Nelson Lam

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.

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.

Time-history analysis of URM walls in out-of-plane flexure

Abstract This paper introduces a single degree-of-freedom analytical model which has been used by the authors to investigate the out-of-plane performance of unreinforced masonry walls in vertical one-way bending subject to seismic actions. The objective of the modelling is to reduce costs on physical experimentation by supplementing and extending the limited experimental database on dynamic wall behaviour with analytical data. The procedure necessitates sufficient physical testing of specimens with representative boundary conditions to enable the analytical model to be calibrated against experimental results. A computer program which requires minimal data preparation effort and computational time has been written to conduct time–history analysis on the model. Importantly, key parameters have been incorporated in the program to facilitate calibration of the model. Details in developing representative tri-linear elastic force-displacement relationships and non-linear damping relationships to define the model are described in the paper. The developed model has been verified by comparing the computed acceleration and displacement response time-histories with results obtained from an extensive series of shaking table experiments. Seven of such time-history comparisons are shown in the paper to demonstrate the accuracy of the analytical model.

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.

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.

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.

A recommended earthquake response spectrum model for Australia

Abstract Response spectrum provisions in the current Australian Earthquake Loading Standard (AS1170.4) were based on recommendations from the 1991 edition of the Uniform Building Code for the United States. Since the implementation of AS1170.4 in 1993 there have been significant developments internationally and in Australia in the area of response spectrum modelling on both rock and soil sites. In this paper, a new normalised design response spectrum is recommended for rock sites in Australia based on a response spectral velocity of 1.8 times the peak ground velocity, and corner periods of T1=0.35secs and T2=1.5secs (based on a magnitude 7 earthquake event). The recommended rock response spectra for 500 year return period is generally consistent with the AS1170.4 spectrum in the low period range, however, more accurately represents the displacement demand expected in the higher period range. Recommendations for the development of response spectra for soil sites have also been presented in ADRS format for use with the capacity spectrum method, to assess the seismic performance of new and existing structures in Australia. A brief introduction to the method has been presented in the paper.

A SIMPLE DISPLACEMENT-BASED MODEL FOR PREDICTING SEISMICALLY INDUCED OVERTURNING

The displacement-based modelling methodology which has been applied extensively to buildings and bridges is extended herein to model the over-turning behaviour of rigid free-standing objects. The acceleration-displacement relationship associated with the overturning motion is linearised in order that the maximum displacement experienced by the object can be estimated using the elastic displacement response spectrum of the building floor. Whilst overturning motion is characterised by highly nonlinear acceleration-displacement properties, it was observed that modelling errors arising from nonlinear behaviour can be effectively controlled through limiting the maximum displacement of the object to some 50% of the ultimate displacement for overturning. The 50% safety margin is one of the key features in the proposed model. Three rigid rectangular objects with depths of 100 mm, 300 mm and 500 mm were used initially to illustrate the use of the model. The height of these objects was 0.5 m, 1.5 m and 2.5 m respectively in order that every object has a common aspect ratio of 1:5. Despite that the aspect ratios of the objects were the same, they have very different levels of vulnerability to overturning. The proposed model was evaluated by nonlinear time-history analyses involving pulse-type excitations, recorded earthquake excitations and computer simulated earthquake excitations. Linear elastic models of buildings have also been used to simulate floor motions at the upper levels in the building. Predictions using the proposed linearised model based on the use of elastic response spectrum of the building floor was found to be very consistent with results obtained from nonlinear time-history analyses. Sufficient verification analyses have been carried out to provide the initial indications that the proposed linearised model seems to work well despite its simplicity.