Seismic vulnerability assessment indices for buildings: Proposals, comparisons and methodologies at collapse limit states

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Abstract

Seismic vulnerability assessment of existing buildings is an essential process for earthquake disaster management. It can support decision-makers to estimate critical response parameters and improve them. While several seismic vulnerability assessment techniques have been introduced over the past years, a detailed formulation to measure the vulnerability index of existing buildings along with the operative procedure is still lacking. Current standards and literature worldwide mostly propose simplified approaches (e.g. based on limited engineering analyses, visual inspection) to perform a rapid vulnerability assessment that can be useful when a large number of buildings should be analyzed. This paper presents a methodology to perform the seismic vulnerability assessment of existing buildings at collapse, analyzing a standard vulnerability index, and proposing alternative formulas. The methodology combines field investigations and experimental tests with nonlinear structural analyses. It has been applied to a case study that consists of an existing reinforced concrete school building to analyze and compare the proposed indices with respect to different ground motion selection procedures. Furthermore, a detailed comparison with alternative vulnerability indices from literature has been also performed.

Introduction

The seismic vulnerability assessment of existing buildings is an essential step for earthquake disaster management that allows to optimize resources allocation, provide a fast response after a disaster, and mitigate consequences of earthquakes[1]. The “seismic vulnerability” is defined as the “susceptibility of a population of buildings to undergo damage due to seismic ground motion”[[1], [2], [3], [4]] and it depends on several key parameters such as the seismic hazard level, the quality of the constitutive materials, the building age, the level of maintenance, the building archetype [5], etc. However, often this information might not be available. Furthermore, the scale of observation is an important element in vulnerability assessments. Indeed, the spatial scale of analysis implies the selection of certain structural details which are more refined for individual building-scale. On the contrary, urban and regional-scale of observation require higher computational effort which entails the use of typological building data. In the following, the state of the art of existing methods for the vulnerability assessment of buildings has been reported, also making reference to the current standards.

In the literature, different methods for seismic vulnerability assessment have been proposed[[6], [7], [8], [9], [10], [11], [12]]. They can be classified into two main categories: empirical and analytical. However, both categories can be merged in hybrid approaches[6,13]. Empirical methods allow evaluating seismic vulnerability by correlating seismic intensity with the level of observed damage (statistical approach), while analytical approaches use mechanical models that reproduce the main characteristics of buildings and estimate the capacity of the structure [14] and the demand level imposed by an earthquake scenario (quantitative approach) [[15], [16], [17], [18], [19], [20], [21], [22]]. Vulnerability assessment may be also addressed based on simple evidence without significant calculation or modeling (qualitative approach). An example of a qualitative approach is given by the Rapid Visual Screening (RVS) procedures that require a visual evaluation and limited information on the building under study; therefore, they are classified as First Level procedures. Among the others, Islam et al. [23] proposed a simplified vulnerability index based on the rapid screen evaluation of the existing Reinforced Concrete (RC) buildings' capacity using data from past earthquakes. The seismic vulnerability index was evaluated as the ratio between the average lateral load-carrying capacity and the mean shear stress on the resistant members [24]. proposed an RVS method for evaluating the safety index for hospital buildings. The methodology was tested on two hospitals located in different seismic areas of Italy. Later [25], developed an RVS methodology to assess the seismic risk of RC school buildings. The vulnerability was assessed through a survey, while the hazard and exposure data were also taken into account. Although RVS methods provide a quick evaluation of the seismic building vulnerability, they are addressed at identifying those structures that require further technical investigation[26].

Second Level procedures require limited engineering analyses based on the site measurement, structural drawings, and visual inspection data[27]. Several simplified vulnerability assessment methods of existing buildings have been carried out during the last two decades. Thermou and Pantazopoulou [28] proposed a vulnerability assessment method for existing RC buildings under the design earthquake. They estimated the inter-story drift from the period of the structure by relating the structural stiffness to the area ratio of the vertical elements. Asteris et al. [29] presented a methodology to evaluate the seismic capacity of masonry buildings using case studies from historical masonry structures in the European area. Later on, Formisano and Marzo [30] assessed the seismic vulnerability of a masonry building that was damaged during the 2012 central Italy earthquake. They proposed a simplified method following the instruction provided by the Italian Guidelines on Cultural Heritage. Uma, Dhakal, and Nayyerloo [31] used a displacement-based method to carry out the vulnerability assessment of Christchurch buildings following the 2012 earthquake event. They used the data from the building safety evaluation surveys to determine the damage level. The observed damages were linked with drift limit states adopted in the theoretical approach. Focusing on reinforced concrete (RC) structures, Kassem, Nazri, and Farsangi [32] proposed a simplified method modifying the existing Italian and European Macro-seismic approaches.

