Performance-based Seismic Risk Assessment of Urban Systems

ABSTRACT

Disaster risk mitigation has become a urgent global need. Similar to other natural hazards, earthquakes may cause significant damage on a large scale. In Europe and in other regions with dense urbanization, seismic events can heavily impact historical city centers due to the several structural fragilities. These centers are often part of the worldwide cultural heritage and their preservation is considered a strategic issue. Furthermore, earthquakes may have severe negative short-term economic effects on the impacted communities and adverse longer-term consequences for economic growth. For this reason, the development of an efficient approach for urban seismic risk assessment becomes essential. An original approach is proposed, based on performance concepts and multidisciplinary perspectives. The procedure is applied for validation to the city center of Concordia Sulla Secchia (Italy), damaged by the 2012 Pianura Padana Earthquake (PPE), comparing predicted damage scenarios with the actual post-seismic survey data.     

Evaluation of Seismic Risk on UNESCO Cultural Heritage sites in Europe

ABSTRACT

Earthquakes have been responsible for the destruction of hundreds of monuments throughout human history. Due to their size, conservation state, and lack of seismic provisions, monuments are particularly vulnerable to the effects of geological hazards. The first step toward the mitigation of the earthquake threat consists of understanding the existing seismic risk and evaluating possible strategies to reduce it. This study presents a simplified assessment to evaluate the probability of damage due to ground shaking on UNESCO World Heritage cultural sites throughout Europe. The seismic hazard model (SHARE) has been employed to derive hazard curves, which were combined with a fragility model to calculate the annual probability of damage or collapse. These calculations were performed assuming different soil conditions, and the resulting risk metrics can be used for risk awareness, to inform the prioritization of the sites in need of structural interventions, and to support additional risk analysis.     

Seismic behavior of two Portuguese adobe buildings: Part I - in-plane cyclic testing of a full-scale adobe wall

This article addresses the laboratory testing of a full-scale double-T shaped adobe wall, under in-plane horizontal cyclic loading of increasing amplitude, with the aim of contributing to the understanding of the seismic behavior of adobe structures. The wall was built with lime adobes taken from an existing building and mortar with a traditional composition. The behavior of the wall was assessed in terms of: shear stress versus horizontal drift and moment versus rotation relationships; maximum lateral strength; drift and rotation at peak stress; evolution of stiffness, lateral displacements, dissipated energy, and natural frequency; and damage pattern. The wall exhibited brittle behavior and in-plane strength corresponding to 56% of the vertical load. Cracking was observed with an X-shaped pattern whereas no sliding occurred at its base. This research supported the subsequent development of a repair and retrofit solution and also of numerical models to simulate the seismic behavior of two adobe buildings.     

Masonry Italian Code-Conforming Buildings. Part 2: Nonlinear Modelling and Time-History Analysis

ABSTRACT

The unreinforced masonry (URM) buildings designed to be conforming with the Italian building code, as illustrated in the companion paper, were analyzed by performing time-history analyses on models realized using an equivalent frame approach and by adopting two different constitutive laws. Both the effect of record-to-record variability and of epistemic and aleatory uncertainties in modelling were explored. The achieved results constitute the basis for the evaluation of the risk level implicit in Italian code-conforming buildings. Two main performance conditions are considered, namely usability-preventing damage and global collapse limit states.     

Modeling and Seismic Response Analysis of Italian Code-Conforming Single-Storey Steel Buildings

ABSTRACT

This article describes the structural design, nonlinear modeling, and seismic analysis of prototype single-storey non-residential steel buildings made of moment-resisting portal frames in the transverse direction and concentric braces in the longitudinal direction. Various design parameters (building geometry, seismic hazard, foundation soil category) and different modeling assumptions (bare frame model, model including cladding elements, ground motions including vertical accelerations, and modeling uncertainties) were considered to investigate their effects on the simulated seismic performance.     

HINTS ABOUT SITE AMPLIFICATION EFFECTS COMPARING MACROSEISMIC HAZARD ESTIMATE WITH MICROTREMOR MEASUREMENTS: THE AGRI VALLEY (ITALY) EXAMPLE

We approach from a new standpoint the problem of estimating seismic hazard for some towns and villages located in Val d'Agri area (Southern Italy) that in the past have been affected by several seismic events. The estimates are carried out using a method that is based on the analysis of site seismic history extracted from macroseismic catalogues. To study the influence of site effects two different procedures have been performed: in the first, seismic hazard estimates have been deduced from epicentral data only, in the second, intensity data actually observed at the site are also considered. The difference between the two estimates can be correlated with local variations of seismic response due to local geological features which are responsible for possible cases of amplification. In order to validate the presence of such correlation, seismic hazard estimates have been compared to site amplification measurements obtained by using the HVSR (Horizontal to Vertical Spectral Ratio) technique. Our findings reveal a good correlation between seismic hazard enhancements and the presence of site amplification effects. The application of this kind of analysis to the Val d'Agri area has pointed out that the joint estimates of site seismic hazard enhancement and HVSR measurements could be a helpful tool to identify problems related to seismic microzonation.     

SEISMIC REHABILITATION OF BEAM-COLUMN JOINTS USING FRP LAMINATES

An innovative and practical technique for the seismic rehabilitation of beam-column joints using fiber reinforced polymers (FRP) is presented. The procedure is to upgrade the shear capacity of the joint and thus allow the ductile ftexural hinge to form in the beam. An experimental study is conducted in order to evaluate the performance of a full-scale reinforced concrete external beam-column joint from a moment resisting frame designed to earlier code then repaired using the proposed technique. The beam-column joint is tested under cyclic loading applied at the free end of the beam and axial column load. The suggested repair procedure was applied to the tested specimen. The composite laminate system proved to be effective in upgrading the shear capacity of the nonductile beam-column joint. Comparison between the behaviour of the specimen before and after the repair is presented. A design methodology for fibre jacketing to upgrade the shear capacity of existing beam-column joints in reinforced concrete moment resisting frames is proposed.     

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.     

COMPLEX MODE SUPERPOSITION ALGORITHM FOR SEISMIC RESPONSES OF NON-CLASSICALLY DAMPED LINEAR MDOF SYSTEM

This paper deals with a complex mode superposition method for the seismic responses of general multiple degrees of freedom (MDOF) discrete system with complex eigenvectors and eigenvalues. A delicate general solution, completely in real value form, for calculat-ing seismic time history response of the MDOF system which cannot be uncoupled by normal modes, is deduced based on the algorithms of the complex superposition method. This solution comprises of two parts which are in relation to the Duhamel integration to sine and cosine function respectively. The related term of the Duhamel integration to sine function is actually the displacement response of the oscillator with corresponding modal frequency and the damping ratio. The other can be transferred into a combina-tion of the displacement and velocity responses of the same oscillator. In order to meet the practical needs of seismic design based on code design spectra for various kinds of structures equipped by viscous dampers, the complex complete quadratic combination (CCQC) method is deduced following similar procedures such as the well-known CQC method, in which a new modal velocity correlation coefficient, together with a new modal displacement-velocity correlation coefficient are involved besides the modal displacement correlation coefficient in normal CQC formula. The new algorithm of CCQC is not only as concise as that of the normal CQC but also has explicit physical meaning. The results obtained from complex mode superposition approaches are discussed and verified in some examples through step by step integration computation under a prescribed earth-quake motion input. From these examplary analyses, it may be pointed that the CCQC algorithm normally yields conservative outcome and that the forced mode uncoupling approach has good approximation even the discussed examplary structures are strongly non-proportional.