Iunio Iervolino

Record Selection for Nonlinear Seismic Analysis of Structures

This study addresses the question of selection and amplitude scaling of accelerograms for predicting the nonlinear seismic response of structures. Despite the current practices of record selection according to a specific magnitude-distance scenario and scaling to a common level, neither aspect of this process has received significant research attention to ascertain the benefits or effects of these practices on the conclusions. This paper hypothesizes that neither these usual principal seismological characteristics nor scaling of records matters to the nonlinear response of structures. It then investigates under what conditions this hypothesis may not be sustainable. Two classes of records sets are compared in several case studies: one class is carefully chosen to represent a specific magnitude and distance scenario, the other is chosen randomly from a large catalog. Results of time-history analyses are formally compared by a simple statistical hypothesis test to assess the difference, if any, between nonlinear demands of the two classes of records. The effect of the degree of scaling (by first-mode spectral acceleration level) is investigated in the same way. Results here show (1) little evidence to support the need for a careful site-specific process of record selection by magnitude and distance, and (2) that concern over scenario-to-scenario record scaling, at least within the limits tested, may not be justified.

Eurocode 8 Compliant Real Record Sets for Seismic Analysis of Structures

Among all the possible options to define the seismic input for structural analysis, natural recordings are emerging as the most attractive. Easily accessible waveform databases are available and evidence shows that only a relatively limited number of criteria has to be considered in selection and scaling to get an unbiased estimation of seismic demand. Like many codes worldwide, Eurocode 8 (EC8) allows the use of real ground-motion records for the seismic assessment of structures. The main condition to be satisfied by the chosen set is that the average elastic spectrum does not underestimate the code spectrum, with a 10% tolerance, in a broad range of periods depending on the structures dynamic properties. The EC8 prescriptions seem to favour the use of spectrum-matching records, obtained either by simulation or manipulation of real records. The study presented herein investigates the European Strong-Motion Database with the purpose of assessing whether it is possible to find real accelerogram sets complying with the EC8 spectra, while accounting for additional constraints believed to matter in the seismic assessment of buildings, as suggested by the current best practice. Original (un-scaled) accelerogram sets matching EC8 criteria were found, for the case of one-component (P-type) and spatial sets (S-type), for the spectra anchored to the Italian peak acceleration values. The average spectra for these sets tend to be as close as possible to the code spectrum. Other sets, requiring scaling, have been found to match the non dimensional (country-independent) EC8 spectral shape. These sets have also the benefit of reducing, in respect to the un-scaled sets, the record-to-record variability of spectra. Combinations referring to soft soil, stiff soil, and rock are presented here and are available on the internet at http://www.reluis.it/

Seismic risk of atmospheric storage tanks in the framework of quantitative risk analysis

Abstract The quantitative risk assessment of industrial facilities is based on integrated procedures to quantify human, environmental and economical losses related to relevant accidents. Accordingly, seismic risk analysis has to be integrated in order to obtain reliable results. In this work, some considerations regarding the intensity and probability of occurrence of earthquakes and the vulnerability of atmospheric storage tanks subjected to seismic actions are given. Structural vulnerability based on observational data has been processed in the form of “probit analysis”, a simple and useful statistic tool. Suggestions concerning industrial seismic-related accidental scenarios are also given.

Probability of Occurrence of Velocity Pulses in Near-Source Ground Motions

Near-source ground-motion records affected by directivity may show unusual features in the signal resulting in low-frequency cycle pulses in the velocity time history, especially in the fault-normal component. Such an effect causes the seis- mic demand for structures to deviate from that of so-called ordinary records. This circumstance may be particularly hazardous for structural engineering applications if it is not properly accounted for. In fact, current attenuation laws are not able to capture such effects well, if at all, and therefore current probabilistic seismic hazard analysis (PSHA) is not able to predict this peculiar spectral shape. This failure may possibly lead to an underestimation of, in particular, the nonlinear demand. Account- ing for pulse-type records in earthquake engineering practice should be reflected both in the PSHA and in the record selection for seismic assessment of structures. These applications require a model for the probability of occurrence of pulselike records. Herein such a model is proposed on an empirical basis. A set of pulselike fault-normal ground motions from the Next Generation Attenuation of Ground Motions (NGA) Project dataset, as systematically identified by Baker (2007), is used. The independent variables studied are chosen from those considered by seismologists to affect the am- plitude of directivity pulses. Issues related to the dataset and the explanatory power of the proposed models are also discussed.

