DESIGN OF PARTS OF THE STRUCTURE 213 



although the portal bracing is sometimes made continuous. Stiff brac- 

 ing is often placed between the trusses in the plane of the center of 

 the building and materially stiffens the structure (see Fig. I75J. 



PURLINS AND GIRTS. Purlins are made of channels, angles, 

 Z-bars and I beams, Fig. in, where simple shapes are used. Channel 

 and angle purlins should be fastened by means of angle lugs as shown 

 in Fig. 107. I beam purlins are very often fastened as shown in the 

 A. T. & S. F. R. R. shops, Fig. 175. Z-bar purlins are bolted direct- 

 ly to the upper chords of the trusses. The channel purlin is the most 

 economical, and the I beam purlin is the most rigid. Girts are made 

 of channels, angles, and Z-bars, and are fastened as shown in Fig. HI. 

 Where the distance between trusses is more than 15 or 16 feet the pur- 

 lins and girts should be kept from sagging by running ^ or ^2 -inch 

 rods through the centers to act as sag rods, the ends of the rods being 

 fastened to the eaves and ridge (see Fig. 81). 



-Purlin 



Purlin Clip 

 FIG. 107. 



Where the columns and trusses are placed so far apart that the 

 use of simple rolled shapes is no longer economical, purlins and girts are 

 trussed. 



DESIGN OF PARTS OF THE STRUCTURE. The methods 

 of determining the sizes of the various members in a mill building will 

 be illustrated by a few examples. For a more detailed treatment of this 

 subject, see "Modern Framed Structures" by Johnson, Bryan and 

 Turneaure ; "Roofs and Bridges" by Merriman and Jacoby ; and other 

 standard works on bridge design. 



Manufacturers of structural material issue handbooks which con- 



