Konstantinos BAKALIS
Konstantinos was born in Larissa, Greece in 1986. He holds a Degree in Civil Engineering from the Aristotle University of Thessaloniki (AUTh, 2010) and an MSc with merit in Earthquake Engineering with Disaster Management from the University College London (UCL, 2011). Following the completion of his MSc studies, he worked as a Discipline Engineer for the oil & gas engineering consultancy DeepSea UK as part of their Subsea Structures group. In June 2013 he joined the Institute of Steel Structures at the National Technical University of Athens (NTUA), where he obtained his PhD (2018). He then served as a Research Associate for the NTUA jointly with his Postdoctoral research activities at the University of Thessaly. He is currently a Swiss Federal Commission Scholar and Marie Skłodowska-Curie Fellow at École Polytechnique Fédérale de Lausanne (EPFL) where he explores data-driven methods that can be used for the performance-based design and assessment of infrastructure assets under uncertainty. His research is focused on the seismic risk assessment of steel structures and community critical facilities, using nonlinear analysis methods, as well as probabilistic concepts. He has also participated in several national and international research projects, funded by the EU Research Executive Agency, the EU Research Fund for Coal and Steel, the Hellenic General Secretariat for Research and Technology and the Swiss Federal Commission for Scholarships.
JOURNAL PAPERS
Now - 2014
Melissianos V.E., Karaferis N.D., Bakalis K., Kazantzi A.K., Vamvatsikos D. (2024). Operational status effect on the seismic risk assessment of oil refineries. International Journal of Disaster Risk Reduction, 113: 104842.
Abstract | The operational status of an oil refinery (type and scale of operations that take place at any time instance) largely determines the amount of fuel produced, circulated within the facility, and stored in tanks. This status is affected by seasonality, periods of peak or low demand, as well as periods of routine maintenance. However, it is an aspect that is typically neglected even though it stands out among the factors that determine the seismic performance of several critical industrial assets, such as the storage tanks, as well as the consequences of any potential failure. An open-source refinery testbed is employed herein to demonstrate the effect of the refinery’s operational status on the seismic risk estimates. Alternative realistic operational scenarios are developed following typical industry practices and are arranged over a time period between two refinery major maintenance shutdown events. The most probable damage state is selected for each asset to identify the most vulnerable ones. Based on the type and importance of the impacted assets, the potential consequences are determined at the facility level. Resulting estimates are very different if an earthquake strikes during a regular/high/low-demand period, or a maintenance period. The framework can be utilized to identify the locations within the refinery that may trigger cascading failures and secondary damages, should their assets be damaged by a seismic event. The outcomes can be exploited by stakeholders, risk engineers, and emergency action planners for developing customized and businesslike procedures to enhance the seismic resilience of the facility.
Melissianos V.E., Karaferis N.D., Bakalis K., Kazantzi A.K., Vamvatsikos D. (2024). Hazard, exposure, fragility, and damage state homogenization of a virtual oil refinery testbed for seismic risk assessment. Earthquake Spectra. 2024;0(0).
Abstract | A virtual mid-size oil refinery, located in a high-seismicity region of Greece, is offered as a testbed for developing and testing system-level assessment methods due to direct impact from seismic shaking and without considering geohazards, such as liquefaction and surface faulting. Its characterization is offered in a dedicated repository (https://doi.org/10.5281/zenodo.11419659) and it comprises (a) a comprehensive probabilistic treatment of seismic hazard tied to an open-source seismological model; (b) a hazard-consistent set of ground motion records; (c) a full geolocated exposure model with all pertinent critical assets, namely tanks, pressure vessels, process towers, chimneys, equipment-supporting buildings, and a flare; (d) the corresponding record-wise asset demands and summarized fragilities derived via nonlinear dynamic analyses on reduced-order numerical models. Background information is provided on all refinery assets to delineate their role in the refining process. Furthermore, an explicit homogenization of the damage states is proposed, translating them from the asset level to the refinery system level considering the importance of each asset on the overall operational and structural integrity of the refinery. The results can form the basis of any follow-up study that seeks to characterize the effects of cascading failures (fires, explosions), mitigation measures, seismic sequences, and operational constraints on the functionality, risk, and resilience of refining facilities.
