Punching of Concrete Slabs With Shear Reinforcement

Titre

Punching of Concrete Slabs With Shear Reinforcement

(Poinçonnement de dalles en béton armé avec armature d’effort tranchant)

 

     

Thesis director

Prof. Dr Aurelio Muttoni  

Doctorant

Luca Tassinari

 

Projet summary

Reinforced concrete flat slabs are cost-efficient structural systems that are extensively used in buildings and parking garages since the 1950’s. The design of flat slabs is typically governed by deflections in service conditions and by the punching shear strength  at failure.

Failure by punching shear of a column in a flat slab without transverse reinforcement is an extremely brittle phenomenon. Furthermore, punching shear failure of one column propagates to adjacent columns of a flat slab, eventually leading to the total collapse of the structure. This phenomenon occurs with very limited (almost none) advanced warning signs, since deflections and crack widths in the flat slab remain small prior to failure. Over the past decades, several collapses due to punching shear failures have occurred in Switzerland, resulting in various human casualties (the latter at Gretzenbach, Switzerland, 2004) and in millions of Swiss Francs of damages.

The use of punching shear reinforcement in flat slabs enhances the structural performance in two ways:

  • The punching shear strength of flat slabs is increased.
  • The ductility at failure is significantly increased. Thus, the structure presents some capacity to redistribute the loads and to avoid a progressive collapse.
  • For some kind of punching reinforcement (bent-up bars) the post-punching behaviour is also improved

Gretzenbach parking collapse (2004)

Behaviour of slabs with and without punching reinforcement

 

 

These circumstances are currently encouraging designers and contractors to increasingly use punching shear reinforcement in flat slabs. However, most design models found in the scientific literature or in codes of practice rely on empirical relationships. This type of methods do not provide the engineer with a clear understanding of the phenomenon and their application to cases not directly treated by the models may be misleading or not possible.

Another problem of current theoretical models is that the various punching shear reinforcement systems (see Figure 2) are usually treated and designed in the same manner. However, significant differences in their behaviour and strength can been observed.

Currently, there is no generally accepted sound theory explaining the behaviour and strength of punching shear in flat slabs with transverse reinforcement. As a consequence, punching shear reinforcement is designed in practice according to the various models available in codes of practice. Both the European (EC-2) and North-American (ACI-318, CSA A23.3) codes rely on empirical or semi-empirical models. Such models have been shown to present many deficiencies and to neglect the role of some instrumental physical parameters.

In Switzerland, the Swiss Code for Structural Concrete (SIA 262) is based for design of punching shear reinforcement on the theory of plasticity. This theory is physically sound, however its application to punching shear reinforcement is performed by introducing a very coarse simplification (the contribution of concrete in carrying shear forces is neglected), leading to very conservative amounts of shear reinforcement.

With respect to the design of flat slabs without punching shear reinforcement, the Swiss Code is based on a different theory, named the critical shear crack theory. This theory is based on a physical model accounting for the development of a shear crack whose width governs the punching shear strength.  This research project proposes to extend the critical shear crack theory to the case of flat slabs with punching shear reinforcement.

In order to develop a rational approach to understand the problem of punching shear, empirical test have to be performed. The test are carried out on actual scale specimens in a test rig to reproduce the static system of punching shear.

 

Casting of test specimen

 

Test rig

 

Test laboratory (EPFL - ENAC - IS)