G.L. Hutchinson

GENERATION OF SYNTHETIC EARTHQUAKE ACCELEROGRAMS USING SEISMOLOGICAL MODELLING: A REVIEW

A stochastically based seismological model is described in this paper and used to generate synthetic accelerograms that are considered representative of intraplate earthquake events recorded on rock. The model, whilst well publicised in the seismological literature, is discussed and reviewed in this paper from an engineering perspective. The key factors influencing the frequency properties of the Fourier spectrum of the earthquake ground motion are presented and the differences observed between regions such as between eastern and western USA axe discussed. A procedure to generate representative artificial accelerograms from the Fourier spectrum is described. Average response spectra derived from these synthetic accelerograms are then compared with a selection of response spectra derived from recorded accelerograms to test and substantiate the model. The application of the seismological modelling approach to regions outside North America and the implications of the Displacement Based prinicples on the future development of seismological modelling have been addressed and discussed in the paper.

Lateral performance of cold-formed steel-framed domestic structures

Abstract This paper presents key outcomes of an investigation in the performance of domestic structures with cold formed steel frames. The primary objective of this research project was to assess the performance and behaviour of these structures when subjected to earthquake loading. The research involved an extensive racking and dynamic testing program on both two- and three-dimensional framing configurations. A variety of construction details was tested to identify the critical components and assess the contributions from the non-structural components, particularly the plasterboard lining. It is concluded that the steel frames perform very well under earthquake loads. Non-structural components, such as plasterboard lining, make a significant contribution to the lateral bracing of the frames. The failure mechanisms and the load sharing between the various components are also identified and discussed.

RESPONSE SPECTRAL RELATIONSHIPS FOR ROCK SITES DERIVED FROM THE COMPONENT ATTENUATION MODEL

The seismological model was developed initially from the fundamental relationship between earthquake ground motion properties and the seismic moment generated at the source of the earthquake. Following two decades of continuous seismological research in the United States, seismological models which realistically account for both the source and path effects on the seismic shear waves have been developed and their accuracy rigorously verified (particularly in the long and medium period ranges). An important finding from the seismological research by Atkinson and Boore and their co-investigators is the similarity of the average frequency characteristics of seismic waves generated at the source between the seemingly very different seismic environments of Eastern and Western North America (ENA and WNA, respectively). A generic definition of the average source properties of earthquakes has therefore been postulated, referred to herein as the generic source model. Further, the generic ‘hard rock’ crustal model which is characteristic of ENA and the generic ‘rock’ crustal model characteristic of WNA have been developed to combine with the generic source model, hence enabling simulations to be made of the important path-related modifications to ground motions arising from different types of crustal rock materials. It has been found that the anelastic contribution to whole path attenuation is consistent between the ENA and WNA models, for earthquake ground motions (response spectral velocities and displacements) in the near and medium fields, indicating that differences in the ENA and WNA motions arise principally from the other forms of path-related modifications, namely the mid-crust amplification and the combined effect of the upper-crust amplification and attenuation, both of which are significant only for the generic WNA ‘rock’ earthquake ground motions. This paper aims to demonstrate the effective utilization of the latest seismological model, comprising the generic source and crustal models, to develop a response spectral attenuation model for direct engineering applications. The developed attenuation model also comprises a source factor and several crustal (wave-path modification) component factors, and thus has also been termed herein the component attenuation model (CAM). Generic attenuation relationships in CAM, which embrace both ENA and WNA conditions, have been developed using stochastic simulations. The crustal classification of a region outside North America can be based upon regional seismological and geological information. CAM is particularly useful for areas where local strong motion data are lacking for satisfactory empirical modelling. In the companion paper entitled ‘response spectrum modelling for rock sites in low and moderate seismicity regions combining velocity, displacement and acceleration predictions’, the CAM procedure has been incorporated into a response spectrum model which can be used to effectively define the seismic hazard of bedrock sites in low and moderate seismicity regions. This paper and the companion paper constitute the basis of a long-term objective of the authors, to develop and effectively utilize the seismological model for engineering applications worldwide.

Response spectrum predictions for potential near-field and far-field earthquakes affecting Hong Kong: Soil sites

A pressure gauge comprises a housing, a main chamber in the housing partly filled with a liquid and a body floating in the liquid. A portion of the body extends through an opening in the housing. A fluid to be measured is introduced into the chamber above the liquid and its pressure is exerted on the surface of the liquid thus causing the body to assume a floated position. A scale is provided outside the housing and the position of the body, and thus the pressure in the measuring chamber, is indicated by the position of the body portion relative to the scale.

