International Concrete Abstracts Portal

Editors: Carlos E. Ospina, Denis Mitchell and Aurelio Muttoni

fib Bulletin 81 reports the latest information available to researchers and practitioners on the analysis, design and experimental evidence of punching shear of structural concrete slabs. It follows previous efforts by the International Federation for Structural Concrete (fib) and its predecessor the Euro-International Committee for Concrete (CEB), through CEB Bulletin 168, Punching Shear in Reinforced Concrete (1985) and fib Bulletin 12, Punching of structural concrete slabs (2001), and an international symposium sponsored by the punching shear subcommittee of ACI Committee 445 (Shear and Torsion) and held in Kansas City, Mo., USA, in 2005.

This bulletin contains 18 papers that were presented in three sessions as part of an international symposium held in Philadelphia, Pa., USA, on October 25, 2016. The symposium was co-organized by the punching shear sub-committee of ACI 445 and by fib Working Party 2.2.3 (Punching and Shear in Slabs) with the objectives of not only disseminating information on this important design subject but also promoting harmonization among the various design theories and treatment of key aspects of punching shear design. The papers are organized in the same order they were presented in the symposium. The symposium honored Professor Emeritus Neil M. Hawkins (University of Illinois at Urbana-Champaign, USA), whose contributions through the years in the field of punching shear of structural concrete slabs have been paramount.

The papers cover key aspects related to punching shear of structural concrete slabs under different loading conditions, the study of size effect on punching capacity of slabs, the effect of slab reinforcement ratio on the response and failure mode of slabs, without and with shear reinforcement, and its implications for the design and formulation in codes of practice, an examination of different analytical tools to predict the punching shear response of slabs, the study of the post-punching response of concrete slabs, the evaluation of design provisions in modern codes based on recent experimental evidence and new punching shear theories, and an overview of the combined efforts undertaken jointly by ACI 445 and fib WP 2.2.3 to generate test result databanks for the evaluation and calibration of punching shear design recommendations in North American and international codes of practice. Sincere acknowledgments are extended to all authors, speakers, reviewers, as well as to fib and ACI staff for making the symposium a success and for their efforts to produce this long-awaited bulletin. Special thanks are due to Laura Vidale for preparing the bulletin for publication.

Note: The individual papers are also available. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP-315

Current building regulations for design against progressive collapse normally use prescriptive rules and risk-based qualitative scales, which are insufficient to cover current design needs. Structural robustness of concrete flat slab structures is examined using different theoretical models to capture the dynamic behavior under accidental events. In such extreme events, the large dynamic reactions at the connections could potentially lead to punching and progressive collapse. Punching formulae based on load-deformation response relationships such as the Critical Shear Crack Theory (CSCT) are particularly useful in dynamic situations. The Ductility-Centred Robustness Assessment developed at Imperial College London is also used in this paper to derive simple design formulae to assess punching of adjacent columns in the sudden column removal scenario, which is commonly adopted in practice. The approach can be extended to assess flat slab systems when considering membrane action in the slab and post-punching behavior in the connections. Analytical models for tensile membrane are used in combination with the CSCT to demonstrate that the tying forces required in codes of practice cannot be achieved without prior punching of the connections. It is also shown that numerical modelling of post-punching is a promising tool to review detailing provisions for integrity reinforcement.

Flat slab structures are a very common structural solution nowadays, due to their architectural and economic advantages. However, flat slab-column connections may be vulnerable to punching failure, especially in the event of an earthquake, with potentially high human and economic losses. This type of structural solution is adequately covered by design codes and recommendations in North America, due to the large amount of experimental research that has been carried out. In Europe, the situation is different: specific guidance to flat slab design under earthquake action is missing from most European codes. The ACI 318-14 prescriptive approach to the gravity shear ratio-drift ratio relationship shows good agreement with experimental results. Following a similar approach and, based on a databank containing cyclic horizontally loaded tests of slab-column connections found in the literature, proposals are made that are applicable to Eurocode 2 and the fib Model Code 2010.

The focus of this research is on developing new punching shear retrofit techniques for slab-column connections to improve the seismic response of flat-plate systems. Previous tests have shown the effectiveness of using shear reinforcement to enhance the shear strength and ductility of individual slab-column connections. However, the advantage of ductility in reducing the earthquake impact on structures is accompanied by an increase in the base shear, due to increased stiffness. Herein, a new type of punching shear retrofit element, shear bolts with flexible washers, is introduced. The flexible washers allow for shear crack opening during the lateral displacements, while at the same time providing control of the crack width by controlling the washer thickness and/or stiffness. The results show that this technique increases the ductility of the connections, without a commensurate increase in stiffness. The effect of this type of shear reinforcement on the response of an assembled structure is investigated through dynamic analysis, to check how energy dissipation within individual connections affects the overall energy dissipation of a flat-plate system. The presented system was designed for slab retrofit. However, it can be anticipated that similar concepts could be used in the construction of new slabs in seismic zones.

This paper examines the influence of flexural continuity on punching resistance at edge columns of braced flat slabs under gravity loading, making use of experimental data, nonlinear finite element analysis (NLFEA) and the Critical Shear Crack Theory (CSCT) as presented in the fib Model Code 2010 (MC2010). According to the CSCT, punching resistance reduces with increasing rotation y of the slab relative to its support area due to loss of aggregate interlock in the critical shear crack. NLFEA shows that as loads are increased to failure, moment redistribution from edge column supports to the span causes the loading eccentricity at edge columns to reduce below its initial elastic value. The resulting rotation y and peak shear stress are less than they are in comparable isolated test specimens with fixed loading eccentricity. Consequently, the CSCT predicts punching resistance at edge columns of flat slabs to be significantly influenced by flexural continuity, which is unaccounted for in the design methods of ACI 318 and EC2. Both NLFEA and the CSCT suggest that providing surplus flexural reinforcement in the span can be more effective at increasing punching resistance at edge columns than the common UK practice of providing surplus hogging flexural reinforcement.