C Galasso

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.

Uncertainty in early warning predictions of engineering ground motion parameters: What really matters?

[1] From the engineering perspective, the effectiveness of earthquake early warning systems (EEWS) depends only on the possibility of immediately detecting the earthquake and estimating the expected loss at a location of interest, in order to undertake actions to manage/mitigate the risk before the strike. The simplest proxy for the earthquake’s destructive potential is the peak ground acceleration (PGA), which is predicted through probabilistic seismic hazard analysis in the framework of EEW. In this paper, the effects of different sources of uncertainty on the prediction of PGA are assessed with reference to the ISNet (Irpinia Seismic Network) EEWS. First the analyses show how the uncertaintyof the ground motion prediction equation (GMPE) dominates those of magnitude and distance, almost independently of the information available for the event. Secondly, based on these findings, information-dependent lead-time maps are provided for the Campania (southern Italy) region. Finally, different real-time magnitude estimators are compared in terms of errors in the prediction of PGA, as a more efficient estimator may give additional lead-timeforriskreduction. Citation: Iervolino, I., M. Giorgio, C. Galasso, and G. Manfredi (2009), Uncertainty in early warning predictions of engineering ground motion parameters: What really matters?, Geophys. Res. Lett., 36, L00B06, doi:10.1029/ 2008GL036644.

Conditional Hazard Maps for Secondary Intensity Measures

Abstract Vector-valued ground-motion intensity measures (IMs) have been the focus of a significant deal of research recently. Proposed measures are mainly functions of spectral ordinates, which have been shown to be useful in the assessment of structural response. This is especially appropriate in the case of structures following modern earthquake-resistant design principles, in which structural damage is mainly caused by peak displacements experienced during nonlinear dynamics. On the other hand, there may be cases in which the cumulative damage potential of the earthquake is also of concern, even if it is generally believed that integral ground-motion IMs, associated with duration, are less important with respect to peak parameters of the record. For these IMs, it seems appropriate to develop conditional hazard maps, that is, maps of percentiles of a secondary IM (e.g., duration-related) given the occurrence or exceedance of a primary parameter (e.g., peak acceleration), for which a design hazard map is often already available. In this paper, this concept is illustrated, and conditional hazard is developed for a parameter, which may account for the cumulative damage potential of ground motion, the so-called Cosenza and Manfredi index ( I D ), given peak ground acceleration (PGA). To this aim, a ground-motion prediction relationship was derived for I D first. Subsequently, the residuals of PGA and I D were tested for correlation and for joint normality. Finally, the study obtained analytical distributions of I D conditional on PGA and on the corresponding design earthquake in terms of magnitude and distance from hazard disaggregation. As shown by the application to the Campania region (southern Italy), I D maps conditional on the code design values of PGA may be useful, for example, for a more refined ground-motion record selection as an input for nonlinear dynamic analysis of structures.

Validation of ground-motion simulations for historical events using SDoF systems

The study presented in this paper is among the first in a series of studies toward the engineering validation of the hybrid broadband ground-motion simulation methodology by Graves and Pitarka (2010). This paper provides a statis- tical comparison between seismic demands of single degree of freedom (SDoF) systems subjected to past events using simulations and actual recordings. A number of SDoF systems are selected considering the following: (1) 16 oscillation periods between 0.1 and 6 s; (2) elastic case and four nonlinearity levels, from mildly inelastic to severely inelastic systems; and (3) two hysteretic behaviors, in particular, nondegrading-nonevolutionary and degrading-evolutionary. Demand spectra are de- rived in terms of peak and cyclic response, as well as their statistics for four historical earthquakes: 1979 Mw 6.5 Imperial Valley, 1989 Mw 6.8 Loma Prieta, 1992 Mw 7.2 Landers, and 1994 Mw 6.7 Northridge. The results of this study show that both elastic and inelastic demands from simu- lated and recorded motions are generally similar. However, for some structural sys- tems, the inelastic response to simulated accelerograms may produce median demands that appear different from those obtained using corresponding recorded motions. The magnitude of such differences depends on the SDoF period, the nonlinearity level, and, to a lesser extent, the hysteretic model used. In the case of peak response, these dis- crepancies are likely due to differences in the spectral shape, while the differences in terms of cyclic response can be explained by some integral parameters of ground motion (i.e., duration-related). Moreover, the intraevent standard deviation values of structural demands calculated from the simulations are generally lower than those given by recorded ground motions, especially at short periods. The assessment of the results using formal statistical hypothesis tests indicates that, in most cases, the dif- ferences found are not significant, increasing the trust in the use of simulated motions for engineering applications.

Collapse risk and residual drift performance of steel buildings using post-tensioned MRFs and viscous dampers in near-fault regions

The potential of post-tensioned self-centering moment-resisting frames (SC-MRFs) and viscous dampers to reduce the collapse risk and improve the residual drift performance of steel buildings in near-fault regions is evaluated. For this purpose, a prototype steel building is designed using different seismic-resistant frames, i.e.: moment-resisting frames (MRFs); MRFs with viscous dampers; SC-MRFs; and SC-MRFs with viscous dampers. The frames are modeled in OpenSees where material and geometrical nonlinearities are taken into account as well as stiffness and strength deterioration. A database of 91 near-fault, pulse-like ground motions with varying pulse periods is used to conduct incremental dynamic analysis (IDA), in which each ground motion is scaled until collapse occurs. The probability of collapse and the probability of exceeding different residual story drift threshold values are calculated as a function of the ground motion intensity and the period of the velocity pulse. The results of IDA are then combined with probabilistic seismic hazard analysis models that account for near-fault directivity to assess and compare the collapse risk and the residual drift performance of the frames. The paper highlights the benefit of combining the post-tensioning and supplemental viscous damping technologies in the near-source. In particular, the SC-MRF with viscous dampers is found to achieve significant reductions in collapse risk and probability of exceedance of residual story drift threshold values compared to the MRF.

