I. Vanzi

TO RETROFIT OR NOT TO RETROFIT

Abstract Under what conditions is it economically convenient to seismic retrofit a structure? This paper present a procedure whose results allow to give a simple answer to the above question, central in earthquake engineering and, in a broader context, in any man-made activity. The procedure uses, as a starting point, the results of a standard reliability analysis conducted on the structure in its present state and after retrofitting. Once these are known, expressed in terms of mean rate of exceedance of specified limit states, it is shown that it is possible to compute whether the upgrading should be made after all and how convenient it is. The assumptions to make the problem tractable are clearly listed and appear, in the authors’s viewpoint, quite reasonable. The final results are presented both in diagrams and with a simple formula. The method is finally applied to two cases, already studied and presented by the authors, which had been developed up to the computations of the mean rate of failure: bridges on an Italian highway stretch (Donferri et al. Autostrade 1998;2:7-15) and a hospital in Tuscany (Ferrini et al. XII World Conference on Earthquake Engineering, Auckland, New Zealand, Paper No. 2102, 2000.).

Assessment of post‐earthquake availability of hospital system and upgrading strategies

SUMMARY A model for the regional hospitals system behaviour in case of a seismic event is developed. The aim is the evaluation of the vulnerability of the system as well as the selection of the best intervention strategy for the retrofitting of the hospitals so as to minimize the cost benefit ratio and to evaluate the e⁄ect of di⁄erent post-earthquake emergency measures like the use of camp hospitals. The eƒciency of the system is measured in terms of the mean distance to be cured for persons injured by the earthquake and by damages to the system. Some simplifying assumptions are used and discussed; these can be easily removed if necessary. Results allow to clearly indicate the most convenient interventions. ( 1998 John Wiley & Sons, Ltd.

Structural upgrading strategy for electric power networks under seismic action

This paper presents a procedure to identify an optimal retrofitting strategy for electric power networks (EPN) subjected to seismic events. The optimization consists of the minimization, under economic constraints, of the probability of power cut-off at the nodes where electric energy is most needed in the post-earthquake situation; these nodes are referred to as critical nodes. The EPN model used herein has been presented earlier and is briefly reviewed in the text. The method to individuate the critical nodes, based upon the value of an index proposed in this study, is presented first; then the optimization procedure to select which elements of the EPN to upgrade is examined based upon a standard reliability study. Its effectiveness is tested against routinely used upgrading schemes for an existing EPN (the one of Sicily in Italy). It is shown that the optimization procedure is effective and leads to a significant saving of economic resources.

Speedup of post earthquake community recovery: the case of precast industrial buildings after the Emilia 2012 earthquake

The Emilia, May–July 2012, earthquake hit a highly industrialized area, where some tens thousands industrial buldings, mainly single storey precast structures, are located. Due to the likelihood of strong after shocks and the high vulnerability of these structures, the authorities first asked for a generalized seismic retrofit after the strong shakings of May 20th. In order to accelerate community recovery, this requirement was later loosened, leaving out the buildings which had undergone a strong enough shaking without any damage; the strong enough shaking was defined with reference to the ultimate limit state design earthquake. To the authors’ knowledge, it is the first time that the information on the earthquake intensity and structural damage is used for such a large scale post earthquake simplified safety assessment. In short, the earthquake was used as large experimental test. This paper shows the details of the models and computations made to identify the industrial buildings which have been considered earthquake tested and therefore not compelled to mandatory seismic retrofit. Since earthquake indirect (e.g. due to economic halt) costs may be as large the direct ones, or even larger, it is believed that this method may considerably lower the earthquake total costs and speed up the social and economic recovery of a community.

Seismic safety of network structures and infrastructures

Realistic assessment of network structural safety requires modelling of a reasonably large part of the network itself. Although this statement may appear too demanding, both for modelling and computing reasons, there are clear motivations and technological possibilities to do complex network analyses. In this paper, the safety analyses of three infrastructures that have been shaken by an earthquake are described, modelled and computed: the electric power, water and road systems. For each network, results extracted from real networks, some of which have been studied in the past by the authors, are presented and discussed. No inter-network analysis is carried out, although it is recognised that this would be the most complete approach. The common parts in the procedure to model and analyse each network, via Monte Carlo simulations, are detailed at the beginning of the paper, thus showing the many common points that show up in any network analysis.

A simplified procedure to assess the seismic response of nonlinear structures

The paper presents an improvement of the iterative procedure proposed in the ATC 40 document to evaluate seismic response of nonlinear structures in terms of maximum displacement and acceleration, given the structural initial elastic period, the yielding acceleration and the hardening ratio in the plastic range. As a second issue the results of the procedure are compared with nonlinear step by step dynamic integration, with the bilinear and Takeda model, and with the assumptions of equal energy and equal displacement. Two case studies of existing reinforced concrete buildings are further examined. For the cases examined, the ATC 40 procedure is overall as or less precise than the simpler equal energy and equal displacement principles.