Kazantzi A.K., Karaferis N.D., Melissianos V.E., Bakalis K., Vamvatsikos D. (2022). Seismic fragility assessment of building-type structures in oil refineries. Bulletin of Earthquake Engineering, 20: 6853–6876.
Abstract | A seismic fragility assessment methodology is presented for equipment-supporting reinforced concrete and steel buildings that are typically encountered in oil refineries. Using a suite of hazard-consistent ground motions and reduced-order models, incremental dynamic analysis is performed to obtain the seismic demand of the structural systems examined. Appropriate drift- and floor acceleration-sensitive failure modes are considered to define the limit state capacities of the supporting structure and the nested non-structural process equipment. Special care is exercised on the demand and capacity representation of structural and non-structural components, offering a transparent roadmap for undertaking analytical fragility assessment for equipment-supporting buildings typical to an oil refinery. The findings and the proposed methodology can be exploited by designers and facility managers for mitigating the risk of failure prior to the occurrence of an earthquake event, for designing the pertinent structures and their non-structural components by means of a risk-aware performance-based methodology, or as feed data in early warning systems.
Karaferis N.D., Kazantzi A.K., Melissianos V.E., Bakalis K., Vamvatsikos D. (2022). Seismic fragility assessment of high-rise stacks in oil refineries. Bulletin of Earthquake Engineering. 20:6877–6900.
Abstract | The seismic fragility is assessed for typical high-rise stacks encountered in oil refineries, namely process towers, chimneys, and flares. Models of varying complexity were developed for the structures of interest, attempting to balance computational complexity and accuracy regarding the structural dynamic and strength properties. The models were utilized along with a set of hazard-consistent ground motions for evaluating the seismic demands through incremental dynamic analysis. Demand/capacity-related uncertainties were explicitly accounted for in the proposed framework. Damage states were defined for each of the examined structure considering characteristic serviceability and ultimate limit states. Τhe proposed resource-efficient roadmap for the analytical seismic fragility assessment of typical high-rise stacks, as well as the findings of the presented research work are available to be exploited in seismic risk assessment studies of oil refineries.
Bakalis K, Karamanos SA. (2021). Uplift mechanics of unanchored liquid storage tanks subjected to lateral earthquake loading. Thin-Walled Structures, 158: 107145
Abstract | Motivated by the seismic response of unanchored liquid storage tanks, their uplift mechanism under strong lateral loading is examined. Using three-dimensional finite element models, nonlinear static analysis is conducted to define the moment-rotation relationship of uplifting tanks resting on rigid foundation and describe the evolution of critical response parameters with increasing level of lateral loading. Meridional and hoop stress, as well as their distribution and evolution with increased uplift are computed. Comparing the numerical results with the corresponding results from anchored tanks, striking differences are observed on the values of compressive meridional stresses and their distribution around the tank circumference. Cyclic analysis, associated with repeated uplift at both sides of the tank, is also performed, to obtain the corresponding hysteretic response and verify the assumption of nonlinear-elastic tank behaviour, used in several previous works. Finally, an analytical solution is developed, capable of describing tank uplift in an efficient manner. The analytical solution accounts for the special features of uplift, obtained from the finite element solution, and can be used for simple and reliable assessment of seismic performance in unanchored liquid storage tanks.
Tsarpalis P., Bakalis K., Thanopoulos P., Vayas I., Vamvatsikos D. (2020). Pre-normative assessment of behaviour factor for lateral load resisting system FUSEIS pin-link. Bulletin of Earthquake Engineering, 18(6): 2681–2698
Abstract | A pre-normative assessment is presented for the q-factor of the FUSEIS pin-link steel lateral load resisting system for use in low/mid-rise buildings, within Eurocode 8. It is achieved by using the INNOSEIS methodology, a performance based methodological procedure to define behaviour factors for innovative systems. Applying this methodology, consistent behaviour factors can be obtained based on the definition of a set of structures to represent each class of buildings, with the use of nonlinear static and dynamic analysis methods and the incorporation of the effect of aleatory and epistemic uncertainty on the actual systems’ performance, to reach a uniform level of safety across the entire building population.
