Overview
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.
Evidence of shear capacity of prestressed concrete beams in accordance with standard insufficient creep resistance.
There were carried out tests on several prefabricated prestressed concrete beams were approximately 21 m length and 35 t weight taken from bridge replacement in Uri.
Funding
Results
Zwicky, D., und Vogel, T.: “Bruchversuche an ausgebauten Brückenträgern aus Spannbeton”, Institut für Baustatik und Konstruktion, ETH Zürich, IBK Bericht Nr. 258, Nov. 2000, 167 pp.
Zwicky, D.: "Structural Evaluation of 30-year-old Prestressed Concrete Girders", Proceedings, 3rd International PhD Symposium in Civil Engineering, Ed. K. Bergmeister, Institute of Structural Engineering, University of Agricultural Sciences, Vienna, Oct. 5-7, 2000, V. 2, pp. 343-354.
Zwicky, D.: "Structural Evaluation of 30-year-old Prestressed Concrete Girders", Proceedings, 3rd International PhD Symposium in Civil Engineering, Ed. K. Bergmeister, Institute of Structural Engineering, University of Agricultural Sciences, Vienna, Oct. 5-7, 2000, V. 2, pp. 343-354.
Zwicky, D.: “Zur Tragfähigkeit stark vorgespannter Betonbalken”, Institut für Baustatik und Konstruktion, ETH Zürich, IBK Bericht Nr. 275, Sept. 2002, 229 pp.
Zwicky, D.: "Schubwiderstand minimal verbügelter Balken", Beton- und Stahlbetonbau, 96. Jahrgang, Heft 4, Apr. 2001, pp. 342.
Zwicky, D., and Vogel, T.: "A Concept for checking structural safety of existing girder bridges and comparison with full scale tests", Proceedings and CD-ROM, 1. International Conference on Bridge Maintenance, Safety and Management, IABMAS'02, Barcelona, July 14-17, 2002, pp. 455-456.
Zwicky, D., and Vogel, T.: "Checking the Limit State of Existing T-Beam Girder Bridges", Proceedings, 6th International Conference on Short & Medium Span Bridges SMSB VI, Vancouver, BC, July 31, - Aug. 2, 2002, V. II, pp. 691-698.
Zwicky, D.; Vogel, T.: "Critical Inclination of Compression Struts in Concrete Beams", submitted for consideration for publication in the ASCE Journal of Structural Engineering, 2003.
Zwicky, D.; Vogel, T.: "Full-Scale Tests on Dismantled Bridge Girders", submitted for consideration for publication in the ASCE Journal of Bridge Engineering, 2003.