Functions of Flanges and Web. Riveted girders are made up of two general parts (a) - the top and bottom members - which are termed, respectively, the top and bottom flanges; and one or more vertical plates (6), called the web-plate, connecting the top and bottom flanges.

Girders of one web-plate are called single-web girders; of two plates, double-web girders; of three plates, triple-web girders. Figs. 240, 241, and 242 illustrate these different types.

The function of the flanges is to take the compression and tensile stresses developed in the outer fibers by the beam action. The function of the web is to unite these two flanges and to take care of the

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Fig. 240.

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Fig. 241.

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Fig. 242 shear. These functions are distinct. In a rolled beam, the stresses are considered to be distributed over the whole cross-section just as in a rectangular wooden beam; and this stress varies uniformly from the neutral axis. A rolled beam, therefore, is proportioned by using the beam formula, and determining from it the required moment of inertia. A riveted girder, however, is not a homegeneous section; the flanges are separate from the web, except as they are united to it at intervals by rivets. For this reason the stress in the extreme fibers on the compression and tension sides is considered as concentrated at the center of gravity of the flange, and the flanges are considered as taking all the compression and tension stress.

The bending moments caused by the vertical loads acting on the girders are considered as resisted, therefore, by their tension and compression stresses, which form a couple whose arm is the distance between the centers of gravity of the two flanges, as illustrated by Fig. 243.

Proportioning Flanges. Referring to Fig. 244, if the bending moment on the girder

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Fig. 243.

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Fig. 244 is M, and h is the distance between centers of gravity of flanges then


M/h =F= the tension and compression forces necessary to balance the bending moment.

If fc,. = Allowable Stress per square inch in compression, and if ft = Allowable Stress per square inch in tension; then

F/fc= Area required in compression flanges, and

F/ft = Area required in tension flange.

The values of fc and ft vary with the class of construction in which the girders are used. These are generally specified in each case. The usual values for different classes of construction are as follows: