28. Steel trusses are built on exactly the same principles as wooden trusses, and any truss that can be built of wood can also be built of steel, but owing to the different nature of the two materials, the types of trusses best adapted to wooden construction are not the most economical for steel.
Steel trusses are generally built of angles, channels, plates and eye bars, and as any of these shapes are better adapted to resisting a tensile stress rather than a compressive stress, economy requires that the form of the truss or arrangement of the members, shall be such that there shall be as few members in compression as practicable, and that the shorter web members shall be in compression and the longer ones in tension.
Steel roof trusses having a span of less than 100 feet can generally be built more cheaply with riveted connections, or joints, and most of the arched trusses for wide spans, are also riveted together. For some types of trusses, however, the pin connection may be cheaper or more advisable construction.
Riveted trusses are almost always built of angles for the ties, and of a pair of angles or channels for the struts, the angles often being reinforced by a web plate. In pin-connected trusses eye bars are generally used for the ties, and a pair of channels, latticed or reinforced by plates for the struts.
The best form for a steel truss, and the most economical number of braces will depend in a great measure upon the inclination and construction of the roof, as well as upon the span. If purlins are used to support the jack rafters then the distance between the struts may be as great as 12 feet, but if there are no jack rafters and the planking of the roof is nailed directly to the purlins, then the latter should not be placed more than 8 feet apart, and if the roof is covered with corrugated iron secured to the purlins, then the purlins can not be more than 5 feet on centres. Whenever the purlins are more than 4 feet apart, they should come over the end of a strut or brace, to avoid bending moments, consequently the spacing of the purlins will generally determine the number of struts in each half of the truss. For this reason the same form of truss may be required for a span of 40 feet as for a span of 80 feet, but, of course, the members will not be as heavy in the forty-foot truss as in the one with greater span. The trusses shown in this chapter are mostly drawn from actual cases and give a pretty good idea of the most economical division for different spans.
When the truss rafter is subject to a transverse strain, that is, when it is loaded between the joints, the distance between the joints should not exceed 9 feet and preferably 7 or 8 feet, depending somewhat on the distance the trusses are apart.
Believing that a knowledge of the types of trusses best adapted to different conditions can be most readily obtained by means of practical examples, the author has prepared illustrations showing nearly every type of truss commonly used, which, with the explanations given, should enable the reader to select the one most economical for the support of any particular roof.