RIVETED GIRDERS 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 ver tical plates (b), 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 func tion of the web is to unite these two flanges and to take care of the 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 re sisted, t h e r e f or e, by their tension and compression stresses, which form a couple whose arm is the distance be tween the centers of gravity of the two flanges, as illustrat ed by Fig. 243.
Proportioning Flanges. Referring to Fig. 244, if the bending moment on the girder The values of f, and vary with the class of construction ill which the girders are used. These are generally specified in each ease. The usual values for different classes of construction are as follows: (tension) = 15,000 pounds per square inch, net area.
(compression) = 12,000 pounds per square inch, gross area, re duced for ratio of unsupported length to width of flange.
(shearing stress) — 12,000 pounds per square inch, net area.