When there is no ceiling to support, trusses of the type shown by Figs. 35 and 36, may be built with wooden rafters and wrought iron ties. Such trusses present a light appearance, offer a very practical form of construction, and are just about as desirable, for wooden roofs with moderate spans, as steel trusses while they are much cheaper.
Fig. 39 - For Spans Up to 30 Feet.
Figs. 39 and 41, show examples of such trusses, suitable for many places. The dimensions given in Fig. 41, are for yellow pine or Oregon pine timbers and wrought iron rods and are ample for a slate roof, the trusses to be spaced from 12 to 14 ft. on centres.
Trusses like Fig. 39 are sometimes seen with the rods C and D continuous. They should not be made in this way, however, as the stress in C is greater than that in D. The best way of making the connection at joint B is shown by Fig. 40, a cast-iron shoe being fitted to the end of the strut to receive the pin. For the truss shown by Fig. 41, a shoe made as shown in the detail drawing will make a better connection for the rods, two of the rods being placed outside of the brackets and three between the brackets. For a truss with a single strut, a turnbuckle on the rod E. Fig 39, will be sufficient to tighten the rods. When there are three struts, there should be five turnbuckles, as in Fig. 41.
Fig. 40. - Detail of Joint B.
A cast-iron shoe should be made to receive the foot of the rafter, and the rods secured to a pin passed through the shoe and the rafter. At the apex of the truss, a cast-iron shoe and pin should also be used when the rods are in pairs, but when single rods are used, as in Fig. 39, they may be crossed and passed through a cast-iron washer, as shown. The pins which receive the ties should be computed for shearing, bearing and bending moment.