Studies on the shear capacity of a freeway bridge
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Project data
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Titel | Title |
Report in the yearbook 2009
A freeway bridge has been saved
Test setup with measurement matrix
Previous codes clearly deviate from the current version concerning shear design of reinforced concrete. This leads to the critical situation of many old bridge superstructures in which existing stirrups are insufficient according to current code standards. One particular bridge was analysed during reconstruction, and, unfortunately, engineers were unable to approve it as structurally safe relative to shear strength. Consequently strengthening measures were necessary. The effectiveness of the shear strengthening method used was simultaneously studied in the Otto-Mohr-Laboratory as was the actual bearing capacity of the superstructure.
A detailed finite element model was set up to analyze the bearing behaviour of the bridge. Critical areas of the structure were modeled in great detail so as to fully replicate the precise geometry of each individual reinforcing bar. Material parameters for concrete and reinforcement, determined from field tests of the structure, were used for the non-linear simulation. As a result, a prediction of the superstructure’s bearing behaviour, in particular shear cracking and deformation behaviour, could be made. The essential result, evidence of sufficient shear capacity, was unattainable using traditional engineering design analysis.
Because the predicted behaviour deviates significantly from standard assumptions and limits of current codes and guidelines, a test specimen of comparable bearing behaviour – in terms of crack formation and failure mechanism, was designed at a scale of 1 : 3. Static and dynamic tests conducted at the Otto-Mohr-Lab of TU Dresden have shown that the finite element prediction is closely aligned with the actual bearing behaviour.
Specially formed stirrups were included in the superstructure to close the loop of the normal, upward sloping open stirrups. According to code, the increased reinforcing ratio at the connection overlap cannot be taken into account in static analysis. The finite element analysis showed that this overlap contributes considerably to the shear capacity of the structure as clearly substantiated by experiments confirmed this. This means even if shear cracks form in this bridge under traffic load, then there is no potential danger within the following designated inspection time period, and the superstructure will safely resist traffic loads.