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In New Zealand, time history analysis is either the required or preferred method of assessing seismic demands for torsionally sensitive and other important structures, but the criteria adopted for the selection of ground motion records and their scaling to generate the seismic demand remains a contentious and debatable issue. In this paper, the scaling method based on the least squares fit of response spectra between 0.4-1.3 times the structure’s first mode period as stipulated in the New Zealand Standard for Structural Design Actions: Earthquake Actions (NZS1170.5) [1] is compared with the scaling methods in which ground motion records are scaled to match the peak ground acceleration (PGA) and spectral acceleration response at the natural period of the structure corresponding to the first mode with 5% of critical damping; i.e. Sa(T1, 5%). Incremental dynamic analysis (IDA) is used to measure the record-to-record randomness of structural response, which is also a measure of the efficiency of the intensity measure (IM) used. Comparison of the dispersions of IDA curves with the three different IMs; namely PGA, Sa(T1, 5%) and NZS1170.5 based IM, shows that the NZS1170.5 scaling method is the most effective for a large suite of ground motions. Nevertheless, the use of only three randomly chosen ground motions as presently permitted by NZS1170.5 is found to give significantly low confidence in the predicted seismic demand. It is thus demonstrated that more records should be used to provide a robust estimate of likely seismic demands.
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A full scale slice of a 7 story reinforced concrete building was tested on the shake table at the UCSD Engelkirk Structural Research Centre in 2006. As part of the research project, a blind prediction contest was sponsored to assess the capability of currently available analysis procedures to predict the seismic response of cantilever reinforced concrete shear wall structures. This paper describes an entry based on a nonlinear finite element model, using macro elements to represent both the shear and the flexural modes of behaviour. A comparison of the predicted response with the test results showed that the analysis procedure produced reasonable predictions of deformations for the lowest and highest of the four earthquakes but under-estimated response for the two moderate earthquakes by approximately 30%. For all earthquakes, the analysis base moment was much lower than the test value. Modifications to the procedure to improve the correlation were identified and implemented but did not remedy the deficit in base moments. Detailed results of the test program revealed that the causes for this discrepancy were the contribution to overturning results of gravity columns and the flange wall, neither of which had been included in the model. When these were incorporated the average error between test and analysis results was less than 10% for all earthquakes, well within acceptable limits for a design office type of model. The correlation of tests and analysis also provided useful information on design aspects for shear walls, such as the influence of secondary components and dynamic magnification factors.
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In the late 1970’s it was recognised that the seismic provisions of the current NZS 4203:1976 did not readily apply to the types of structures normally used within the land based processing facilities of the “heavy industries” such as petrochemical and oil and gas processing plants impending under the “Think Big” regime.
Since the 1984 revision to NZS 4203, there have not been any publicly available New Zealand guidelines on how to interpret the earthquake provisions of the various versions of NZS 4203 (and now AS/NZS 1170) that would update the 1981 publication created by the Ministry of Works for the Ministry of Energy, “Seismic Design of Petrochemical Plants”.
There are overseas publications that have considered the differences in the typical structural systems necessary to support the equipment and distributive systems needed to process industrial feedstock. How they behave seismically has been reviewed and recommendations made on the methods to be used to determine the design seismic actions. Such standards as ASCE 7 and FEMA 450 incorporate these in a specific manner relating to the design of industrial plant.
With the advent of new oil and gas processing plants in Taranaki, this paper takes the opportunity to review AS/NZS 1170 and adapt these overseas guidelines for the seismic design of new industrial plant in New Zealand. The background for these guidelines will be presented with examples of typical industrial structural systems and their seismic actions. This is with the aim of re-establishing a basis of seismic design for industrial plant within the framework of the new standards AS/NZS 1170.0 and NZS 1170.5.
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The Modified Mercalli intensities of the 1956 Mw 6.3 Bay of Plenty and 1987 Mw 6.5 Edgecumbe earthquakes have recently been reviewed and about one-third of them were found to be erroneous. The resulting revisions to their isoseismal maps are substantial, and both new maps now show the strong influence of the high attenuation in the Taupo Volcanic Zone (TVZ). An analysis of the causes of the errors in the intensities is given. The new maps will help improve the modelling of attenuation in the TVZ, and will contribute to improvements in assessments of seismic hazard and risk in that region. An important implication is that the mean damage ratios estimated from studies of damage costs in the Edgecumbe earthquake by Dowrick and Rhoades are likely to be erroneously low, and need to be reviewed.
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The Hutt Valley is an alluvial basin that hosts the city of Lower Hutt, in the North Island, New Zealand. The basin is bounded by the Wellington Fault on its northwest side, and exhibits ground motion amplification factors up to about 15, measured by several seismic experiments using weak motion and portable seismic arrays during 1990-1991. Synthetic seismograms computed by using local 1D stratigraphic models under each station reproduce qualitatively the amplitudes and durations of the corresponding observed seismograms at most of the soft site stations of the arrays. Amplification factors estimated from spectral ratios of the synthetic seismograms are up to about 9. The authors present comparisons of amplification between synthetics and observations, allowing a “calibration” of the model so that it could be used to determine more realistic ground amplifications for earthquake scenarios.
