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The Canterbury earthquakes have afforded the author a unique opportunity to view the state of engineering from a different perspective. The development of the Detailed Engineering Evaluation (DEE) procedure and the related activities of the Engineering Advisory Group have required thorough consideration of structural engineering practice. This has extended to an overview of the outputs from the DEEs completed by a wide range of engineers, over a wide range of buildings. From these and more general observations of engineering practice in contrast with that of other countries, a view on the state of earthquake engineering in New Zealand is offered with some thoughts on future direction and development needs.
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Wellington Central Police Station was built in the late 1980s. Its 10-storey tower block featured robust X-bracing around the perimeter and strong floor diaphragms to distribute earthquake actions amongst the perimeter frames, thus making it extremely stiff. To provide the ductility required by the 1984 Loadings Standard, the tower block was supported on a base isolation system that allowed up to 400 millimetres of movement in any horizontal direction.
This paper describes investigations into the continuing serviceability of the base isolation parts, and the remedial maintenance required.
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The need for Territorial Authorities (TA) to compile an earthquake-prone building register has been highlighted by the Canterbury Earthquakes Royal Commission and with this in mind the following research was undertaken to enable the characterisation and assessment of potentially earthquake-prone historic unreinforced masonry (URM) buildings in Dunedin. To achieve the research goals, associated technical literature was reviewed and an earthquake-prone building register containing data on 226 URM buildings located in the Dunedin central business district (CBD) area was compiled. Additionally, structural performance of these buildings was also assessed using both the literature suggested initial evaluation procedure and the proposed risk based assessment method. It was estimated that 680 of the existing 750 Dunedin URM buildings are likely to be earthquake-prone and merit detailed assessment. It was also established that the earthquake risk in the city is primarily based on the fact that it has a significant number of URM buildings built prior to the introduction of building code, of which a large proportion is concentrated in the CBD. These not only pose a safety risk to their users but also to pedestrians on the adjacent footpaths.
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Pounding damage in major earthquakes has been observed frequently in the form of aesthetic, minor or major structural cracks and collapse of buildings. These observations have attracted many numerical and experimental studies that led to analytical models for simulating seismic pounding. This study considers pounding between two steel portal frames without a seismic gap. The first frame has a constant natural period while the second frame has variable stiffness and mass values. Five different ground motions are applied to eight combinations of adjacent frames using a shake table. Numerical simulations for the same configurations are carried out with five pounding force models, viz. linear viscoelastic model, modified linear viscoelastic model, nonlinear viscoelastic model, Hertzdamp model and modified Hertzdamp model. The contact element stiffness and coefficient of restitution for numerical models are determined experimentally. The amplification of maximum displacement of the first frame predicted by the numerical simulations is compared with the shake table results. It was found that the Hertzdamp model always overestimated the responses while the other four models also frequently overestimated the amplifications. The predictions from the four models were not significantly different. Since the linear viscoelastic model requires substantially less computation, compared with the other models this model is more suitable for numerical modelling of pounding responses. However, more study is required to refine the numerical models before building pounding can be modelled with enough confidence.
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During seismic assessments of bridges where there is a lack of construction documentation, one method of determining likely structural detailing is to use historic design standards. An overview of the New Zealand bridge seismic standards and the agencies that have historically controlled bridge design and construction is presented. Standards are grouped into design era based upon similar design and loading characteristics. Major changes in base shear demand, ductility, foundation design, and linkage systems are discussed for each design era, and loadings and detailing requirements from different eras were compared to current design practices. Bridges constructed using early seismic standards were designed to a significantly lower base shear than is currently used but the majority of these bridges are unlikely to collapse due to their geometry and a preference for monolithic construction. Bridges constructed after the late 1970s are expected to perform well if subjected to ground shaking, but unless bridges were constructed recently their performance when subjected to liquefaction and liquefaction-induced lateral spreading is expected to be poor.
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Following both the Cook Strait earthquake (Mw 6.6; 21 July, 2013) and the Lake Grassmere earthquake (Mw 6.6; 16 August, 2013) reconnaissance visits were made of the epicentral regions to document the general distribution and extend of landslides, liquefaction, and other ground damage effects generated by these earthquakes. The extent of landsliding generated in central New Zealand by these two earthquakes was at the lower end of the expected range for shallow earthquakes of these magnitudes. Liquefaction effects generated by the Cook Strait and Lake Grassmere earthquakes in central New Zealand were substantially less than those generated by the 2010-2011 Canterbury earthquakes in the Christchurch area, despite the fact that the Cook Strait and Lake Grassmere earthquakes were of comparable size and proximity, and impacted grossly similar geological settings. There is no evidence of primary ground-surface fault rupture during the Lake Grassmere earthquake.
