[Back]

F. Untermarzoner:
"Experimental Investigations on the Influence of the Shear Reinforcement Ratio in RC Beams with Plain Bent-Up Bars";
Supervisor: T. Huber, J. Kollegger; Institut für Tragkonstruktionen, 2021; final examination: 2021-06-18.

Short slab bridges (2-20 m) are substantial for a well-functioning transport system. The Austrian Assessment Standard for bridge structures defines certain cases in which the load capacity needs to be recalculated. According to recalculations, there may be problems to meet the requirements regarding shear forces in tier 1 and 2, although these bridges have been part of the infrastructural network for several decades without signs which indicate a reduced load capacity. According to that standard it is further possible to assess shear capacity using the model of the potential shear crack (PSC-model), which was developed at TU Wien (Institute of Structural Engineering), if the assessment has led to negative results on tier 1 and 2. In case of good bond behavior, this model allows for a combination regarding the proportion of steel and concrete, which might lead to a higher resistance for shear compared to current standards (Eurocode 2). This model can further be used for the evaluation of slab structures containing plain bars. However, in that case only shear resistance provided by shear reinforcement can be taken into account due to a probable reduced contribution of the concrete. This thesis contributes to the understanding of the load-bearing behaviour in slab structures containing plain bent-up bars with regard to the combination of steel and concrete contribution.
The main part of this thesis consists in the description and evaluation of a comprehensive experimental program, which was designed by Dr. Tobias Huber at TU Wien. 19 shear tests have been conducted in the Institute for Structural Engineering´s laboratory at TU Wien in the course of a research project on behalf of ÖBB, DB Netze AG and ASFINAG. By using a photogrammetric measurement system the crack widths and crack slidings at the level of bent-up bars have been evaluated. This data allowed for a calculation of reinforcement stresses under the application of constitutive laws and, thus, an estimation of the contribution of the concrete in the investigated shear crack. In addition, the strains of bent-up bars were determined directly by using strain gauges and continuous fiber optic measurements which confirmed the potential of this technique. In this thesis those experiments are analyzed which show a variation in the shear reinforcement ratio. In combination with the evaluation of failure loads and the associated crack propagation explanations for the level of activation of shear reinforcement could be found.
The results show that the activation of bent-up bars and, therefore, the constribution of steel is strongly dependent on crack position, crack geometry, and crack width. This correlation was shown to be particularly prominent in those beams containing solely bent-up bars as shear reinforcement. In only one experiment the bent-up bars could be fully activated through an advantageous crack formation. In all other cases the beam collapsed beforehand because of large crack widths due to the failure of the compression zone of the concrete. The fast propagation of the crack is likely to be enhanced by the application of smooth bars. In the experiments with the additional use of single legged stirrups and/or a third bent-up bar the beams could be loaded until yielding of the flexural reinforcement because the crack propagation was decreased by the additional steel inserts.
Further evaluations in the ongoing research project will be necessary to enable an extension of the PSC-model for smooth rods.