Adrian M. Chandler

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.

Seismic displacement response spectrum estimated from the frame analogy soil amplification model

Abstract This paper describes the development of a simple and rational manual procedure, termed the “frame analogy soil amplification” (FASA) model, which can be used to construct realistic seismic soil response spectra that account completely for the effects of soil resonance. FASA is based on the analogy of the dynamic response behaviour of a building to represent the dynamic behaviour of a horizontally layered soil column, when subject to earthquake excitations transmitted from bedrock. The theoretical foundation of FASA requires relatively few data for verification and calibration purposes. Thus, it is particularly suited to applications in regions of low and moderate seismicity where field data are typically limited. The procedure becomes even more powerful when its capability is extended to predict the response spectral displacement (RSD). This paper presents (1) a summarised account of the conception and development of the FASA-RSD model, (2) a comparison of the model predictions with response spectra computed from shear wave analysis (using the computer program SHAKE) and with response spectra recorded from the 1989 Loma Prieta earthquake, and (3) the application of FASA including commentaries and an illustrative example.

Performance-based design in earthquake engineering: a multi-disciplinary review

Abstract This paper reviews the contributions of research towards the development of the methodologies associated with Performance-Based Seismic Engineering (PBSE). Research undertaken in various related disciplines is reviewed, under the broad section headings of (i) Engineering Seismology and Geology (Seismic Activity Modelling), (ii) Engineering Seismology (Seismic Hazard Modelling), (iii) Soil Dynamics, (iv) System Dynamics, and (v) Mechanics of Materials (Concrete used as example). The sequence of the discussion is consistent with a typical seismic assessment procedure, which commences with seismic activity modelling in the ‘upstream’ end of the procedure and finishes with consideration of structural mechanics behaviour at the ‘downstream’ end. Each section provides an outline of historical research and development, leading to a review of the state-of-the-art approaches. Particular emphasis is given to the inter-linking of the disciplines, and the paper refers to such links as ‘Nodal Points’. An example of a nodal point is the definition of probabilistic seismic hazard coefficients that are used to define seismic hazard in terms of elastic response spectra, for example the response spectral accelerations at key periods of 0.3 and 1.0 s. Each of the Nodal Points associated with the various disciplines has been critically reviewed, and shortcomings have been identified. For example, the inability of a probabilistic approach to fully represent an earthquake event as a physical process is highlighted. Also, the importance of putting emphasis in future research on determining the Maximum Credible (or Considered) Earthquake, MCE, is emphasised. The paper brings to light the fact that, although significant achievements have been made in each of the related disciplines and in the connection of the Nodal Points, there has been relatively little change in substance at the Nodal Points themselves. An important outcome of this multi-disciplinary review is the identification of some key limitations in current procedures. The source of these limitations was traced upstream, and thence to the Nodal Points that provide the inter-disciplinary links. This process has been referred to herein as Upstream Feedback. A review of the problems at these links sows the seeds for further development, which would not have been possible had all the recent contributions been confined within the individual disciplines. Such an Upstream Feedback process, enabling improvements to the multi-discipinary links, would be instrumental in enhancing the overall effectiveness of PBSE in the future.

Regional and local factors in attenuation modelling: Hong Kong case study

Seismic attenuation behaviour is controlled by a large number of wave modification mechanisms. The characteristics of some of these mechanisms are specific to a local area, whilst the remainder can be generalised to the entire seismic region. Factors representing these mechanisms are often not resolved. A new attenuation modelling approach is demonstrated in this paper (using Hong Kong as a case study), to evaluate individual regional and local wave modification factors. Shear wave velocity (SWV) information for the four prevalent geological formations found in Hong Kong was first obtained: (a) at shallow depths from instrumented boreholes; (b) at depths of up to 100–200 m from measurements using the Microtremor SPatial Auto-Correlation (SPAC) technique; (c) at depths of up to 1.5 km from the monitoring of quarry blasts; and (d) at depths from 1.5 to 8 km in the hard basement rock layers from results of seismological refraction surveys. The upper-crust amplification factor calculated from the four modelled rock SWV profiles was then combined with predicted attenuation parameters to determine the upper-crust modification factor (filter function) incorporating the local wave modification characteristics associated with Hong Kong geological formations. Such functions may then be combined with the regional attenuation characteristics in that part of the South China region. A seismic attenuation model was developed by combining the upper-crust modification factor with the regional source function of intra-plate earthquakes, based on stochastic simulations. The ground shaking model developed from the presented methodology is supported by the comparison with macro-seismic data of seven historical earthquake events affecting Hong Kong.

