GIRDERS 40. Description and Classification. A girder may be designated as a girder or a plate-girder, and each of these classes are again divided into simple and box girders. These and all girders may be divided into deck and through girders. A girder is a beam which consists of a channel, or an I-beam, or any other single rolled shape.
A plate-girder is a beam which consists of a plate called a web plate, with a combination of angles, or one or more plates and angles, riveted on the outer edges. These are called the flanges. The angles are called the flange angles, and the plates are called flange-plates or cover-plates. In case the web is deep, angles are riveted ver tically on its sides at intervals along its length. These are called stiffener angles. Fig. 50 gives sections of plate-girders; and Fig. 51 gives a side view of a plate-girder with cover-plates. These cover-plates are not always the same length, for reasons that will be explained later. At the ends of one of the plate-girders, are small plates which are placed between the girder and whatever it rests upon. These are called masonry plates or wall-plates.
When two girders or plate-girders are placed close together and riveted up, they form a box girder. Fig. 52 gives sections of box girders, and (C 152-156, 161, and 162) are tables giving the carrying capacity of box girders when loaded uniformly. Box girders are much used in building work to carry walls and elsewhere where uniform loads come upon them; but they are seldom used for concentrated loads, on ac count of the fact that beams cannot be connected to their webs, as the riveters cannot get tools inside to rivet up the connections, and, if the beams rest on top of them, one side is liable to bend more than the other, and the stress will be far greater than that for which they were de signed.
41. Design of Girders. Since box girders should be used for uniform loads only, and since the unit-stress (C 152-162) of 15,000 pounds per square inch is good practice, no other method except that of the handbook will be given here.
Suppose that the distance center to center of columns was 20 feet, and that the girder was to carry an 18-inch brick wall 30 feet high. In (C 58) it is found that such a wall weighs 168 pounds for each square foot of wall surface.
The total weight carried is, then, 30 X 20 x 168 =100,800 pounds. To reduce this to tons, divide by 2,000, the number of pounds in a ton. This . gives 100,800 -4- 2,000 ----- 50.4 tons. In (C 154) under the 15-in. 42-pound box girders with two 14-in. by plates, it is found that for a span of 20 feet the girder will carry 53.04 tons, includ ing its own weight, which, according to the sec ond column, is 1.47 tons. This gives a net carry ing capacity of 53.04-1.47 =51.57 tons, which is as close as we can get. This beam will be used.
For a good example in design, see (C 151). An inspection of the box plate-girders shows that they are all too heavy for the load and span as given above.
For example, suppose that the wall to be car ried in a certain case was 381,620 pounds, and that the span was 35 feet. The load is 381,620÷ 2,000=190.81 tons. On the right-hand side of (C 162), across from the span of 35 feet, is found 188.10 tons in the first column. The girder weighs 6.43 tons, so that the net carrying capa city is 188.10-6.43=181.67 tons. In the third column it is seen that if the flange plates are in creased in thickness 1/16th inch, the girder will carry 10.45 tons more, thus making a net capa city of 181.67+10.45=192.12, which, being slightly greater than the required capacity of 190.81, shows the section is all right. The girder section will now be: 2 web plates 42 in. by .1 in.; 4 angles 5 in. by 3-1. in. by in.; 2 cover-plates 30 in. by in.
The increasing of the cover-plates 1/16th inch, also, as noted in the extreme right-hand column, increases the weight of the girder 0.22 ton. This is too small to be considered, and should be used only in computing the weight of the girder to see how much it would cost.