Influence of punching shear reinforcement on the flexural response of flat slabs

Physical punching shear models such as the Critical Shear Crack Theory (CSCT), which is the basis of Model Code 2010 (MC 2010), describe generally the punching strength as a function of the slab deformation. Thus, the prediction of an accurate load-deformation curve is essential for the prediction of the punching strength as well as the deformation capacity of flat slabs. In the vicinity of columns, the flexural response of flat slabs depends not only on the flexural moments but also on the concentrated shear forces. In the case of slender slabs, slabs with low amounts of flexural reinforcement, or slabs without shear reinforcement, the shear forces are limited and thus their influence in the flexural response is low. However, in the case of thick slabs with shear reinforcement, large shear forces are carried influencing the flexural response of the flat slab. Despite the importance of an accurate prediction of the load-rotation response, little research has yet been performed on this subject. This paper presents an investigation on the flexural response of 6 full-scale flat slab specimens. The specimens presented herein belong to an extensive experimental campaign of 16 full-scale flat slab specimens performed at the Ecole Polytechnique Fédérale de Lausanne (Switzerland) with the aim to investigate the punching strength and the rotation capacity of flat slabs with and without shear reinforcement. The investigated parameters were the column size, the slab thickness, the amount of shear reinforcement, and the type of shear reinforcement system. The investigation of the flexural response consists of the comparison of the experimentally obtained load-rotation responses to the predictions by the CSCT, MC 2010, and a modified approach based on the CSCT.

Experimental investigation of reinforced concrete slabs with punching shear reinforcement

Stefan Lips, Aurelio Muttoni

Over the past century, the use of flat slabs in buildings and especially in parking garages has been growing as it is an economic and efficient solution. Flat slabs are easy to build and have, through their smaller depth, an economical and architectural advantage compared to slabs on girders. Because of their limited depth, flat slabs are especially sensitive to deflections and to punching shear, which are their main design criteria. Furthermore, flat slabs without punching shear reinforcement have a rather brittle failure mode with little deformation capacity. This behavior potentially limits the redistribution of the internal forces in flat slabs in the case of punching of an isolated column and can thus lead to progressive collapse of the entire structure. To increase both the strength and the deformation capacity of flat slabs, punching shear reinforcement can be provided in the vicinity of the columns. Although the influence of punching shear reinforcement on the strength of flat slabs has been intensively investigated, there are still fundamental uncertainties such as the contribution of the shear reinforcement and the strength of the concrete struts close to the column. The paper presents the results of an extensive experimental campaign performed at the Ecole Polytechnique Fédérale de Lausanne (EPFL). Sixteen full-scale slab specimens (3.0 x 3.0 m in plane) with varying parameters such as the column size, the slab thickness, the shear reinforcement ratio, and the shear reinforcing system have been investigated. The performance of these specimens is analyzed and compared to modern design codes and to the critical shear crack theory.

Proceeding of the 8th fib-PhD Symposium2010

Shear strength of RC slabs under concentrated loads near clamped linear supports

Miguel Fernández Ruiz, Francisco Manuel Maciel Natário, Aurelio Muttoni

Reinforced concrete slabs without shear reinforcement subjected to concentrated loads near linear sup- ports are usually designed or assessed in shear using design provisions that have been calibrated on the basis of tests on one-way slabs or beams with rectangular cross-section (slabs loaded over their full width). This approach may however be inconsistent, as the actual behavior of slabs under concentrated loads reflects a two-way slab response, with the shear forces not developing in a parallel manner in the failure region and with potential redistribution of the internal forces for increasing levels of load. In this research, a series of 12 tests on 6 full scale slabs (3.00 m 3.00 m 0.18 m) is presented. The slabs were centrally supported on an aluminum profile and were subjected to two concentrated symmet- rical loads. The support profile was equipped with vertical strain gauges, allowing to trace the distribu- tion of reactions at the supported line and its redistributions as the value of the concentrated loads increased. Significant redistributions of the reactions were actually measured prior to failure, which are presented and discussed within the paper. The aim of these measurements was to improve the under- standing of the mechanical behavior of such members and the role of shear cracking on the distribution of internal forces. A number of parameters was investigated, such as the location of the concentrated loads and the presence of ducts in the slab. This latter case (typical of prestressed concrete balanced cantilever bridges and residential buildings) has not been investigated in the past, despite its potential influence on the shear strength. The performance of these specimens is finally investigated and compared to the prediction of the fib - Model Code 2010 and the Critical Shear Crack Theory. This analysis shows that accurate predictions of the strength can be obtained if the influence of direct load strutting (for loads acting near supports) and redistribution of internal forces is accounted.

Engineering structures2014

Shear strength of RC slabs under concentrated loads near clamped linear supports

Miguel Fernández Ruiz, Francisco Manuel Maciel Natário, Aurelio Muttoni

Engineering structures2014