TYPES OF TRUSSES 181 



those caused by an equivalent uniform dead load, thus simplifying the 

 calculation of stresses (see Table VI). 



The outline of the truss will depend upon the spacing of the pur- 

 lins, and upon whether or not the purlins are placed at the panel points 

 of the truss. The most economical and pleasing arrangement is to 

 make a panel point in the truss under each purlin. Taking the normal 

 wind load on the roof at from 25 to 30 Ibs. per sq. ft., it will be seen 

 in Fig. 112 that for Nos.2O and 22 corrugated steel, when used without 

 sheathing, the purlins should be spaced from 4 to 5 feet. If this spac- 

 ing is exceeded corrugated steel roofing supported directly on the pur- 

 lins is almost certain to leak. Where sheathing is used the purlin spac- 

 ing can be made greater. Many designers, however, pay no attention 

 to the matter of placing the purlins at the panel points, the upper chord 

 of the truss being stiffened to take the flexural stress. 



In Fig. 83, (a) shows the form of a Fink truss for a span of 30 

 feet; (b) for a span of 40 feet; (c) for a span of 50 feet; (d) for a 

 span of 60 feet ; and (e) for a span of 80 feet, on the assumption that the 

 purlins are spaced from 4 to 5 feet, and come at the panel points of 

 the truss. If trusses with vertical posts are desired the triangular 

 trusses (h) and (j), or Fink truss (f) may be used. The truss shown 

 in (i) is occasionally used for long spans, although it has little to rec- 

 ommend it except novelty. The truss shown in (k) is used where 

 there is ample headroom. The quadrangular truss shown in (1) and 

 the camels back truss shown in (m), are used for long spans where the 

 appearance of the truss is an important feature, as in convention halls 

 and train sheds. The lower chords of mill building trusses are usually 

 made horizontal, but by giving the lower chord a camber, as in (g), the 

 appearance from the side is greatly improved. 



The "saw tooth" or "weaving shed" roof shown in (a) Fig. 84, 

 has been used abroad for many years and is now coming into quite 

 general use in this country for shops and factories as well as for weav- 

 ing sheds, as indicated by the name. The short leg of the roof is made 

 inclined as in (a), or vertical as in (b), and is glazed with glass or 



