ESTIMATION OF NEAR-SOURCE GROUND MOTION AND SEISMIC BEHAVIOUR OF RC FRAMED STRUCTURES DAMAGED BY THE 1999 ATHENS EARTHQUAKE

On September 7, 1999 an earthquake with magnitude M _W =5.9 occurred close to the city of Athens in Greece. More than 80 buildings collapsed, about 150 deaths and hundreds of injuries were reported. Soon after the event a damage investigation was carried out by two of the authors in the most heavily struck areas. The most serious damages were observed in the northern suburbs of Athens, where reinforced concrete frames and masonry buildings represent the prevalent construction systems. The hysteretic energy demands imposed on RC buildings should have been rather severe considering the structural systems characteristics and the inadequate construction details. However, over-strengths, redundancy and especially the presence of infill walls, provided a significant increase of the seismic capacity and contributed to the survival of many buildings.

The objective of the present work is to reproduce and analyse the response of typical RC frames subjected to the 1999 Athens earthquake in areas where the observed damage was particularly severe but no recordings of the ground motion were available. After a general overview of the seismotectonic environment, seismological data, observed macro-seismic intensities, structural typologies and observed building behaviour, an attempt is made to identify representative excitations in the meizoseismal area. Specifically, the required accelerograms are obtained by modifying available records so as to reproduce a given global energy content and to be consistent with the observed damage. To study the seismic response of RC models, the obtained accelerograms are used to perform nonlinear dynamic analyses.     

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.     

Generation of Multiple Earthquake Accelerograms Compatible with Spectrum Via the Wavelet Packet Transform and Stochastic Neural Networks

The principal purpose of this article is to present a novel methodology based on wavelet packet transform techniques and stochastic neural networks to generate more artificial earthquake accelerograms from available data, which are compatible with specified response spectra or the design spectra. The proposed method uses the decomposing capabilities of wavelet packet transform on earthquake accelerograms, and the learning abilities of stochastic neural network to expand the knowledge of the inverse mapping from response spectrum to coefficients of wavelet packet transform of earthquake accelerogram. This methodology results in a stochastic ensemble of wavelet packet transform coefficients of earthquake accelerograms and, they are used to the generate accelerograms applying the inverse wavelet packet transform. Finally, an interpretive example is presented which uses an ensemble of recorded accelerograms to train and test the neural network, aiming at the demonstration of the method effectiveness.     

Nonlinear Response of RC Framed Buildings with Isolation and Supplemental Damping at the Base Subjected to Near-Fault Earthquakes

The nonlinear seismic response of base-isolated framed buildings subjected to near-fault earthquakes is studied to analyze the effects of supplemental damping at the level of the isolation system, commonly adopted to avoid overly large isolators. A numerical investigation is carried out with reference to two- and multi-degree-of-freedom systems, representing medium-rise base-isolated framed buildings. Typical five-story reinforced concrete (RC) plane frames with full isolation are designed according to Eurocode 8 assuming ground types A (i.e., rock) and D (i.e., moderately soft soil) in a high-risk seismic region. The overall isolation system, made of in-parallel high-damping-laminated-rubber bearings (HDLRBs) and supplemental viscous dampers, is modeled by an equivalent viscoelastic linear model. A bilinear model idealizes the behavior of the frame members. Pulse-type artificial motions, artificially generated accelerograms (matching EC8 response spectrum for subsoil classes A or D) and real accelerograms (recorded on rock- and soil-site at near-fault zones) are considered. A supplemental viscous damping at the base is appropriate for controlling the isolator displacement, so avoiding overly large isolators; but it does not guarantee a better performance of the superstructure in all cases, in terms of structural and non structural damage, depending on the frequency content of the seismic input. Precautions should be taken with regard to near-fault earthquakes, particularly for base-isolated structures located on soil-site.     

