Dina D’Ayala

PERPETUATE Project: The Proposal of a Performance-Based Approach to Earthquake Protection of Cultural Heritage

The paper describes the methodology proposed in the PERPETUATE Project (funded by the Seventh Framework Programme – Theme ENV.2009.3.2.1.1). The methodology proposed in PERPETUATE uses a displacement-based approach for the vulnerability evaluation and design of interventions. The use of safety verification in terms of displacement, rather than strength, orients to new strengthening techniques and helps in the comprehension of interaction between structural elements and unmovable artistic assets. The procedure is based on the following fundamental steps: definition of performance limit states, specific for the cultural heritage assets (considering both structural and artistic assets); evaluation of seismic hazard and soil-foundation interactions; construction knowledge (non-destructive testing, material parameters, structural identification); development of structural models for the seismic analysis of masonry structures and artistic assets and design of interventions; application and validation of the methodology to case studies. Two main scales are considered: the seismic risk assessment at territorial scale and at the scale of single historic building or artistic assets. The final aim of the project is to develop European Guidelines for evaluation and mitigation of seismic risk to cultural heritage assets.

Review of Out-of-Plane Seismic Assessment Techniques Applied To Existing Masonry Buildings

ABSTRACT Observations after strong earthquakes show that out-of-plane failure of unreinforced masonry elements probably constitutes the most serious life-safety hazard for this type of construction. Existing unreinforced masonry buildings tend to be more vulnerable than new buildings, not only because they have been designed to little or no seismic loading requirements, but also because connections among load-bearing walls and with horizontal structures are not always adequate. Consequently, several types of mechanisms can be activated due to separation from the rest of the construction. Even when connections are effective, out-of-plane failure can be induced by excessive vertical and/or horizontal slenderness of walls (length/thickness ratio). The awareness of such vulnerability has encouraged research in the field, which is summarized in this article. An outline of past research on force-based and displacement-based assessment is given and their translation into international codes is summarized. Strong and weak points of codified assessment procedures are presented through a comparison with parametric nonlinear dynamic analyses of three recurring out-of-plane mechanisms. The assessment strategies are marked by substantial scatter, which can be reduced through an energy-based assessment.

Damage assessment and analysis of the 1997 Umbria-Marche earthquakes

This paper reports the results of damage surveys carried out after the September 1997 Umbria-Marche earthquakes in collaboration with colleagues from the Italian Servizio Sismico Nazionale and the UK Earthquake Field Investigation Team. The extent of damage in the epicentral area and its relationship to the form of construction are described. The results of systematic damage studies in three towns damaged by the earthquake are presented, and conclusions are drawn on the relationship between damage and previous strengthening of masonry buildings. The paper also discusses the observed relationship between the level of ground motion recorded and the level of damage sustained, and the number of human casualties in comparison with estimations from existing models.

Sensitivity analysis for setting up the investigation protocol and defining proper confidence factors for masonry buildings

At present, the codified procedures for the seismic assessment of existing structures do not consider explicitly the use of sensitivity analysis that, on the contrary, is an essential tool due to the intrinsic and epistemic uncertainties involved. In particular, results of a sensitivity analysis allow, on one hand, to optimize the plan of investigations and tests, limiting it to what is really useful, and, on the other hand, to be aware of the propagation of uncertainties in the final outcome of the assessment. The paper proposes how to implement the sensitivity analysis in a systemic way, in order to define proper confidence factors, which are used within the semi-probabilistic approach of the performance-based assessment procedure. In relation to the procedures adopted by current standards, this method is aimed to overcome the following drawbacks: (1) the a priori definition of values for the confidence factor; (2) the a priori selection of parameters to which confidence factor has to be applied; (3) the weak connection between investigation and assessment. Despite the generality of most aspects of the proposed procedure, main attention is given in the paper to its application in the case of masonry existing buildings and, in particular, of those with a significant cultural value, where the minimization of invasiveness of investigations on the construction together with that of costs plays a crucial role for the aim of conservation.

Evaluation of Existing Fragility Curves

There is a wealth of existing fragility curves for buildings and infrastructure. The main challenge in using these curves for future applications is how to identify and, if necessary, combine suitable fragility curves from a pool of curves with different characteristics and, often unknown, reliability. The present chapter aims to address this challenge by developing a procedure which identifies suitable fragility curves by firstly assessing their representativeness to the needs of the future application and then assessing the reliability of the most relevant relationships. The latter is based on a novel procedure which involves the assessment of the most significant factors affecting the robustness and quality for each fragility assessment methodology, also presented here. In addition, a decision-tree approach is adopted in order to combine more than one suitable fragility curves. The proposed selection and combination procedures are illustrated here with a simple case study which appraises the impact of different weighting schemes and highlights the importance of a deep understanding of the existing fragility curves and their limitations.

Non linear push over assessment of heritage buildings in Istanbul to define seismic risk

The objectives of the Istanbul Seismic Risk Mitigation and Emergency Preparedness Project are to improve Istanbul’s preparedness for a potential earthquake. Within this framework “Risk Assessment of Cultural Heritage Buildings” was designed to address the vulnerability of cultural assets, specifically buildings with global cultural heritage value. One of the components of the project was the vulnerability and risk assessment of 170 historical buildings. After a discussion of the choice of the most appropriate earthquake scenario, the methodology used for assessing the effects of local site conditions on the seismic performance of selected cultural heritage buildings is presented. The purpose is to estimate the earthquake characteristics on the ground surface based on the earthquake characteristics on the engineering bedrock outcrop obtained from the probabilistic and deterministic hazard studies. The site specific elastic design spectra for each site are then further manipulated to obtain site specific non linear displacement spectra, so that these can be directly compared with capacity curves for the buildings obtained by using plasticity based limit state analysis. The procedure for obtaining the capacity curves is described and the choice of the most appropriate level of ductility and the equivalent reduction coefficient are discussed. A procedure to evaluate performance points and to define safety factors based on lateral acceleration, drift or expected damage level, is presented. The process of arriving at a risk evaluation and hence recommendation for strengthening or otherwise, is finally highlighted with respect to two comparable case studies.

