A constitutive model for confined concrete in slender rectangular RC sections incorporating compressive energy

Reinforced concrete (RC) structural walls can provide superior stiffness in tall buildings while maintaining notable ductility when subjected to seismic demands. In flexure-dominated walls, this ductility is based on the ability of the end regions (also known as boundary elements) to maintain load carrying capacity through increasing tension and compression cycles. In order to achieve this behavior, modern design practice aims to force substantial yielding in tension prior to loss of compressive strength. As such, a sound understanding of the compressive behavior of wall end regions is paramount to understanding RC wall ductility. A database of both rectangular RC prism (axial) and RC wall tests was assembled, and the results were compared with a popular analytical constitutive model for confined concrete. The existing model assumes that compressive behavior is based on fracture of the transverse reinforcement, however the data indicates that bar buckling and/or crushing of the core can occur well before transverse bar fracture. Therefore, the existing model significantly overestimates both compressive strength and strain capacity in many conditions consistent with modern design. Formulations for compressive strength and strain capacity, as well as a monotonic constitutive model, are proposed to capture the compressive behavior observed in the data. Both the number of longitudinal bars and the use of crossties in place of closed hoops were found to have a significant impact on the compressive behavior of confined concrete, and are therefore integral to the proposed model. The model was developed based on tests performed by the authors, and then validated using the aforementioned database.