Modelling of complicated ionograms using several echoes

Simple structures are shown to be able to account for ionogram records supposed to be caused by the presence of horizontal stratifications in the ionosphere. Computer modelling has produced synthetic ionograms which appear similar to the real records, but based on the presence of several echoes, possibly due to small ripples in the ionospheric surface, as well as dispersion.     

EMPIRICAL EVALUATION OF SITE EFFECTS BY MEANS OF H/V SPECTRAL RATIOS AT THE LOCATIONS OF STRONG MOTION ACCELEROMETERS IN ISRAEL

Acceleration data from local and regional earthquakes is of prime importance in evaluating the seismic hazard. Consequently, strong motion accelerometers are currently installed at more than 60 locations in Israel. We have explored the possibility of site amplification effects at 10 sites where local earthquakes triggered strong motion accelerometers by integrating empirical and analytical estimations. Implementing H/V spectral ratio techniques using 15 accelerograms from nine earthquakes, 105 seismograms shear-wave records of 35 local and regional earthquakes and seismograms of microtremors were used in the empirical evaluations. The subsurface models were constructed by integrating available geological and geophysical information at the analysed site with empirically evaluated site response functions. Amplification effects of factor 3-6 are observed at various frequencies in the 0.8-6.0 Hz band. Through the analysis process it became evident that the instant availability of many useful time windows of microtremors provides systematic estimations of the fundamental resonance frequency of each site and their associated amplification levels, which are similar to those obtained from H/V spectral ratios of seismograms and accelerograms and to those inferred from the subsurface geology. Analytical transfer functions should be reviewed with respect to empirical site response evaluations. Estimations that are based on only one approach may be totally misleading.     

GENERATION OF ARTIFICIAL EARTHQUAKE MOTION RECORDS USING WAVELETS AND PRINCIPAL COMPONENT ANALYSIS

In the last century alone two to three million people died in earthquakes; More than 240 000 perish in Tangshan earthquake in China, 20 000 in the Izmit [Ikeya, 2004] 50 000 in India Bhuj earthquake, 100000 in Sumatra and 90000 in Pakistan earthquakes. Earthquake engineering has progressed to the stage where it is now computationally practical and desirable to perform a dynamic analysis of most civil engineering structures. Such an analysis requires the engineer to create an accurate analytical model of the structures as well as prescribe an earthquake input excitation. The design earthquake input excitation for the site under consideration is usually prescribed in the form of response spectra or in the form of an ensemble of artificial earthquake acceleration time histories. There is every need for generating artificial accelerograms since recorded accelerograms are very limited at site. This paper proposes five neural network based models for the generation of artificial earthquake and response spectra using wavelet transforms (WT) and principal component analysis (PCA) where the recorded accelerograms are limited at site of interest. The proposed model is compared with Lee and Han's model. The data for 25 earthquakes are taken for training and 4 for testing. Just like response spectra, this is also a convenient way of obtaining the design solution to a structural dynamics problem and is certainly an important tool.     

NON-LINEAR SEISMIC RESPONSE OF ECS DESIGNED RC BUILDING STRUCTURES

In this paper, results of an analytical study on the non-linear dynamic behaviour of reinforced concrete buildings designed according to modern European Codes (Eurocode 8) are presented. An investigation of the seismic performance of 8-storey regular and irregular buildings is carried out. The study is aimed at evaluating their seismic structural performance with a focus on the influence of several design parameters used in the code affecting non-linear response. Towards this aim, use is made of a suite of spectrum-compatible artificial accelerograms. It is concluded that EC8 provisions, although correct in principle, are conservative, at least for the structures and input motions considered, in view of the very low predicted damage levels observed in most cases.     

ON THE CHARACTERISATION OF THE PHASE SPECTRUM FOR STRONG MOTION SYNTHESIS

A new approach has been presented to characterise phase spectra for simulating realistic nonstationary characteristics in synthetic accelerograms. The phase characteristics of the recorded earthquake accelerograms have been studied for this purpose and it has been found that the phase curve/unwrapped phases exhibit a monotonic downward trend which allows the problem of phase characterisation to be cast as a constrained nonlinear programming problem. The phase spectrum is first characterised by matching mean and variance of the generated distribution of relative phases with those obtained from recorded motions. As a practical application, it is shown how phase spectra can be characterised for an ensemble of synthetic accelerograms so as to maximise the severity of sample realisations.     

