Reconstruction of earthquake hazard in regions of sparse seismic monitoring

Studying strong motion records and the spatial distribution of ground shaking is of great importance in understanding the underlying causes of damage in earthquakes. Many regions in the world are either not instrumented or are sparsely instrumented. As such, significant opportunities for motion-damage correlations are lost. Two recent and damaging earthquakes belong to the class of lost opportunities, namely the Kashmir (Pakistan) earthquake of October 2005 and the Yogyakarta (Indonesia) earthquake of May 2006. In this paper, an overview of the importance of supply and demand studies in earthquake-stricken regions is given, followed by two examples of investigative engineering seismology aimed at reconstructing the hazard from sparse data. The paper closes with a plea for responsible authorities to invest in seismic monitoring networks in the very near future.     

Effect of random material variability on seismic design parameters of steel frames

Whereas an increase in material yield stress beyond the code specified characteristic value enhances plastic capacity, it may cause a reduction in overall ductility and energy absorption capability of steel frames. Since quality control of various shapes of sections used on site is difficult to impose, the effect of this random variability on design response parameters should be accounted for in earthquake-resistant regulations. Moreover, the required weak-beam/strong-column design principle in particular, and failure mode control in general, could be undermined if the yield stresses in beam and column assume two opposite extremes in a random sample. This paper addresses the problem of defining the expected range of response parameters in a steel frame with randomly varying yield stress. A simple portal frame is designed using code specified characteristic values and verified by non-linear transient dynamic analysis. The influence of yield stress variability, including the degree of correlation between beam and column material properties, on several response parameters is assessed through a Monte Carlo simulation study. Results are presented from both univariate and bivariate statistical analyses that quantify the relationship between input (material) and output (response) parameters. Assessment of the interdependence of output parameters given a particular model for yield stress variability is also undertaken. It is shown that certain response parameters exhibit more favourable statistical properties than others. Thus, the implications for seismic code design are discussed in the light of these results.     

Inelastic spectra for displacement-based seismic design

In recognition of the emergence of displacement-based seismic design as a potentially more rational approach than force-based techniques, this paper addresses derivation of inelastic displacement spectra and associated topics. A well-constrained earthquake strong-motion dataset is used to derive inelastic displacement spectra, displacement reduction factors and ductility–damping relationships. These are in a format amenable for use in design and assessment of structures with a wide range of response characteristics.     

Performance of composite steel/concrete members under earthquake loading. Part II: Parametric studies and design considerations

In this paper, an advanced analytical model proposed and verified in a companion paper (Part I: Analytical model) is used to conduct parametric investigations of ductile partially-encased composite beam-columns. Particular attention is given to a new configuration of composite member developed and tested at Imperial College. A detailed account of the analytical results is given, and the most significant observations are highlighted and discussed. Quantitative assessments of plastic moment capacity, rotation and displacement ductility and plastic hinge length are undertaken. The implications of the main findings on the seismic design process are also presented. Finally, a procedure for ductility-based design according to modern codes of practice is outlined.     

Experimental behaviour of reinforced concrete walls under earthquake loading

The experimental work and first results of a recently completed experimental research programme investigating the response of reinforced concrete (RC) walls under earthquake (EQ) loading are discussed in this paper. A brief literature review is given as a prelude to the outline of research objectives. The tests are presented in two groups according to the scale of models. For the 1:5 scale tests, a modified similitude relation for small scale reinforced concrete dynamic modelling is developed. Based on the chosen model parameters, the design of the isolated RC walls is given. The test-rig set-up and the EQ input signals suitable for testing the model on the Imperial College shake-table are also discussed. Preliminary observations regarding stiffness, strength and failure modes of the RC wall models are given. Experimental results from the shake-table are compared to tests, at the same scale, under static cyclic conditions. For the scale 1:2–5 cyclic tests a different test-rig assembly is designed. The test results are given in three pairs of flexurally similar walls followed with general observations and discussion. Finally, conclusions are drawn regarding experimental procedures and behaviour patterns of the tested models.     

An analytical solution for the probabilistic response of sdof non-linear random systems subjected to variable amplitude cyclic loading

Design for a specific ductile failure mode is assuming a rǒle of increasing importance for earthquake-resistant structures. This necessitates an accurate assessment of the distribution of overstrength in the structure, in order that the predefined failure mode can be realized. Consequently, the variability of the response for a given variability in the salient material properties, such as yield strength for steel structures, should be assessed and accounted for. In this paper an analytical method is proposed for the evaluation of the probability density function of the response of a single-degree-of-freedom hysteretic system with random parameters subject to a variable amplitude cyclic load history. A simple algorithm is derived which may be used to obtain the system response as a function of the system parameters. This response function may then be used to evaluate the displacement response probability density function when given the probability density function of the system parameters. Results derived from this procedure are verified against Monte Carlo simulation. It is shown that accurate response statistics are obtained at a fraction of the computing cost of simulation techniques.     

Experimental observations on the seismic shear performance of RC beam-to-column connections subjected to varying axial column force

The paper presents results from the first series of an ongoing experimental study aimed at quantifying the effect of axial column load on the shear capacity of beam-to-column connections. This is deemed important due to the recent evidence showing that vertical earthquake ground motion, when combined with high overturning moments, may cause reduced column compression or even tension. In which case, the concrete contribution to shear resistance in the panel zone is diminished, which may lead to failure prior to the attainment of the full resisting capacity of the beam section. The results first show that the failure mode of the models was, as intended, shear failure of the panel zone. It is further observed that the axial column load has a marked effect on the shear deformation capacity, yield point, cracking pattern, ultimate capacity and ductility of the panel zone. Differences in the range of 30 per cent in capacity and 50 per cent in deformability were recorded. The preliminary results are useful in providing design guidance for structures located in areas of potential high vertical ground motion component. Also, for high-rise structures, where there are large overturning moments, the results may be of use in ensuring a uniform safety factor (or overstrength) in various non-dissipative parts of the structure.     

Performance of composite steel/concrete members under earthquake loading. Part I: Analytical model

This paper presents an advanced non-linear model developed for the analysis of composite steel/concrete frame structures subjected to cyclic and dynamic loads. The formulation consists of beam-column cubic finite elements accounting for geometric non-linearities and material inelasticity. The non-linear cyclic concrete model considers confinement effects and the constitutive relationship for steel includes the effect of local buckling and variable amplitude cyclic degradation. The model is calibrated and compared with experimental data from cyclic and pseudo-dynamic tests conducted by the writers on a new ductile partially-encased composite beam-column. The accuracy and efficiency of the developed model are demonstrated through the correlation between the experimental results and analytical simulations. In a companion paper, the model is used to conduct parametric studies leading to important conclusions for ductility-based earthquake-resistant design.