The Viaduct Works' Handbook

The girders are not always under equally distributed loads. On the approach of a train to the centre, only half, or a part, of the girder is loaded, the other half being without, causing variation in the direction and amount of strain on the diagonals. Some of those which are wholly in tension when the girder is equally loaded being brought into compression, and vice versâ. These diagonals may be seen by careful inspection of diagram, Fig. 2, which represents a girder loaded at two points. (To avoid fractions, I have here fixed upon 18 as a unit, instead of 15, as before.) The effect of @ is shown by the Figs, marked O; 4th being supported by one end of the girder must pass through all the diagonals to the left, the re- maining §ths passing to the right; the effect of the next weight is shown by the plain figures. It should be stated that the results thus arrived at are not exactly correct; but are sufficiently near for a rough approximation. The amount of strain on top and bottom flanges can be found from the principle of the bent lever. The proportions of such lever are as the depth of girder being one arm, and half the length of span being the other arm ; thus, supposing the depth of girder to be 15-5 feet, and the half span 75 feet, the leverage is in the pro- portion of 4'8 to 1, and this multiplied by 1th of the distributed load on a girder, gives the strain at centre. Example.—A pair of girders carry a distributed load of 270 tons, including their own weight, or 135 tons for one girder; therefore, 135 = 33-7 x 4.8 = 161 tons strain. 4 PERMANENT LOADS ON BRIDGES, &c. For rough calculations the weight of the bridge itself may be assumed to be (in wrought-iron bridges) :— For 30 feet spans, single line, 5 cwt. per foot run 60 „ „ 6 „ 100 „ » 9 150 „ „ 12 200 „ » 15 Dense crowds average 120 lbs. per square foot.