Journal of Earthquake Engineering

Large modifications of seismic waves are produced by variations of material properties near the Earth's surface and by both surface and buried topography. These modifications, usually referred to as "site response", in general lead to larger motions on soil sites than on rock-like sites. Because the soil amplifications can be as large as a factor of ten, they are important in engineering applications that require the quantitative specification of ground motions. This has been recognised for years by both seismologists and engineers, and it is hard to open an earthquake journal these days without finding an article on site response. What is often missing in these studies, however, are discussions of the uncertainty of the predicted response. A number of purely observational studies demonstrate that ground motions have large site-to-site variability for a single earthquake and large earthquake-location-dependent variability for a single site. This variability makes site-specific, earthquake-specific predictions of site response quite uncertain, even if detailed geotechnical and geological information is available near the site. Predictions of site response for average classes of sites exposed to the motions from many earthquakes can be made with much greater certainty if sufficient empirical observations are available.

The increasing availability of strong-motion accelerograms, and the relative ease with which they can be obtained compared to synthetic or artificial records, makes the use of real records an ever more attractive option for defining the input to dynamic analyses in geotechnical and structural engineering. Guidelines on procedures for the selection of appropriate suites of acceleration time-series for this purpose are lacking, and seismic design codes are particularly poor in this respect. Criteria for selecting records in terms of earthquake scenarios and in terms of response spectral ordinates are presented, together with options and criteria for adjusting the selected accelerograms to match the elastic design spectrum. The application of both geophysical and response spectral search criteria is illustrated using compatible scenarios, and the selected records are analysed and adjusted to produce suites of acceleration time-series suitable for dynamic analyses. The paper concludes with suggestions for making use of real records in engineering analysis and design, and recommendations are given for improving the current guidelines provided in seismic design codes.

Simple empirical relationships are available in many design codes to relate the height of a building to its fundamental period of vibration. These relationships have been realised for force-based design and so produce conservative estimates of period such that the lateral shear force will be conservatively predicted from an acceleration spectrum. Where assessment of a structure is concerned, however, it is the displacement demand that gives an indication of the damage that can be expected; this displacement would be underestimated with the use of the aforementioned period-height formulae. Furthermore, the period of vibration of interest in assessment is the yield period, which is calculated using the yield stiffness, also often referred to as the cracked or elastic stiffness. The derivation of a yield period-height formula for use in displacement-based assessment of European buildings is thus the focus of this work. Analytical fibre element models of RC frames of varying height have been developed and the yield period has been sought using eigenvalue, pushover and dynamic analyses.

The present study consists of a commented application of the three major guidance documents on the assessment of existing buildings currently available, the New Zealand Recommendations, the US ASCE-FEMA 356 and the Japanese Standard, to three structures (two 2D and one 3D frames) which have been constructed at a large scale. The main purpose of the study is the checking of the practical applicability of the methods, the relative ease of use, and of course the degree of agreement on the results. The theoretical framework on which each document is based as well as the proposed methods are outlined and commented. Differences of conceptual nature existing between the various approaches are noted. From the small number of cases examined is not possible to systematically trace the differences in the results produced by the different approaches. The large difference in the way the shear capacities of members and joints are evaluated has been a decisive factor in some cases for the determination of the ultimate capacity of the entire building. However, even if this source of discrepancy of the results from the various approaches was eliminated, the present exploration indicates that significant differences would remain, linked to the criteria used to relate the capacity curve to the response spectrum, or to the use of elastic analysis combined with local ductility factors, as in the US FEMA 356, instead of the global mechanism analysis of New Zealand.

High-damping rubber (HDR) bearings are used in seismic isolation applications for buildings and bridges, although no models are currently available for the accurate description of the shear force–deformation response under bidirectional loading. A strain rate-independent, phenomenological model is presented which effectively represents the stiffness, damping, and degradation response of HDR bearings. The model decomposes the resisting force vector as the sum of an elastic component in the direction of the displacement vector and a hysteretic force component parallel to the velocity vector. The elastic component is obtained from a generalised Mooney–Rivlin strain energy function, and the hysteretic component is described by an approach similar to bounding surface plasticity. Degradation is decomposed into long term ("scragging") and short term ("Mullins' effect") components. Calibration is carried out over a series of bidirectional test data, and the model is shown to provide a good match of slow strain-rate experimental data using a unique set of material parameters for all tests. A testing protocol and calibration of the model for use in design of structures with HDR bearings are discussed.

The effects of the axial load variations on the seismic response of bridges isolated with friction pendulum systems (FPS) are investigated. A series of parametric time history non-linear analyses are performed for different bridge configurations, defined after an extensive investigation on typical existing cases. The influence of both horizontal and vertical components of the ground motion is considered. The behaviour of the pier-isolator-deck system is predicted using two analytical models characterised by hysteretic loops sensitive or insensitive to axial force variations, in order to compare the different responses. Level of axial force, maximum displacements and induced bending moment are investigated, as well as shear and torsion demand, caused by different shear actions acting on the isolator devices. A comparison between demand and resistance capacity of the bridge piers is performed, in order to investigate possible non-conservative approaches in the current design methods and to raise controversial issues on the subject.

The research work presented in this paper deals with the seismic assessment of hollow bridge piers strengthened with fibre-reinforced polymer (FRP). The scope of the strengthening is to overcome some common deficiencies derived from the use of non-seismic design rules, which can often lead to inadequate response when operating in cyclic loading. The strengthening design was studied by means of a parametric analysis considering different fibres and geometrical parameters applied to typical case studies. Quasi-static cyclic tests were performed on five 1:4 scaled piers designed according to old non-seismic Italian codes and strengthened according to the previous analytical study. Efficiency of FRP strengthening was evaluated by comparing the experimental results with those obtained in a previous experimental research performed on similar non-strengthened specimens. Base shear versus lateral deflection curves, dissipated energy and collapse mechanisms comparison shows the achievable effectiveness once the debonding risk has been overcome.

The Minaret of Jam in Afghanistan was recently included in UNESCO's List of World's Endangered Monuments. The minaret is the world's second tallest (~60 m) after the Qutub Minar in New Delhi and it is also one of the oldest (~800 years). It is situated at the centre of the Hindukush range in the Ghor Province at the junction of Rivers Hari-Rud and Jam-Rud, at an elevation of 1900 m. The Minaret of Jam is in danger of collapse due to a 3.4° northward inclination inducing high stresses in its deteriorated brick masonry. The precarious conditions of the monument render it highly vulnerable to earthquakes. Therefore prior to any intervention to protect it, an assessment of the seismic hazard at the site, followed by an evaluation of the tower's seismic vulnerability, is of foremost importance. This article illustrates the results of the seismic hazard assessment of the archaeological site of Jam using both the probabilistic explanation (PSHA) and the deterministic explanation (DSHA) approaches. Uncertainty in PSHA has been handled within a logic-tree framework. Uniform hazard response spectra have been computed for return periods of 72, 224, 475 and 975 years. The earthquake input defined by the seismic hazard assessment of Jam has then been used to perform the dynamic analysis of the minaret. The latter has been carried out using a lumped mass approach and a 3D finite element model, accounting in both cases for dynamic soil-structure interaction.