A procedure for the identification of the seismic vulnerability at territorial scale. Application to the Casbah of Algiers

One of the objectives of the PERPETUATE EU-FP7 project was to determine a sound approach to the quantification of the seismic vulnerability of historic centres at territorial scale. The procedure presented herein provides a ten steps guideline from how to select building samples in the area of study, to how to compute the buildings’ seismic performance and finally how to evaluate rehabilitation decisions to reduce the seismic fragility of the studied typologies over an entire district or city. The procedure is illustrated in this paper by way of application to The Casbah of Algiers, a world heritage site composed of building clusters from the Ottoman to the French period. Pre-existing seismic damage, decay due to lack of maintenance and environmental factor and a urban irregular and complex lay-out make this application particularly challenging. A total number of around 150 houses, characterised by the most representative features of the Algerian historic construction, have been selected in collaboration with the Directorate of the Office for Management of Cultural Property of the Ministry of Culture,. The seismic Vulnerability Assessment Method (VAM) underpinning the 10-step procedure is is FaMIVE (Failure Mechanism Identification and Vulnerability Evaluation), a mechanical approach based on limit state analysis and kinematics, which allows computing collapse load factor, deriving capacity curves and determining fragility functions. As the approach identifies also the collapse mechanisms, it provides a base for choosing and evaluating the effects of strengthening interventions, which are rolled out at territorial level to improve the seismic performance of the whole sample. The effectiveness of the present procedure for the identification of the seismic vulnerability at territorial scale is discussed in the conclusions.

The value of multiple earthquake missions: the EEFIT L'Aquila Earthquake experience

In November 2012 EEFIT launched its first ever return mission to an earthquake affected site. The L’Aquila Earthquake site was chosen as this is a recent European event of interest to the UK and European earthquake engineering community. The main aims of this return mission were to document the earthquake recovery process and this paper presents an overview of the post-disaster emergency phase and transition to reconstruction in the Aquila area after the earthquake. It takes an earthquake engineering perspective, highlighting areas mainly of interest to the fields of structural/seismic engineering and reconstruction management. Within the paper, reference is made to published literature, but also to data collected in the field during the return mission that would not otherwise have been available. The paper presents some specific observations and lessons learned from the L’Aquila return mission. However, in light of current international efforts in conducting return missions, the paper ends with some reflections on the value that return missions can provide to the field of earthquake engineering in general, based on the EEFIT L’Aquila experience.

Bayesian Networks and Infrastructure Systems: Computational and Methodological Challenges

This chapter investigates the applicability of Bayesian Network methods to the seismic assessment of large and complex infrastructure systems. While very promising in theory, Bayesian Networks tend to quickly show limitations as soon as the studied systems exceed several dozens of components. Therefore a benchmark study is conducted on small-size virtual systems in order to compare the computational performance of the exact inference of various Bayesian Network formulations, such as the ones based on Minimum Link Sets. It appears that all formulations present some computational bottlenecks, which are either due to the size of Conditional Probability Tables, to the size of clique potentials in the junction-tree algorithm or to the recursive algorithm for the identification of Minimum Link Sets. Moreover, these formulations are limited to connectivity problems, whereas the accurate assessment of infrastructure systems usually requires the use of flow-based performance indicators. To this end, the second part of the chapter introduces a hybrid approach that presents the merit of accessing any type of system performance indicator: it uses simulation-based results and generates the corresponding Bayesian Network by counting the outcomes given the various combinations of events that have been sampled in the simulation. The issue of the system size is also addressed by a thrifty-naive formulation, which limits the number of the components that are involved in the system performance prediction, by applying a cut-off threshold to the correlation coefficients between the components and system states. A higher resolution of this thrifty-naive formulation is also obtained by considering local performance indicators, such as the flow at each sink. This approach is successfully applied to a realistic water supply network of 49 nodes and 71 pipes. Finally the potential of this coupled simulation-Bayesian approach as a decision support system is demonstrated, through probability updating given the observation of local evidences after an event has occurred.

Evaluation of environmental impact on historical stone masonry through on-site monitoring appraisal

This paper presents the design and application of an environmental monitoring system on the historical site of Odda’s Chapel in Deerhurst, Gloucestershire, UK, and the determination of hygroscopic behaviour of the original building materials to obtain long-term moisture content variation. The monitoring system provides a comprehensive profile of hygrothermal conditions in the walls at the locations where moisture conditions are potentially causing decay and damage. The system makes use of temperature and relative humidity sensing technology placed on the wall construction, providing continuous data including rainfall, wind-driven rain and runoff rain measurements. The relative merits and results of the designed system are evaluated and discussed, as well as the implications of its application in the conservation of historical structures. The environmental monitoring results show general deterioration in the shell of the building. Internal wall surface measurements are essential for the documentation of wall interior response and should be conducted across the wall section at several locations. The fact that 91% of in-wall recorded relative humidity (RH) values are greater than 75% RH throughout the monitoring period demonstrates the need for measures to conserve this historical stone masonry.