4_ _OM. %L IOW /Otis -AL PIG. 13. Continuous Bridge Truss.
Movable Bridges. One type of movable bridge has already been mentioned, i.e., the swing bridge. A circular track provides the two centre supports, A and B, Fig. 12, and the bridge may be rotated on this track, becoming a balanced cantilever when opened, while when closed it is generally continuous for the live load.
Another type in which the movable span is raised at one or both ends is the bascule bridge. One form of bascule bridge is illus trated in Fig. 14. By means of the operating pinion which is geared to the strut, and to which in turn is geared the operating mechanism, the bridge may be raised or lowered, the bal .
. c,...., ,ii..; 10.... . _, .. ;.....:,:, op.,' Al nu. 14. Bascule Bridge.
ante being maintained by the concrete counter weight. When closed this bridge acts as a simple span under live load, and in this par ticular the single leaf type of bascule is pref erable to the double leaf, operated from both ends; for without expensive interlocking de vices the latter cannot be made to act as a simple span, but becomes either an arch or cantilever structure under live load. The truss of a bridge of this kind may have various arrangements of the web members. The direct lift bridge (Fig. 15) is more economical than a bascule except for comparatively short spans, while its operating mechanism is more simple. When the movable span is only part of the whole bridge structure, the towers may be con structed upon and as part of the trusses of the adjacent spans.
pi eldity II el II C0010b#7000;90fr li II 4 il S;00,, Mr11111',111101111/1 A A ? A Fie. 1 Direct Lift Bridge. ' Cantilever Trusses. The development of the cantilever bridge was an attempt to avoid the disadvantages of the continuous structure, and unlike the latter, a slight settlement of the supports is not followed by a great increase in stresses. It is not suitable for short spans where the live load is large in comparison with the dead load, for in such a structure the stresses in the anchor arm as the load travels on the structure require a large amount of metal. As the ratio of dead load to live load increases, the cantilever becomes more eco nomical, for some of the dead load stresses are large enough to prevent reversal by the live load. Hence the cantilever system is best
adapted for use in long spans, or where the simple truss or arch is difficult of erection, for the cantilever may be erected with very little falsework by building it out panel by panel.- In form the chords of the anchor and cantilever arms may be straight, like those of Fig. 16, or curved to give the appearance of .94.4r G4460. Ca014.. ogmAlr oresp Alin a suspended structure, and various arrangements of the web members are to be found in modern trusses. The suspended span is usually one of types of simple trusses heretofore described.
Girder Bridges.-- The simplest form of a girder bridge consists of two timbers spanning an opening and supporting a plank floor. For more permanent construction, longer spans or heavy traffic, modern girder bridges are built either of steel or reinforced concrete.
Rolled steel shapes, such as I-beams and channels, are used as girders or beams for short 'lays P441 - 1 Z Beam? Cho,Ave/ 'Amps Avyk ',boys Phis FIG. 17. Section.
spans, and built up sections known as plate girders consisting of rolled steel plates and angles have been used for spans as high as 130 feet. (See Fig. 17). The plate-girder indi cated in the figure is a comparatively light sec tion. For heavy traffic additional angles and plates may be added to the flange and web plates may be increased in number.
Awing 1.
- p&oreill ...cmaPatam je...e.spAr t .bee/ Rrilimeenne L I /3 :FIG. 18. Reinforced Concrete Deck Girder Bridge.
In the last decade reinforced concrete girders (see Figs. 18 and 19) have been used exten sively for short spans. They make a more rigid and more permanent structure than those built of steel, although their first cost is usually somewhat greater. The sections illustrated are such as are used for heavy country highway traffic. Reinforced concrete slabs without girders are often constructed for the shortest spans. The: deck type may be widened to any desired width, sidewalks may be supported on canti lever brackets or additional girders, and pro vision may be made to carry street railway tracks. Reinforced concrete slabs and girders have also been used for short-span railroad bridges, where a permanent and artistic ture was desired, but where the vertical dear- ...