Vamvatsikos D., Bakalis K., Kohrangi M., Pyrza S., Castiglioni C., Kanyilmaz A., Morelli F., Stratan A., D’ Aniello M., Calado L., Proença J.M., Degee H., Hoffmeister B., Pinkawa M., Thanopoulos P., Vayas I. (2020). A risk-consistent approach to determine EN1998 behaviour factors for lateral load resisting systems. Soil Dynamics and Earthquake Engineering, 131: 106008
Abstract | A risk-consistent approach is proposed for the evaluation of behaviour factors that are compatible with Eurocode 8, using nonlinear static and dynamic analysis. It comprises seven discrete steps, involving hazard assessment and record selection at multiple sites, designing and modelling multiple archetype buildings and assessing their performance vis-à-vis target safety objectives. In all cases, uncertainty is incorporated and propagated to the final results whereby a flexible verification procedure is offered to account for the confidence of the investigator on the data available. An example application is offered on steel concentrically braced frames, highlighting the need for selecting appropriate performance targets for Europe. The overall added value goes beyond the current state of art, offering a consistent risk basis for the seismic design of different systems that is compatible with current uniform hazard design spectra and can benefit from future risk-targeted hazard maps.
Bakalis K., Kazantzi A.K., Vamvatsikos D., Fragiadakis M. (2019). Seismic performance evaluation of liquid storage tanks using nonlinear static procedures. ASME Journal of Pressure Vessel Technology, 141(1), 010902
Abstract | A simplified approach is presented for the seismic performance assessment of liquid storage tanks. The proposed methodology relies on a nonlinear static analysis, in conjunction with suitable “strength ratio-ductility-period” relationships, to derive the associated structural demand for the desired range of seismic intensities. In the absence of available relationships that are deemed fit to represent the nonlinear-elastic response of liquid storage tanks, several incremental dynamic analyses are performed for variable post-yield hardening ratios and periods in order to form a set of data that enables the fitting of the response. Following the identification of common modes of failure such as elephant’s foot buckling (EFB), base plate plastic rotation, and sloshing wave damage, the aforementioned relationships are employed to derive the 16%, 50%, and 84% percentiles for each of the respective response parameters. Fragility curves are extracted for the considered failure modes, taking special care to appropriately quantify both the median and the dispersion of capacity and demand. A comparison with the corresponding results of incremental dynamic analysis (IDA) reveals that the pushover approach offers a reasonable agreement for the majority of failure modes and limit states considered.
Bakalis K., Kohrangi M., Vamvatsikos D. (2018). Seismic intensity measures for liquid storage tanks. Earthquake Engineering and Structural Dynamics, 47(9): 1844-1863
Summary | A series of scalar and vector intensity measures is examined to determine their suitability within the seismic risk assessment of liquid storage tanks. Using a surrogate modelling approach on a squat tank that is examined under both anchored and unanchored support conditions, incremental dynamic analysis is adopted to generate the distributions of response parameters conditioned on each of the candidate intensity measures. Efficiency and sufficiency metrics are used in order to perform the intensity measure evaluation for individual failure modes, while a comparison in terms of mean annual frequency of exceedance is performed with respect to a damage state that is mutually governed by the impulsive and convective modes of the tank. The results reveal combinations of spectral acceleration ordinates as adequate predictors, among which the average spectral acceleration is singled out as the optimal solution. The sole exception is found for the sloshing‐controlled modes of failure, where mainly the convective period spectral acceleration is deemed adequate to represent the associated response due to their underlying linear relationship. A computationally efficient method in terms of site hazard analysis is finally proposed to serve in place of the vector‐valued intensity measures, providing a good match for the unanchored tank considered and a more conservative one for the corresponding anchored system.
