TUNING OF SEISMIC HAZARD ESTIMATES BY MEANS OF OBSERVED SITE INTENSITIES

Probabilistic seismic hazard is usually assessed by means of computer codes using seis-mogenic sources, parametric catalogues, seismicity rates and attenuation relationships. All these ingredients are conditioned by expert judgement that influences the final results. Even the attenuation relationships, though strictly based on experimental data, are considered a weak point due to the difficulty of modelling the interaction between seismic energy radiation and site response, and because earthquakes do not usually repeat themselves according to one theoretical model. Recently, methods making wide use of site intensity data have been developed in regions such as Italy, where the observed seismic history at selected sites is quite exhaustive. We analyzed these observations to assess seismic hazard at about 600 sites. We used a probabilistic counting technique, integrating the observations (when necessary), with computed shakings obtained from a logistic-type attenuation model. The results were then compared with the estimates provided by the recent seismic hazard map of Italy, compiled according to the traditional probabilistic seismic hazard approach. The match shows significant differences for some sites. A tentative explanation which seems to point to three alternatives is provided: (1) The mismatch between the two methodologies might appear because the stationary assumption has a poor fit with reality (at least in certain areas); (2) Some sites show a response that is systematically different from the average values predicted using attenuation relationships; (3) The definition of seismogenic zones leads to a bias in the seismicity rate estimate.     

A PROCEDURE FOR ESTIMATING INPUT ENERGY SPECTRA FOR SEISMIC DESIGN

In this paper, the damage potential of an earthquake ground motion is evaluated in terms of the total power of the acceleration of the ground motion. By assuming an appropriate spectral shape for the input energy spectrum, and using the well-known Parseval theorem for evaluating the total power of a random signal, the peak amplification factor for the equivalent input energy velocity spectrum can be determined. It is shown that the peak amplification factor for the input energy spectrum depends on the peak-ground-acceleration to peak-ground-velocity ratio and duration of the strong motion phase of the ground motion. Values for the equivalent input energy velocity amplification factor vary from about 2 to 10 for most of the recorded ground motions used in this study. Although a considerable scatter of data is observed in this study, the peak amplification factor predicted by the Fourier amplitude spectrum of the ground acceleration provides a fairly good estimate of the mean value of the peak input energy compared to that determined from inelastic dynamic time history analyses, particularly for systems with high damping and low lateral strength. The peak amplification factor derived in this paper provides a more consistent approach for estimation of seismic demand when compared to an earlier empirical expression used for the formulation of duration-dependent inelastic seismic design spectra, even though only a slight difference in the required lateral strength results from the use of the new formula.     

ON THE RESOLUTION OF H/V MEASUREMENTS TO DETERMINE SEDIMENT THICKNESS,A CASE STUDY ACROSS A NORMAL FAULT IN THE LOWER RHINE EMBAYMENT,GERMANY

In recent years, H/V measurements have been increasingly used to map the thickness of sediment fill in sedimentary basins in the context of seismic hazard assessment. This parameter is believed to be an important proxy for the site effects in sedimentary basins (e.g. in the Los Angeles basin). Here we present the results of a test using this approach across an active normal fault in a structurally well known situation. Measurements on a 50 km long profile with 1 km station spacing clearly show a change in the frequency of the fundamental peak of H/V ratios with increasing thickness of the sediment layer in the eastern part of the Lower Rhine Embayment. Subsequently, a section of 10 km length across the Erft-Sprung system, a normal fault with ca. 750 m vertical offset, was measured with a station distance of 100 m. Frequencies of the first and second peaks and the first trough in the H/V spectra are used in a simple resonance model to estimate depths of the bedrock. While the frequency of the first peak shows a large scatter for sediment depths larger than ca. 500 m, the frequency of the first trough follows the changing thickness of the sediments across the fault. The lateral resolution is in the range of the station distance of 100 m. A power law for the depth dependence of the S-wave velocity derived from down hole measurements in an earlier study [Budny, 1984] and power laws inverted from dispersion analysis of micro array measurements [Scherbaum et aL, 2002] agree with the results from the H/V ratios of this study.     

SOIL DAMPING FORMULATION IN NONLINEAR TIME DOMAIN SITE RESPONSE ANALYSIS

Nonlinear time domain site response analysis is used to capture the soil hysteretic response and nonlinearity due to medium and large ground motions. Soil damping is captured primarily through the hysteretic energy dissipating response. Viscous damp-ing, using the Rayleigh damping formulation, is often added to represent damping at very small strains where many soil models are primarily linear. The Rayleigh damping formulation results in frequency dependent damping, in contrast to experiments that show that the damping of soil is mostly frequency independent. Artificially high damp-ing is introduced outside a limited frequency range that filters high frequency ground motion. The extended Rayleigh damping formulation is introduced to reduce the over-damping at high frequencies. The formulation reduces the filtering of high frequency motion content when examining the motion Fourier spectrum. With appropriate choice of frequency range, both formulations provide a similar response when represented by the 5% damped elastic response spectrum.

The proposed formulations used in non-linear site response analysis show that the equivalent linear frequency domain solution commonly used to approximate non-linear site response underestimates surface ground motion within a period range relevant to engineering applications. A new guideline is provided for the use of the proposed formulations in non-linear site response analysis.     

