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Structures may be irregular due to non-uniform distributions of mass, stiffness, strength or due to their structural form. For regular structures, simple analysis techniques such as the Equivalent Static Method, have been calibrated against advanced analysis methods, such as the Inelastic Dynamic Time-History Analysis. Most worldwide codes allow simple analysis techniques to be used only for structures which satisfy regularity limits. Currently, such limits are based on engineering judgement and lack proper calibration. This paper describes a simple and efficient method for quantifying irregularity limits. The method is illustrated on 3, 5, 9 and 15 storey models of shear-type structures, assumed to be located in Wellington, Christchurch and Auckland. They were designed in accordance with the Equivalent Static Method of NZS 1170.5. Regular structures were defined to have constant mass at every floor level and were either designed to produce constant interstorey drift ratio at all the floors simultaneously or to have a uniform stiffness distribution over their height. Design structural ductility factors of 1, 2, 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 a suite of code design level earthquake records. Irregular structures were created with floor masses of magnitude 1.5, 2.5, 3.5 and 5 times the regular floor mass. These increased masses were considered separately at the first floor level, mid-height and at the roof. The irregular structures were designed for the same drifts as the regular structures.
The effect of increased mass at the top or bottom of the structure tended to increase the median peak drift demands compared to regular structures for the record suite considered. When the increased mass was present at the mid-height, the structures generally tended to produce lesser drift demands than the corresponding regular structures. A simple equation was developed to estimate the increase in interstorey drift due to mass irregularity. This can be used to set irregularity limits.
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Steel members subject to axial compression and inelastic cyclic displacements, such as may occur during earthquake excitation, exhibit axial shortening due to material inelastic deformation irrespective of the occurrence of buckling. This column axial shortening can cause undesirable effects in the building, especially if it occurs to a different extent in different columns of a seismic-resisting system. This paper summarizes experimental and finite element studies to quantify the axial shortening of columns with known axial forces pushed to inelastic cyclic displacements. A flexural hinge model for a frame analysis program is developed and calibrated against that from experimental and analytical studies. Then, to quantify the effect of axial shortening on realistic moment and eccentrically-braced frames during earthquakes, inelastic dynamic time history analyses were conducted. While axial shortening of more than 7% of the column length was obtained during experimental testing, the axial shortening was always less than 1% of the column interstorey height in the steel frames studied. A method to estimate the axial shortening as a function of the expected inelasticity is developed. Finally, several new details are described in order to prevent detrimental effects due to axial shortening.
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Many new and existing buildings have insufficient weight to resist overturning loads due to earthquakes without uplift. Previous versions of the New Zealand loadings code allowed simplified procedures for the design of rocking structures provided the ductility factor was limited to not more than two. The new loadings code, NZS 1170.5, removed this exemption and requires that a special study be performed whenever energy dissipation through rocking occurs. This paper presents a tentative design procedure intended to substitute for the special study required by the code.
The resistance function of rocking walls was developed from the principles of engineering mechanics. The results from a series of time history analyses were used to develop a procedure to estimate maximum seismic displacements and empirical equations were derived to estimate the dynamic amplification of inertia forces. A substitute structure approach, using spectral displacements at an effective period calculated from the ductility factor, provided accurate predictions of the displacements from more sophisticated nonlinear analyses.
Four example designs were completed and the predicted response compared to time history results. The procedure provided a satisfactory estimate of response for regular structures, but it was less accurate where torsional effects were significant.
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On 12 May 2008 at 2.28 pm Beijing Time, an Ms 8.0 earthquake occurred in the Wenchuan County of Sichuan province, China. The associated fault ruptured over 240 km on the ground surface. The resulting damage was very severe and widespread, with casualties of almost 70,000, another 18,000 missing and 370,000 injured. The New Zealand Society for Earthquake Engineering reconnaissance team observed the effects and the recovery from this massive earthquake. The team studied the damages caused to the natural and the built environment due to fault rupture, seismic shaking, huge landslides and rockfalls. Maximum shaking intensity of MM XI significantly exceeded design intensity of MM VII for the area. Earthquake induced landslides had a major and catastrophic impact on development and infrastructure in this earthquake. Site selection was demonstrated to be critical. Brittle or non-ductile and irregular buildings performed very poorly especially in a seismic overload situation. Well engineered structures and dams performed well. Lifeline facilities were severely damaged, which resulted in interruptions to key transportation routes, inhibited rescue and recovery operations.
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The Mw 7.6 Dusky Sound earthquake of July 15th, 2009, was the largest magnitude earthquake in New Zealand since the devastating 1931 Hawke’s Bay event (Ms 7.8). The earthquake was sufficiently large to generate at least a 2.3 m wave at Passage Point. Despite its large magnitude, this event resulted in relatively minimal damage when compared to worldwide events of a similar size. This can be explained as a fortunate combination of the sparse population of the area and the specific physical characteristics of the earthquake. Centroid Moment Tensor (CMT) solutions define the rupture surface as a low-angle plane and finite fault inversions confirm the slip occurred on the interface between the eastward-subducting Australian plate and overriding Pacific plate, initiating at about 30 km depth and rupturing upward and southwestward to about 15 km depth. The oceanward rupture directivity likely contributed to the lower intensity of measured ground motion than might be expected for such a large, shallow event. The amount of radiated seismic energy from the earthquake was relatively small, and far fewer landslides were triggered from this event than from the 2003 Mw 7.2 Fiordland event.
