Querkraftwiderstand und Verformungsvermögen von Stahlbetonplatten

Abstract

Reinforced concrete slabs are among the most important and most frequently used structural concrete members. However, the shear strength and the deformation capacity of reinforced concrete slabs with and without transverse reinforcement have not yet been investigated in a sufficient way. In particular, the questions whether the slab thickness as well as deviations of the principal shear and moment directions from the directions of the in-plane reinforcement influence the shear strength and the deformation capacity, so far remained unanswered. Also of interest is the minimum transverse reinforcement ratio that eliminates the size effect in thick slabs, leading to ductile failures. Within the framework of the research project “Deformation Capacity of Structural Concrete” an extensive series of tests on reinforced concrete slab specimens with and without transverse reinforcement was conducted at the Institute of Structural Engineering of the Swiss Federal Institute of Technology ( ETH ) in Zurich. The test results served as a basis to develop the extended sandwich model described in this thesis. In the first part of the thesis, relevant steel and concrete properties are introduced and the interaction between steel and concrete is treated with the so-called tension chord model. Concerning the behaviour in shear, a simple linear shear stress-crack width relationship is suggested to describe the shear transfer in the cracked core. The second part of the thesis addresses important aspects of the theory of plasticity. After an explanation of the theory of the plastic potential and the theorems of limit analysis, the modified Coulomb yield criterion, suitable to describe the concrete behaviour, as well as the basics of static and kinematic discontinuities are discussed. In the third part of the thesis an overview of the common design procedures for reinforced concrete slabs is given. The static and kinematic relationships are summarised and the normal moment yield criterion as well as the sandwich model for orthogonally reinforced concrete slab elements are presented. In particular, different design methods for reinforced concrete elements in plane stress are introduced with regard to the design of the covers and the core of the sandwich. The thickness of the covers, having a major influence on the strength and the deformation capacity as well as the crack width, is defined. With the so-called extended sandwich model the fourth part of the thesis introduces a new general mechanical model to describe the strength and the deformation capacity of reinforced concrete slabs with and without transverse reinforcement. The cracks in the core are assumed to be rotating and capable of transferring shear stresses, resulting in the total shear resistance being subdivided into a concrete and a steel contribution. Unlike the normal moment yield criterion, the extended sandwich model permits to consider the significant influences on the flexural behaviour of deviations of the principal shear and moment directions from the the in-plane reinforcement directions. Regarding shear strength, the extended sandwich model allows a general treatement of the principal shear force and the shear strains in the core can be calculated. The experimental evidence relating to size effects as well as to deviations of the principal shear direction from the inplane reinforcement directions is confirmed. A comparison of experimental data and theoretical predictions according to the extended sandwich model generally shows a good agreement regarding both strength and deformation behaviour. In the fifth part of the thesis, the main results are summarised, conclusions are drawn and future research projects are suggested.