Six months after the 4 September 2010 Mw 7.1 Darfield (Canterbury) earthquake, a Mw 6.2 Christchurch (Lyttelton) aftershock struck Christchurch on the 22 February 2011. This earthquake was centred approximately 10km south-east of the Christchurch CBD at a shallow depth of 5km, resulting in intense seismic shaking within the Christchurch central business district (CBD). Unlike the 4 Sept earthquake when limited-to-moderate damage was observed in engineered reinforced concrete (RC) buildings [35], in the 22 February event a high number of RC Buildings in the Christchurch CBD (16.2 % out of 833) were severely damaged. There were 182 fatalities, 135 of which were the unfortunate consequences of the complete collapse of two mid-rise RC buildings. This paper describes immediate observations of damage to RC buildings in the 22 February 2011 Christchurch earthquake. Some preliminary lessons are highlighted and discussed in light of the observed performance of the RC building stock. Damage statistics and typical damage patterns are presented for various configurations and lateral resisting systems. Data was collated predominantly from first-hand post-earthquake reconnaissance observations by the authors, complemented with detailed assessment of the structural drawings of critical buildings and the observed behaviour. Overall, the 22 February 2011 Mw 6.2 Christchurch earthquake was a particularly severe test for both modern seismically-designed and existing non-ductile RC buildings. The sequence of earthquakes since the 4 Sept 2010, particularly the 22 Feb event has confirmed old lessons and brought to life new critical ones, highlighting some urgent action required to remedy structural deficiencies in both existing and “modern” buildings. Given the major social and economic impact of the earthquakes to a country with strong seismic engineering tradition, no doubt some aspects of the seismic design will be improved based on the lessons from Christchurch. The bar needs to and can be raised, starting with a strong endorsement of new damage-resisting, whilst cost-efficient, technologies as well as the strict enforcement, including financial incentives, of active policies for the seismic retrofit of existing buildings at a national scale.
The 22 February 2011 M w 6.2 Christchurch (Lyttelton) earthquake was a particularly severe test for both modern seismically designed and existing non-ductile reinforced concrete (RC) buildings. Some 16.2 % of 833 buildings with RC systems within the Christchurch central business district (CBD) were severely damaged. There were 182 fatalities, 135 of which were the unfortunate consequences of the complete collapse of two medium-rise RC buildings. As with the post-Northridge 1994 earthquake, the design performance of "modern" structures is being scrutinized - with the inevitable question: is "life safety" but irreparable damage still a valid performance target? This brief paper presents a summary of RC building damage from a broad performance-based earthquake engineering perspective. Several preliminary lessons, not all of them surprising, and the issues that have arisen will be discussed using case study buildings, with suggestions for urgently needed research areas
SUMMARY 7This paper presents an innovative set of high-seismic-resistant structural systems termed Advanced FlagShaped (AFS) systems, where self-centering elements are combined in series and/or in parallel with 9 alternative forms of energy dissipation (yielding, friction and viscous damping). AFS systems is developed using the rationale of combining velocity-dependent with displacement-dependent energy dissipation for 11 self-centering systems, particularly to counteract near-fault earthquakes. Non-linear time-history analyses (NLTHA) on a set of four single-degree-of-freedom (SDOF) systems under a suite of 20 far-field and 13 20 near-fault ground motions are used to compare the seismic performance of AFS systems with the conventional systems. It is shown that AFS system with a combination of hysteretic and viscous energy 15 dissipations achieved greater performance in terms of the three performance indices. The use of friction slip in series of viscous energy dissipation is shown to limit the peak response acceleration and induced 17 base-shear. An extensive parametric analysis is carried out to investigate the influence of two design parameters, 1 and 2 on the response of SDOF AFS systems with initial periods ranging from 0.2 to 3.0 s 19 and with various strength levels when subjected to far-field and near-fault earthquakes. For the design of self-centering systems with combined hysteretic and viscous energy dissipation (AFS) systems, 1 is 21 recommended to be in the range of 0.8−1.6 while 2 to be between 0.25 and 0.75 to ensure sufficient self-centering and energy dissipation capacities, respectively.
This paper describes observations of damage to reinforced concrete buildings from the September 2010 Darfield (Canterbury) earthquakes. Data was collated from first-hand earthquake reconnaissance observations by the authors, post-earthquake surveys, and communications and meetings with structural engineers in Christchurch. The paper discusses the general performance of several reinforced concrete building classes: pre-1976 low-rise, pre-1976 medium rise, modern low- and mid-rise, modern high-rise, industrial tilt-up buildings, advanced seismic systems and ground-failure induced damaged and retrofitted RC buildings. Preliminary lessons are highlighted and discussed. In general, reinforced concrete buildings behaved well and as expected, given the intensity of this event.
Strength hierarchy assessment is a method that can be utilized to identify the weakest structural element at a reinforced concrete (RC) beam-column joint. The method was extensively used in various research activities at the University of Canterbury, which mainly involved beam-column joint subassembly tests. However, this method required improvements and refinements in order to be adopted in multi-story building applications. In this paper, the improvements made to the method are reported. In the improved method, capacity of the weakest element at every beam-column joint in an RC frame building can be related to the corresponding global base shear demand. The method has been illustrated via two example applications: an RC frame lacking joint shear reinforcement and a modern RC frame with adequate joint shear reinforcement. The case study examples confirmed the accuracy and the effectiveness of the method
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