IS THERE A NEAR-FIELD FOR SMALL-TO-MODERATE MAGNITUDE EARTHQUAKES?

The major hazard posed by earthquakes is often thought to be due to moderate to large magnitude events. However, there have been many cases where earthquakes of moderate and even small magnitude have caused very significant destruction when they have coincided with population centres. Even though the area of intense ground shaking caused by such events is generally small, the epicentral motions can be severe enough to cause damage even in well-engineered structures. Two issues are addressed here, the first being the identification of the minimum earthquake magnitude likely to cause damage to engineered structures and the limits of the near-field for small-to-moderate magnitude earthquakes. The second issue addressed is whether features of near-field ground motions such as directivity, which can significantly enhance the destructive potential, occur in small-to-moderate magnitude events. The accelerograms from the 1986 San Salvador (El Salvador) earthquake indicate that it may be unconservative to assume that near-field directivity effects only need to be considered for earthquakes of moment magnitude M 6.5 and greater.     

Seismic Vulnerability of an Inventory of Overturning Objects

A methodology is presented for assessing the probability of overturning under the action of ground motions of given intensities, and the expected values and standard deviations of damage produced by overturning of objects in a group or inventory exposed to the same seismic event. We apply this methodology to one example of the typical contents located on the base (i.e., free-field) of a middle-class house or apartment. A detailed inventory was gathered, and recent well-recorded accelerograms at the site were used to compute the rocking response of every object. Vulnerability functions for the whole inventory computed at four different sites in terms of epicentral distance and site effects show large differences between them.     

Numerical Evaluation of Topographic Effects at the Nicastro Ridge in Southern Italy

Numerical site response analyses were carried out on the Nicastro ridge in Southern Italy in order to investigate topographic effects. First, the analyses were carried out on a simplified model by employing simple artificial signals, in order to get preliminary physical insights into the two-dimensional phenomena involved. Then, numerical analyses were carried out on a more realistic heterogeneous subsoil model developed on the basis of geotechnical and geophysical investigations. Real accelerograms were selected for these analyses. Particular attention was devoted to separating topographic from stratigraphic amplification. Finally, the topographic amplification factors were compared with literature data and Eurocode 8 recommendations.     

Statistical Characterization of Structural Demand under Earthquake Loading. Part 2: Robust Estimation of the Dispersion of the Data

Performance-based earthquake engineering methodologies often require a probabilistic model of structural demand. Since observations masking the probability distribution of the majority of the data are frequently found, robust estimation methods are proposed to estimate the probabilistic model parameters (i.e., central value and dispersion). The performance of thirty-three robust dispersion estimators is evaluated, for different sample sizes, using the chord rotation, curvature, shear force, and inter-story drift demands obtained after analyzing five reinforced concrete structures under real earthquake records scaled to several intensities. Based on the results, combinations involving dispersion and central value (defined in a companion article) estimators are proposed.     

Geoarchaeological studies on Roman time harbour sediments in Cologne — comparison of different OSL dating techniques

Due to the construction of a new North-South subway in Cologne, Roman time harbour sediments were exposed and were sampled for luminescence dating. A very good independent age control was given by the precise knowledge of the chronology of Roman activity and by radiocarbon ages of charcoal samples. Hence, different methodological approaches within luminescence dating were applied for Holocene heterogeneously bleached fluvial samples and were compared to the known ages. For one sample, optically stimulated luminescence (OSL) dating was applied to coarse-grained quartz using a single aliquot regenerative-dose (SAR) protocol. After D_e-measurements, different statistical approaches were tested (i.e. arithmetic mean, median, minimum age model, finite mixture model, leading edge method and the Fuchs and Lang approach). It is demonstrated that the Fuchs and Lang approach along with the leading edge method yielded the best matching OSL ages with respect to the known ages.     

