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The Darfield earthquake of 3rd September 2010 UT and its aftershocks have yielded New Zealand’s richest set of strong-motion data since recording began in the early 1960s. Main-shock accelerograms were returned by 130 sites, ten of which had peak horizontal accelerations in the range 0.3 to 0.82g. One near-fault record, from Greendale, had a peak vertical acceleration of 1.26g. Eighteen records showed peak ground velocities exceeding 0.5 m/s, with three of them exceeding 1 m/s. The records included some with strong long-period directivity pulses, some with other long-period components that were related to a mixture of source and site effects, and some that exhibited the effects of liquefaction at their sites. There were marked differences between records on the deep alluvium of Christchurch City and the Canterbury Plains, and those on shallow stiff soil sites. The strong-motion records provide the opportunity to assess the effects of the earthquake in terms of the ground motions and their relationship to design motions. They also provide an invaluable set of near-source motions for seismological studies. Our report presents an overview of the records and some preliminary findings derived from them.
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The Darfield moment magnitude (Mw) 7.1 earthquake of September 2010 is the first heavily damaging earthquake to strike New Zealand since the surface wave magnitude (MS) 7.8 Hawkes Bay earthquake in 1931. Although the earthquake has a clear strike-slip surface expression characterised by the Greendale Fault, seismological evidence suggests it is a complex event beginning as a reverse faulting earthquake. Evidence for complexity of the mainshock includes a well constrained epicentre north of the surface fault trace, high near-source vertical accelerations, first-motion and regional moment tensor focal mechanisms which differ from teleseismic solutions, and a complex aftershock pattern. The earthquake and aftershock sequence were very well recorded by the GeoNet sensor networks in the region, and provide an exceptional dataset for understanding the earthquake rupture process and reducing damage from future earthquakes. This was the most significant test of GeoNet since its inception in 2001, and the first such New Zealand event in the “internet age”. GeoNet data proved important for the response and the interaction with emergency management, media and the public. The GeoNet website sustained continued heavy load over the weeks and months following the earthquake but continued to deliver timely information because of significant improvements carried out as the aftershock sequence continued.
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This Bulletin of the New Zealand Society for Earthquake Engineering (NZSEE) is a collaboration with the New Zealand Geotechnical Society (NZGS) and the Structural Engineering Society New Zealand (SESOC), with papers on the preliminary observations of the 2010 September 4, 04:35 (NZST; September 3, 16:35 UTC) Darfield (Canterbury) earthquakes.
This Introductory paper summarises preliminary observations of the earthquakes and the performance of ground, structures, non-structural elements, and lifelines; the assessments of usability; and the communication of information amongst the science and engineering communities.
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In New Zealand, the performance of instrumented structures has rarely been tested by earthquake events with design-level motions to enable verification of the code design recommendations and related design assumptions. In the Darfield event, Rutherford building at the University of Canterbury was the only instrumented building that recorded its earthquake response. Lessons from overseas earthquakes, in particular the 1994 Northridge event, have demonstrated the invaluable use of information from instrumented buildings. In order to derive similar benefits from any future New Zealand events, steps were initiated to install modern digital accelerographs in structures. The new building instrumentation programme aims to install earthquake strong-motion instruments within up to 30 structures (mainly buildings and bridges) across New Zealand so as to measure their responses to future earthquake-induced ground motions. This article describes the objective of the instrumentation programme, highlighting the expected benefits to various end-users, the progress made so far and the future scope of the ongoing programme.
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Bridges are built in a variety of locations, many of which are susceptible to multiple extreme hazards (earthquakes, vehicle collisions, tsunamis or storm surges, and blasts as a minimum for some locations). In addition, they must be built to achieve the objectives of both accelerated bridge construction (ABC) and rapid return to service following a disaster. Meeting some or all of these demands/objectives drives the development of innovative multi-hazard design concepts. This paper presents recent research on structural fuses and concrete-filled steel shapes strategies developed for this purpose. The structural fuse concept considered here for seismic resistance was developed and experimentally validated for implementation in a composite multi-column pier using double composite rectangular columns of Bi-Steel panels. Experimental results from another series of tests on the blast resistance of concrete-filled-steel-tubes support the blast resistance of the concept. In parallel, the development and design of a conceptual multi-hazard resistant steel plate shear wall box pier concept considered each of the four aforementioned hazards by use of simplified analyses for design, and of advanced nonlinear finite element analyses to confirm that the proposed steel plate shear wall box system provides adequate ductile performance and strength for each of the hazards.