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This paper investigates the practicality of using base-isolation devices to protect lightweight buildings, such as timber-framed houses, against earthquakes.
As the timber-framed buildings considered are designed to a lateral seismic force of 0.24W in Wellington (where W is building weight), it was considered that the appropriate isolation level should be significantly lower, say 0.1W, and lower still in Auckland where such buildings are designed for a lateral force of 0.12W. An analysis showed that houses which had a base-isolation yield set to yield at 0.1 W would have unacceptable deformations under the design wind load if the isolators were located beneath a timber floor, but may be satisfactory if located beneath a concrete slab. A large-scale test using isolators beneath pre-cast floor slabs showed the method used would work even if it was unduly expensive. However, an analysis indicated that there might be little protection for some building contents.
A literature survey of alternative base-isolation solutions showed a wide range of innovative, but often impractical, concepts have been proposed. However, one concept showed promise for timber-framed structures. This used two layers of synthetic sheet beneath a concrete floor slab to provide a slip layer. The sheet materials recommended reputably gave a dynamic friction coefficient of 0.07. BRANZ measured the friction coefficient using large samples of both these and other sheet materials. It was concluded that the measured friction was too high for use for the planned buildings, although it may have application for low-rise heavy-brittle construction.
Alternative base-isolation concepts are presented which will be useful to others interested in this topic.
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Several on-ground cylindrical liquid storage tanks experienced strong ground motion during the “Silakhor” earthquake of March 31, 2006 in western Iran, and some of the tanks suffered minor to moderate damage. In this study two of the affected tanks that were located close to the station of recording the time history of the earthquake were investigated. Responses of these tanks to the earthquake were estimated using published methods and also non-linear time history analysis, for both rigid foundation and flexible foundation assumptions. Theoretical results were compared and were generally in good agreement with the observed performance of tanks during the earthquake. For the broad tank uplift displacements observed from the earthquake matched quite closely the predictions of numerical analysis and some of the published methods, although there was a significant variation in the predictions of various methods. It was also shown that axial stresses in tank shells uplifting under earthquake are very dependent on the rigidity of the foundation.
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During the past two decades, the focus has been on the need to provide communities with structures that undergo minimal damage after an earthquake event while still being cost competitive. This has led to the development of high performance seismic resisting systems, and advances in design methodologies, in order respect this demand efficiently.
This paper presents the experimental response of four pre-cast, post-tensioned rocking wall systems tested on the shake-table at the University of Canterbury. The wall systems were designed as a retrofit solution for an existing frame building, but are equally applicable for use in new design. Design of the wall followed a performance-based retrofit strategy in which structural limit states appropriate to both the post-tensioned wall and the existing building were considered.
Dissipation for each of the four post-tensioned walls was provided via externally mounted devices, located in parallel to post-tensioned tendons for re-centring. This allowed the dissipation devices to be easily replaced or inspected following a major earthquake. Each wall was installed with viscous fluid dampers, tension-compression yielding steel dampers, a combination of both or no devices at all – thus relying on contact damping alone. The effectiveness of both velocity and displacement dependant dissipation are investigated for protection against far-field and velocity-pulse ground motion characteristics.
The experimental results validate the behaviour of ‘Advanced Flag-Shape’ rocking, dissipating solutions which have been recently proposed and numerically tested. Maximum displacements and material strains were well controlled and within acceptable bounds, and residual deformations were minimal due to the re-centring contribution from the post tensioned tendons. Damage was confined to inelastic yielding (or fluid damping) of the external dampers.
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On 16 July 2007, an earthquake of magnitude 6.8 occurred with an epicentre off the west coast of Niigata Prefecture (Japan), causing widespread damage to buildings and other types of civil engineering structures due to ground shaking and earthquake-induced ground failures. Landsliding and soil liquefaction occurred extensively in various parts of the affected region. This paper presents the preliminary results of the post-earthquake damage investigation conducted at the affected areas after the earthquake, with emphasis on the seismic-induced ground failures and their effects on the built environment.
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The 8th October 2005 Kashmir Earthquake was one the largest earthquakes in Northern Pakistan in its recorded history. It caused an unprecedented level of damage and destruction in Pakistan Administered Kashmir (PAK) and the North Western Frontier Province (NWFP). It damaged or collapsed more than 0.6 million buildings - leaving 3.5 million people shelter less as winter approached. A large part of the earthquake-affected area is difficult to access and highly snow-prone, with rugged terrain and scattered settlements. It posed unique challenges and efforts on a massive-scale for reconstruction. For residential buildings, the Pakistan government adopted a house-owner driven approach. The reconstruction policy stated that the government and other agencies would provide equal technical assistance and subsidy to each family, without differentiating between who lost what. To increase capacity in earthquake-resistant construction, large-scale training of artisans, technicians, engineers, and community mobilisers has been conducted. Campaigns to “build back better” have raised awareness in the communities. Local Housing Reconstruction Centres have been established for training, advice, and dissemination of earthquake-resistant technology. This decentralised approach has helped in achieving reconstruction smoothly. This paper will present the authors’ first-hand experience in the reconstruction effort, and the opportunities and unique challenges faced.
