Art. 118. 



TYPES OF ROOF TRUSSES. 



207 



is, therefore, not a stable structure unless the joints are so con- 

 structed as to offer sufficient resistance. In properly constructed 

 trusses, it is assumed that the joints do not offer enough resist- 



ance to the small changes of angles between members to affect 

 the stresses in them. 



If there were a diagonal member DE (in tension) or CF (in 

 compression), the truss would be stable, because it would be 

 composed of triangles. Every truss, therefore, in order to be a 

 stable structure, should be an assemblage of triangles. Any de- 

 formation of such a truss will be accompanied by axial stresses, 

 that is, direct tension and compression. 



118. Types of Roof Trusses. Roof trusses may be 

 made in a great variety of forms, varying with the character 

 of the building, the clearance requirements and the materials 

 of construction. Fig. 149 shows some common types used in 

 mill building construction. The lower chords are usually hori- 

 zontal. The number of panels depends upon the purlin spacing, 

 the span length, and the loads. 



For timber construction, the Howe truss, Fig. 149 (d), or 

 the quadrangular truss, Fig. 149 (h), is best adapted. All of the 

 members may be made of timber except the verticals, which 

 are made of steel rods with screw ends, nuts, and washers. 



The eight-panel Fink truss, Fig. 149 (/), is by far the com- 

 monest in use; it may be made into a sixteen-panel truss by 

 subtrussing each panel. 



The triangular truss, Fig. 149 (i), is commonly used for flat 

 roofs. 



Stresses in roof trusses are usually gotten graphically, but 

 since, for a given pitch of roof, all roof trusses of a certain type 

 are similar figures, whatever the span may be, the stresses will 

 be proportional to the panel loads. A table of stresses due to 

 one-pound panel loads will, therefore, answer for all spans of 