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Structures may have vertical stiffness or strength irregularity for many reasons. In many practical cases, a change in storey stiffness, results a change in strength at the same storey. In this paper, the effect of a change in interstorey height is quantified. In order to do this, relationships between storey stiffness and strength resulting due to a modified interstorey height for a few common lateral force resisting systems was considered. It was applied to simple shear-type structures of 3, 5, 9 and 15 storeys, assumed to be located in Wellington. All structures were considered to have a constant mass at every floor level. Both regular and irregular structures were designed in accordance with the Equivalent Static method of the current New Zealand seismic design Standard, NZS 1170.5. Regular structures were designed to either (i) produce a constant target interstorey drift ratio at all the storeys simultaneously or (ii) to have uniform stiffness distribution over the height of the structure, with the target interstorey drift ratio at the first storey. An “interstorey height ratio” was defined as the ratio of modified to initial interstorey height, and applied separately at the first storey, mid-height storey and at the topmost storey by amounts between 0.5 and 3. The modified structures were then redesigned until the target interstorey drift ratio was achieved at the critical storey/storeys. Design structural ductility factors of 1, 2, 3, 4 and 6, and target (design) interstorey drift ratios ranging between 0.5% and 3%, were used in this study. Inelastic dynamic time-history analysis was carried out by subjecting these structures to code design level earthquake records, and the maximum interstorey drift ratio demands due to each record were used to compare the responses of regular and irregular structures.
It was found that structural types in which only the storey stiffness was modified due to a change in the interstorey height produced the maximum increase in drift demands rather than structural forms with other stiffness-strength coupling cases. Shorter structures having an increased first storey height, and taller structures with an increased middle storey height generally produced greater interstorey drift demands than regular structures. For cases of increased storey stiffness due to decreased storey heights, the shorter structures with a decreased middle storey height resulted in higher median peak ISDR due to irregularity. A simple equation describing the maximum increase in response due to modifications to a storey height was developed. The equation was used along with the realistic correlations between storey stiffness and strength to obtain the governing code regularity limit.
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Occasionally-observed resonances in the vertical components of earthquakes recorded at the Wainuiomata, New Zealand, soft site, are likely to be manifestations of the Airy phase of fundamental-mode Rayleigh waves which traverse the site. These packets of waves exist only when a soft, water-saturated layer of soil overlies a substrate with a much higher velocity. Other soft sites in Wellington also show the phenomenon, which may have implications for hazard estimates.
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A distributed-source model, recently developed by the authors, was used to study the spatial distribution of Modified Mercalli (MM) intensities and peak ground accelerations (PGA) in characteristic earthquakes, of Mw7.5 and 8.1 respectively, on the 75 km long Wellington fault and the 413 km long Alpine fault. In each event the predicted intensities reach MM10 and the PGAs reach 0.8g near the fault trace over much of its length, varying along it depending on the location of asperities. PGAs are related to MM intensity using a quadratic expression derived using New Zealand data. Comparisons are made between the PGA patterns estimated indirectly from the distributed-source MM intensity model and those estimated directly from a PGA model, which defines site-source distance as the shortest distance from the site to the fault. There are many similarities and some differences, the latter being attributable largely to the different methods of measuring site-to-source distances. Finally selected seismic risk issues for people and the built environment, including lifelines, are considered for Alpine fault earthquakes.
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Earthquake engineering is facing an extraordinarily challenging era. These challenges are driven by the increasing expectations of modern society to provide low-cost, architecturally appealing structures with high seismic performance. Modern structures need to be able to withstand a design level earthquake with limited or negligible damage such that disruption to business be minimised because of the economic consequences of such downtime.
Technological solutions for seismic resisting structural systems are emerging. However, within the goal of developing a seismic-resisting building, not only the structural skeleton of the building but the entire system must be fully protected from damage. This includes the non-structural components of the building such as the claddings, ceilings and contents. Substantial studies are still required to develop technological solutions and design methods capable of achieving such an earthquake resistance structure.
This paper presents a review of current technology for facades, including design guidelines for seismic-resistant non-structural components and the steps made towards a performance-based design framework. Alternative conceptual strategies and technical solutions to reduce the damage to non-structural elements will also be introduced.
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Eccentrically Braced Frames (EBFs) are widely used seismic-resisting systems, as they allow both strength and stiffness to be optimised while providing good ductility capacity. However, in theory they have a low damage threshold in severe earthquakes and post-earthquake repair of conventional EBFs will be difficult and expensive.
This paper presents the Numerical Integration Time-History (NITH) analysis of two ten storey EBF buildings; one with a conventional active link and the other with a new form of low damage active link based on rotational sliding bolted plates. The low damage active link can be designed to allow rotation only, or to allow both rotation and axial extension. The conventional active link response in terms of displacement, rotation and inelastic demand was well within the range of the rotational active links under the records considered. The analysis shows that average maximum displacement of the building and rotation of the link for both the rotational and the rotational+extension active links was almost identical. The extension of the rotational active link permitting axial extension was less than 1.5 mm. Axial load demands on the collector beams and braces were similar in the case all three active links.
It can be concluded from the analysis that the rotational active link with extension is not required, as the lateral extensions can be accommodated within the rotational plates with nominal clearances in the bolt holes to accommodate the lateral extension.