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The Cook Strait earthquake sequence occurred in a region of known high seismicity. However, this was the strongest shaking felt in decades for the Wellington region and the top of the South Island. The location and size of the earthquake meant that the ground shaking was of rather short duration and moderate intensity, except for the epicentral region of the Lake Grassmere earthquake where a PGA of 0.7g was recorded, and for part of the Wellington foreshore where up to 0.2g was recorded in both earthquakes. The level of shaking in terms of response spectra was, in general, moderate except for very high “spiked” response at particular Wellington sites (WNKS and VUWS) at periods of 0.4-0.5 seconds. Amplification and polarization in the NE-SW direction at approximately ~1 s period at many Wellington stations is likely due to basin resonance effects, whereas dominant polarization in the NW-SE direction at shorter periods is consistent with a directivity effect, and is particularly evident in the Lake Grassmere earthquake. The earthquakes were not only a real-life test on the level of preparedness for the population but also on the behaviour of recently-built structures in the Wellington region that had not yet experienced a moderate earthquake. The ability to measure, analyse and understand the intensity and characteristics of the ground shaking coupled with well-documented damage to the buildings and building array recordings will hopefully foster collaboration across earthquake engineering disciplines.
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This paper presents various aspects of the preliminary damage observations caused by ground motions in the Marlborough region following the Mw6.6 Lake Grassmere earthquake on 16 August 2013. To emphasize the severity of the ground shaking, the observed pseudo-acceleration response spectra are compared to those from the 21 July 2013 Mw6.5 Cook Strait earthquake and the NZS1170.5:2004 design spectrum. The near-source damage to State Highway 1 roads, bridges and buildings is presented within. Stainless steel wine storage tanks showed various damage states that were consistent with observations from previous earthquake events. The performance of wine tanks and other winemaking infrastructure are discussed with future design considerations. Eleven water storage dams within 12 kilometres of the earthquake source were inspected and preliminary observations are discussed. A 250,000 cubic metre dam located 10 kilometres southwest of Seddon suffered moderate damage following the 21 July event while significant further damage was sustained following the 16 August event and emergency earthworks were undertaken to reduce the risk of dam failure (to those living downstream). The performance of residential housing in rural townships of Seddon and Ward was satisfactory with respect to preserving life safety however there was moderate levels of damage which are presented within. Post-earthquake business disruption was minimal as commercial buildings in the Blenheim central business district sustained either minor or no damage.
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Moment-resisting frames made of laminated veneer lumber (LVL) in combination with unbonded post-tensioning have recently been proposed for multi-storey timber buildings. Prefabricated and post-tensioned timber frames can be designed to have enhanced re-centering and energy dissipation after seismic loading. The unbonded post-tensioning provides re-centering capacity while energy is dissipated through the addition of special dissipating devices which also act as external reinforcing.
As part of a research program on multi-storey timber structures, this paper describes experimental and analytical studies to investigate the behaviour of post-tensioned LVL columns under uni-and bi-directional seismic loading. The results show excellent seismic performance, characterized by negligible damage of the structural members and small residual deformations, even under the combined effect of loading in two directions. Energy is dissipated mostly through yielding of external mild steel dissipaters connecting the column and the foundation, which can be easily removed and replaced after an earthquake. Since post-tensioning can be economically performed on site, the system can be easily implemented for either column-to-foundation connections in multi-storey timber buildings as well as for pier-to-foundation and/or pier-to-deck connections in timber bridges.
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The fundamental site period, T, is a key parameter for site classification in NZS 1170.5:2004. Many sites in New Zealand will fall into site classes C and D, where the boundary between the site classes is T = 0.6 seconds. NZS 1170.5 offers several methods of determining site classification. The intent of this paper is to expand on NZS 1170.5 and guide practising engineers towards more accurate and efficient methods for determining site period. We review methods to calculate the shear-wave velocity, then give specific examples for calculating the site period for five types of soil profile (uniform layer, shear-wave velocity increasing as a power of depth, shear modulus increasing linearly with depth, two-layer profile and three-layer profile). We find that NZS 1170.5 clause 3.1.3.7 for calculating site period at layered sites is unconservative and inconsistent with two other well-accepted methods for calculating site period. We consider the most accurate and efficient method of calculating site period for layered sites is to represent the profile as a lumped mass system, then calculate the fundamental frequency from the eigenvalues of the system. The successive application of the two-layer closed form solution is also considered an acceptable method.
