Drywalls are the typical infill or partitions used in new structures. They are usually located within structural frames and/or between upper and lower floor slabs in buildings. Due to the materials used in their construction, unlike masonry blocks, they can be considered as light non-structural infill/partition walls. These types of walls are especially popular in New Zealand and the USA. In spite of their popularity, little is known about their in-plane cyclic behaviour when infilled within a structural frame. The cause of this lack of knowledge can be attributed to the typical assumption that they are weak non-structural elements and are not expected to interact with the surrounding structural system significantly. However, recent earthquakes have repeatedly shown that drywalls interact with the structure and suffer severe damage at very low drift levels. In this paper, experimental test results of two typical drywall types (steel and timber framed) are reported in order to gather further information on; i) their reverse cyclic behaviour, ii) inter-storey drift levels at which they suffer different levels of damage, iii) the level of interaction with the surrounding structural frame system. The drywall specimens were tested using quasi-static reverse cyclic testing protocols within a full scale precast RC frame at the Structures Laboratory of the University of Canterbury.
La pericolosità associata ad un dato fenomeno costituisce uno dei fattori più importanti e difficili da quantificare nelle analisi di rischio, a maggior ragione quando si tratta di fenomeni complessi come nel caso della liquefazione sismica. Il presente lavoro illustra sinteticamente uno studio della pericolosità indotta al suolo da liquefazione basato su un caso campione statistico particolarmente significativo, il terremoto (Mw 6.2) che ha colpito Christchurch, Nuova Zelanda, del 2011. La notevole mole di dati disponibili, relativi alle caratteristiche geotecniche del sottosuolo, unitamente al rilievo dei danni ha consentito innanzitutto di caratterizzare la suscettibilità a liquefazione dell’area, indipendentemente dall’evento sismico, quindi di correlare statisticamente le diverse grandezze e di derivare delle curve di vulnerabilità del suolo.
This article presents a quantitative case study on the site amplification effect observed at Heathcote Valley, New Zealand, during the 2010-2011 Canterbury earthquake sequence for 10 events that produced notable ground acceleration amplitudes up to 1.4g and 2.2g in the horizontal and vertical directions, respectively. We performed finite element analyses of the dynamic response of the valley, accounting for the realistic basin geometry and the soil non-linear response. The site-specific simulations performed significantly better than both empirical ground motion models and physics based regional-scale ground motion simulations (which empirically accounts for the site effects), reducing the spectral acceleration prediction bias by a factor of two in short vibration periods. However, our validation exercise demonstrated that it was necessary to quantify the level of uncertainty in the estimated bedrock motion using multiple recorded events, to understand how much the simplistic model can over- or under-estimate the ground motion intensities. Inferences from the analyses suggest that the Rayleigh waves generated near the basin edge contributed significantly to the observed high frequency (f>3Hz) amplification, in addition to the amplification caused by the strong soil-rock impedance contrast at the site fundamental frequency. Models with and without considering soil non-linear response illustrate, as expected, that the linear elastic assumption severely overestimates ground motions in high frequencies for strong earthquakes, especially when the contribution of basin edge-generated Rayleigh waves becomes significant. Our analyses also demonstrate that the effect of pressure-dependent soil velocities on the high frequency ground motions is as significant as the amplification caused by the basin edge-generated Rayleigh waves.
Predicting building collapse due to seismic motion is critical in design and more so after a major event. Damaged structures can appear sound, but collapse under following major events. There can thus be significant risk in decision making after a major seismic event concerning the safe occupation of a building or surrounding areas, versus the unknown impact of unknown major aftershocks. Model-based pushover analyses are effective if the structural properties are well understood, which is not valid post-event when this risk information is most useful. This research combines Hysteresis Loop Analysis (HLA) structural health monitoring (SHM) and Incremental Dynamic Analysis (IDA) methods to determine collapse capacity and probability of collapse for a specific structure, at any time, a range of earthquake excitations to ensure robustness. The nonlinear dynamic analysis method presented enables constant updating of building performance predictions using post-event SHM results. The resulting combined methods provide near real-time updating of collapse fragility curves as events progress, quantifying the change of collapse probability or seismic induced losses for decision-making - a novel, higher resolution risk analysis than previously available. The methods are not computationally expensive and there is no requirement for a validated numerical model. Results show significant potential benefits and a clear evolution of risk. They also show clear need for extending SHM toward creating improved predictive models for analysis of subsequent events, where the Christchurch series of 2010-2011 had significant post-event aftershocks after each main event. Finally, the overall method is generalisable to any typical engineering demand parameter.
