Análise crítica dos critérios normativos de dimensionamento à punção em lajes lisas

Abstract

The Brazilian code NBR 6118/2003 introduces significant changes in the criteria for the punching shear strength of reinforced and prestressed concrete flat slabs. In this scenario, the objective of this work is to evaluate these punching shear design procedures in terms of safety, precision and economy with respect to test results of reinforced and prestressed concrete flat slabs, with and without transversal reinforcement built with normal strength concrete (fc 50MPa) and high-performance concrete (fc > 50MPa). The effects of the presence or not of punching reinforcement, of longitudinal reinforcement rates, of concrete compressive strength, yield strength of shear reinforcement and the magnitude of the initial prestressed force are also investigated. EUROCODE 2 (2004) and ACI 318 (2005) punching shear design criteria are also analyzed. The data basis is composed of 354 experimental results of slabs with symmetrical internal columns. The overall analysis indicates that all design code criteria are safe, conservative and not economical. A revision of NBR 6118 design equation for the transversal reinforcement contribution in the punching shear strength of reinforced concrete flat slabs is recommended. The results also show that, the value of k, in EUROCODE 2, shall not be limited to 2 and that the longitudinal reinforcement rate be included in ACI 318 design equations. For prestressed slabs all design criteria are extremely conservative and consequently not economical.