This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
One plate must, of course, be made the full length of the girder. The length of the other plate is determined as previously described, and a length added at each end sufficient 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 columns 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 practical 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 portion 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 previously 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 between 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 somewhat 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 H and G to the side webs, it is necessary to provide 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 5/8, 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 distribution. The actual number of rivets may vary in the different portions, because of angles being used which may allow of only one line of rivets, as in the case shown in Fig. 260.
The detail of connection of floor beams to girder is made special because of the awkward relation of beams to girder flanges, which relation could not be changed; only a single angle could be used for the connection if this was to be riveted on, and this had to be shipped riveted to girder rather than beam. It would have been possible to have a double-angle connection by using an intermediate plate and two side plates; but this would have added to the expense of erection, and sufficient rivets for the reaction were obtained by the single angle.
It will be noted that some rivets near these connections are shown flattened in the bottom flange to clear the flange of beams; also, in the elevation, some rivets are shown countersunk to clear the angle connection. Rivets are also shown countersunk where the cover-plates are left off, because there is not room to extend the plate beyond the last rivet without interfering with the next rivet. All such cases of countersinking or flattening rivets to avoid stiffeners or ends of flange plates, are to be avoided wherever possible, as they are objectionable and expensive. They can generally be avoided by changing the rivet spacing somewhat at such points. In the case shown in Fig. 260, the girder is such a heavy one, and the rivet spacing so close, that it was better to countersink rather than have the wide spacing otherwise necessary.
The end view shows open holes for riveting angles to the main column angles as shown in Fig. 261. This practice is objectionable for light girders, as previously noted in Part II, and where it is possible to properly brace the girder and column connection in any other way. In the case of a heavy girder such as this, where the deflection would be slight, it is not so objectionable, especially if these rivets are not driven until after the columns are carried up and the dead weight of construction is put upon the girder.
The bill of material should be carefully followed through as illustrating points previously mentioned.
Fig. 263 shows a single web-plate girder which carries the wall section over an entrance doorway, and also a column line on its cantilever end.
The center lines of the supporting column and of the column above, are shown on the plan of bottom flange. Fig. 264 shows the girder in its relation to the stonework, and the method of securing same to the girder.
The stiffeners G are arranged to come directly over the line members, and the shear angles on column below. The stiffeners A, E, and F are similarly arranged with respect to the column above carried on the end of the girder. It will be noted that this girder is not symmetrical about its center line, and therefore the detail of the whole girder is shown. It should be noted also that the concentration of loading at one end makes it necessary to increase the web greatly to provide for the shear. For this reason a 1/2-inch plate is riveted on each side over the flange angles and carried to a point beyond the cen ter of column bearing where the area of the web alone is sufficient for the shear. This end being the point of maximum moment, also, is the reason for the increased flange area here.
Floor beams frame to this girder in the same relation as in the case of the three-web girder shown in Fig. 260; but as this is only a single web, the connection angles can be riveted to the beam. As the beam must be cut to clear the bottom flange angle, this necessitates a filler between the web and the connection angles on beam.
Note that where brackets or similar riveted members occur on a girder, it is better to give a separate section for the details of riveting of these members. The end view, and sections A, B, C, and D, show the details for these brackets supporting the stonework, and show the various details necessary to conform to the position and spacing of stiffeners on the girder.
In a girder loaded as this is, there should be sufficient area in each set of stiffeners coming under the column above and over the supporting column, to provide for the shear; and these stiffeners should be fitted to top and bottom flanges.