This paper concerns the explicit consideration of near-fault directivity in conventional ground motion prediction models, and its implication for probabilistic seismic hazard analysis (PSHA) in New Zealand. The proposed approach utilises recently developed models by Shahi & Baker (2011), which account for both the 'narrowband' nature of the directivity pulse on spectral ordinates, and the probability of pulse occurrence at the site of interest. Furthermore, in order to correctly consider directivity, distributed seismicity sources are considered as finite-faults, as opposed to their (incorrect) conventional treatment as point-sources. The significance of directivity on hazard analysis results is illustrated for various vibration periods at generic sites located in Christchurch and Otira, two locations whose seismic hazard is comprised of notably different seismic sources. When compared to the PSHA results considering directivity and distributed seismicity as finite faults, it is shown that the NZS1170.5:2004 directivity factor is notably unconservative for all vibration periods in Otira (i.e. high seismic hazard region); and unconservative for Christchurch at short-to-moderate vibration periods ( < 3s); but conservative at long periods ( > 4s).
This paper develops representative ground motion ensembles for several major earthquake scenarios in New Zealand. Cases considered include representative ground motions for the occurrence of Alpine, Hope, and Porters Pass earthquakes in Christchurch, and the occurrence of Wellington, Wairarapa, and Ohariu, fault ruptures in Wellington. Challenges in the development of ground motion ensembles for subduction zone earthquakes are also highlighted. The ground motions are selected based on the generalized conditional intensity measure (GCIM) approach, ensuring that the ground motion ensembles represent both the mean, and distribution of ground motion intensity which such scenarios could impose. These scenario-based ground motion sets can be used to complement ground motions which are often selected in conjunction with probabilistic seismic hazard analysis, in order to understand the performance of structures for the question “what if this fault ruptures?”
This paper presents the ongoing development of a new 3D seismic velocity model of Canterbury, New Zealand. The model explicitly represents the Canterbury sedimentary basin, and other significant geologic horizons, which are expected to have important implications on observed ground motions. The model utilizes numerous sources of data, including 3D regional tomography with a variable-depth inferred Moho, seismic reflection survey lines, geotechnical boreholes and well logs, spectral analysis of surface waves, and CPT logs which provide velocity constraints over their respective ranges of application. The model provides P- and S-wave velocity and density (i.e. Vp, Vs and p) over a grid of input points, and is presently being utilized in broadband ground motion simulations of the 2010-2011 Canterbury earthquakes. Comparison of simulated ground motions with those observed in the 2010-2011 Canterbury earthquakes will help provide a better understanding of the salient physical processes which characterized the unique set of strong ground motions recorded in this sequence of earthquake events.
The abundance of cone penetration test (CPT) data from subsurface explorations in Christchurch and the surrounding areas provides a useful source of information for a characterization of the near surface shear wave velocity ( ) profile for the region. A portion of the investigations were conducted using seismic CPT, enabling the comparison of measured shear wave velocity with CPT data, and subsequently the evaluation of existing CPT- correlations for applicability to Canterbury-specific soils. The existing correlations are shown to be biased, generally over-predicting the observed with depth, thus demonstrating the need for a Canterbury-specific CPT- correlation.
The 2010–2011 Canterbury earthquake sequence began with the 4 September 2010, Mw7.1 Darfield earthquake and includes up to ten events that induced liquefaction. Most notably, widespread liquefaction was induced by the Darfield and Mw6.2 Christchurch earthquakes. The combination of well-documented liquefaction response during multiple events, densely recorded ground motions for the events, and detailed subsurface characterization provides an unprecedented opportunity to add well-documented case histories to the liquefaction database. This paper presents and applies 50 high-quality cone penetration test (CPT) liquefaction case histories to evaluate three commonly used, deterministic, CPT-based simplified liquefaction evaluation procedures. While all the procedures predicted the majority of the cases correctly, the procedure proposed by Idriss and Boulanger (2008) results in the lowest error index for the case histories analyzed, thus indicating better predictions of the observed liquefaction response.
