232 APPLIED MECHANICS 
and diagonal members alternately. The diagonals are usually, though 
not necessarily, placed at 45°. The shorter vertical members are struts, 
and the longer diagonals ties. The truss is shown inverted in Fig. 337 
to get the deck on the top. 
A modification of the Pratt truss, with the diagonals sloping the 
other way, and known as the Howe truss, is used in America. It is 
usually constructed mainly of timber. The verticals, which are now 
ties, are wrought-iron or steel bolts, and the diagonals, which are now 
struts, are of wood. 
For long and heavy spans, duplicate systems of web bracing are used. 
Fie. 336. Fig. 337. Fig. 338. Fie. 339. 
If two Warren girders of the form shown in Fig. 334 be taken, and one 
is inverted and superposed on the other, a Lattice girder (Fig. 338) is 
formed. If two N trusses of the form shown in Fig. 336 be similarly 
treated, the lattice girder shown in Fig. 339 is obtained, which is the 
girder of Fig. 338 with the verticals of Fig. 334 left in. The function 
of these verticals is to equalise the load between the two systems. In 
the type shown in Fig. 338, two diagonals in the same bay do not carry 
the same stress ; in the type shown in Fig. 339, they should. In actual 
bridges it is doubtful if these verticals really act as they are supposed 
to do. 
If two N trusses be superposed, one being moved half a bay along 
relative to the other, a 
Linville truss (Fig. a. & 
340)is obtained. Fig. z be 
341 shows the same Fia@. 340. Fi. 341. Fig. 342. 
truss with a slightly 
different end post. Fig. 342 shows the Linville truss inverted to get the 
deck on the top. 
In designing girders with duplicate web systems it is usual to separate 
the two systems, and to design each on the assumption that it carries 
one half of the load, though this assumption is not strictly correct. The 
two are then again superposed, and the stresses combined’ in those mem- 
bers which are made to coincide. 
Sometimes two girders of the type shown in Fig. 338 are combined, 
one being moved half a bay along relative to the other. Such a combina- 
tion is termed a double lattice girder. 
In small girders the web is often composed of a number of diagonal 
bars lattice braced, as shown in 
Fig. 343. Such a web may be Sth oe 
looked upon as a multiple lattice 
girder. The bracing usually con- Fic. 343 
sists of flat bars, which are made Lis 
wider and thicker toward the points of support. The usual assumption 
when designing such a web is to make the diagonals cut by any vertical 
section of such size that the vertical shear force at the section will be 
