On the need for classification criteria for cast in situ RC beam–column joints according to their stiffness

Abstract

The purpose of this paper is to draw attention to the lack, in current technical specifications, (i) of guidelines on the modelling of cast in situ reinforced concrete beam–column (RCBC) joints and, more specifically, (ii) for a given RCBC joint, of simplified procedures for evaluating if the fully rigid joint model is acceptable or not, having in view its effective stiffness. This is an important issue because, even though it is widely recognized that RCBC joints are a major source of deformation of cast in situ unbraced RC frames, there is no acknowledged simple procedure to assess the relevance of joint flexibility to the overall structural behaviour—as a consequence, the fully rigid joint model is commonly used without any reference to joint stiffness minimum required values. On the other hand, for steel frames, classification criteria for joints according to their stiffness have already been developed. This paper (i) shows that EN 1993-1-8: 2005 simplified classification criteria for steel joints according to their stiffness are inadequate for cast in situ RCBC joints, (ii) develops a modified version of these criteria, appropriate only for very particular conditions, which maintains their basic hypotheses but considers the specificities of RCBC joints and frames and (iii) in agreement with recently published results, concludes that, in some cases, RCBC joints should not be considered nominally rigid and thus their flexibility should be explicitly accounted for in the analysis of unbraced RC frames. The developed simplified classification criteria apply only to very particular cases—some guidance is therefore also provided on the development of general simplified classification criteria for RCBC joints.