234 APPLIED MECHANICS 
sloping the other way to the ordinary-diagonal ties, are called counter- 
braces. The dotted lines in Fig. 344 represent counterbraces. 
In duplicate systems of web bracing a member of the second system 
will carry the reversed stress. In Linville trusses the bracing near the 
centre takes the form shown in Fig. 347. 
208. Booms of Open Web Girders.—The booms or flanges of open 
web girders are usually somewhat similar in form to those of plate web 
girders, being made up of a number of horizontal plates. They differ, 
however, in having one or more vertical plates, called stringer or cwrtaim 
plates, which are connected to the flange plates by angles, and which 
form convenient attachments for the web bracing. The boom, in fact, is 
usually of a T or LJ section, as shown in Figs. 348 and 349. In the 
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e elee 
ee { ee 
_] ie ele ie ee } ee 
Fig. 348. Fig. 349. Fie. 350. 
compression boom the lower edges of the stringer plates are often stiffened 
by angles, as shown in Fig. 351, to prevent it from buckling. Occasion- 
ally channels are used instead of these plates and angles, as shown in 
Fig. 350. To prevent distortion LJ sections may be fitted with diaphragm 
plates, as shown in Fig. 352. —— 
A type of boom sometimes ies? aa 
adopted is shown in Figs vir Plates 
353, 354, and 365. Instead Flange lés. 
of placing the flange plates ee 7 
horizontally they are placed a Stringer Plates 
vertically. Combinations with Stiffening Angles 
angles and channels are also Bre, a6 seer: 
used. In American practice 
these vertical plates become eye-bars in the tension flange, as shown 
in Fig. 375, p. 245. This type possesses the advantage that it is a 
ie 
Plate. 
| Plan 
Fig. 353. Fig. 354. 
most convenient form to get a really good connection with the web 
bracing and for the attachment of the cross girders. Also, it does not 
eS a es 