Bakalis K., Vamvatsikos D. (2018). Seismic fragility functions via nonlinear response history analysis. ASCE Journal of Structural Engineering, 144(10): 04018181
Abstract | The estimation of building fragility, i.e., the probability function of seismic demand exceeding a certain limit state capacity given the seismic intensity, is a common process inherent in any seismic assessment study. Despite this prolific nature, the theory and practice underlying the various approaches for fragility evaluation may be opaque to their users, especially regarding the handling of demand and capacity uncertainty, or the generation of a single fragility curve for multiple failure conditions, using either an intensity measure or engineering demand parameter basis. Hence, this paper provides a comprehensive guide that compiles all necessary information for generating fragility curves of single structures based on the results of nonlinear dynamic analysis. Although various analysis methods are discussed, incremental dynamic analysis is invoked to clearly outline different methodologies that rely either on response parameter or intensity measure ordinates. Step-by-step examples are presented for each case, under both a deterministic and an uncertain limit state capacity framework, using limit states that range from simple structural damage to the global collapse of the structure.
Bakalis K., Vamvatsikos D., Fragiadakis M. (2017). Seismic risk assessment of liquid storage tanks via a nonlinear surrogate model. Earthquake Engineering and Structural Dynamics, 46(15): 2851-2868
Abstract | A performance‐based earthquake engineering approach is developed for the seismic risk assessment of fixed‐roof atmospheric steel liquid storage tanks. The proposed method is based on a surrogate single‐mass model that consists of elastic beam‐column elements and nonlinear springs. Appropriate component and system‐level damage states are defined, following the identification of commonly observed modes of failure that may occur during an earthquake. Incremental dynamic analysis and simplified cloud are offered as potential approaches to derive the distribution of response parameters given the seismic intensity. A parametric investigation that engages the aforementioned analysis methods is conducted on 3 tanks of varying geometry, considering both anchored and unanchored support conditions. Special attention is paid to the elephant’s foot buckling formation, by offering extensive information on its capacity and demand representation within the seismic risk assessment process. Seismic fragility curves are initially extracted for the component‐level damage states, to compare the effect of each analysis approach on the estimated performance. The subsequent generation of system‐level fragility curves reveals the issue of nonsequential damage states, whereby significant damage may abruptly appear without precursory lighter damage states.
Bakalis K., Fragiadakis M., Vamvatsikos D. (2017). Surrogate modeling for the seismic performance assessment of liquid storage tanks. ASCE Journal of Structural Engineering, 143(4): 04016199
Abstract | A three-dimensional surrogate model is presented for the seismic performance assessment of cylindrical atmospheric liquid storage tanks. The proposed model consists of a concentrated fluid mass attached to a single vertical beam-column element that rests on rigid beam-spokes with edge springs. The model is suitable for rapid static and dynamic seismic performance assessment. Contrary to other simplified models for tanks, its properties are determined through a simple structural analysis that can be performed in any nonlinear analysis software, without the need for complex finite-element models. The results compare favorably to those of three-dimensional finite-element models on three tanks of varying aspect ratios. A step-by-step example of the modeling procedure is presented for a squat unanchored tank, and a sensitivity analysis is conducted in order to investigate the effect of various modeling parameters on the seismic response of the proposed tank model.
CONFERENCE PAPERS
Now - 2014
Grajales-Ortiz C., Melissianos V.E., Bakalis K., Kohrangi M., Vamvatsikos D., Bazzurro P. (2024). Relationships Between Earthquake-induced Damage And Material Release For Liquid Storage Tanks. Proceedings of the 18th World Conference on Earthquake Engineering, Milan, Italy.
