International Concrete Abstracts Portal

SP-232 The development of high-performance materials and advanced computational tools has allowed the building of relatively thin concrete slabs supported on columns. The simple appearance of such structural systems and ease of their construction make them economically attractive and popular. However, these structures develop complex three-dimensional stresses in the slab, at the columns, which can eventually lead to a potentially catastrophic brittle punching shear failure. Although structural failures are rare, in part due to high safety factors, understanding punching shear phenomenon is crucial for safe and rational design of flat reinforced concrete slabs supported on columns. As part of the activities of the ACI/ASCE Committee 445, Shear and Torsion, members of Subcommittee 445-C, Punching Shear, organized a symposium in conjunction with this special publication devoted to the state of the art on punching shear. The symposium will be held during the ACI Fall 2005 Convention under the sponsorship of ACI/ASCE Committee 445. The last significant and comprehensive overviews on this topic were completed several years ago by the fédération internationale du béton (fib) in their state-of-the-art report on punching shear (2001)1 and in the Proceedings of the International Workshop on Punching Shear Capacity of RC Slabs (2000)2 published by the Royal Institute of Technology in Sweden. Subcommittee 445-C has compiled a series of papers that present updated developments in the state of the art and research regarding behavior, rational design, and evaluation of code provisions related to punching shear. The papers are grouped into two parts. The first part contains a report, coauthored by members of Subcommittee 445-C, on issues related to design philosophy, code provisions, contributions of flexural and shear reinforcements, and seismic and blast loads. The second part includes papers on new developments obtained from different research centers from around the world. The aim is to present comprehensive and objective information on the topic of punching shear. It is hoped that this publication will be important for the engineering design community in its efforts to improve long-term strength and ductility of slab-column structural systems.

The effects of the column dimensions on the punching shear strength andductility of slab-column connections are investigated. For this purpose, five edgecolumn-slab connections were tested. The parameters were the column aspect ratioand the perimeter length of the critical shear section. The test results in addition to testresults from the literature for interior columns are compared with the shear strengthprovisions of the ACI Code. The validity of applying the shear strength equationsoriginally developed for interior connections is investigated, in particular with respect tothe effect of the column dimensions.

The paper presents an overview of different shear reinforcing methods forflat concrete slabs supported on columns. Typical reinforcing systems are described forboth new construction and for retrofitting existing slabs. These systems have beenstudied, tested and used in construction in the last several decades. The description ofthe reinforcement types, properties, methods of design, and construction issuesrelated to placement in slabs is included in this paper. The aim is to show theimportance of providing shear reinforcement in flat concrete slabs in the vicinity ofcolumns in terms of both strength and ductility. The theoretical background and themost important requirements for the effective shear reinforcement are emphasized anddiscussed. Selected experimental results are presented to illustrate the performanceof slabs with different types and placement configurations of shear reinforcements inslabs.

The ACI Code design equation for concentric punching shear is often criticized for failing to account for the effect of flexural reinforcement. This criticism is based on the poor performance of the code equation as a predictor of test results. However, in thecontext of design, the code works as well as any detailed analytical model available, witha fraction of the computation effort. This is because for non-prestressed slabs the codeequation correctly identifies the transition between flexural failure and shear failure,assuming the slab has been correctly designed for flexure. It is concluded that a strongargument can be made for retaining the current equation for concentric punching.

Reinforced concrete slabs exhibited interesting behavior under severedynamic loads associated with impact and explosions. During the 1980s, high explosivesimulation techniques (HEST) were used to study the behavior of shallow-buriedreinforced concrete boxes under the effects of nuclear explosions. Those tests showedthat, under specific conditions, the roofs could fail in a direct shear mode within aboutthe first millisecond of loading. Those incidents were studied also numerically, andgood behavioral models were developed to predict this phenomenon. The behavior ofstructural concrete slabs under localized impact was studied more recently bycombining numerical simulations and precision impact tests, as well as by addressingthe pressure-impulse characteristics for such structures. These studies enabled abetter understanding of structural concrete slabs behavior under severe dynamic loads,as presented and discussed in this paper.