CHAPTER XV 
DESIGN OF STRUCTURES—BRACED GIRDERS 
205. Open Web or Braced Girders.— Open web girders include all those 
in which the web is constructed of separate bars or members instead of 
a continuous plate. In such girders the tensile and compressive stresses 
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_ to which the shear has been shown to be equivalent are carried by ties 
and struts specially designed to take them. 
Open web girders are lighter than corresponding plate web girders. 
__ The metal in the open web is better disposed, and the girder presents less 
_ surface to the force of the wind. 
Above 60 to 80 feet span open web girders are preferable in most 
cases to plate web girders, and in very large spans they are a necessity. 
Very light girders also are often made of the open web type. 
Open web girders are, however, more costly per ton than plate web 
girders, and the latter are therefore less expensive for small spans carry- 
ing heavy loads. 
It is sometimes convenient to construct the web of a girder partly as 
a plate web and partly as an open web. If the shear is very large, say, 
at the ends, the bracing and connections are sometimes very difficult to 
design. In such cases it may be more convenient to use a plate web. 
Near the centre, however, or where the shear is small, it may be more 
economical to carry it by means of ties and struts. Such a girder is 
termed a semi-plate web girder. This form is, however, not much used, 
except in special cases. 
206. Types of Open Web Girders.—The web bracing takes many 
diverse forms, from which the various types mainly take their names. 
Examples are shown in Figs. 332 to 347. 
In the type known as the Warren girder (Figs. 332 to 335), the web 
braces form the sides of isosceles triangles, whose bases are parts of the 
booms. ‘The web members are inclined at 60° to the booms in Figs. 332 
Fig. 332. Fie. 333. Fig. 334. Fig. 335. 
and 333, and at 45° in Figs. 334 and 335. Vertical members shown 
dotted in Figs. 334 and 335 are introduced to add further support to 
the roadway. In Figs. 332 and 334 the floor or deck of the bridge is 
at the bottom, and the traffic would pass between the main girders. In 
Figs. 333 and 335 the deck is on the top. 
A Pratt or Whipple-Murphy truss is shown in Fig. 336. This is 
sometimes called an N truss. The web bracing is composed of vertical 
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