Abstract | Liquid storage tanks are critical components in the industrial sector, as large amounts of toxic, volatile, or flammable substances are stored in them. Motivated by their underlying vulnerability to physical damage caused by earthquakes, an empirical relationship is proposed to link the specific type of earthquakeinduced structural damage with the extent of material release that can potentially be triggered. To achieve this goal, an extensive database of recorded seismic-related failures of industrial tanks is examined and suitable damage states are identified. For each tank in the database, material release levels consistent with industry standard procedures for risk assessment are associated with the pertinent damage states. This is done with the consideration of characteristics that affect the seismic behaviour of tanks, such as aspect ratio and filling level. As a result, we propose a series of event trees for industrial liquid storage tanks that associate damage states and combinations thereof with material release levels. These event trees can be exploited for consequence analysis (e.g., to analyse the propagation of damage, or potential domino effects) within the context of seismic risk assessment for industrial facilities.
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Bakalis K., Kazantzi A.K. (2023). Composite floors under human-induced vibrations. Proceedings of the 10th Hellenic National Conference on Steel Structures, Athens, Greece. (in greek)
Abstract | Composite steel-concrete floor systems are widely used in modern construction for achieving long-spans with a low number of intermediate columns. The design of such slender and lightweight floor systems is typically governed by the serviceability limit state requirements, associated with deformations, human comfort perception, and vibration tolerances. To guide designers through the process of delivering floors that are not prone to human-induced vibrations, and hence imposing a feeling of discomfort to their users, a number of design guidelines of variable complexity have been developed in the past few decades [1,2]. In their simplest form, such guidelines adopt several deterministic assumptions regarding the floor damping, the imposed loads, the connection rigidity under service loads, the step frequency, the footpath and the human weight. In this study, sources of uncertainty are discussed. A numerical grillage-based floor model is also presented, that could be utilised for extracting the needed engineering demand parameters for undertaking an assessment of such floor systems when subjected to walking-induced vibrations.
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Melissianos V.E., Kazantzi A.K., Karaferis N., Bakalis K., Vamvatsikos D. (2023). Reduced-order models of steel structures for the seismic risk assessment of oil refineries. Proceedings of the 10th Hellenic National Conference on Steel Structures, Athens, Greece.
Abstract | Ensuring the structural and operational integrity of oil refineries in case of an earthquake event is of utmost importance for the society, the environment, and the economy. A potential failure in such critical facilities may trigger a number of undesirable situations, such as fire, injuries, environmental pollution, etc. Hence, improving safety plan and increasing seismic resilience is a necessity that requires the development of reliable models and seismic risk assessment tools. Towards this direction, this paper presents a seismic fragility study of two characteristic steel high-rise stacks encountered in oil refineries, namely a relatively low-rise chimney and a process tower. The developed of reduced-order numerical models, the selection of appropriate engineering demand parameters to capture the seismic response of the structures, the calculation of the fragility curves, and finally the evaluation of the overall seismic response are presented. The results could be exploited in the context of a seismic risk assessment study of an oil refinery, as an integrated system.
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Melissianos V.E., Karaferis N.D., Kazantzi A.K., Bakalis K., Vamvatsikos D. (2023). Towards seismic resilience of industrial facilities: the case study of an oil refinery. Proceedings of the SECED 2023 Conference, Cambridge, UK.
Abstract | Crude oil refineries are high-importance infrastructure that play a key role in the energy supply chain. Securing the operational and structural integrity of refineries in the aftermath of an earthquake is crucial for avoiding the undesirable consequences of a Natural-Technological (NaTech) incident, such as injuries, environmental pollution, business interruption, and monetary losses. Refineries are designed, constructed, maintained, and operated under a strict framework of standards and regulations. Still, seismic-related NaTech incidents are occurring. Thus, to assess with more confidence and consequently improve, if needed, their seismic resilience, a coherent performance-based framework needs to be utilised, that accounts for the refinery as an integrated system comprising a variety of structural typologies, such as buildings, tanks, and high-rise stacks. These structures have very diverse dynamic properties and hence seismic responses. Towards this objective, a virtual crude oil refinery is examined herein as a case study. The aim is to showcase the steps of a seismic risk assessment framework when applied to such infrastructures, focusing on the evaluation of the seismic hazard, the development of the exposure model, the numerical analysis of the structures, and the preliminary damage assessment of the facility using different earthquake scenarios.
