Severe damage to six out of a total of 21 subway stations in the Kobe area during the 1995 Hyogoken-nanbu earthquake indicated a need for more attention to be given to the earthquake design of rectangular underground structures. This paper presents work undertaken to extend the present knowledge of the dynamic interaction of box-section structures with the surrounding soil, and a design method for predicting the earthquake loads on underground structures such as basement walls, tanks, subways, utility boxes, highway underpasses, and culverts.
In this study, we present an inelastic demand spectrum for the design of seismically-isolated structures using lead-rubber bearings or other types of isolators with bi-linear hysteresis loops and the inelastic spectrum can be used in the design of seismically-isolated structures in a very similar manner to capacity spectrum method. The inelastic demand spectrum is a very useful design tool for visual selection of optimal isolation parameters, and eliminates the use of equivalent linear-elastic substitute structures as the displacement demand is obtained from nonlinear time history analysis. The responses of seismically-isolated structures subjected to near-source ground motions with either large forward-directivity pulses or fault-fling pulses are presented. Our analyses suggest that seismic isolation can be used to protect structures subjected to recorded ground motions currently available to us, with acceptable levels of base shear coefficient and isolator displacement, except for one component of the TCU068 record from the 1999 Chichi, Taiwan, earthquake (which contained a large permanent displacement of nearly 10 m).
A magnitude (Mw) 7.6 earthquake occurred at 8.55 am (local time) on 8 October 2005 causing extensive damage to buildings, bridges and roads and killing in excess of 87,000 people in the Kashmir region of northern Pakistan. Damage and deaths were also reported from Indian Administered Kashmir and eastern Afghanistan. The most severely affected region was in the epicentral area around Muzaffarabad in Pakistan Administered Kashmir. Reverse or thrust fault rupture on or near the Main Boundary Thrust of the Himalayas has been reported or observed from Chennari in the Jhelum River valley upstream of Muzaffarabad through to Muzaffarabad and over into the Kaghan valley as far north as Balakot, a distance of approximately 60 km. A notable feature of the effects of this earthquake was the asymmetric distribution of landslides across the fault rupture zone. On the downthrown or footwall side (to the southwest) landslide damage was relatively minor - the road from Manshera to Muzaffarabad was open to traffic within 8 hours of the earthquake and required the clearance of only one landslide. On the up-thrown or hanging wall side of the fault rupture zone (to the northeast) the road from Balakot to Kagan required the clearance of 253 landslides and took 24 days. These observations are consistent with the findings of recent strong motion studies.
Supplemental dampers are a means of repeatedly dissipating energy without damage to the underlying structure, increasing life-safety and helping provide better serviceability of structures following a major earthquake. High performance (small size) lead dampers are designed and tested to characterise their force-displacement behaviour and produce trade-off curves relating device geometry to force capacity, to parameterise the design space to enable further devices to be designed for structural applications. Peak forces of 120-350 kN were obtained for devices that were all able to fit within standard structural connections.
Results show that prestressing the working material is critical to obtain optimal energy dissipation. Although previously characterised as extrusion dampers it is shown that classical extrusion modelling formulations do not strictly work well for this class of damper. Instead a coulomb type of stress-based model is proposed, with relationships presented that are independent of device scale. Empirical reduction factor equations are applied to the New Zealand Structural Design Actions to enable lead extrusion devices to be incorporated into structural design analyses. The overall results indicate that repeatable, optimal energy dissipation can be obtained in a compact device to minimise damage to critical buildings and infrastructure.
Placing reinforced concrete jackets or layers to strengthen or repair and strengthen concrete columns is a normal construction practice but there are many unresolved issues regarding the capacity of the strengthened elements. In the absence of any guidance, engineering judgement is often used. This paper sets out to assist the engineer when considering some of these unresolved issues. Revised values for factors of safety are proposed for design. A procedure to guarantee a sufficient connection between contact surfaces and to determine the performance of retrofitted columns is presented, considering the strengthened columns as “composite” elements. The parameters affecting the main mechanisms for the transfer of shear stress at the interface between new and old concrete are described and practical design considerations are given. An approximate procedure is presented, based on the design of monolithic elements, supplemented by the use of specific modification factors (monolithic factors), in order to evaluate the capacity of a strengthened element. Available experimental results are processed to derive appropriate values for monolithic behaviour factors and an extended analytical analysis is used to fill in gaps in the experimental work. Although this paper has particular relevance to seismic strengthening, its contents will have a wider application to strengthening in general. The object of this paper is to provide guidance so that the engineer is better equipped to deal with the practical design needs of today.
Reinforced concrete frames infilled with masonry panels constitute an important part of the high-risk structures in different regions of high seismicity. In some developing countries, they are still used as main structural system for low to medium rise buildings. Consequently, reliable methods to analyse infilled frames are required in order to reduce the loss of life and property associated with a possible structural failure.
The equivalent strut model, proposed in the 1960s, is a simple procedure to represent the effect of the masonry panel. Several improvements of the original model have been proposed, as a result of a better understanding of the behaviour of these structures and the development of computer software. This paper presents a new macro-model for the evaluation of the global response of the structure, which is based on a multi-strut formulation,. The model, implemented as 4-node panel element, accounts separately for the compressive and shear behaviour of masonry using a double truss mechanism and a shear spring in each direction. The principal premises in the development of the model are the rational consideration of the particular characteristics of masonry and the adequate representation of the hysteretic response. Furthermore, the model is able to represent different modes of failure in shear observed for masonry infills. The comparison of analytical results with experimental data showed that the proposed model, with a proper calibration, is able to represent adequately the in-plane response of infilled frames.
In the late 1970’s it was recognised that the seismic provisions of then 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.
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.
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-torecord 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.
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.
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.
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.
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.