The reinforced-concrete bridge shown in Fig. 212 was constructed near Allentown, Pennsylvania, in 1907.

Fig. 212, Girder Bridge near Allentown, Pa.

Fig. 212, Girder Bridge near Allentown, Pa.

This type of bridge has been found to be economical for short spans. Worn-out wooden and steel highway bridges are in general being replaced with reinforced-concrete bridges, and usually at a cost less than that of a steel bridge of the same strength. Steel bridges should be painted every year; and plank floors, as commonly used in highway bridges, require almost constant attention, and must be entirely renewed several times during the life of a bridge. A reinforced-concrete bridge, however, is entirely free of these expenses, and its life should at least be equal to that of a stone arch. From an architectural standpoint, a well-finished concrete bridge compares very favorably with a cut-stone arch.

The bridge shown in Fig. 212 is 16 feet wide, and has a clear span of 30 feet. It was designed to carry a uniformly distributed load of 150 pounds per square foot, or a steel road-roller weighing 15 tons, the road-roller having the following dimensions: The width of the front roller is 4 feet; and of each rear roller, 20 inches; the distance apart of the two rear rollers is 5 feet, center to center; and the distance between front and rear rollers is 11 feet, center to center; the weight on the front roller is 6 tons, and 4.5 tons on each of the rear rollers.

In designing this bridge, the slab was designed to carry a live load of 4.5 tons on a width of 20 inches, when placed at the middle of the span, together with the dead load consisting of the weight of the macadam and the slab. The load considered in designing the crossbeams, consisted of the dead load - weight of the macadam, slab, and beam - and a live load of 6 tons placed at the center of the span of the beam, which was designed as a T-beam. In designing each of the longitudinal girders, the live load was taken as a uniformly distributed load of 150 pounds per square foot over one-half of the floor area of the bridge. The live load was increased 20 per cent over the live load given above, to allow for impact.

In a bridge of this type, longitudinal girders act as a parapet, as well as the main members of the bridge. The concrete for this work was composed of 1 part Portland cement, 2 parts sand, and 4 parts 1-inch stone. Corrugated bars were used as the reinforcing steel.

When there is sufficient headroom, all the beams can be constructed in the longitudinal direction of the bridge, and are under the slab. The parapet may be constructed of concrete; or a cheaper method is to construct a handrailing with 1 1/2-inch or 2-inch pipe.

CONNECTICUT AVENUE BRIDGE OVER ROCK CREEK, WASHINGTON, D. C.

CONNECTICUT AVENUE BRIDGE OVER ROCK CREEK, WASHINGTON, D. C.

Largest concrete bridge in world without steel reinforcement. The five principal arches have spans of 150 feet; highest point of bridge above gorge, 150 feet; each abutment pier comprises two smaller arches. Total length between abutments, 1,341 feet.