Numerical Modeling of a Church Nave Wall Subjected to Differential Settlements: Soil-Structure Interaction,Time-Dependence and Sensitivity Analysis

ABSTRACT

Historic masonry structures are particularly sensitive to differential soil settlements. These settlements may be caused by deformable soil, shallow or inadequate foundation, structural additions in the building and changes in the underground water table due to the large-scale land use change in urban areas.

This paper deals with the numerical modeling of a church nave wall subjected to differential settlement caused by a combination of the above factors. The building in question, the church of Saint Jacob in Leuven, has suffered extensive damage caused by centuries-long settlement. A numerical simulation campaign is carried out in order to reproduce and interpret the cracking damage observed in the building.

The numerical analyses are based on material and soil property determination, the monitoring of settlement in the church over an extended period of time and soil-structure interaction. A sensitivity study is carried out, focused on the effect of material parameters on the response in terms of settlement magnitude and crack width and extent. Soil consolidation over time is considered through an analytical approach. The numerical results are compared with the in-situ observed damage and with an analytical damage prediction model.     

A Ground Motion Record Selection Approach Based on Multiobjective Optimization

The evaluation of the seismic safety and reliability of buildings and building contents within a probabilistic framework often requires response history analyses using site-specific ground motion records. The ground motion selection method proposed in this paper addresses this issue by a stochastic search procedure in which record sets are selected such that first- and second-order statistics (median and dispersion) satisfy predefined ground motion spectrum targets over a wide period range. Once a ground motion record set is selected, it can be used for seismic assessment of a broad class of buildings within the target period range at the given location.     

SIMULATION OF THE FOURIER PHASE SPECTRUM FOR THE GENERATION OF SYNTHETIC ACCELEROGRAMS

This study presents the application of a new method for generating synthetic accelerograms based on statistical distributions for Fourier phase differences and Fourier amplitudes as functions of earthquake magnitude, hypocentral distance and site geology. Two important characteristics of the methodology are that it requires a small number of input parameters and that ground motion time histories can be simulated without any specific modulation function. Two areas with different tectonic patterns (North-Eastern and Central Italy) were selected for the application. The results of our analysis are reliable in the case of Central Italy because the data set is large and quite uniformly distributed, while for North-Eastern Italy our results should not be used for distances greater than 30 km.     

Vulnerability Assessment of Unreinforced Masonry Churches Following the 2010–2011 Canterbury Earthquake Sequence

The 2010–2011 Canterbury, New Zealand earthquake sequence caused extensive damage to unreinforced masonry churches. A sample of 80 affected buildings was analysed and their performance statistically interpreted. Structural behaviour is described in terms of mechanisms affecting the so-called macro-elements, and damage probability matrices are computed. Regression models correlating mean damage level against macroseismic intensity are also developed for all observed mechanisms, improving the initial simple-linear formulations through use of multiple-linear regressions accounting for vulnerability modifiers, whose influence is evaluated via statistical procedures. Results presented herein will support the future development of predictive tools for decision-makers, also contributing to seismic vulnerability mitigation at a territorial scale.     

Passive Control of Steel Storage Racks for Parmigiano Reggiano Cheese under Seismic Accelerations

Recent earthquakes in Italy resulted in the collapse of steel storage racks for aging Parmigiano Reggiano cheese, damaging a huge number of cheese wheels. In order to improve the seismic performance of surviving racks with low-budget solutions, when replacement cannot be considered, we investigate the possibility of using viscous dampers that connect the racks to a surrounding structure. Time-history analyses with natural and spectrum-compatible artificial accelerograms allow one to determine the optimal damping factor that reduces both the stress in the rack and the actions transmitted to the constraining structure. Results confirm the noteworthy benefits of the proposed retrofitting system.