This section is from the book "Notes On Construction In Mild Steel", by Henry Fidler. Also available from Amazon: Notes On Construction In Mild Steel.
The design of the struts forming portion of the intermediate bracing in trussed principals will be governed by the laws of long columns, and they will generally be found to be free from the transverse stresses which may sometimes affect the sectional area required in the main rafters; but, considered as columns, due allowance must be made for the imperfect seating or fixing of the ends of the struts, due to the exigencies of design in certain types of construction. Where a bond-fide pin end can be obtained at both ends of the strut, the strut can then be more certainly classed under pin- or round-ended columns, and calculated accordingly. But it frequently happens that the line of thrust is, from the nature of the connections, not axial, and the loss in strength should be allowed for accordingly.
These considerations lead to the adoption of a low working resistance, or of a large factor of safety, if the strength of the strut is calculated from the usual formulae.
The variation of stress in the bracing caused by unequal loading will, of course, have received attention in the preparation of the stress diagram.
The form of section for struts will vary with the dimensions of the principal, and the position occupied in the truss. The section may be a simple angle or tee, two tees back to back, kept apart by cast-iron distance pieces, and riveted through, two flats treated in similar manner (see Fig. 141); while a section consisting of four angles, arranged as shown in Fig. 144, and kept in position by special castings, has been used with success in some of the largest examples of the bow-string truss in this country. Tubular struts are occasionally used, but require connections at their ends of a somewhat special character. The use of cast iron for struts was frequent in roofs of old-fashioned design, but has been replaced in modern roof-work in wrought iron or steel by the types above referred to.
The ties are usually constructed as pure tension members, and may be of any of the sections previously alluded to and used for the main tie, as round or flat bars.
In some cases, however, angles are employed both for struts and ties, with riveted connections, and in this way an effective and economical truss (economical, that is, in cost of construction) is obtained. As all the bracing members are thus capable of resisting compressive stresses, the changes of sign in stress in the bracing arising from unequal loading are met.