A large number of floors are built as indi cated by Fig. 24, with the I-beam stringers spaced somewhat closer together than in the type shown by Fig. 23, and designed as in the C60442.7 CA. Alta' PIG. Plain Concrete Floor.
case of railroad structures to carry the whole load, the concrete serving merely as protection to the beams. The concrete may be covered with earth for a depth of a few inches or any type of modern pavement may be used.
As shown by Figs. 18 and 19, the floor of a reinforced concrete girder bridge is similar to one of the floors described above, the slab being part of the bridge itself.
Bracing.— Between the two or more trusses or girders forming a bridge a large number of struts, or ties and struts, are required to hold the truss in position and make the structure stiffer and less subject to vibration. In modern practice most of the bracing is designed to take both tension and compression as experience has shown that greater stiffness is obtained in this manner.
The bracing between the end posts of through bridge trusses is known as the °portal bracing° while the vertical or inclined bracing between other web members of one truss and the corresponding members in the other truss is called the °sway bracing.° The horizontal brac ing between the chords of the trusses is re ferred to as the °upper lateral system° or °lower lateral system° respectively.
In a through truss bridge, when the depth of the trusses is not sufficient to permit the construction of the portal, sway and upper lateral bracing, the trusses are known as "pony bridge trusses.° Such bridges have very little lateral stiffness and consequently are used only for light traffic and short spans.
In through girder bridges the floor system itself is constructed to serve the function of the lower lateral system and the upper flanges of the girders are stiffened by riveting heavy brackets or gussets to the tops of the floor beams and the sides of the girders. In deck bridges the portal and sway bracing are often referred to as "cross-frames,° and extend the full depth of the trusses or girders.
Suspension Fig. 25 shows the skeleton diagram of a suspension bridge. Cables anchored at the ends are carried over towers and the bridge floor is hung from the cables by means of vertical suspenders. In the mod
si:(2r21171:1:311r\; Fm. 25.— Suspension Bridge.
ern suspension bridge the bridge floor cannot be hung directly from the cables, for the mov ing of a heavy load across the bridge would cause the cables to sag first at one point and then another, giving a kind of wave motion as the load passed over the structure. A more equal distribution of the load is effected by the use of stiffening trusses. (See Fig. 26). These trusses are hung from the cables and the bridge floor or floors is supported on them. When well braced together such a system resists the horizontal swinging which may be caused by the wind.
AlOWW,WIALIPXWAIIAVA Pio. 26.— Stiffened Suspension Bridge.
The lack of rigidity of a suspended struc ture generally renders it unsuitable for short spans except for light traffic. While the addi tion of stiffening trusses and bracing may result in sufficient stiffness, the increased ex pense usually makes the cost of the structure greater than that of simple trusses. Even for longer spans the proper degree of stiffness can not be secured except at such an expense as to render it more costly than a cantilever. Since long spans cannot economically be built for light traffic, the suspension system seems to have its principal economic advantages for long spans under heavy traffic where the dead load is large in comparison with the live load, and consequently assists in making the structure a rigid one.
The suspension bridge can often economi cally be constructed for foot traffic, or where the possibility of future heavy traffic is de batable. In the latter instance considerable ex pense may be saved by constructing this less rigid type, which may satisfactorily care for the traffic for many years.
Arch Bridges.—A beam or truss bridge under vertical loads has only vertical reactions, provided proper provision is made at one or more of the supports for a horizontal move ment due to the deflection of the bridge and to changes in temperature. When the reactions are inclined under vertical loads the bridge be comes an arch. The vertical loads on an arch produce a horizontal thrust at the endi equal to the horizontal component of each inclined reaction. The thrust may be resisted either directly by the abutments or by a tie uniting the two supports.