The empirical liquefaction triggering chart of Idriss and Boulanger (2008) is compared to direct measurements of the cyclic resistance of Christchurch silty sands via undisturbed and reconstituted lab specimens. Comparisons suggest that overall there is a reasonable agreement between the empirical triggering curve and the interpreted test data. However, the influence of fines on cyclic resistance appears to be over-predicted by the empirical method, particularly for non-plastic silty sands that are commonly encountered in flood over-bank deposits in Christchurch and nearby settlements
The 2010-2011 Canterbury earthquake sequence was extremely damaging to structures in Christchurch and continues to have a large economic and social impact on the city and surrounding regions. In addition to strong ground shaking (Bradley and Cubrinovski 2011 SRL; Bradley 2012 SDEE), extensive liquefaction was observed, particularly in the 4 September 2010 Darfield earthquake and the 22 February 2011 Christchurch earthquake (Cubrinovski et al. 2010 BNZSEE; 2011 SRL). Large observed vertical ground motion amplitudes were recorded in the events in this sequence, with vertical peak ground accelerations of over 2.2g being observed at the Heathcote Valley Primary School during the Christchurch earthquake, and numerous other vertical motions exceeding 1.0g (Bradley and Cubrinovski 2011 SRL; Bradley 2012 SDEE; Fry et al 2011 SRL). Vertical peak ground accelerations of over 1.2g were observed in the Darfield earthquake.
The performance of buildings in recent New Zealand earthquakes (Canterbury, Seddon and Kaikōura), delivered stark lessons on seismic resilience. Most of our buildings, with a few notable exceptions, performed as our Codes intended them to, that is, to safeguard people from injury. Many buildings only suffered minor structural damage but were unable to be reused and occupied for significant periods of time due to the damage and failure of non-structural elements. This resulted in substantial economic losses and major disruptions to our businesses and communities. Research has attributed the damage to poor overall design coordination, inadequate or lack of seismic restraints for non structural elements and insufficient clearances between building components to cater for the interaction of non structural elements under seismic actions. Investigations have found a clear connection between the poor performance of non-structural elements and the issues causing pain in the industry (procurement methods, risk aversion, the lack of clear understanding of design and inspection responsibility and the need for better alignment of the design codes to enable a consistent integrated design approach). The challenge to improve the seismic performance of non structural elements in New Zealand is a complex one that cuts across a diverse construction industry. Adopting the key steps as recommended in this paper is expected to have significant co-benefits to the New Zealand construction industry, with improvements in productivity alongside reductions in costs and waste, as the rework which plagues the industry decreases.
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In practice, several competing liquefaction evaluation procedures (LEPs) are used to compute factors of safety against soil liquefaction, often for use within a liquefaction potential index (LPI) framework to assess liquefaction hazard. At present, the influence of the selected LEP on the accuracy of LPI hazard assessment is unknown, and the need for LEP-specific calibrations of the LPI hazard scale has never been thoroughly investigated. Therefore, the aim of this study is to assess the efficacy of three CPT-based LEPs from the literature, operating within the LPI framework, for predicting the severity of liquefaction manifestation. Utilising more than 7000 liquefaction case studies from the 2010–2011 Canterbury (NZ) earthquake sequence, this study found that: (a) the relationship between liquefaction manifestation severity and computed LPI values is LEP-specific; (b) using a calibrated, LEP-specific hazard scale, the performance of the LPI models is essentially equivalent; and (c) the existing LPI framework has inherent limitations, resulting in inconsistent severity predictions against field observations for certain soil profiles, regardless of which LEP is used. It is unlikely that revisions of the LEPs will completely resolve these erroneous assessments. Rather, a revised index which more adequately accounts for the mechanics of liquefaction manifestation is needed.
We present preliminary observations on three waters impacts from the Mw7.8 14th November 2016 Kaikōura Earthquake on wider metropolitan Wellington, urban and rural Marlborough, and in Kaikōura township. Three waters systems in these areas experienced widespread and significant transient ground deformation in response to seismic shaking, with localised permanent ground deformation via liquefaction and lateral spreading. In Wellington, potable water quality was impacted temporarily by increased turbidity, and significant water losses occurred due to damaged pipes at the port. The Seaview and Porirua wastewater treatment plants sustained damage to clarifier tanks from water seiching, and increased water infiltration to the wastewater system occurred. Most failure modes in urban Marlborough were similar to the 2010-2011 Canterbury Earthquake Sequence; however some rural water tanks experienced rotational and translational movements, highlighting importance of flexible pipe connections. In Kaikōura, damage to reservoirs and pipes led to loss of water supply and compromised firefighting capability. Wastewater damage led to environmental contamination, and necessitated restrictions on greywater entry into the system to minimise flows. Damage to these systems necessitated the importation of tankered and bottled water, boil water notices and chlorination of the system, and importation of portaloos and chemical toilets. Stormwater infrastructure such as road drainage channels was also damaged, which could compromise condition of underlying road materials. Good operational asset management practices (current and accurate information, renewals, appreciation of criticality, good system knowledge and practical contingency plans) helped improve system resilience, and having robust emergency management centres and accurate Geographic Information System data allowed effective response coordination. Minimal damage to the wider built environment facilitated system inspections. Note Future research will include detailed geospatial assessments of seismic demand on these systems and attendant modes of failure, levels of service restoration, and collaborative development of resilience measures.
