Research to address the performance of lightly reinforced concrete walls during the 2010/2011 Canterbury earthquakes

Abstract

During the 2010/2011 Canterbury earthquakes, several reinforced concrete (RC) walls in multi-storey buildings formed a limited number of cracks at the wall base with fracture of vertical reinforcement occurring. This failure mode is typical of lightly reinforced concrete members, where the area of reinforcing steel is insufficient to develop the tension force required to form secondary cracks. The minimum vertical reinforcement limits for RC walls in different concrete design standards were compared, and a series of numerical analyses were used to investigate the behaviour of an example RC wall designed according to the minimum requirements of each standard. The analysis results confirmed the observed failure mode of an RC wall with less than the current minimum vertical reinforcement that was damaged during the Canterbury earthquakes. Furthermore, RC walls built in accordance with current minimum vertical reinforcement requirements in both ACI 318-11 and NZS 3101:2006 are still susceptible to limited flexural cracking and premature bar fracture. The ductility of RC walls with concentrated reinforcement at the wall ends, such as that required by Eurocode 8, CSA 2004 and GB 50010, was significantly improved. A comprehensive experimental program is also currently underway to verify the seismic performance of lightly reinforced concrete walls, and a test setup has been developed to subject RC wall specimen to loading that is representative of a multistorey building. Numerical analyses of the test walls further confirmed that RC walls designed according to current NZS 3101:2006 minimum vertical reinforcement requirements may be susceptible to limited flexural cracking and premature reinforcement fracture. Additionally, the analyses indicated that drift capacity of RC walls with minimum vertical reinforcement improved as the aspect ratio or axial load ratio was increased.