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

Seventeen large-scale interior reinforced concrete slab-column connections were tested to study the effect of different shear stud layouts and the percentage of slab flexural reinforcement. They were divided into two series M (twelve specimens) and S (five specimens) based on their dimensions. Each specimen in Series M had a 6 ft by 6 ft (1830 mm by 1830 mm) and 8 in. (200 mm) thick slab and a 6 in. by 6 in. (150 mm by 150 mm) column cross-section, while each specimen in Series S had a 10 ft by 10 ft (3050 mm by 3050 mm) and 10 in. (250 mm) thick slab and a 12 in. by 12 in. (300 mm by 300 mm) column cross-section. The percentage of slab flexural tension reinforcement was approximately either 0.8% or 1.2%, and shear studs were arranged in either an orthogonal or radial layout. Test results showed that shear strength equations in the ACI Building Code (ACI 318, 2014) overestimated the strength of some test specimens. Also, specimens with a radial layout of shear studs typically had higher strength and more ductile behavior than specimens with an orthogonal stud layout. Recommendations to improve the design of flat plate systems are presented.

The shear capacity of slabs under concentrated loads is particularly of interest for bridge decks under concentrated live loads. Often, one-way shear will be analyzed by considering the slab as a wide beam (without taking advantage of the transverse load redistribution capacity of the slab) and two-way shear by considering the punching area around the load. Since experiments have shown that the failure mode of slabs under concentrated loads is a combination of one-way and two-way shear as well as two-way flexure, a method was sought that bridges the gap between the traditional one-way and two-way shear approaches. The proposed method is a plasticity-based method. This method is based on the Strip Model for concentric punching shear and takes the effects of the geometry into account for describing the ultimate capacity of a slab under a concentrated load. The model consists of “strips” that work with arching action (one-way shear) and slab “quadrants” that work in two-way shear. As such, the resulting Extended Strip Model is suitable for the design and assessment of elements that are in the transition zone between one-way and two-way shear.

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.

Size effect has been theoretically and experimentally acknowledged as a phenomenon influencing the shear and punching shear strength of concrete structures, with reducing unitary shear strength for increasing member sizes. For members failing in shear, as beams or one-way slabs without transverse reinforcement subjected to uniform loading, size effect has been shown to have a variable influence, with low significance for failures governed by limit analysis (strength or yield criterion) and large influence for members failing in a brittle manner. When the response of a member failing in shear can be reasonably approximated by a linear behaviour (i.e. linear relationship between the acting shear force and the crack widths), the predictions of Linear Elastic Fracture Mechanics (LEFM) can be applied to asymptotically large specimen sizes. This phenomenon can for instance be demonstrated by the Critical Shear Crack Theory (CSCT) and leads to a dependence of the shear strength with the power-1/2 of the size of the specimen. Nevertheless, in actual structures failing in shear (such as slabs or shells), the structural response is normally characterized by some level of redundancy and capacity to redistribute internal forces in the longitudinal and transversal directions. In this case, the relationship between the acting shear force and the crack widths is not linear (with lower crack widths associated with larger shear strengths) and the influence of size effect on the shear strength is milder than that predicted by LEFM. With respect to punching (shear failures due to concentrated loads in two-way slabs), a similar behaviour is observed with respect to size effect. A low dependency can be observed when limit analysis governs whereas for brittle failures, size effect becomes significant. In this case, it can be observed that the behaviour of slabs is highly nonlinear (as for redundant members failing in shear), and the crack openings are to a large extent dependent on local and structural tension-stiffening effects. This deviates the actual behaviour from the one predicted by LEFM and modifies the influence of size effect, which becomes less significant than the predicted behaviour according to LEFM. In this paper, this phenomenon is investigated by means of the CSCT, providing a consistent frame to analyse size and strain effects accounting for realistic slab responses.