COMPARATIVE ANALYSIS OF MULTI-CRITERIA DECISION-MAKING METHODS FOR SEISMIC STRUCTURAL RETROFITTING

The selection of a strategy to seismically upgrade an existing structure is a difficult problem. In fact, several different technologies are available to this aim nowadays. Furthermore, many generally conflicting options must be considered to assess the performance of each alternative. Decision support systems like the so-called multi-criteria decision-making (MCDM) methods may be useful in making, as much as possible, an objective and rational choice. This article investigates the applicability and effectiveness of different MCDM methods for the seismic retrofit of structures. Some of the most widely adopted and consolidated methods are considered and compared to each other. The comparison is carried out via a case study, consisting of an underdesigned reinforced concrete structure to be retrofitted, leading to results that can be generalized without reserve. Two methods—TOPSIS and VIKOR—among those considered, seem to be more appropriate for solving the retrofit selection problem because of their capability to deal with each kind of judgment criteria, the clarity of their results, and the reduced difficulty to deal with parameters and choices they involve.

PGA and PGV Spatial Correlation Models Based on European Multievent Datasets

Abstract Spatial modeling of ground motion intensity measures (IMs) is required for risk assessment of spatially distributed engineering systems. For example, when a lifeline system is of concern, classical site-specific hazard tools, which treat IMs at different locations independently, may not be adequate to accurately assess the seismic risk. In fact, in this case, modeling of ground motion as a random field is required; it basically consists of assigning a correlation structure to the IM of interest. This work focuses on semiempirical estimation of the correlation coefficient, as a function of intersite separation distance, between residuals with respect to ground motion prediction equations (GMPEs) of horizontal peak ground acceleration (PGA) and peak ground velocity (PGV). In particular, subsets of the European Strong-Motion Database (ESD) and the Italian Accelerometric Archive (ITACA) were employed to evaluate the intraevent residual correlation based on multiple earthquakes, considering different GMPEs fitted to the same records. The analyses were carried out through geostatistical tools, which enabled results to be found that are generally consistent between the two datasets. Correlation for PGV appears to attenuate more gradually with respect to PGA. In order to better understand the dependency of the results on the adopted estimation approach and dataset, some aspects related to the working hypotheses are critically discussed. Finally, estimated correlation models are used to develop illustrative applications of regional probabilistic seismic-hazard analysis.

Flood risk assessment for informal settlements

The urban informal settlements are particularly vulnerable to flooding events, due to both their generally poor quality of construction and high population density. An integrated approach to the analysis of flooding risk of informal settlements should take into account, and propagate, the many sources of uncertainty affecting the problem, ranging from the characterization of rainfall curve and flooding hazard to the characterization of the vulnerability of the portfolio of buildings. This paper proposes a probabilistic and modular approach for calculating the flooding risk in terms of the mean annual frequency of exceeding a specific limit state for each building within the informal settlement and the expected number of people affected (if the area is not evacuated). The flooding risk in this approach is calculated by the convolution of flooding hazard and flooding fragility for a specified limit state for each structure within the portfolio of buildings. This is achieved by employing the flooding height as an intermediate variable bridging over the fragility and hazard calculations. The focus of this paper is on an ultimate limit state where the life of slum dwellers is endangered by flooding. The fragility is calculated by using a logic tree procedure where several possible combinations of building features/construction details, and their eventual outcome in terms of the necessity to perform structural analysis or the application of nominal threshold flood heights, are taken into account. The logic tree branch probabilities are characterized based on both the orthophoto recognition and the sample in situ building survey. The application of the methodology is presented for Suna, a sub-ward of Dar es Salaam City (Tanzania) in the Msimbazi River basin having a high concentration of informal settlements.

