It has already been mentioned that the ends of iron girders, especially those of cast iron, must not be rigidly fixed unless they have been designed as girders to be fixed at the ends, for otherwise stresses will come upon them which they were not designed to bear.
The ends of girders resting upon walls should be supported by hard stone templates, and may be bedded upon sheet lead or upon two thicknesses of asphalted or tarred felt, and if they are required to afford a tie to the structure, they may be secured by a bolt in a similar manner to the roof in Fig. 373.
Girders of over 50 feet span should have cast-iron shoes upon the ends so that they may slide when the girder alters length under changes of temperature, and large girders should have one end supported by rollers under a casting.
When the ends of girders meet over columns or piers they should be connected as shown in the Figs, below.
Fig. 263 shows two plate girders meeting over the head of a column, on which is a special casting leading to the floor above, through which casting the ends of the girders are connected by bolts as shown.
A rolled cross girder resting upon the lower flange of the main girder may be secured by a piece of angle iron, as in Fig. 263a;.
When, however, the level of the floor or other reason requires that the cross girder should be higher, it may rest upon the top flange of the main girder, when it may meet another cross girder running in the same direction and be connected to it by two flat plates on the web, as in Fig. 263b.
When the cross girders are shallower than the main girder they may be connected, as in Fig. 264, or if the construction of the plan requires that they should be higher, they may be supported upon an angle iron bracket, as in Fig. 265, a.
If the cross girders are of the same height as the main girder they may be connected, as in Fig. 265, b.
As a rule girders should be connected by the webs, not the flanges. The latter arrangement would often be inconvenient, and would in many cases be equivalent to fixing the ends, or making the girders continuous, which would be objectionable.
When possible, the holes for bolts, etc., should be made as near as possible to the neutral axis or centre line of the girder, so that they may be clear of the direct stresses upon it.
Further Particulars regarding Wrought-iron Girders.
The depth of plate girders varies from 1/10 to 1/15 the span; 1/12 is said to be the most economical proportion.
The width of the flange under compression should not be less than 1/30 to 1/40 of the span, or it will be liable to buckle sideways. Both flanges must be wide enough for the rivets and for the ends of stiffeners where they are used (see Fig. 260).
There should be as few joints as possible, especially in the tension flange and web.
Care should be taken to use market sizes of plates and angle irons (see Part III.) as far as possible. For example, the web in small girders should where possible "be made an even multiple of 2 inches in order that market widths of plates may be used, to avoid the extra cost of shearing."1
Angle irons should not be specified of peculiar size and thickness exactly to suit the calculated dimensions, or the difficulty in fitting them will cause not only expense but delay. Expense is incurred by using extra sizes. It may, however, be cheaper in some cases to use extra lengths, in order to reduce the number of joints.
Plate girders should be constructed with a camber of about 1/240 to 1/480 of the clear span, so that when loaded they may not sag or appear to do so, and their ends should be bedded on lead or felt.
Plate II. Contract Drawing of Plate Girder, etc., to support Floor.
Figs. 266 to 269, Plate II., are reduced copies of the actual contract drawings for a girder to support a floor recently erected for a workshop in which heavy machines are used.
A plate girder of 35 feet span, a part elevation of which is shown in Fig. 266, supports rolled joists 6 feet 10 inches apart; upon these rest the timber joists carrying the wooden floor.
At the point where the cross section on AB, Fig. 267, is taken, there are three plates in each flange. A joint occurs near this point, and the ends of the cover plates at x x, and of the cover strips and angle iron covers for this joint, are seen in elevation.
From a to 6 (in which portion the stress is less than in the centre of the girder) there are only two plates; and from b to the end of the girder, in which portion the stress is still less, there is only one plate, as shown in the section, Fig. 268, taken through CD.1
Fig. 269 is a longitudinal section of part of the floor, and shows how the rolled joists are attached to the main girder.
It will be seen that the positions of the rivets are shown in section by their centre lines and in elevation by intersecting lines showing their centres. This is more accurate and less troublesome than showing the rivet shanks in section and their rivet heads in plan and elevation.
1 The methods by which the length and thickness of the plates and covers are found, and the other calculations necessary for designing such a girder, cannot be given here, but are fully described in Part IV.