P. E. Pinto

On the role of road networks in reducing human losses after earthquakes

A road-network reliability analysis for a scenario seismic event is performed for a region of southern Italy characterised by a large number of small to medium municipalities quite close to each other and served by a dense network of roads. Among the many functions of the road network, whose links may fail after an earthquake due to the collapse of the bridges within them, the one selected for the present study is that of allowing rescue operations to be carried out at the sites of collapsed schools. For this to be possible, connection must be maintained between schools that survived, rescue centres and hospitals. Required elements for the study are the fragility curves of the bridges, the schools, the hospitals and the rescue centres. Output of the study is the expected value of the fraction of the total population in the area that is in need of assistance and cannot be hospitalised due to either failure of the network or other vulnerable components.

Allowing traffic over mainshock-damaged bridges

A criterion is proposed for deciding whether, after a damaging mainshock, a bridge can still be open for either emergency or ordinary traffic. The criterion is based on the comparison between the collapse risk of the mainshock-damaged structure and the pre-mainshock risk of the intact structure. The approach requires fragilities for multiple damage states for the intact structure, and transition probabilities from these states to collapse for the damaged structure. The aftershock risk decreases with time, hence a decision for reopening might have to wait until the risk level goes down to an acceptable value. A realistic application demonstrates the approach.

Seismic fragility of reinforced concrete structures using a response surface approach

A statistical approach for time-variant system-reliability problems has been developed and investigated in this study. The basic proposal is to use a response surface, characterised by a statistical model of the mixed type, to represent the capacity part in an analytical limit state function. The fragility of the system is then calculated by SORM analysis, with the constructed empirical limit state function as input. The developed method has been applied to a reinforced concrete frame: investigations have been carried out to check the stability and accuracy of the suggested procedure.

Risk analysis of a medieval tower before and after strengthening

Abstract The objective of this analysis has been the evaluation of the probability of failure under gravity load (Pf) of the Fraccaro tower in Pavia before and after a recent strengthening intervention. Both Pf, and its sensitivity to the parameters of the distributions of the random quantities controlling the system response have been evaluated. A probabilistic model of the spatial variability and of the uncertainties of the material properties typical of ancient conglomerate masonry constructions has been proposed. It accounts for both the continuous variability of material parameters and the drastic strength reductions due to localized areas of degraded material and construction defects. A 3-D nonlinear finite element model allowing for local crushing and fracturing of the masonry has been adopted for the numerical analyses. An extended version of the Response Surface (RS) method, coupled with a SORM algorithm, allowed for an efficient treatment of the several thousands of random variables of the model. The control variables of the RS have been reduced to a few major sources of uncertainty while the effects of all other variables, grouped in appropriate subsets, have been considered in a cumulative manner with a statistical treatment of the results of suitably planned numerical experiments.

Seismic Vulnerability of the Italian Roadway Bridge Stock

This study focuses on the evaluation of the seismic vulnerability of the Italian roadway bridge stock, within the framework of a Civil Protection sponsored project. A comprehensive database of existing bridges (17,000 bridges with different level of knowledge) was implemented. At the core of the study stands a procedure for automatically carrying out state-of-the-art analytical evaluation of fragility curves for two performance levels—damage and collapse—on an individual bridge basis. A WebGIS was developed to handle data and results. The main outputs are maps of bridge seismic risk (from the fragilities and the hazard maps) at the national level and real-time scenario damage-probability maps (from the fragilities and the scenario shake maps). In the latter case, the WebGIS also performs network analysis to identify routes to be followed by rescue teams. Consistency of the fragility derivation over the entire bridge stock is regarded as a major advantage of the adopted approach.

SEISMIC RISK ASSESSMENT OF RC STRUCTURES WITH THE "2000 SAC/FEMA" METHOD

The applicability of a new, fully probabilistic approach to seismic design and assessment of reinforced concrete (RC) structures is investigated. Fundamental advantages of the method are mathematical simplicity and comparatively light computational effort. The original formulation, which was developed for steel structures, is first illustrated; ah extension which allows consideration of multiple failure mechanisms, typical of RC structures, is then proposed. The applicability of the method is demonstrated through an example: the seismic risk of a four storey RC building that was not designed for seismic resistance is evaluated. Three failure mechanisms are considered: joint failure, column shear failure and drift failure.

Performance-based seismic design of integral abutment bridges

Integral abutment bridges (IAB) are experiencing increasing diffusion in the short to mid-range lengths, where they offer some advantages over traditional girder bridges with non-monolithic connection at the abutments. One challenging problem with their analysis and design is that consideration of the interaction between foundation soil, structure and backfill is unavoidable, also for the deck design. Further, the end of the construction is only one of the conditions that need to be verified during design. Cyclic deformations, such as those occurring during ground shaking, typically lead to an increase in stresses in the abutments and connections, due to progressive compaction (ratcheting) of the backfill soil. This problem is magnified when the bridge is comprised between two embankments, whose response may amplify the input motion and drive the deformation of the bridge. Performance-based design aims at superseding current design procedures by explicitly checking that the target performances set out are achieved, and not overly exceeded. Such a design paradigm naturally calls, on the one hand, for improved accuracy in response determination and more refined analyses, and, on the other, for taking into account the uncertainties entering into the problem by means of an explicitly probabilistic approach. With this objective in mind, the paper presents an inelastic dynamic model for the seismic analysis and design of IABs. The model, that features a balanced compromise between the setup and evaluation effort on one hand, and accuracy on the other, has been developed for implementation in typical commercial analysis packages. It builds on 1D site-response analysis and on inelastic Winkler-like modeling, to reproduce the main physical aspects of the seismic response of IABs. One example application to a highway overpass in Italy illustrates the model and the relevance of a fully probabilistic approach to performance-based design. The application offers also important insight into the choice of an efficient intensity measure for this type of structure.

Increased accuracy of vector-IM-based seismic risk assessment?

The vector-valued ground motion intensity measure (IM) consisting of spectral acceleration at two different periods is considered for seismic risk assessment of structures. The first component of the IM is the spectral acceleration at the first-mode structural period T 1. The second period is selected to increase efficiency in the estimation of seismic risk (i.e., minimizing dispersion). A method to assess vector structural fragility using a scalar global measure of structural performance is proposed. With reference to an example RC frame structure, the accuracy of prediction of the seismic risk using the considered vector IM vs. a conventional scalar IM is presented. In both cases, probabilistic seismic hazard analysis (scalar and vector) is carried out by means of a subset simulation approach that employs a stochastic model of ground motion. Results show that an effective choice of the second period T2 leads to an estimate of the seismic risk close to that obtained employing the scalar IM consisting of Sa(T1) only, while reducing the associated dispersion in the estimate. For the examined example structure, however, the reduction is negligible in light of the effort required for switching from a scalar to a vector IM.

Issues in the Upgrade of Italian Highway Structures

The last 15 years have seen fundamental advances in both the definition of the hazard and on the criteria in codified seismic design, creating a situation whereby even relatively modern constructions may represent an unchecked risk. This article deals with a case of particular relevance, i.e., the seismic risk of bridges and viaducts on the Italian highway network. After a brief outline of the present state of the network, the article concentrates on the solutions adopted in the upgrading, for increasing both the traffic and the seismic capacities, through the illustration of two case studies. The second, main part of the article deals in more detail with one of the case studies, examining alternative design options and the corresponding analytical aspects. Finally, the opportunity is taken to investigate the relevance of two aspects not commonly taken into account in design practice, i.e., differential support motion and soil-structure interaction.

Comparison of different approaches for seismic assessment of existing buildings

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.