In situ conditions affecting the ductility capacity of lightly reinforced concrete wall structures in the Canterbury earthquake sequence

Abstract

Following the 2010-2011 Canterbury (New Zealand) earthquake sequence, lightly reinforced wall structures in the Christchurch central business district were observed to form undesirable crack patterns in the plastic hinge region, while yield penetration either side of cracks and into development zones was less than predicted using empirical expressions. To some extent this structural behaviour was unexpected and has therefore demonstrated that there may be less confidence in the seismic performance of conventionally designed reinforced concrete (RC) structures than previously anticipated. This paper provides an observation-based comparison between the behaviour of RC structural components in laboratory testing and the unexpected structural behaviour of some case study buildings in Christchurch that formed concentrated inelastic deformations. The unexpected behaviour and poor overall seismic performance of ‘real’ buildings (compared to the behaviour of laboratory test specimens) was due to the localization of peak inelastic strains, which in some cases has arguably led to: (i) significantly less ductility capacity; (ii) less hysteretic energy dissipation; and (iii) the fracture of the longitudinal reinforcement. These observations have raised concerns about whether lightly reinforced wall structures can satisfy the performance objective of “Life Safety” at the Ultimate Limit State. The significance of these issues and potential consequences has prompted a review of potential problems with the testing conditions and procedures that are commonly used in seismic experimentations on RC structures. This paper attempts to revisit the principles of RC mechanics, in particular, the influence of loading history, concrete tensile strength, and the quantity of longitudinal reinforcement on the performance of real RC structures. Consideration of these issues in future research on the seismic performance of RC might improve the current confidence levels in newly designed conventional RC structures.

Fragility Functions for RC Shear Wall Buildings in Australia

“…Although Hazus (FEMA 2010) has building parameters for “Pre-Code” buildings, which correspond to structures that have not been seismically designed, it is possible that the findings from Edwards et al (2004) will also hold true for the comparisons made between the fragility curves derived from the generic building parameters provided by Hazus (FEMA 2010) and those derived from an extensive number of capacity curves, which better reflect the RC structural wall building stock in Australia. This is primarily because of the poor performance observed from lightly reinforced and unconfined concrete walls in recent earthquake events (Beca 2011, Canterbury Earthquakes Royal Commission 2012, Henry 2013, Morris et al 2015, Sritharan et al 2014, Wallace et al 2012). Because of the low standard of detailing required in the current materials standards in Australia and the low earthquake return period (RP) typically used in design, it is anticipated that most of the RC walls and cores embedded within structures around Australia are lightly reinforced and unconfined, and this is likely to lead to brittle behavior in an earthquake (Hoult et al 2015).…”