CHAPTER X. 



ROOF TRUSSES. 



116. Roof Construction. Where trusses are used to 

 support the roofs of buildings they are usually spaced from 

 10 to 20 ft. apart and receive their load from beams called 

 purlins, which rest directly on the top chords of the trusses. 

 The roof covering is carried by the purlins. 



The trusses may either rest on walls or on columns at the 

 sides of the building. When the trusses are supported on 

 masonry walls, provision for the expansion and contraction of 

 the trusses due to temperature changes must be made. Other- 

 wise these changes would either crack the walls or produce 

 excessive temperature stresses in the trusses. When the trusses 

 are supported on columns the}'' are usually fastened rigidly to 

 the tops of the columns, and the movements due to tempera- 

 ture changes are taken up by the elasticity of the columns, 

 producing bending stresses in them. 



The loads from the roof are more or less uniformly dis- 

 tributed along the top chords or rafters of the roof trusses, and 

 in calculating the stresses in the trusses are assumed to be 

 concentrated at the upper panel points as shown in Figs. 50 

 and 53. This assumption gives the stresses with sufficient accu- 

 racy, but in designing the top chord or rafter the bending due 

 to the purlins not coming exactly at the panel points must be 

 taken into account. The rafter is usually subjected to both direct 

 stress and bending stresses (115). 



117. Truss Elements. The simplest truss is one made 

 of three members in the form of a triangle as shown in Fig. 48, 

 Art. 50. 



Such a truss can not be deformed without changing the 

 length of its members, that is, without inducing stresses in them. 

 It is, therefore, a stable structure under loads if properly de- 

 signed (2). 



Fig. 148 shows a truss which may be deformed without 

 changing the lengths of the sides of the rectangle CDEF, and 



206 



