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

The assessment of existing flat slabs and bridges often shows insufficient punching shear resistance in the area of the support regions. The reasons for this are modified requirements of use or more restrictive design rules. There are many different methods to increase the punching shear resistance, but most of them are expensive and require access to the upper surface of the structure. Thus the construction work is only possible under restricted operation. In addition, difficult detail issues arise with regard to the rearrangement of the structure sealing. This paper deals with a new strengthening system using concrete screws, which are installed vertically into pre-drilled holes from the soffit of the slab. The results of two test series with a total of nine specimens are presented in this paper. It turns out that this strengthening method leads to a significant increase in the shear punching capacity and to a less brittle failure mode.

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.

Recommendations are presented for proposed revisions to the punching shear provisions of ACI 318-14 in order to recognize the limitations imposed on punching shear capacity by low amounts of slab flexural reinforcement and by slab depth effects. The influence of those two factors is investigated by examining relevant experimental results from tests on slab-column assemblages and slab systems.

Punching shear failure of reinforced concrete slabs has been examined by many researchers through laboratory experiments. However, the existing punching shear testing database cannot address all aspects of the punching shear stress transfer mechanism. Advanced 3-D finite element analysis (FEA) can be used to supplement the existing testing background and for parametric investigations. In this way, different aspects of punching shear failure may be explored in detail, to enable understanding of the phenomena that control the response and to support drafting design code requirements. This paper describes research on calibrating constitutive and finite element models in ABAQUS to capture punching shear behavior of concrete slabs. The coupled damaged-plasticity model is used for modeling the concrete. Two interior reinforced concrete slab-column connections previously tested under static loading are presented: one slab is without shear reinforcement (SB1) and the other slab is with shear bolts (SB4). The developed formulation is calibrated using the results for specimen SB1, where the tension stiffening response, the damage parameters and the support conditions are examined. Then, the adopted FEA and concrete model are used for the analysis of slab SB4, which was retrofitted with shear bolts. Finally, both test and numerical results are compared to the ACI 318-14 provisions.