Distribution of loads in beam-and-slab bridges

Holcomb, Robert
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Civil, Construction, and Environmental Engineering

A new procedure for predicting the strains and deflections of the beams in simple-span beam-and-slab bridges of the usual proportions has been developed. It divides the calculations into two primary steps: (1) Temporary reactions are assumed at the beams to prevent deflections of the beams, and the loads are distributed to these reactions by the slab acting as a continuous beam. (2) The temporary reactions are removed and the consequent effects on the beams are computed;Since no deflections or moments are produced in the beams in step 1, the entire effect on the beams is found in step 2. This effect on a beam is assumed to be that of a loading consisting of (1) A concentrated or narrowly distributed force, the temporary reaction reversed, and (2) a widely distributed force produced by the resistance of the slab to deformation;Part 2 of the beam loading has been assumed to be sinusoidal, but any other form could be assumed. For the bridges tested the effects of part 2 are relatively small; so the precision of the predictions of maximum strains and deflections is not sensitive to changes in the form assumed;It is suggested that, pending further study, the use of the procedure be limited to bridges having a ratio of span to beam spacing of 2 or more, and also a ratio of beam to slab stiffness, H, of 2 or more;To obtain checks on the predictions by the proposed procedure, by the present (1953) AASHO specifications, and by the tentative revisions' (T-15-50), four bridges were tested. Two are full-size bridges in use on a highway; their spans are 41.25 ft and 71.25 ft, end their roadways are 30 ft wide. The other two were built in a laboratory. They include crown, curbs, and diaphragms; their spans are 10 ft end 25 ft, and their roadways are 10 ft wide. Each of the four bridges has four beams equally spaced, has the interior beams large than the exterior, and is of composite construction. Among the four bridges the span to spacing ratio varied from to 78, and the beam stiffness to slab stiffness ratio varied from 3.0 to 10.7. The loads on the laboratory bridges were either single-axle or tandem-axle trucks; either one truck, alone, or two side by side. The load on the highway bridges was a single semi-trailer truck having tandem rear axles;Strains and deflections were measured at a number of locations at each bridge for various positions of the loads. Of these test results, those of most interest to designers and those directly comparable to the predictions under the specifications are the maximum strains caused by a given loading when it may be placed in any position. (Abstract shortened by UMI.)