H. H. Tsang

SIMULATIONS OF RESPONSE TO LOW VELOCITY IMPACT BY SPREADSHEET

A program which can be used to simulate the elastic response of lumped mass single-degree-of-freedom systems to impact actions is introduced in this paper. The underlying governing equations and the implementation of their solutions on a spreadsheet are described. As illustrated in the paper, the program can be extended to obtaining approximate solutions for the inelastic response of a reinforced column to the impact by a vehicle. Solutions obtained from the developed spreadsheet have been compared with those obtained by other means. Few engineers have the skills of undertaking their own sanity checks on analyses involving transient actions including that of the impact of an object. Codes of practices often resort to the use of equivalent static loading for representing transient actions and this may give misleading results. Portable and inexpensive computational tools like the ones presented herein enable engineers to conveniently undertake their own sanity checks on computer output from 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.

Modelling earthquake ground motions by stochastic method

The prediction of earthquake ground motions in accordance with recorded observations from past events is the core business of engineering seismology. An attenuation model presents values of parameters characterising the intensities and properties of ground motions estimated of projected earthquake scenarios (which are expressed in terms of magnitude and distance). Empirical attenuation models are developed from regression analysis of recorded strong motion accelerograms. In situations where strong motion data are scarce the database of records has to cover a very large area which may be an entire continent (eg. Ambrasey model for Europe) or a large part of a continent (eg. Toro model for Central & Eastern North America) in order that the size of the database has statistical significance (Toro et al., 1997; Ambrasey, 1995). Thus, attenuation modelling based on regression analysis of instrumental data is problematic when applied to regions of low and moderate seismicity. This is because of insufficient representative data that has been collected and made available for model development purposes.

Geotechnical seismic isolation system - Experimental study

In this study, an experimental investigation program on a newly proposed seismic isolation technique, namely “Geotechnical Seismic Isolation (GSI) system”, is conducted with an aim of simulating its dynamic performance during earthquakes. The testing procedure is three-fold: (1) A series of cyclic simple shear tests is conducted on the key constituent material of the proposed GSI system, i.e., rubber-sand mixture (RSM) in order to understand its behavior under cyclic loadings. (2) The GSI system is then subjected to a series of shaking table tests with different levels of input ground shakings. (3) By varying the controlling parameters such as percentage of rubber in RSM, thickness of RSM layer, coupled with the weight of superstructure, a comprehensive parametric study is performed. This experimental survey demonstrates the excellent performance of the GSI system for potential seismic hazard mitigation.

A refined design spectrum model for regions of lower seismicity

Abstract This paper presents a refined site classification scheme and a design spectrum (DS) model for different site conditions, with site natural period as the key classification parameter. The refined model has a particular emphasis on the phenomenon of resonant-like amplification behaviour in soil sites, which have not been explicitly considered in any existing code models. The need to address the effects of soil resonance and increased displacement demand is particularly justified in regions of lower seismicity, where structures are typically of limited ductility with low energy dissipation capability. Significantly, the mitigating effects of a very flexible soil site resulting in reduction in the level of seismic demand on low-rise buildings is a distinctive feature of the model which has been well validated by comparison with results obtained from computational site response analysis of soil columns derived from real borehole records, as well as from strong motion data recorded in the 1994 Northridge earthquake.