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When the sand compaction pile (SCP) method is implemented to improve loose deposits of sandy soils, its effect is evaluated generally in terms of increase in density, which is beneficial for reducing the liquefaction potential of the deposits during earthquakes. An additional advantage can be expected to occur due to concurrent increase in lateral stress. When the resistance to liquefaction is evaluated on the basis of SPT N-value or CPT qc-value, the increased resistance to penetration due to the sand compaction has been interpreted conventionally as being associated mainly with the increase in density. Therefore, in order to properly evaluate the effectiveness of ground improvement in compacted soils, it is necessary to quantify the effect of lateral stresses on the penetration resistance and liquefaction strength. In this paper, based on the results of SPT and CPT performed in a chamber box in the laboratory, the relationships between penetration resistance, liquefaction resistance and relative density were re-examined and the influence of lateral stress, expressed in terms of KC, was investigated. Although the results indicated that generally the resistance to liquefaction increases with increasing KC–value, little difference was noted when the density of the deposit was high. Based on the results, recommended charts incorporating the effect of KC were proposed.
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Some standards for the design of steel structures attempt to encourage column yielding at the member ends, rather than along the column length during inelastic action such as may occur during strong earthquake shaking. In this paper, effects of residual stress and stability are considered separately and then combined to obtain equations that are more simple and more accurate than those in current standards.
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By recording microtremors simultaneously using arrays having two apertures, the effect of incoherent noise, which can act to depress coherency values, may be reduced, leading to better estimates of azimuthally-averaged coherency, and hence to improved shear-wave velocity profiles at sites. The method is exemplified by the use of 30 m and 40 m triangular arrays at McEwan Park, Lower Hutt, New Zealand, where the method is shown to result in better fits to theoretical coherency. Adequate correction is confined to low frequencies (less than 4.5 Hz in this case). Estimates of Vs are modified for greater depths (50 to 200m in this example) but unaltered for near-surface materials.
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The Padang earthquake is a timely reminder to New Zealand structural engineers of a number of things with respect to seismic design and construction practice of steel structures. These include: The importance of implementing the latest seismic loadings and design technology into new and existing structures without undue delay; The need to maintain an effective Building Code enforcement and audit process, including the keeping of publicly transparent compliance records; The important role of the design engineer in observing and auditing the interpretation and implementation of the design is essential, to prevent improper substitution of materials and ill-considered design changes; The need for ongoing continuing professional development and education for design, construction and building code enforcement officials to develop and maintain technical competency; The separation of non-structural elements from interfering with the primary seismic resisting system needs to be carried through diligently from design and into construction. Where structural separation is not achieved then design models for integrating unreinforced brickwork panels within moment resisting frames need to be developed, particularly for retrofit situations; The design for weak-axis bending of two way moment resisting steel frames requires careful attention to secondary effects, and should be avoided where possible; Non-self centring structural elements need to be identified at design stage and designed to minimise inelastic behaviour during ultimate limit state earthquakes; Diagonal bracing rods should be designed to avoid failure within couplings. Consideration should also be given to the dynamic response of the roof level bracing system to heavy wall induced lateral loads; Connections at the interface of steel work with concrete and masonry sub-trades need to be carefully monitored to ensure intended design performance is achieved; Unreinforced masonry without lateral tiebacks should be avoided on lintels over egress-ways; A guide of typical structural repair methods would also be a useful tool for post-earthquake use, to quickly identify appropriate repair strategies and allow repair estimates to be developed. At a philosophical level, should a post-earthquake repair be required to simply allow a resumption of functionality? Alternatively should the repair be required to reinstate the structural performance to its pre-earthquake strength? Or should the repair improve the seismic resisting performance of the structure in line with current earthquake engineering knowledge?
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This paper presents an exploratory case study based analysis of the seismic performance of multi-storey passive and semi-active tuned mass damper (PTMD and SATMD) building systems are investigated for 12-storey moment resisting frames modelled as ‘10+2’ storey and ‘8+4’ storey. Segmented upper stories of the structure are isolated as a tuned mass, and a passive viscous damper or semi-active resetable device is adopted for energy dissipation. Optimum TMD control parameters and appropriate matching SATMD configurations are adopted from a companion study on a simplified two degree of freedom (2-DOF) system. Log-normal statistical performance results are presented for 30 probabilistically scaled earthquake records. The time history analysis and normalised reduction factor results show the response reductions for all seismic hazards. Thus, large SATMD systems can effectively manage seismic response for multi degree of freedom (MDOF) systems across a broad range of ground motions in comparison to passive solutions. This research demonstrates the validity of the TMD building systems for consideration in future design and construction. It also provides a template for the design and analysis of passive or semi-active TMD buildings utilising large masses, or more efficiently, added storys, for improving seismic response performance.
