In the 1960's and 1970's prefabricated concrete beams were often used for road bridges with spans up to approx. 30 m in Switzerland. The road deck was extended with cast-in-situ concrete. The girders were frequently prestressed with wires and post-tensioned with tendons.
At that time, the beams were designed for shear according to the concept of inclined principal stresses. In addition, high prestressing ratios were applied in order to reduce tensile stresses. Therefore, the shear reinforcement consisted of secondary reinforcement only. Applying existing models, e.g. general truss models with restricted inclination of the compression diagonals, this kind of girder exhibits insufficient shear strength.
Due to the replacement of a bridge, several prefabricated prestressed beams of approx. 21 m length and weighing approx. 35 tons are available for load tests up to failure. For this purpose an outdoor testing facility was built. With the help of ground anchors, beams with spans of 15-25 m can be loaded.
The research project originally aimed at providing a method of strengthening girder bridges consisting of prefabricated beams under traffic for shear forces. Applying several different test set-ups, shear failure could not be reached before the reliably predictable bending failure.
The shear strength of prestressed beams is obviously higher than the value calculated with existing standards. The research project aims at predicting the effective shear strength of this kind of concrete girder.
Higher shear strength in beams can be calculated by accepting flatter inclinations of the compression diagonals than allowed by standards. Also, it is worth considering the shear resistance of the flexural compression zone in beams with wide flanges. This resistance depends on the tensile strength of the concrete, similar to the punching shear strength of concrete slabs.