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OPINION: Associate Professor MARK QUIGLEY, from the University of Canterbury's department of geological sciences, and Dr MATTHEW HUGHES, from its department of civil and natural resources engineering, survey the changing landscape of post-quake Christchurch.
In order to provide information related to seismic vulnerability of non-ductile reinforced concrete (RC) frame buildings, and as a complementary investigation on innovative feasible retrofit solutions developed in the past six years at the University of Canterbury on pre-19170 reinforced concrete buildings, a frame building representative of older construction practice was tested on the shake table. The specimen, 1/2.5 scale, consists of two 3-storey 2-bay asymmetric frames in parallel, one interior and one exterior, jointed together by transverse beams and floor slabs. The as-built (benchmark) specimen was first tested under increasing ground motion amplitudes using records from Loma Prieta Earthquake (California, 1989) and suffered significant damage at the upper floor, most of it due to lap splices failure. As a consequence, in a second stage, the specimen was repaired and modified by removing the concrete in the lap splice region, welding the column longitudinal bars, replacing the removed concrete with structural mortar, and injecting cracks with epoxy resin. The modified as-built specimen was then tested using data recorded during Darfield (New Zealand, 2010) and Maule (Chile, 2010) Earthquakes, with whom the specimen showed remarkably different responses attributed to the main variation in frequency content and duration. In this contribution, the seismic performance of the three series of experiments are presented and compared.
An as-built reinforced concrete (RC) frame building designed and constructed according to pre-1970s code design construction practice has been recently tested on the shake table at the University of Canterbury. The specimen, 1/2.5 scaled version of the original prototype, consists of two 3-storey 2-bay asymmetric frames in parallel, one interior and one exterior, jointed together by transverse beams and floor slabs. Following the benchmark test, a retrofit intervention has been proposed to rehabilitate the tested specimen. In this paper, detailed information on the assessment and design of the seismic retrofit procedure using GFRP (glass fibre reinforced polymer) materials is given for the whole frame. Hierarchy of strength and sequence of events (damage mechanisms) in the panel zone region are evaluated using a moment-axial load (M-N) interaction performance domain, according to a performance-based retrofit philosophy. Specific limit states or design objectives are targeted with attention given to both strength and deformation limits. In addition, an innovative retrofit solution using FRP anchor dowels for the corner beam-column joints with slabs is proposed. Finally, in order to provide a practical tool for engineering practice, the retrofit procedure is provided in a step-by step flowchart fashion.
Recent advances in timber design at the University of Canterbury have led to new structural systems that are appropriate for a wide range of building types, including multi-storey commercial office structures. These buildings are competitive with more traditional construction materials in terms of cost, sustainability and structural performance. This paper provides seismic design recommendations and analytical modelling approaches, appropriate for the seismic design of post-tensioned coupled timber wall systems. The models are based on existing seismic design theory for precast post-tensioned concrete, modified to more accurately account for elastic deformation of the timber wall systems and the influence of the floor system. Experimental test data from a two storey post-tensioned timber building, designed, constructed and tested at the University of Canterbury is used to validate the analytical models.
The 4th of September 2010 Mw 7.1 Darfield (Canterbury) earthquake had generated significant ground shaking within the Christchurch Central Business District (CBD). Despite the apparently significant shaking, the observed structural damage for pre-1970s reinforced concrete (RC) buildings was indeed limited and lower than what was expected for such typology of buildings. This paper explores analytically and qualitatively the different aspects of the "apparent‟ good seismic performance of the pre-1970s RC buildings in the Christchurch CBD, following the earthquake reconnaissance survey by the authors. Damage and building parameters survey result, based on a previously established inventory of building stock of these non-ductile RC buildings, is briefly reported. From an inventory of 75 buildings, one building was selected as a numerical case-study to correlate the observed damage with the non-linear analyses. The result shows that the pre-1970s RC frame buildings performed as expected given the intensity of the ground motion shaking during the Canterbury earthquake. Given the brittle nature of this type of structure, it was demonstrated that more significant structural damage and higher probability of collapse could occur when the buildings were subjected to alternative input signals with different frequency content and duration characteristics and still compatible to the seismicity hazard for Christchurch CBD.