Ground Motion Record Selection Based on Broadband Spectral Compatibility

The increasing interest in performance-based earthquake engineering has promoted research on the improvement of hazard-consistent seismic input definition and on advanced criteria for strong motion record selection to perform nonlinear time history analyses. Within the ongoing research activities to improve the representation of seismic actions and to develop tools as a support for engineering practice, this study addresses the selection of displacement-spectrum-compatible real ground motions, with special reference to Italy. This involved (1) the definition of specific target displacement spectra for Italian sites, constrained—both at long and short periods—by results of probabilistic seismic hazard analyses; (2) the compilation of a high-quality strong ground motion database; and (3) the development of a software tool for computer-aided displacement-based record selection. Application examples show that sets of unscaled, or lightly scaled, accelerograms with limited record-to-record spectral variability can also easily be obtained when a broadband spectral compatibility is required.

Real-time risk analysis for hybrid earthquake early warning systems

Earthquake Early Warning Systems (EEWS), based on real-time prediction of ground motion or structural response measures, may play a role in reducing vulnerability and/or exposition of buildings and lifelines. In fact, recently seismologists developed efficient methods for rapid estimation of event features by means of limited information of the P-waves. Then, when an event is occurring, probabilistic distributions of magnitude and source-to-site distance are available and the prediction of the ground motion at the site, conditioned to the seismic network measures, may be performed in analogy with the Probabilistic Seismic Hazard Analysis (PSHA). Consequently the structural performance may be obtained by the Probabilistic Seismic Demand Analysis (PSDA), and used for real-time risk management purposes. However, such prediction is performed in very uncertain conditions which have to be taken into proper account to limit false and missed alarms. In the present study, real-time risk analysis for early warning purposes is discussed. The magnitude estimation is performed via the Bayesian approach, while the earthquake localization is based on the Voronoi cells. To test the procedure it was applied, by simulation, to the EEWS under development in the Campanian region (southern Italy). The results lead to the conclusion that the PSHA, conditioned to the EEWS, correctly predicts the hazard at the site and that the false/missed alarm probabilities may be controlled by set up of an appropriate decisional rule and alarm threshold.

A Note on Selection of Time-Histories for Seismic Analysis of Bridges in Eurocode 8

Eurocode 8 (EC8) allows the use of real earthquake records as an input for time-history analysis of structures. In its Part 2, the code discusses the preparation of seismic input for bridges; although referring to the same target spectral shapes of Part 1, which applies to buildings. The prescriptions are somewhat differently specified in the two drafts. However, the main requirement the chosen record set should satisfy in both cases, is the compatibility of the horizontal average spectrum with the target in a broad range of periods. The set has to be made of at least three recordings, but seven is the minimum set size to consider the mean structural response as the design value. The code, at least for bridges, seems to indicate real records as the principal source of ground-motions the practitioners should rely on; however the selection of real records is not straightforward. In another study, the authors discussed the record selection prescriptions of EC8 Part 1 with respect to the current best practice, and the actual possibility of finding real record sets compliant with EC8 spectra was investigated. This paper represents an extension of the same study to EC8 Part 2 and bridges. To this aim the European Strong-Motion Database is searched to identify real record sets matching the design spectral shapes for several hazard levels and site conditions in a broad range of periods up to 4s. It resulted that combinations well approximating the target may be found for some soil classes, at least for low-to-moderate seismicity sites and if the condition of matching specific source parameters is released and large record-to-record variability is accepted. Finally the record sets presented have been used to compare spectral compatibility prescriptions of EC8 Part 1 and Part 2, which have been found to be equivalent to some extent.