the trusses. Occasionally it is desirable to span the common rafters from truss to truss, but as a general rule it is better construction to support them by means of large beams or purlins which span from truss to truss, as shown by Fig. 16. The trusses can be designed so that the purlins need not be more than 10 feet apart, and very often not more than 6 or 8 feet apart, so that the common rafters need not be more than 2"X4" or 2"X6" in cross-section wliile the trusses may be spaced 12, 14, or 16 feet on centres. As a rule a spacing of about 14 ft. for the trusses, and of 9 ft. 6 in. for the purlins, will be found most economical Another advantage in the use of purlins is tha: where the purlins are placed at the truss joints no cross-str;»in is brought on the truss rafters or chords, and hence the latt«r may be made much lighter than if they supported the common rafters. For wooden trusses of 60 feet span or more, purlins should ill ways be used. Th purlins should atirays be located over or close to a joint oj 'lie truss, so as not to produce a bending-moment in the trussrifter or chord. Purlins may be placed with their sides either vertical or at right angles to the plane of the roof, but the author prefers placing them with the sides vertical, as shown in Figs. 2 and 4. The best method of supporting the ends of the purlins is by means of duplex hangers, described in Chapter XXI; they may also be supported by...