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Bridges 55

bridge, trusses, feet, spans, plate, simple, design and stresses

BRIDGES 55. Use of Bridge Trusses. Since 1854 the metal bridge truss has been in general use for all spans from 10 or 15 feet up to several hun dred feet. The huge Firth of Forth bridge in Scotland, and the Quebec bridge crossing the St. Lawrence river near Quebec, have spans of about 1,800 feet.

Bridges may be considered as trussed beams. A truss is a combination of members which take stresses in direction of their length only. An other definition, perhaps, is that a truss is a combination of tension and compression mem bers. Two or more trusses constitute a bridge. The trusses constituting a bridge are connected by a system of beams on which the load is car ried. This is the floor system of the bridge, and has no purpose except to carry the loads to the trusses.

56. Classes of Bridges. Bridges, like beams, may be divided into continuous, simple, and overhanging. In the case of overhanging bridges, there may be one or both ends overhanging. Such bridges are called cantilever bridges from the fact that the overhanging end is a cantilever (Plate 7). Other classes of bridges are the suspension, swing, rolling-lift, and trunnion-lift.

Bridges 55

Probably 95 per cent of all bridges built are of the simple type—that is, having each end resting on a support. The simplest forms are the king-post and queen-post trusses (Fig. 84). Since the panel length—that is, the distance be tween any two joints—is limited to about 20 feet, the greatest spans of these classes of bridges are 40 and 60 feet respectively.

Other classes of simple trusses are the Howe, Warren, Pratt (Plate 8), Camel Back (Plates 9 and 10), Baltimore, and Petit (Plate 11). The first three and the last were named after their inventors; the fourth, because of its humped back; and the fifth, because the Balti more Bridge Works was its first builder. The camel back is frequently called the bowstring.

The limiting economical span for a simple truss is between 300 to 500 feet, according to the class. The Pratt is built to a greater extent than any of the others, probably 85 per cent of all simple trusses are of the Pratt type. The Petit and Baltimore are for long spans only— say from 250 to 500 feet. The rolling-lift (Plates 12 and 13) and trunnion-lift (Plates 14 and 15) are the outcome of the attempt to do away with the old swing bridge or drawbridge (Plate 16). The latter was costly, and also had a pier in the center; and this, especially in narrow streams, was a great obstruction to navigation.

The rolling and trunnion lifts have their foundations and supports on the banks, the full width of the stream being available for naviga tion. These bridges are balanced about their rolling or trunnion centers, by huge counter weights of cast-iron blocks or masses of con crete, and are opened or closed by a pull or push on the operating strut, which is usually sup plied with power from electric motors in a small house as indicated.

All bridges shown in Plates 9 to 16 are called through bridges, since the traffic moves through them, between the trusses. Any of them shown, except the suspension bridge (Plate 17), might be turned up the other way, allowing the traffic to move over the top. They are then called deck bridges. Plate 18 shows a deck bridge of the Pratt type.

Continuous bridges have the same objec tions as continuous beams. A slight settlement of the trusses causes the stresses to be greater than computed. They are not used in this country, except in a very few cases.

Cantilevers and suspension bridges should be used for long spans only, or where their form offers ease of erection. They are very un economical for short spans. Their special use is for spans of about 500 to 1,800 feet for canti levers, and 1,000 to 3,000 feet for suspension bridges.

In Fig. 84, the heavy lines indicate compres sion members, while the light lines indicate ten sion members. Dotted lines indicate that the member sometimes takes compression and some times tension. The names of the various mem bers are also put upon them, in many cases. In simple trusses, the top chord is always in com pression, and the bottom chord always in tension.

Design of Bridges 57. The determination of stresses in steel bridges is one of the functions of the designing engineer, and calls for a high grade of technical skill and mathematical training.

Once the stresses are computed, however, the design is simple. The design of the sections is made in the same way that the tension and compression members are designed in this text. The economical design and the getting the parts to fit together in the best manner, can be done only after much experience. The student is ad vised to study plans of actual bridges and more advanced texts, before attempting design.

Both ends of a bridge should be bolted to the supports so that they cannot move. In warm weather a bridge is slightly longer than in cold weather, and any change of temperature causes it to change in length. If both ends were fixed, the stresses in the bridge would change consid erably from those computed on the basis of the loads, and may in extreme cases cause danger. One end should be fixed to the support, and the other end should either be on rollers or on a planed plate so that it may move backwards and forwards when the temperature changes.