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Kazantzi A.K., Karaferis N.D., Melissianos V.E., Bakalis K., Vamvatsikos D. (2022). Seismic fragility assessment of two low-rise equipment-supporting RC industrial buildings. Proceedings of the 3rd European Conference on Earthquake Engineering and Seismology (3ECEES), Bucharest, Romania.
Abstract | Open-frame reinforced concrete (RC) buildings for supporting essential mechanical/electrical equipment are encountered in almost all industrial plants. Hence, to ensure the undisrupted operation of an industrial facility, the integrity of such structural assets along with their nested nonstructural components should be verified against a spectrum of natural and man-made hazards. Focusing on the earthquake peril, this study presents an analytical seismic fragility assessment framework for two RC equipment-supporting buildings that are deemed typical to an oil refinery. The proposed fully-probabilistic fragility concept, utilises reduced-order building models for the evaluation of the induced seismic demands and accounts for both drift and acceleration-sensitive failure modes in the definition of the damage states. The findings can be exploited by designers and facility managers for developing efficient pre- and post-event risk-aware mitigation/response strategies and are delivered in a manner that can be readily integrated into the seismic performance assessment framework of an entire industrial facility.
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Melissianos V.E., Karaferis N.D., Kazantzi A.K., Bakalis K., Vamvatsikos D. (2022). An integrated model for the seismic risk assessment of an oil refinery. Proceedings of the 3rd International Conference on Natural Hazards & Infrastructure ICONHIC 2022, Athens, Greece.
Abstract | Oil refineries play a key role in the energy supply chain. Safeguarding the integrity of such high-importance facilities against natural hazards is crucial because a potential failure may result in a sequence of unwanted events, spanning from business disruption to uncontrolled leakage and/or major accidents. Despite the strict criteria enforced during the design, construction, maintenance, and operation of an oil refinery, Natural-Technological events caused by earthquakes still occur.
Oil refining is a complex process that involves a variety of structural typologies, such as buildings, tanks, chimneys, pipe-racks, pressure vessels, and process towers. These structures have fundamentally different dynamic properties and seismic responses. A comprehensive seismic risk assessment framework is thus required to account for the refinery as an integrated system and provide information about both the structural and operational integrity of the individual assets and the system. In the present study, a virtual crude oil refinery is examined as a case study to demonstrate the steps of a preliminary seismic risk assessment framework, consisting of the seismic hazard calculation, the development of the exposure model, the analysis of the structures at risk, and the damage assessment of the facility. Scenario-based results are presented for the refinery and the critical assets are identified.
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Vamvatsikos D., Melissianos V., Kostaridis A., Kazantzi A.K, Karaferis N., Chatzidaki A., Diagourtas D., Bakalis K. (2021). The PANOPTIS-INFRASTRESS framework for infrastructure risk assessment. Proceedings of the 8th International Conference on Civil Protection & New Technologies (SafeGreece 2021), Athens, Greece.
Melissianos V., Karaferis N., Vamvatsikos D., Kazantzi A., Bakalis K. (2021). An integrated model for the seismic risk assessment of an oil refinery. Proceedings of the ICONHIC2022 – Preparatory Workshop 2021, Athens, Greece.
Abstract |
Bakalis K, Lignos DG. (2021). Effect of Modelling Uncertainty on Nonlinear Simulations of a 4-Storey Steel Frame Building Tested Through Collapse. Proceedings of the 13th International Conference on Structural Safety and Reliability (ICOSSAR 2021), Shanghai, P.R. China.
