Many existing RC structures around the world were designed to sustain gravity and wind loads only. Past earthquake reconnaissance have shown that strong earthquakes can lead to substantial damage ranges to non-seismically designed RC buildings, particularly to beam-column joints. This paper presents a novel retrofit method using buckling restrained haunches (BRHs) to improve the seismic performance of such joints. A numerical model for RC joints is introduced and validated. Subsequently, a new seismic retrofit strategy using BRHs is proposed, aimed at relocating plastic hinges and increasing energy dissipation. The results indicate the retrofit method can effectively meet the performance objectives.

Introduction

Seismic safety of a large number of existing non-seismically designed reinforced concrete (RC) structures is a major concern around the world. Evidences from past earthquake reconnaissance (Northridge 1994, Kobe 1995, Kocaeli 1999, Christchurch 2011) demonstrated that non-seismically designed RC buildings are vulnerable to substantial damage or even collapse under moderate to severe earthquakes. In general, non-seismically designed RC structures can be classified into two categories: one has the construction time prior to the introduction of seismic design provisions with ductility concept, such as the buildings constructed in California before the mid-1970s according to the 1967 Uniform Building Code [Liel et al., 2011]; the other is in regions of low-to-moderate seismicity where structures were traditionally designed with no or little consideration of seismic resistance, such as Hong Kong and Singapore whose design philosophy had followed the British standard BS8110 for a long time [Kuang and Wong, 2006; Pam and Ho, 2010; Li and Pan, 2007]. Buildings designed and detailed taking into account only gravity and wind loads would have to rely greatly on its inherent ductility if a severe earthquake were to strike. Non-seismically designed structures mainly feature inadequate reinforcement details, such as a lack of joint transverse reinforcement, insufficient transverse reinforcement in columns, column lap splice located in potential plastic hinge regions, and inadequate anchorage detailing [NIST, 2013]. Typical reinforcement details of non-seismically designed beam-column interior and exterior joints are shown in Fig. 1. In this paper, the deficiencies in beam-column joints are of particular interest because they are likely to fail prior to the formation of plastic hinges in the beams, thereby leading to the severe deterioration or premature collapse of buildings. For these reasons, ACI-ASCE Committee 352 stated that methods for improving the performance of older joints need to be studied because only limited knowledge was available on connection repair and retrofitting [ACI-ASCE Committee 352, 2002].