DESIGN OF STRUCTURES 208 
. ‘other direction, as shown in Fig. 284. The verticals, except the centre 
one, then become struts, and the diagonals ties. 
oaks, 
Fra. 284. Fira. 285. 
_ Figs. 285-288 show the more frequent, types of trussed rafter roofs. 
‘The principal rafters are supported by trusses, consisting of struts and 
Fig. 286. Fic. 287. 
ion The upper ends of the struts divide the rafters into panels, and 
the lower ends are supported by the tie rods. The trusses mipportang 
the two opposite raf- . 
_ ters are held together 
_ by the main tie rod. 
This type of truss has 
the advantage that 
the struts, which are 
usually perpendicular 
to the rafters, are 
_ short. For a given span and a given system of loads, this type of truss 
probably makes a lighter roof than any other type of principal. 
he design shown in Fig. 285 is 
_ used for small spans up to, say, 20 or | 
30 feet. The design shown in Fig. 
286 may be used for spans up to 40 
feet, and that in Fig. 287 up to 
45 feet. The truss shown in Fig. 288 Bae 
is known as the French, Belgian, or Fink truss. It is a very common 
design for trusses of from 40 to 60 feet span. A light suspension rod, 
Fie. 288, 
Fra. 290. 
q shown dotted, is often introduced to give support to the main tie rod. 
______ Two types of curved roof truss are shown in Figs. 289 and 290 ; that 
