Effect of Varying Material Properties on the Available Rotation Capacity of a One-Way Slab

Abstract

The influence of reinforcing steel properties on the behavior of a one-way slab is investigated. A number of European and American reinforcing steels are evaluated at different effective reinforcing ratios and their influence on the rotation capacity of the slab is determined. The effect of high strength concrete in a slab with large effective reinforcing ratios also is addressed. Analytical work is performed using a nonlinear finite element approach in a three-dimensional model. To simulate the behavior of concrete in tension, the smeared crack approach is used, while compression softening is approximated with the Willam-W arnke model. The stressstrain behavior of the reinforcing steel is incorporated using a multilinear elastic strain-hardening model and a model for bond-slip is adapted to take the structural response at the steel-concrete interface into account. During preliminary studies different models for compression softening and concrete behavior under tension were investigated. Also, the effects of the mesh, convergence tolerances and load step size were studied. Analyses are performed for three point bending and the load is applied by imposing displacements on the system. To calibrate the finite element model, the results are compared to experimental data. Load-displacement curves are generated to measure the response of the system and the maximum amounts of carrying capacity obtained from the finite element analysis are compared to predictions using ACI 318-95 (1995) design equations. The results of this study suggest that the influence of the ductility characteristics of the reinforcing steel on the rotation capacity of the structure depend on the effective reinforcing ratio of the specimen. At low reinforcing levels, the structural behavior is governed by the ductility characteristics of the steel, thus, the amount of deflection at failure depends on the ultimate elongation capability of the steel. By increasing the amounts of steel in the structure, the overall behavior is shifted from a ductile to a brittle failure mode. Therefore, in structures with moderate amounts of reinforcement the choice of reinforcing steel determines whether the structural behavior is governed by the steel characteristics or the concrete behavior. At high reinforcing levels, the concrete characteristics completely determine the structural behavior and generally all specimens with high effective reinforcing ratios exhibit a failure of the compression strut in the concrete - a brittle failure mode while the steel does not reach its yield capacity. The carrying capacity for all analysis cases with large amounts of reinforcement was found significantly under the capacities calculated with the ACI equations. Finally, all analyzed cases exhibited insensitivity to the effects of bond-slip.