Institute of Structural Concrete

Experimental and theoretical investigations on the influence of the loading and support conditions

Reason and purpose

Reinforced concrete slabs represent a large proportion of the overall concrete volume used in structures. In contrast to RC beams, RC slabs can usually be realised without shear reinforcement which is beneficial in terms of building practice. To avoid applying shear reinforcement to slabs made of cast-in-place concrete, larger slab heights may be required. Consequently, the concrete volume and, hence, the total building costs increase. The current shear design according to Eurocode 2 (EC 2) for members without shear reinforcement is based on a semi-empirical approach. For its calibration, a database was evaluated which includes mainly flexural shear tests on simply supported beams under single loads. In usual buildings, RC slabs feature a lateral restraint due to adjacent slab segments. Furthermore, slabs are usually subjected to distributed loading. Neglecting beneficial effects resulting from the structural system and loading conditions leads to an overconservative and, thus, uneconomic design.

Examples from building practice show that thin existing RC slabs often resist higher shear stresses than predicted based on common design codes. A possible reason is the activation of arching forces (Compressive Membrane Action (CMA)) due to lateral and rotational restraints. The influence of these compressive membrane stresses on shear resistance of slabs has not been investigated, yet.

Figure 1: Principle of Compressive Membrane Action

Taking the beneficial effects from loading and support conditions into account, by means of compressive membrane stresses, can lead to a more resource-efficient and economic shear design and construction of RC slabs.

Experimental investigations

In order to enhance existing design approaches for shear in RC members without shear reinforcement, theoretical, experimental and numerical investigations on the shear behaviour of RC slabs resp. slab segments are conducted in the AiF-project IGF 21756 N/1. In two test series on RC slab segments without shear reinforcement, the influence of loading conditions (distributed instead of single loads, series 1) as well as CMA (series 2) will be investigated. Test parameters are chosen in accordance with the completed AiF-project IGF 17732 N/1 to allow a cross-comparability between the tests. Shear slenderness as a representative factor for loading and support conditions is the emphasised parameter of investigation. To this end, the location of the point of inflexion of the moment curve is varied systematically by varying the degree of rotational restraint at the intermediate support. The supplementary numerical investigations shall give an insight into the predicted magnitude of CMA as well as a more detailed investigation on the direct load transfer in the vicinity of the supports.

The experimental investigations in the first test series have been completed. In total, five test specimens were cast and tested in two subsequent partial tests until failure. In the tests, the influence of the structural system, the degree of rotational restraint resp. the location of the point of inflexion for continuous members and the loading conditions on shear behavior were investigated. One test specimen featured a doubling of the width and the longitudinal reinforcement ratio was smaller. Part of the longitudinal reinforcement was shorter to investigate the influence of a staggered reinforcement on the crack pattern and load-carrying behavior. It was observed that the position of the failure-critical shear crack shifted to the region where the shorter bars ended. As there were no tests with concentrated load planned in this research programme, the tests of the completed research project IGF 17732 N/1 were used as a reference for the investigation of the influence of the loading conditions. The distributed loading in the tests was introduced via equidistant smeared concentrated loads.

Figure 3: Casting and testing of the test specimens in series 1

The shear capacities achieved in the experimental investigations were approximately 20‑50 % higher than those predicted with the shear design approach in current Eurocode 2. Two tests (SV2-2 and SV3-2 with rotational restraints of d_r = 100 % and d_r = 75 %, respectively) did not fail in shear despite high loading.

The influence of a direct load transfer to the support for distributed loading was so far considered in two different ways: On the one hand, the control section was set to 1.0 d from the support edge (this is equivalent to the specifications in current Eurocode 2) and on the other hand, the concentrated loads that were close to the support (0.5 d < a_v ≤ 2.0 d) were reduced with the reduction factor β according to current Eurocode 2. The direct load transfer will be evaluated more in detail and through additional numerical simulations.

 

 

Figure 4: Comparison of the experimental shear capacities and the calculated shear capacities according to current Eurocode 2

The current investigations are a supplement to the completed AiF research programme. The influence of the shear slenderness on the shear capacity can be confirmed by the current investigations. For concentrated loads, the reduction of the shear capacity with increasing shear slenderness is more pronounced than for distributed loading. The influence of a direct load transfer to the support is not yet depicted in the figure.

Figure 5: Normalized shear capacities with respect to shear slenderness

Prospects

Considering mechanisms that increase the shear strength of members without shear reinforcement in design can lead to a more precise shear design of RC slabs. In comparison to simply supported RC beams, which were the calibration basis for shear design in the current EC 2, the loading and support conditions of existing RC slabs differ. Resulting beneficial effects on shear capacity will be determined to be the groundwork for an enhanced design procedure for flexural shear.  This leads to both, a resource-efficient design of new structures, and an extended lifetime of existing structures.

 

Acknowledgements

The IGF project 21756 N/1 of the German Society for Concrete and Construction Technology (DBV), Kurfürstenstraße 129, 10785 Berlin, is funded within the programme for Sponsorship by the German Federation of Industrial Research Associations (AiF, IGF number 21756 N/1) of the German Federal Ministry for Economic Affairs and Climate Action on the basis of a decision by the German Bundestag.

Research associate

Annkathrin Sinning, M.Sc.