Performance of reinforced concrete frames using force and displacement based seismic assessment methods

This paper reviews the traditional force-based (FB) seismic design method and the newly proposed displacement-based (DB) seismic assessment approach. A case study is presented for reinforced concrete (RC) moment-resisting frames designed and detailed according to European and Australian earthquake code provisions, having low, medium and high ductility capacity. The aim is to assess the performance characteristics of these frames, using the well known El Centro NS earthquake ground motion as the seismic input. Overall ductility demands have been computed for the force-based analyses conducted on the typical design frames. In the second part of the paper, the performance of the case study frames has been re-evaluated in the light of displacement-based principles. A recently proposed method for displacement-based seismic assessment of existing RC frame structures has been implemented for this purpose, from which it has been concluded that the displacement-based approach predicts very similar overall displacement demands for such frames. These results, whilst limited to the consideration of a small number of seismic frame structures and a single, typical strong earthquake ground motion, nevertheless give confidence that the displacement-based approach can rapidly and easily facilitate a seismic assessment of an existing RC structure, without the necessity to undertake detailed inelastic dynamic analyses.

Generic Approach for Modelling Earthquake Hazard

The earthquake attenuation behaviour is a critical part of seismic hazard modelling for regions of low and moderate seismicity. This paper presents a new approach for attenuation modelling, which does not involve the use of strong motion data, and is based on taking measurements of the shear wave velocity (SWV) in bedrock by a noninvasive technique to characterize the transmission of seismic waves. The developed filters are then applied to the generic source model for intraplate earthquakes for calculating the frequency content of seismic waves at the bedrock surface and was used as input to soil response analyses for the determination of site seismic hazard. Ground motion parameters and response spectra are then obtained from the stochastically simulated accelerograms to develop representative attenuation models for rock conditions. The described approach of obtaining a seismic attenuation relationship based on modelling the rock and soil crustal properties is not constrained to any particular environment. Thus, the approach is described as generic. The northern suburbs of the city of Melbourne, Australia, have been used as the study area to illustrate the modelling procedure. The peak ground velocity values (PGVs) predicted from attenuation relationship developed from this study are well correlated with the PGVs inferred from MMI Intensity data of historical earthquakes felt in Melbourne over the past hundred years.

Assessment of low-rise building with transfer beam under seismic forces

An overview of the structural performance of a transfer structure in Hong Kong under potential seismic actions is presented. A hypothetical but realistic low-rise building model has been developed comprising a seven-storey reinforced concrete frame structure with reinforced concrete transfer beams at first floor level. Structural design has been based on the British Standard BS8110 and local practices. Parametric analyses of the moment–curvature relationship of each component have been conducted. By adopting the displacement-based (DB) approach, various seismic assessment methodologies, including response spectrum analysis (RSA), manual calculation, pushover analysis (POA) and equivalent static analysis (ESA) have been implemented. The deformations induced by the predicted seismic actions in Hong Kong are compared with those arising from POA in terms of average lateral drift ratios and maximum interstorey drift ratios arising in the building. Factors influencing the performance of this form of transfer structure are highlighted and discussed. This paper also provides a general indication of seismic vulnerability of common low-rise transfer structures in regions of low to moderate seismicity.

Property Loss Estimation for Wind and Earthquake Perils

This article describes the development of a generic loss assessment methodology, which is applicable to earthquake and windstorm perils worldwide. The latest information regarding hazard estimation is first integrated with the parameters that best describe the intensity of the action of both windstorms and earthquakes on building structures, for events with defined average return periods or recurrence intervals. The subsequent evaluation of building vulnerability (damageability) under the action of both earthquake and windstorm loadings utilizes information on damage and loss from past events, along with an assessment of the key building properties (including age and quality of design and construction), to assess information about the ability of buildings to withstand such loadings and hence to assign a building type to the particular risk or portfolio of risks. This predicted damage information is then translated into risk-specific mathematical vulnerability functions, which enable numerical evaluation of the probability of building damage arising at various defined levels. By assigning cost factors to the defined damage levels, the associated computation of total loss at a given level of hazard may be achieved. This developed methodology is universal in the sense that it may be applied successfully to buildings situated in a variety of earthquake and windstorm environments, ranging from very low to extreme levels of hazard. As a loss prediction tool, it enables accurate estimation of losses from potential scenario events linked to defined return periods and, hence, can greatly assist risk assessment and planning.

Modal Analysis of Steel-Framed Residential Structures for Application to Seismic Design

There has been a significant worldwide increase in the use of cold-formed steel in low-rise resi dential structures as an alternative to timber. A major research project to assess the performance under lateral loading of cold-formed steel frames commenced more than 7 years ago at The University of Melbourne, Australia, in collaboration with industry and other research organizations. This paper reports key findings from an extensive experimental program. It details a particular aspect of the program that uses nondestruc tive dynamic testing to identify the basic dynamic properties of such structures. Laboratory tests, employing a shaking table, have been conducted on unclad wall panels and a model test house to study the influence of different framing connection types and to assess the influence of nonstructural components, namely, plas terboard interior lining and brick-veneer exterior cladding. The findings from these tests were verified and further extended by testing a full-scale prototype house at various stages of construction. This paper presents information concerning typical natural frequencies and mode shapes for such structures. It also details the contribution of nonstructural components to the lateral stiffness, which was found to be significant. The re sults have important implications for seismic design and performance of such structures, even in regions of low to moderate seismicity.