An overview of the 22 February 2011 Christchurch earthquake is presented in the context of characterization of extreme/rare events. Focus is given to the earthquake source, observed near-source strong ground motions, and effects of site response, while structural response and consequences are mentioned for completeness. For each of the above topics comparisons and discussions are made with predictive models for each of phenomena considered. In light of the observations and predictive model comparisons, the author’s opinion on improving the characterization of such extreme/rare events, and their appropriate consideration in seismic design is presented
This paper describes pounding damage sustained by buildings and bridges in the February 2011 Christchurch earthquake. Approximately 6% of buildings in Christchurch CBD were observed to have suffered some form of serious pounding damage. Almost all of this pounding damage occurred in masonry buildings, further highlighting their vulnerability to this phenomenon. Modern buildings were found to be vulnerable to pounding damage where overly stiff and strong ‘flashing’ components were installed in existing building separations. Soil variability is identified as a key aspect that amplifies the relative movement of buildings, and hence increases the likelihood of pounding damage. Pounding damage in bridges was found to be relatively minor and infrequent in the Christchurch earthquake.
This paper describes the performance of (or damage to) ceilings in buildings during the 22nd February 2011 Christchurch earthquake and the subsequent aftershocks. In buildings that suffered severe structural damage, ceilings and other non-structural components (rather expectedly) failed, but even in buildings with little damage to their structural systems, ceilings were found to be severely damaged. The extent of ceiling damage, where the ceilings were subject to severe shaking, depended on the type of the ceiling system, the size and weight of the ceilings and the interaction of ceilings with other elements. The varieties and extent of observed ceiling damage are discussed in this paper with the help of photographs taken after the earthquake.
The University of Canterbury has initialized a research program focusing on the seismic sustainability of structures. As part of this program, the relative seismic sustainability of various structures will be assessed to identify those with the highest sustainability for the Christchurch rebuild and general use in New Zealand. This preliminary case study assesses one reinforced concrete (RC) frame structure and one RC wall structure. The scenario loss is evaluated for two earthquake records considering direct losses only in order to explain and illustrate the methodology.
This paper discusses the seismic performance of the standard RC office building in Christchurch that is given as a structural design example in NZS3101, the concrete structures seismic standard in New Zealand. Firstly the push-over analysis was carried out to evaluate the lateral load carrying capacity of the RC building and then to compare that carrying capacity with the Japanese standard law. The estimated figures showed that the carrying capacity of the New Zealand standard RC office building of NZS3101:2006 was about one third of Japanese demanded carrying capacity. Secondly, time history analysis of the multi-mass system was performed to estimate the maximum response story drift angle using recorded ground motions. Finally, a three-dimensional analysis was carried out to estimate the response of the building to the 22nd February, 2011 Canterbury earthquake. The following outcomes were obtained. 1) The fundamental period of the example RC building is more than twice that of Japanese simplified calculation, 2) The example building’s maximum storey drift angle reached 2.5% under the recorded ground motions. The main purpose of this work is to provide background information of seismic design practice for the reconstruction of Christchurch.
The objective of this project is to collect perishable seismic response data from the baseisolated Christchurch Women's Hospital. The strong and continuing sequence of aftershocks presents a unique opportunity to capture high-fidelity data from a modern base-isolated facility. These measurements will provide quantitative information required to assess the mechanisms at play in this and in many other seismically-isolated structures.
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.
Despite their good performance in terms of their design objectives, many modern code-prescriptive buildings built in Christchurch, New Zealand had to be razed after the 2010-2011 Canterbury earthquakes because repairs were deemed too costly due to widespread sacrificial damage. Clearly a more effective design paradigm is needed to create more resilient structures. Rocking, post-tensioned connections with supplemental energy dissipation can contribute to a damage avoidance designs (DAD). However, few have achieved all three key design objectives of damage-resistant rocking, inherent recentering ability, and repeatable, damage-free energy dissipation for all cycles, which together offer a response which is independent of loading history. Results of experimental tests are presented for a near full-scale rocking beam-column sub-assemblage. A matrix of test results is presented for the system under varying levels of posttensioning, with and without supplemental dampers. Importantly, this parametric study delineates each contribution to response. Practical limitations on posttensioning are identified: a minimum to ensure static structural re-centering, and a maximum to ensure deformability without threadbar yielding. Good agreement between a mechanistic model and experimental results over all parameters and inputs indicates the model is robust and accurate for design. The overall results indicate that it is possible to create a DAD connection where the non-linear force-deformation response is loading history independent and repeatable over numerous loading cycles, without damage, creating the opportunity for the design and implementation of highly resilient structures.
