*old 41.4 463 Pkwir . Beator Pm. 19.— Reinforced Concrete Through Girder Bridge.
ance or the soil conditions would not allow the construction of a concrete arch.
Both steel and reinforced concrete girders may be used in continuous and cantilever con struction, while the former is often used for the leaf of a bascule or the movable span of a direct lift bridge.
Bridge Floors and Floor Systems.—The floor is that part of a bridge which directly supports the live loads. The floor system is that part of the structure which supports the floor and transfers the loads to the trusses, girders or arches. In a "deck bridge" the floor system is attached to the upper chords of the trusses at the panel points, or to the upper flange or top of the girders, while in a "through bridge" the floor system is supported at the lower chord, or near the lower flange or bottom of the girders. There are some instances where a bridge has two floors, and such a structure is known as a "double deck bridge." In railway bridges two main classes of floors are used viz., the open floor and the solid floor. The usual type of open floor consists of cross-ties separated five or six inches and supported on "stringers" in the case of through spans and in most deck truss spans, and on the girders or trusses in the case of short deck spans. Where stringers are used they transfer the loads received from the cross-ties to "floor beams" which are supported either between or upon the trusses or girders. With short panels the stringers may consist of I-beams, but usually both stringers and floor beams arc plate-girders. Fig. 20 is a skeleton diagram of a portion of the open floor and floor system of a through railway truss bridge.
tiiiiyvt T ,y Roy/ untaintitinin.1; '"'' 1111111,11111111111,11 LIN6fittutimen: 77-v_ss FIG. 20.— Open Solid- floors indicate by their name a non open construction, and usually carry ballast such as is used on the rest of the track. Where no provision need be made for drainage a solid flooring of creosoted timbers may be laid on the stringers, girders or trusses and upon this is spread the ballast in which the track ties are laid. Various kinds of so-called trough floors were among the first types of solid floors to be built. These (see Fig. 21) were built
up of different steel shapes and sections and resting upon the stringers, floor beams, girders or trusses, either carried the ties directly or supported the ballast in which the ties were laid. Floors of this kind are seldom constructed at the present time. The latest trend has been toward a type where concrete is the predominant feature.
In through girder bridges where more ex amples of this type of floor are to be found, there are two general systems. In one the floor beams which are rolled I-beams are W"' T , ‘,) t Fin. 21.
spaced very close together (from 12 to 24 inches) and completely encased in concrete. The beams are designed to carry the whole load and the concrete serves as protection and filling only. In the other the floor beams are somewhat farther apart and support a rein forced concrete slab. In either case there is a surface of concrete upon which the ballast and ties may be laid.
Previous to 1900 few highway bridge floors were made of other material than wood. A common type of floor for country highway /167.4div ArCes "'Nod • FIG. Wooden Floor.
bridges is illustrated by Fig. 22. Planks rest ing directly on the stringers were fastened down by nails to nailing pieces which were bolted to the stringers. In some cases a floor was laid as indicated by Fig. 22 and another one, called a wearing floor, was put on top of it. All wooden floors have the disadvantage of being difficult to attach to the stringers so that they will remain secure and not be jarred loose by traffic.
Fig. 23 illustrates another type of floor which allows the pavement of the adjacent - prh-ogre,7‘ 7,-A7 Zrat'W Cam' Pm. Brick Floor.
roadway to be carried without a break across the bridge. Creosoted timbers packed close together are bolted on the stringers and sup port the pavement and intermediate sand cushion. A more modern and more permanent floor can be made by the substitution of a reinforced concrete slab in place of the floor timbers, and except in instances where the addi tional dead load weight of such a slab would be markedly disadvantageous, this type is pre ferred.