|Abstract: ||This research addresses the design of reinforced concrete flat slabs for punching shear. Flat slabs are two way spanning slabs without downstand beams that are directly supported by columns. The thickness of flat slabs is typically governed by considerations of either deflection control or punching shear for which there is still no widely accepted theoretical treatment. The research seeks to develop a better understanding of the effects of in-plane restraint and flexural continuity on the punching resistance of flat slabs. The overriding aim is to determine whether flexural continuity is sufficient to explain the satisfactory performance of existing flat slabs which appear punching shear deficient when assessed against test data from isolated punching shear specimens. The influence of slab continuity on punching resistance is assessed through analysis of test data, nonlinear finite element analysis (NLFEA) and fib MC2010 Level of Approximation (LoA) IV which is based on the Critical Shear Crack Theory (CSCT) of Muttoni.
The CSCT relates shear resistance to the width of the so-called ‘critical shear crack’ which depends on slab rotation. MC2010 has four LoA of which I to III are intended for design. Punching resistance is normally determined from tests on isolated slab-column specimens which are intended to simulate behaviour within the elastic line of contraflexure for radial moments, although the benefits of continuity on punching shear strength is well known. MC2010 LoA IV, which uses nonlinear finite element analysis to determine the slab rotation, is used to investigate and develop a better understanding of the influence of continuity on the punching shear resistance of flat slabs at edge and internal columns. The FE model is calibrated against relevant test data from both isolated and continuous specimens.
Parametric studies are presented which show the superseded UK design code BS8110 to require significantly less punching shear reinforcement than EC2 (2004) and MC2010 LoA II which give reasonable strength predictions for isolated internal column punching specimens. Despite this, slabs designed to BS8110 have performed satisfactorily for many years. MC2010 LoA IV is used to show that punching resistance at internal columns can be increased significantly by restraint from the surrounding slab. The increase in punching resistance due to continuity is shown to be sufficient to explain the satisfactory performance of flat slabs designed to BS8110 at internal columns. However, the strength increase is variable and, in the case of uniformly loaded slabs, already largely included in BS8110 and EC2.
The research also considers the influence of slab continuity on punching resistance at edge columns. Parametric studies are developed to examine the influence of shear force eccentricity, flexural reinforcement distribution between support and span, and modelling assumptions on the punching resistance of flat slabs at edge columns. Relating the punching resistance at edge columns to the elastic unbalanced moment as done in MC2010 LoA II and III is shown to be overly conservative for continuous slabs. The ACI 318 and EC2 practice of making the design punching resistance independent of the unbalanced moment is reviewed and shown to be reasonable particularly for continuous slabs. MC2010 LoA IV is shown to be a powerful tool for assessment, however the predictions for the method are very sensitive to the modelling assumptions and boundary conditions.|