One plate must, of course, be made the full length of the girder. The length of the other plate is determined as previously de scribed, and a length added at each end suffi cient to get rivets equal to one-third the capacity of the plate. In this case, the net area of the plate being about 8.2 inches, the capacity is 123,000 pounds; and the required number of rivets in single shear is 10, or 5 in each line.
It should be noted that in two-web girders it is possible to have flange angles only on the outside of the web, as the only way inside angles could be riveted would be by working a man from the end in between the webs. This is ordinarily impossible on account of the small space between, and would always be too expensive to justify such designs.
Fig. 260 gives the detail of a three-web girder. This girder is in the street front of a modern steel-framed office building, and spans the large store fronts which are made possible by stopping one of the main lines of columns on top of this girder. The girder rests on col umns at each end, as shown by Fig. 261, and is symmetrical with respect to the center line. It will be noted from Fig. 261 that the column carrying the end of this girder is practically made up of two columns riveted together through their flanges. This construction permits the heavy girder to get a bearing directly over the column shaft, and continues in a direct line the axis of the column section above and the portion of this column carrying these upper sections. This girder also carries the floor beams, which frame into the bottom flange as illustrated in Fig. 262.
There are some points of a practical nature which should be noted on this detail. In a heavy girder of three webs, there are prac tical difficulties to be met with in riveting. These must be considered and provided for in making the details.
The steps in assembling this girder would be: (1) Rivet up the central portion, consisting of web and four angles.
(2) Rivet the top and bottom flange plates to this central por tion of the girder.
(3) Rivet up each side portion, consisting of web-plate and two angles.
(4) Rivet each side section to the flange plates, which have pre viously been riveted to the central portion.
It will be noted that the position of stiffeners is somewhat different from what has previously been described. The stiffeners A and B at
the end are placed so as to come down directly over the line members of the column below. The stiffeners C and D are placed so as to come over the shear plate on the column. B and D are also so placed that they can be riveted together and thus form a plate stiffener be tween the three webs. To rivet up B and D, it is necessary to rivet them together first; then rivet D to the side webs and angles C before these side webs are assembled with the central web. After the side webs are assembled, B can be riveted to the central web.
The stiffeners at the center of the girder are arranged to come under the line members of the column resting on the top flange of girder, and also to serve as plate stiffeners for the webs.
The method of procedure for riveting up these stiffeners is some what different from that used in case of the end ones. In this case, B and H would be riveted together, and then B riveted to the central web before the side webs are assembled.
In order to rivet II and G to the side webs, it is necessary to pro vide a hand hole in each side web as shown, so that these rivets can be held on the back side while being driven up after the side webs are assembled.
In three-web girders the distribution of the shear over the three webs depends to a considerable degree on the way in which the loads are applied. It is generally considered that the center web takes the larger proportion, sometimes as much as and the side webs take the remainder equally. These webs should always be stiffened so as to distribute all loads as much as possible over all three webs.
The designer, in choosing his sections, will necessarily make an assumption as regards this distribution; and this should be indicated on the diagram. Practically the pitch in all three webs and flange angles would be made the same, this being determined so as to provide for the maximum shear according to the assumption as regards dis tribution. The actual number of rivets may vary in the different por- . tions, because of angles being used which may allow of only one line of rivets, as in the case shown in Fig. 260.