The choice between the use of bolts or of pins for truss connections is determined in each case by the relative cost of manufacture and erection. Generally for small trusses bolted connections are very much the cheaper, but for trusses of long span, where erection may be difficult or expensive, pin connections can be employed to greater advantage.
The most common form of rafter is one made up of two angles arranged as shown by Fig. 9, and having gusset plates between the adjacent flanges at the panel point connections. If the load is uniformly distributed, this form of rafter should have a continuous web riveted between the angles to resist the bending strain, instead of the separate gusset plates at connections only. When the loads are concentrated at the panel points they should be located as nearly as possible directly over the sub-struts.
For all ordinary cases the bottom chords of roof trusses may be made of two angles placed back to back. If, however, it is desired to have stiff chords to which weights can be safely attached at any point, two channel riveted back to back should be used. When jib cranes are used there should be a horizontal bracing between the roof trusses in the plane of the bottom chords.
Angles, channels, Z-bars and I-beams are all used for purlins. Angles, channels and Z-bars are fastened to the rafters by angle clips, Fig. 10, but I-beams are usually bolted directly to the rafter. An angle trussed with a tension rod and center strut is a form of purlin sometimes employed, but ordinarily simple shapes without trussing are preferable, since too much trussing and bracing injures the appearance of the roof and adds to the cost of manufacture. It is more economical to use simple shapes even at the expense of increasing the weights slightly than it is to introduce trussing. When the distance between rafters is more than about 15 ft. a line of 5/8-in. rods should be run from the ridge through the purlins to prevent them from sagging in the plane of the rafters. At the gable walls a single angle may be built into the masonry and the purlins attached to it by clips as they would be attached to a rafter.
Fig. 10. Clip Connection Between Purlin and Rafter.
The best way of placing angle purlins on a sloping roof is as shown in the sketch, Fig. 10a. In this position it has a greater vertical moment of resistance than if the roof leg were placed in a reverse position, as in Fig 10b. To rivet the overlapping ends of the corrugated iron on both sides of the angle purlin, as shown in the sketch, Fig. 10c, and securing the covering to the angle by means of a bent iron, passing around the purlin, makes altogether a very much tighter piece of work than for a single clinch nail to be driven through the sheathing and bent around one leg of the purlin. In order to protect the overhanging corrugated iron at the eave from teing battered and getting out of shape it is desirable to extend the upper chord angles of the trusses out far enough to receive an outside purlin placed as nearly as possible at the edge of the sheathing. This overhang need not be greater than 12 or 15 ins., and if a slightly better appearance is desired a molded sheet metal cornice may be used.
For dead and for live load stresses a factor of safety of four is sufficient. For greater combinations such as dead, live, wind and crane loads combined, a factor of safety of three should be used. The temporary buildings for the Columbian Exposition at Chicago were proportioned for unit tensile strains of from 20,000 lbs. to 25,000 lbs. per square inch of section.