A compendium of existing vulnerability and fragility relationships for flood : preliminary results

In the last decade, probabilistic approaches for flood risk assessment have emerged, often as an extension of more consolidated methods used in probabilistic seismic risk assessment. Nonetheless, only a few studies deal with best-practice methodologies for flood vulnerability assessment and existing approaches lack of an appropriate guidance for their selection. These concerns underline the need for a rational, integrated and complete compendium of all the existing flood-related vulnerability and fragility relationships to be used in a comprehensive probabilistic flood risk assessment framework. Following the same approach used in the guidelines recently developed by the Global Earthquake Model (GEM) Project, this paper presents a preliminary review of the state-of-art regarding existing empirical vulnerability and fragility curves in the context of flood risk. In particular, a worldwide overview is intended in terms of data sources, assets features and also statistical techniques employed for data collection and fitting. The research aims at providing a complete and flexible guide for selection of vulnerability and fragility curves for building structures. A discussion on data sources, building classification and considered features, and damage scales is presented, in order to evaluate the reliability, and at the same time the limitations, of different approaches and provide recommendation for future studies.

Spectral shape proxies and simplified fragility analysis of mid-rise reinforced concrete buildings

The objective of this study is to identify an optimal intensity measure (IM) for conditioning probabilistic seismic demands of case-study reinforced concrete (RC) frame buildings, representative of mid-rise RC building classes in the Mediterranean region. The prediction is performed via statistical relationship between multiple IMs (particularly advanced scalar parameters accounting for spectral shape over a range of periods) and various displacement-based engineering demand parameters (EDPs). Such statistical relationships are built on data obtained from analysis of the frames subjected to over nine hundred ground motion records by employing an innovative capacity spectrum method, introduced in the paper, which uses inelastic response spectra derived from actual earthquake accelerograms to estimate seismic demand and derive fragility curves. The outcomes of the present work are in a good agreement with previous investigations conducted by other researchers on selecting optimal IMs for predicting structural response by using full nonlinear dynamic analyses for different structural typologies.

Fragility of reinforced concrete framed structures to flow-type landslides

Flow-type landslides are typically triggered by heavy rainfalls and may cause large losses. Landslide risk may be rationally evaluated and mitigated with probabilistic approaches. In this paper, physical vulnerability of reinforced concrete buildings to landslides is assessed. Fragility analysis was carried out by assuming flow velocity as intensity measure, several damage states, and different mechanical models for beams, columns and masonry infill walls. Uncertainties in landslide impact loading, material properties, members geometry, and capacity models were taken into account. Both earthquake-resistant and gravity-load designed buildings were assessed as being representative of two low-rise building subclasses. Analysis results show that landslide fragility significantly depends on the presence and type of infill walls, which influence both out-of-plane and in-plane failure modes as well as yielding and failure of plastic hinges in columns.

Development of empirical vulnerability curves for electrical supply systems subjected to wind hazard

In this paper, we develop a series of empirical vulnerability curves for energy distribution infrastructure in the UK, specifically for overhead line components, when subjected to wind storm hazard. We have achieved this by combining an atmospheric model, driven by reanalysis data, with empirical fault data from 1991 to 2010. The fault data used in this study comes from a national database of electricity distribution faults. While the fault data in this database is comprehensive, it has the deficiency of not recording the exact location of the fault, instead it only indicates which District Network Operator owned the asset. Better fault location information is available, but this is only available from the Operator. We also investigate the sensitivity of vulnerability curves to three different resolutions of the fault information; namely by Operator, Region and Area in order to evaluate the impact that this has to the vulnerability curve. From the results shown in this paper, we can conclude that the spatial resolution of the hazard data can have a significant impact to the vulnerability curve, particularly for large wind storm hazards.

Engineering Analysis of Strong Motion Data from Recent Earthquakes in Sichuan, China

Recent earthquakes in the Sichuan Province have contributed to significantly expand the existing ground-motion database for China with new, high-quality ground-motion records. This study investigated the compatibility of ground-motion prediction equations (GMPEs) established by the NGA-West2 project in the US and local GMPEs for China, with respect to magnitude scaling, distance scaling, and site scaling implied by recent Chinese strong-motion data. The NGA-West2 GMPEs for shallow crustal earthquakes in tectonically active regions are considerably more sophisticated than widely used previous models, particularly in China. Using a mixed-effects procedure, the study evaluated event terms (inter-event residuals) and intra-event residuals of Chinese data relative to the NGA-West2 GMPEs. Distance scaling was investigated by examining trends of intra-event residuals with source-to-site distance. Scaling with respect to site conditions was investigated by examining trends of intra-event residuals with soil type. The study also investigated other engineering characteristics of Chinese strong ground motions. In particular, the records were analyzed for evidence of pulse-like forward-directivity effects. The elastic median response spectra of the selected stations were compared to code-mandated design spectra for various mean return periods. Results showed that international and local GMPEs can be applied for seismic hazard analysis in Sichuan with minor modification of the regression coefficients related to the source-to-site distance and soil scaling. Specifically, the Chinese data attenuated faster than implied by the considered GMPEs and the differences were statistically significant in some cases. Near-source, pulse-like ground motions were identified at two recording stations for the 2008 Wenchuan earthquake, possibly implying rupture directivity. The median recorded spectra were consistent with the code-based spectra in terms of amplitude and shape. The new ground-motion data can be used to develop advanced ground-motion models for China and worldwide and, ultimately, for advancing probabilistic seismic hazard assessment (PSHA).