Third Level assessment procedures require detailed information on the structure and use refined computer methods and analyses. El Khoudri et al. [33] developed a numerical method to evaluate the seismic vulnerability using incremental dynamic analysis to predict the distribution of expected losses due to an earthquake event. However, the proposed approach does not provide an explicit formulation of a vulnerability index but instead employs nonlinear dynamic analysis for the construction of fragility curves. The study developed by Emami and Halabian [34] aims at establishing fragility relationships as well as collapse probability of high-rise RC core-wall buildings under maximum considered earthquake ground motions using the Incremental Dynamic Analysis (IDA) and multi-directional pushover analysis. Also this study, focused on the structural typology of tall buildings, is not focused on the formulation of a vulnerability index, but on the probabilistic assessment of the collapse through fragility analysis. In some cases, third-level analyses can be used to extract general information related to a group of buildings. These methodologies aim at providing useful information which can be adopted in second-level methodologies [35]. performed both static and dynamic nonlinear analyses on a sample of 15 low-rise existing school buildings to assess the associated fragility curves. The proposed methodologies revealed satisfactory results for both individual building-level and regional fragility curves [36]. proposed an alternative Displacement-Based Assessment (DBA) that relates the structural vulnerability to the deformations instead of the forces. The effectiveness of the DBAs was analyzed by Ref. [37] to investigate their practical applications. Both static and dynamic DBAs were adopted in the analysis of a set of 36 RC continuous-deck bridges with different span numbers, pier heights, and transverse stiffness. The New Zeland Society for Earthquake Engineering [38] recommends the use of DBAs that can be based on several analysis procedures to assess the structural capacity. The results of the seismic performance of single structures are then computed as capacity/demand ratio.

Most building codes in the world implicitly propose different seismic vulnerability assessment methods. RVS is the simplest method introduced by FEMA154 [39]. It is based on performing basic structural computations for a quick evaluation of building portfolios in large areas [40]. However, it does not require performing a detailed seismic analysis of individual buildings. Accordingly, buildings are classified into two categories using cutoff scores performing sidewalk surveys: (i) those acceptable for life safety level of performance or (ii) seismically unsafe. Based on this classification, the final needs for rehabilitation can be determined. Later on, the National Research Council of Canada [41] introduced a final cutoff score following the procedure provided by FEMA154 [39] for both structural and non-structural components. It also takes into account the building occupancy class and its importance. Upon the computed score, the appropriate rehabilitation strategy can be made. FEMA310 [42] improved the RVS method by proposing a three-tiered procedure, in which Tier 1 is the screening phase in the form of checklists for a fixed performance level. Tier 2 and Tier 3 are performed where the outcome of Tier 1 is too conservative or a significant economic advantage was verified. Eurocode-8 [43] provides criteria for the seismic vulnerability assessment of existing buildings incorporating a model uncertainty factor to include uncertainties related to the analysis. The New Zealand Guidelines [44] proposes the visual screening procedure of ATC-21 [45] and computes the final building vulnerability score by aggregation of fourteen structural indicators that are representative of the potential building's damage components. Recently, the new Italian standard [46] introduced a method to compute the seismic vulnerability index of existing buildings based on the bearable maximum seismic action. In previous codes, the attention was focusing on the structural verification to withstand an assigned level of seismic demand, while the new Italian code is converging toward the performance level of the building [47]. However, it does not provide any specific procedure for the computation of the maximum bearing capacity [46].

The previously mentioned studies and guidelines on the vulnerability assessment of existing buildings made meaningful advancements in this field of research considering a wide class of conditions and building typologies. However, they do not provide much knowledge on the formulation of specific vulnerability indices for existing buildings, proposing an accurate step-by-step procedure that includes field tests with nonlinear structural analyses (Third Level procedures). On the contrary, this contribution explores these aspects and proposes different indices to quantitatively measure the building vulnerability starting from the new Italian seismic provision. This paper proposes three methods for assessing the vulnerability index with application to a real case study building. The effects of different factors that may affect the computation have been addressed: type of analysis (nonlinear, static or dynamic), collapse drift threshold, record selection, Intensity Measure (IM) parameter for the seismic action. The results are then compared with those obtained through existing alternative first and second-level approaches, while the influence of the motion selection in evaluating the building capacity is also investigated.

The proposed methods to assess the vulnerability indices include both practical approaches such as pushover analysis and more refined approaches through nonlinear time history analyses. Furthermore, the paper provides also comparisons and discussion about the different methodologies that may be useful for engineering practice.

Section snippets

Methodologies for seismic vulnerability assessment

The seismic vulnerability of an existing building depends on different conditions such as aging, poor maintenance, outdated design, materials’ characteristics, the construction place, and natural events, etc. According to the current Italian standard[46], the seismic vulnerability index for an existing building is defined by a new parameter ζE that is the ratio between the maximum bearable seismic action of the existing structure and that one required to design a new one on the same site with

Case study

An existing school building located in Northern Italy has been used as a case study to compare the three proposed vulnerability assessment indices. The building is an RC structure with shear walls in both principal directions. Two types of columns cross-sections have been used (0.35 × 0.40 and 0.40 × 0.40 m) with a mean compressive strength of 31.5 MPa. A steel reinforcement ratio of about 1.5% has been adopted. The horizontal deck is made by reinforced brick-concrete slab at each floor level.