Bazzurro P, Kohrangi M, Bakalis K, Vamvatsikos D. (2020). Bi-directional Conditional-Spectra-Based Record Selection for Horizontal and Vertical Ground Motions. Proceedings of the 17th World Conference on Earthquake Engineering (17WCEE), Sendai, Japan
Abstract | Conditional Spectra (CS) based record selection is a state-of-the-art approach to select sets of records for performing nonlinear response time history analysis consistent with the seismic hazard at a specific site. So far this method has been developed and applied mainly to select horizontal components of the ground motion. There are many structural or nonstructural building components, however, and entire structures that are also sensitive to the vertical component of ground motion, e.g., due to concurrent vertical and horizontal deformability, uplifting and/or rocking. Thus, we aim here to further extend the CS approach to select a set of hazard consistent 3-component records. The proposed method is applied to the risk assessment of a liquid storage tank located at a site of major oil refineries in Elefsina, Greece. Tanks are prone to uplifting due to horizontal excitations, a behavior that can be exacerbated when the vertical component of the ground motion is considered. Therefore, a realistic seismic assessment of such structures cannot be carried out without simultaneously accounting for all three translational components of the ground motion. To this end in the case study we tested different record selection approaches with and without consideration of the vertical motions, as well as with and without vertical ground motions hazard consistency. Overall, there is a non-negligible dependence of the tank response to the effects of the vertical component, which should be included. Neglecting it typically result to an underestimation of about 20%. In addition, we recommend incorporating the hazard consistency of the vertical component in the record selection because it does have an impact on the tank response.
Chatzidaki A., Bakalis K., Vamvatsikos D. (2020). Seismic resilience assessment for the G7 highway bridge in Greece. Proceedings of the 11th European Conference on Structural Dynamics (EURODYN 2020), Athens, Greece
Abstract | The seismic risk is assessed for two twin bridges, one per direction, forming the G7 branch of the Egnatia Odos highway in Greece. These are structurally independent horizontally-curved cantilevered-deck three-span reinforced concrete structures with a monolithic pier-to-deck connection that have been designed circa 2004 according to Greek and European standards. The aim is to develop a tool for pre-event risk assessment and rapid post-event inspection of critical road infrastructure by combining hazard, vulnerability and sensor information (where available) to predict the resulting consequences. To enhance the assessment resolution, a component-based approach is followed, allowing us to evaluate damage scenarios for individual critical components (i.e., piers and bearings) and propagate them to the system-level performance. Consequences are quantified in terms of repair losses, downtime, and traffic capacity losses, the latter identified as the number of closed lanes and the allowable speed limit for the open ones. This allows tracing back the consequences after an event to individual bridge components to help road operators establish bridge inspection prioritization protocols and manage associated incidents, facilitating the rapid assessment of the state of the bridge and optimal recovery to full functionality.
Bakalis K, Karamanos SA. (2020). Exploring the Uplift Mechanics of Unanchored Liquid Storage Tanks Under Seismic Loading. Proceedings of the EURODYN 2020: XI International Conference on Structural Dynamics, Athens, Greece
Bakalis K., Vamvatsikos D., Grant D.N., Mistry A. (2019). Downtime assessment of base-isolated liquid storage tanks. Proceedings of the SECED 2019 Conference, Greenwich, UK
Abstract | Seismic base isolation is examined as a design alternative for supporting industrial facility liquid storage tanks against earthquake loading. A 160,000m3 liquid storage tank is adopted as a case study, for which two designs are assessed, one with and one without base isolation. Using a nonlinear surrogate model and a set of ground motion records selected using the conditional spectrum approach for the average spectral acceleration intensity measure, Incremental Dynamic Analysis is employed to derive seismic fragility curves. Consequences of damage are evaluated in terms of downtime, considering the characteristics of petrochemical storage tanks, whereby any repair requires a lengthy list of actions dictated by health and safety requirements. The results reveal considerable benefits when base-isolation is employed, by drastically reducing downtime when sufficient displacement capacity is provided in the isolators.
