Behaviour and analysis of beam–column joint strengthened by cast-in-situ expansion

Abstract

This paper presents a joint strengthening method using cast-in-situ expansion. The objective of cast-in-situ expansion is to make a joint stronger than adjoining beams and change failure from brittle joint shear to ductile beam flexure. In this method, cast-in-situ expansions are fabricated around corners of the joint. The cast-in-situ expansion increases joint shear capacity through increased effective joint area and increases anchorage bond capacity of beam bars passing through the joint through increased apparent column depth. An experimental programme is conducted. Four half-scale beam–column specimens were tested under quasi-static cyclic load. The specimens were strengthened with triangular and square expansions. Test results indicated brittle joint shear failure in the control specimen and beam flexural failure in the strengthened specimens. The strength, stiffness and energy dissipation of strengthened specimens were increased. The experiment is supplemented by non-linear finite-element analysis to investigate the flow of stresses in the joint panel and the expansion. The non-linear finite-element analysis applies the fixed smeared crack concept to simulate distributed cracks in the beam, column and joint panel. A one-dimensional discrete joint element is applied to represent the interfacial contact between beam and column and between expansions and members where local non-linearity such as bar pull-out and interface shear may occur. Both experiment and finite-element analyses demonstrate parallel cracks along the edge of expansion, indicating a diagonal compressive strut. Based on the principal stress plot, the finite-element method reveals two load-bearing mechanisms. In addition to the primary diagonal compressive strut in the joint panel, the expansion provides the secondary strut mechanism along the edge of the opposite expansions. The size of expansion has a direct influence on the capacity of the secondary strut. The failure of the strengthened specimens depends on the relative bending capacity of beam sections and the combined capacity of the primary and secondary joint strut mechanism.