Girder and Slab Bridges

GIRDER AND SLAB BRIDGES Girder and slab bridges are known as beam bridges; and carry the loads which come upon them by their resistance to bending. They are extensively used for short spans and low, shal low crossings. These bridges are very interest ing as examples of the advantages of reinforced concrete construction, because it is only the use of the steel reinforcement which makes it pos sible for them to resist the bending stresses which the applied loads and their own weight bring upon them. The reinforcement is only an assistance and improvement in the case of an arch bridge; in the case of a beam bridge, it is the essential feature.

The difference between a girder bridge and a slab bridge of reinforced concrete, is shown by Figs. 40, 41, 42, and 43. Figs. 40 and 41 are cross sections of girder bridges, the former a railway, the latter a highway structure. Figs. 42 and 43 are cross-sections of slab bridges, the former for a railway, and the latter for a highway. The slabs are seen to be of uniform depth, and have the advantage of taking up less room than the girder bridges, which are made up of deep, nar row beams supporting a thin reinforced concrete floor.

The flat slab bridge is the simplest to design and construct, and also proves to be the most economical in materials when used for spans up to about 20 feet. For spans from 20 to 35 feet or thereabouts, the girder type is the best to use. Beams longer than 35 feet are rarely built; an arch is generally used for such spans.

The design of reinforced concrete slab and girder bridges is based on the same principles as the design of reinforced concrete slabs and gir ders in general, when the loading is known. The loading for railway bridges ordinarily consid ered may be found below under the heading "Reinforced Concrete Design for Bridge Work." A large manufacturer of reinforcing materials much used in bridge construction gives the fol lowing three classes of loadings for highway bridges: Class No. 1—Light highway specification answering the purposes of ordinary county traffic where the heaviest load may be taken as a twelve-ton road-roller. Uniformly

distributed Load, 100 pounds per square foot.

Class No. 2—Heavy highway specification, designed for localities where heavy road-rollers, up to twenty tons, and electric cars of a maximum weight of forty tons, must be provided for. Uniformly distributed load, 125 pounds per square foot.

Class No. 3—City highway specification, designed for heavy concentrated loads and large interurban ears. This classification should be adopted for all city work; the weight of the maximum car has been taken as sixty tons. Uniformly distributed load, 150 pounds per square foot.

Highway Girder and Slab Bridges. High way bridges are of the greatest practical utility and importance, and the number required in the present state of civilization is extremely large. Good bridges are as necessary as good roads for the advancement of a district, and reinforced concrete is an ideal material for a "good bridge." A concrete bridge is reasonably cheap in first cost, requires the minimum of maintenance, and will last indefinitely. For a structure to be main tained by a public body, as highway bridges usually are, the fewer repairs that are required the better.

Portland cement should always be used for concrete highway bridge construction, because it is stronger and more reliable and hardens more quickly than natural cement.

The cement should be of a standard brand, and not liable to expansion or disintegration, fine, and of uniform quality. It should be free from lumps and stored in a dry place.

For the aggregates in concrete, the sand should be clean and coarse, or a mixture of coarse and fine, the coarse predominating. It should be free from clay, loam, and sticks, organic mat ter, and other impurities.

Screenings or crusher dust from broken stone may be substituted for sand by altering the proportions so as to give a dense mixture and the same relative volume of aggregates.