EFFECTS OF DIAPHRAGMS ON LATERAL LOAD DISTRIBUTION IN BEAM-SLAB BRIDGES

The effect of diaphragms on the lateral distribution of live load in simple-span beam-slab bridges with prestressed concrete I-beams and without skew is presented. The computer-based analysis used the finite-element method for two previously field-tested bridges with span lenghts of 21.8 and 20.9 m (71.5 and 68.5 ft). The first part of the investigation dealt with the extent of the participation of the midspan diaphragms in lateral load distribution. It was found that the reinforced-concrete mid-span diaphragms contribute only about 20 to 30 percent of their stiffness to load distribution. In addition, when all design lanes are loaded the contribution of the diaphragms is negligible. The second phase of the research dealt with the effect of the use of multiple diaphragms on lateral load distribution. Numerical comparisons were made for cases in which the superstructure had a midspan diaphragm and diaphragms at third, quarter, and fifth points. When the vehicle was located so as to produce maximum bending moment in the bridge, it was found that the increase in the number of diaphragms does not necessarily correspond to a more even distribution of loads at midspan. It was also found that, if all the design lanes are loaded, the contribution of diaphragms is negligible regardless of the number of diaphragms used. /Author/