DESIRABLE STRENGTH DISTRIBUTION FOR ASYMMETRIC STRUCTURES WITH STRENGTH-STIFFNESS DEPENDENT ELEMENTS

Recent studies have shown that for many reinforced concrete lateral force-resisting elements (LFRE) stiffness is dependent on strength, and as a result strength assign-ment to these elements would affect both the strength and stiffness distributions in a structure. As a consequence, stiffness distribution cannot be considered known prior to strength assignment. This implies that in assigning strength to LFRE, the designer has the ability not only to prescribe the strength distribution, but also indirectly control the stiffness distribution in the structure. In this paper, a study is made on the seis-mic performance of a number of single-story structures to reconfirm that the “balanced CV-CR location” criterion, previously suggested by the writers, constitutes a desirable strength/stiffness distribution for minimising torsional response of asymmetric reinforced concrete structures.     

A PRACTICAL MODEL FOR SEISMIC ANALYSIS OF REINFORCED CONCRETE SHEAR WALL BUILDINGS

A simple macro-model for reinforced concrete shear walls is proposed, which consists of spring elements representing flexure and shear behaviour. The model for flexural behaviour is based on section analysis, while the model for shear behaviour is based on key parameters of the flexural behaviour. Four wall test specimens are selected to evaluate the reliability of the model. Modelling parameters for the backbone curves and the hysteretic rules are examined by conducting static and time history analyses, with the hysteretic response of a test specimen compared to that calculated using the proposed model. Results show some differences between measured and calculated shear force versus shear distortion relationships, but the model is acceptable because the differences do not significantly affect calculated global response. Parametric studies are also conducted to examine the influence of modelling parameters on seismic demand and capacity, which are the major design parameters for structural performance evaluation. Differences due to variation in modelling parameters are not significant, further indicating that the proposed model is reasonable.     

A DAMAGE MODEL FOR THE ANALYSIS OF THE SEISMIC RESPONSE OF MONUMENTAL BUILDINGS

In this paper the Author proposes a damage model for the analysis of masonry plates and shells, which is based on an improvement of a previous constitutive model. The modifications introduced, connected to the head joint damage, allow us to study the influence of masonry texture on the damage modes once the mechanical characteristics of the elements constituting the masonry and the results of tests on simple assemblages are known. Having a nonlinear constitutive model is certainly one of the basic elements for understanding the damage mechanisms in masonry buildings. If, in fact, an elastic-linear constitutive model may be used under normal loading conditions, in critical situations it is necessary to model the damage and the dissipation mechanisms that occur between the elements, stone (brick) and mortar, in correlation with their characteristics and kind of masonry. To validate the model a comparison is made between the numerical and experimental results, in the case of tests available in the literature in masonry panels subjected to out-of plane loading and in a real structure through the observation of the damage in Umbria (Italy) surveyed after the 1997 earthquake.     

EVALUATION OF CONVENTIONAL AND ADAPTIVE PUSHOVER ANALYSIS I: METHODOLOGY

In this paper, a methodology is suggested and tested for evaluating the relative performance of conventional and adaptive pushover methods for seismic response assessment. The basis of the evaluation procedure is a quantitative measure for the difference in response between these methods and inelastic dynamic analysis which is deemed to be the most accurate. Various structural levels of evaluation and different incremental representations for dynamic analysis are also suggested. This method is applied on a set of eight different reinforced concrete structural systems subjected to various strong motion records. Sample results are presented and discussed while the full results are presented alongside conclusions and recommendations, in a companion paper.     

A Critique in Search of a Corpus: Re-visiting Governance and Re-interpreting Urban Politics

One of the most profound changes of the last two decades has been in the form and the function of local government. Its role as part of the local state has been challenged through the re-scaling and re-configuration of many of its aims. This paper examines the growth of business sector involvement and the ascendancy of the partnership model in urban development, as part of an analysis of the changes and the continuities that stretch across the local government – local governance conceptualization of political relations. I start the paper with a discussion of the main issues in the local governance literature as a precursor to a commentary onImrie and Raco's (1999) recent paper, 'How New is the New Local Governance? Lessons from the United Kingdom'. The wider literature is drawn upon to discuss some of the theoretical approaches used to analyse this much-vaunted transformation and three key themes are used to structure the remainder of the paper. I argue that Imrie and Raco caricature the work of other academics in order to make their claims over omissions from the literature. In doing so, they ignore or under-play how state restructuring and the logic of capital has often been the object of analysis, rather that the voluntarist incorporation of business élites into the local governing apparatus. Using empirical examples from three English cities I argue that while, on the one hand, their examples are revealing and serve to sensitize current debates around governance, the state and regulation, on the other hand, there is still a need to interpret these 'local' politics within a broader and more scale-sensitive framework and in more abstract terms. This demands a clear distinction between local governance as a concept, and its investigation in an empirical way.     

Local Markets and the Polarization of Public-Sector Schools in England and Wales

Notwithstanding the recent change of government, UK educational policy continues to stress school performance tables, parental choice and competition – reflecting an acceptance by New Labour of Tory notions concerning the role that market mechanisms play in improving school performance. In view of the 1997 Education White Paper's insistence that the new government's goal was to 'overcome the spiral of disadvantage … passed from one generation to the next', this implies that the DfEE remains unimpressed by long-standing arguments concerning the socially-divisive consequences of market forces in education.
This paper seeks to add empirical weight to this debate. Arguing that recent analyses of aggregate levels of social stratification have masked polarization occurring at the local level, our analysis places schools in the context of the local markets within which they operate. This yields clear evidence that educational markets tend to exacerbate existing differences between schools in terms of both their performance and social status.