Shear Cracks in Preflex Bridge Girders

The design of preflex beams is mostly focused on limiting steel and concrete stresses near the span center or at the location of the precambering forces. Verification of shear strength is generally limited to the steel beam web and the identification of the connectors, as well as determining the required amount. In several cases of bridges built at the beginning of this century, fine cracks were found in the compressed concrete bottom flange of the beams, corresponding to a herringbone pattern. In the case of a 55-year-old bridge, a wider longitudinal crack was found in the bottom flange. The study explains these cracking cases based on the internal equilibrium of forces. The origins of the cracks are in the burst effect of the additional bonded prestressing, the spreading across the concrete of the high compression force in the steel lower flange, and the very light shear reinforcement typically used in preflex beams. In addition, recommendations are given to avoid such forms of shear cracks.