Comparisons among different GMSM

The vulnerability indices obtained with Methods #1 and #2 assume different values compared the adopted ground motion selection procedures. Even if a detailed comparison of the GMSM approaches is out of the scope of this article, the following discussion is meant to investigate the variability range of the estimated vulnerability indices and the influence of the adopted GMSM. Therefore, the dynamic response dispersion βcollapse around the mean values of the PGAcollapse and the Sa for Method #1

Concluding remarks

This paper presents three different vulnerability assessment methods at collapse limit state using nonlinear dynamic analysis (Method #1 and Method #2) and pushover analysis (Method #3) providing different vulnerability indices. The ground motion selection has been conducted comparing three different approaches from the literature. A reinforced concrete building has been used as a case study to compare the results selecting different sets of drifts associated with the collapse limit states.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The research leading to these results has received funding from the European Research Council under the Grant Agreement n° ERC_IDEAL RESCUE_637842 of the project IDEAL RESCUE - Integrated Design and Control of Sustainable Communities During Emergencies. Furthermore, the authors would like to thank the Municipality of Melzo (Italy) in which the school building belongs as well as Dr. V. Villa (Politecnico di Torino) for her endless support and providing the BIM model. Prof. Farhad Ansari

References (70)

  • Roberto Gentile et al.

    Effectiveness of the displacement-based seismic performance assessment for continuous RC bridges and proposed extensions

    Eng. Struct.

    (2020)
  • Helmut Krawinkler et al.

    Seismic drift and ductility demands and their dependence on ground motions

    Eng. Struct.

    (2003)
  • G. Cockburn et al.

    Earthquake disaster risk index for Canadian cities using Bayesian belief networks

    Georisk

    (2012)
  • N. Alam et al.

    Buildings' seismic vulnerability assessment methods: a comparative study

    Nat. Hazards

    (2012)
  • Marc Hill et al.

    Comparison of building damage scales and damage descriptions for use in earthquake loss modelling in Europe

    Bull. Earthq. Eng.

    (2008)
  • FEMA1999

    Earthquake Loss Estimation Methodology: User's Manual HAZUS 99

    (1999)
  • Martin Bertogg et al.

    Vulnerability functions derived from loss data for insurance risk modelling: findings from recent earthquakes

  • G Michele Calvi et al.

    Development of seismic vulnerability assessment methodologies over the past 30 years

    ISET J. Earthq. Technol.

    (2006)
  • Shigeyuki Okada et al.

    Classifications of structural types and damage patterns of buildings for earthquake field investigation

  • Philippe Guéguen et al.

    A simplified approach for vulnerability assessment in moderate-to-low seismic hazard regions: application to Grenoble (France)

    Bull. Earthq. Eng.

    (2007)
  • Robin Spence et al.

    The Global Earthquake Vulnerability Estimation System (GEVES): an approach for earthquake risk assessment for insurance applications

    Bull. Earthq. Eng.

    (2008)
  • Kerstin Lang et al.

    On the seismic vulnerability of existing buildings: a case study of the city of Basel

    Earthq. Spectra

    (2004)
  • Haluk Sucuoğlu et al.

    A screening procedure for seismic risk assessment in urban building stocks

    Earthq. Spectra

    (2007)
  • Stefano Colombini

    Vulnerabilità sismica di edifici esistenti in cemento armato e in muratura

    (2014)
  • A Zamani Noori et al.

    Smart cities to improve resilience of communities

  • M.C. Djemai et al.

    Seismic vulnerability assessment of bridges using analytical hierarchy process

  • Roberto Gentile et al.

    From rapid visual survey to multi-hazard risk prioritisation and numerical fragility of school buildings

    Nat. Hazard. Earth Syst. Sci. Discus.

    (2019)
  • Ahmet Yakut et al.

    Seismic vulnerability assessment using regional empirical data

    Earthq. Eng. Struct. Dynam.

    (2006)
  • Md Shafiul Islam et al.

    Rapid Seismic Capacity Evaluation Method of RC Buildings with Masonry Infill

    (2017)
  • Daniele Perrone et al.

    Rapid Visual Screening for Seismic Evaluation of RC Hospital Buildings. Paper Read at Structures

    (2015)
  • Sudhir K. Jain et al.

    A proposed rapid visual screening procedure for seismic evaluation of RC-frame buildings in India

    Earthq. Spectra

    (2010)
  • Ravi Sinha et al.

    A National Policy for Seismic Vulnerability Assessment of Buildings and Procedure for Rapid Visual Screening of Buildings for Potential Seismic Vulnerability

    (2004)
  • G.E. Thermou et al.

    Assessment indices for the seismic vulnerability of existing RC buildings

    Earthq. Eng. Struct. Dynam.

    (2011)
  • S.R. Uma et al.

    Evaluation of displacement‐based vulnerability assessment methodology using observed damage data from Christchurch

    Earthq. Eng. Struct. Dynam.

    (2014)
  • M. El Khoudri et al.

    Evaluation de la vulnérabilité sismique des bâtiments en béton arméen utilisant l'analyse dynamique incrémentale (Seismic vulnerability assessment of reinforced concrete buildings using Incremental Dynamic Analysis IDA)

    J. Mater. Environ. Sci.

    (2016)
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