Damage, Vulnerability and Retrofitting Strategies for the Molise Hospital System Following the 2002 Molise, Italy, Earthquake

The earthquake of October 31, 2002, inflicted moderate damage on the region of Molise in southern Italy. However, it attracted much attention because almost all the victims were children killed in the structural failure of their primary school. This paper focuses on a different, but equally important, structural type: hospitals. The earthquake left them largely untouched. In the paper we first give a detailed picture of the structural damage and of the organization of emergency operations. Then we try to give guidance about the most effective retrofitting strategies for the regional hospital system as a whole, following a methodology developed in Nuti and Vanzi (1998b), which has been adjusted for this case study. Depending on the community decision criteria, the hospitals of Campobasso and Bojano appear to have the highest priority for retrofitting, even though they were unscathed by the event.

Generation of Non-synchronous Earthquake Signals

In this chapter, we describe two procedures to generate earthquake asynchronous signals at different space points for the same seismic event. The foundations of long structures, such as bridges, are placed at distant space points. The earthquake signals at these points have different characteristics and their correct evaluation is important to define design actions. However, design codes around the world do not consider this complex type of action in a consistent manner. The point-to-point signal variation is due both to time lag, since the seismic waves move through the soils with a finite velocity among distant points, and to a change of the signal frequency contents. This depends on physical complex soil-wave interaction phenomena during wave propagation (reflection, refraction, filtering, amplification, etc.). In this chapter, two different generation procedures (PR1 and PR2) to determine the non-synchronous actions at different surface points are shown. Both procedures have been implemented in MATLAB. PR1 generates asynchronous signals at the soil surface. It starts from recorded signals at a few surface points for the same seismic event. PR2 produces asynchronous surface signals by amplifying the bedrock signals obtained by a bedrock propagation process. The inputs for the bedrock propagation are obtained via deconvolution of the recorded surface signals. These latter are also the inputs of the PR1 procedure. Detailed knowledge of soil characteristics is required (soil layers, shear wave velocity profiles, soil density, nonlinear materials shear moduli and damping curves), which relies on in situ tests. Deconvolution and amplification processes are performed by Equivalent-Linear Earthquake Site Response (1D soil model, SHAKE91 (Schnabel et al. 1972) and EERA (Bardet et al. 2000)). PR1 and PR2 are then applied to an example case. Asynchronous surface signals are generated at eight foundation points of a bridge placed in the Aterno Valley near the city of L’Aquila in Italy, where recordings are available at different recording stations (AQA and AQV) for the same earthquake. The EW component of the strong main shock of 4-6-2009 in L’Aquila is selected as input for the two procedures. Finally, the comparison between the signals resulting by PR1 and PR2 and the input signals recorded at the same points is discussed in term of effects on the structures (acceleration response spectrum) and characteristics of the generated signals (Fourier amplitude spectra, coherences for each frequency) to evaluate the differences between the two procedures and between the procedures and the actually recorded signals.

Design Of Bridges For Non Synchronous Seismic Motion

this paper aims to develop and validate structural design criteria which account for the effects of earthquakes spatial variability. In past works [1, 2] the two simplest forms of this problem were dealt with: differential displacements between two points belonging to the soil or to two single degree of freedom structures. Seismic action was defined according to EC8 [3]; the structures were assumed linear elastic sdof oscillators. Despite this problem may seem trivial, existing codes models appeared improvable on this aspect. For the differential displacements of two points on the ground, these results are now validated and generalized using the newly developed response spectra contained in the new seismic Italian code [4]; the resulting code formulation is presented. Next, the problem of statistically defining the differential displacement among any number of points on the ground (which is needed for continuos deck bridges) is approached, and some preliminary results shown. It is also shown that the curr...

Seismic vulnerability assessment of historical centers at urban scale

ABSTRACT Seismic prevention and mitigation of historical centers have gained a central position within earthquake engineering topics, particularly in areas such as Italy, Greece, and Portugal. Many historical towns in these countries have been strongly damaged, due to the high quantity of old buildings and urban structures and infrastructures. In this article, these aspects are described, modeled, and investigated in terms of structural safety, the goal being the set-up of a comprehensive strategy for seismic prevention and mitigation of a whole historical center. The proposed approach is based on two relevant parts: the first is an urban risk assessment, the second is a prioritization of retrofitting interventions so as to optimally increase urban safety. The effectiveness of the proposed methodology is shown with reference to a complex case study, the historical center of “Montebello di Bertona”. It is initially modeled as a series/parallel system and then studied by applying seismic reliability methods. Seismic retrofitting interventions are finally prioritized.