The ductility reduction factor in the seismic design of buildings

This paper presents new trends in the relationship between the ductility reduction factor and the ductility demand in the seismic design of buildings. A total of 4860 inelastic time-history analyses were carried out to study this relationship using 60 single-degree-of-freedom models excited by an ensemble of 81 earthquake accelerogram records from around the world. The asymmetrical distribution of the results highlighted the inaccuracies associated with assuming a normal distribution simply described by the mean and standard deviation to represent the data. A probability of exceedence approach has been used based on counting the number of occurrences the ductility demand exceeds a specified level. The ductility reduction factors developed in this study are consistent with other studies in the long-period range but are different in the short-period range. The ductility reduction factor for very short period buildings of limited ductility has been found to be greater than previously predicted.

Torsional coupling effects in the earthquake response of asymmetric buildings

Abstract This paper presents a detailed parametric study of the coupled lateral and torsional response of a partially symmetric single storey building model subjected to both steady state and earthquake base loadings. It is shown that the qualitative effects of the controlling parameters on the maximum translational and torsional responses of the coupled system are not affected by the nature of the loading. The maximum lateral edge displacement of the building arising from the combined response effects is investigated. The related lateral shear forces in vertical resisting elements located on the periphery of the structure may be significantly increased in comparison with the corresponding values for a symmetric building. It is concluded that for particular ranges of the key parameters defining the structural system, typical of the properties of many actual buildings, torsional coupling induces a significant amplification of earthquake forces which should be accounted for in their design.

A review of reference models for assessing inelastic seismic torsional effects in buildings

Abstract This paper reviews the various forms of reference model adopted for studies that evaluate inelastic seismic torsional effects and assess their implications for building design. Both qualitative and quantitative comparisons are presented. The importance of selecting an appropriate reference model is in accordance with the above aims is emphasised. It is found that variations in the reference models adopted in analyses of inelastic seismic torsional effects may lead to significant differences in the results obtained and, hence, to the conclusions drawn from such studies. It is demonstrated that accidental torsional effects, as incorporated in code design provisions, result in significant changes to the distribution of element strengths and the inelastic response behaviour of symmetric and generalised torsionally balanced reference models. Such changes should be considered when employing such models to evaluate the ineslatic response of torsionally unbalanced building systems.

Parametric earthquake response of torsionally coupled buildings with foundation interaction

Abstract A study is made of the effect of soil-structure interaction on the coupled lateral and torsional responses of asymmetric buildings subjected to a series of historical free-field earthquake base motions. It sh shown that for particular classes of actual buildings the equivalent rigid-base responses are significantly increased for structures founded on medium-stiff soils, and hence the assumption of the major building codes that a conservative estimate of response is obtained by considering the structure to be fixed rigidly at its base is shown to be inconsistent with the presented dynamic results. It is shown that foundation interaction produces greatest amplification of torsional coupling effects for structures subjected to a particular class of European strong-motion earthquake records, identified by similarities in their spectral shape, for which the vibrational energy of the ground motion is distributed approximately uniformly over the range of frequencies which are of interest for real structures. It is recommended that provision be made in the torsional design procedures of building codes for the increase in the coupled torsional response due to soil-structure interaction as indicated in this study. Such provision should be based on the results of comprehensive parametric studies employing a wide selection of earthquake records and accounting for expected variations in localized soil conditions.

Seismic response of flexible-edge elements in code-designed torsionally unbalanced structures

Abstract This study investigates the adequacy of static code torsional provisions in controlling the ductility and deformation demands in the flexible-edge element of structures with asymmetric stiffness distributions (termed torsionally unbalanced), excited into inelastic behaviour by intense earthquakes. The model configurations are designed strictly according to seismic building standards applied in the US, New Zealand, Canada, Australia and Europe. Significant additional ductility demand is found to arise in short-period systems designed according to the US and New Zealand provisions. The remaining codes, which amplify the static eccentricity to account for dynamic torsional effects, adequately control the additional ductility demand. However, the Australian and European codes may be considered over-conservative with respect to this response parameter. The deformation demand for the flexible-edge element increases rapidly with the magnitude of the static eccentricity, and for the US, New Zealand and Canadian codes may reach values as high as five or six times that of corresponding torsionally balanced systems. The Australian and European provisions control the flexible-edge element deformation to levels similar to that of the torsionally balanced system for moderate levels of eccentricity, but exhibit significant additional deformation demand for highly eccentric systems. For increasing values of the force reduction factor, code provisions give improved control of both the additional ductility and deformation demands, but both these response parameters are relatively insensitive to variations in the buildings plan aspect ratio above the value 2 : 1.

Definitions of static eccentricity for design of asymmetric shear buildings

Abstract This paper assesses three definitions of eccentricity employed in the description of asymmetric multistorey buildings. Using the three approaches, formulae are developed to evaluate the asymmetry of shear-type buildings where the deformation is governed by the lateral and rotational rigid-floor displacements arising from the seismic design loadings. Simple comparisons and comments on the alternative approaches are presented with the aid of appropriate examples. This clarification of the definition of eccentricity is needed for improved understanding of the application of codified torsional design procedures. It is also prerequisite for advanced studies of the inelastic seismic response of multistorey shear buildings.