Bakalis K, Karamanos SA. (2019). Analytical Methods for the Seismic Response Estimation of Liquid Storage Tanks. Proceedings of the 4th Hellenic Conference on Earthquake Engineering and Engineering Seismology, Athens, Greece (in greek)
Abstract | Σκοπός της παρούσας μελέτης είναι η παρουσίαση μιας εις βάθος διερεύνησης της μηχανικής απόκρισης των ελεύθερα εδραζόμενων δεξαμενών αποθήκευσης υγρών υπό πλευρικά σεισμικά φορτία, καθώς και την επακόλουθη επιρροή του ανασηκώματος σε παραμέτρους όπως η τάση του τοιχώματος. Επιδιώκει επίσης να προσφέρει πληροφορίες σχετικά με την υστερητική απόκρισή τους υπό ανακυκλιζόμενη φόρτιση μεταβλητού εύρους. Ως απώτερος στόχος ορίζεται ο καθορισμός του πλαισίου μιας αναλυτικής λύσης η οποία προσφέρει αξιόπιστες εκτιμήσεις της καμπύλης αντίστασης δεξαμενών, διευκολύνοντας έτσι την αξιόπιστη εφαρμογή μεθόδων για την εκτίμηση της σεισμικής επιτελεστικότητας σε κρίσιμες βιομηχανικές εγκαταστάσεις.
Bakalis K., Vamvatsikos D. (2018). Seismic vulnerability assessment for liquid storage tank farms. Proceedings of the 16th European Conference on Earthquake Engineering, Thessaloniki, Greece
Abstract | A seismic vulnerability estimation procedure is developed for liquid storage tank-farms, specifically ensembles of atmospheric tanks that are interconnected to provide enhanced storage capacity for a given liquid product. All pertinent sources of uncertainty are considered together with associated intra-and inter-structure correlations, while particular attention is paid to the effect of uncertainty on damage state threshold values. Appropriate decision variables are defined in view of enabling decision-making for the mitigation of seismic losses at the level of the system, rather than the individual structure, focusing on (a) the leakage of stored product and (b) the loss of storage capacity. A case study of nine tanks, evenly split in three types, is undertaken. Whenever uncertain damage state thresholds are considered, Monte-Carlo simulations reveal a significant potential for loss of containment for average spectral accelerations (AvgSa) of 0.30g. While storage capacity is proportionately impacted, a remarkable 30% of the total farm storage volume can survive an AvgSa of 0.5g, thus leaving considerable room for the drainage and repair of damaged tanks in typical operation scenarios.
Vamvatsikos D., Castiglioni C., Bakalis K., Calado L., D’ Aniello M., Degee H., Hoffmeister B., Pinkawa M., Proenca J.M., Kanyilmaz A., Morelli F., Stratan A., Vayas I. (2017). A risk-consistent approach to determine behavior factors for innovative steel lateral load resisting systems. Proceedings of the EUROSTEEL 2017 Conference, Copenhagen, Denmark
Abstract | A risk-consistent approach is proposed for the evaluation of behaviour factors that are compatible with Eurocode 8 using nonlinear static and dynamic analysis. The proposed process comprises seven discrete steps, involving hazard assessment and record selection at multiple sites, designing and modelling multiple archetype buildings and assessing their performance vis-à-vis target safety objectives. In all cases, uncertainty is incorporated and propagated to the final results whereby a flexible verification procedure is offered to account for the confidence of the investigator on the data available. The value added goes beyond the current state of art, offering a consistent risk basis for the seismic design of different systems that is compatible with current uniform hazard design spectra and future risk-targeted hazard maps.
Bakalis K., Vamvatsikos D., Pyrza S. (2017). Q-factor verification of a 3-storey concentrically braced frame via the INNOSEIS risk-based approach. Proceedings of the 9th Hellenic National Conference on Steel Structures, Larisa, Greece
Abstract | A case study example is presented to support a methodology that evaluates the design behaviour-factor on a risk-basis, using the code-compatible performance targets for life safety and global collapse. The case study employs a 3-storey concentrically braced frame with a detailed physics-based representation of braces. Nonlinear static analysis is conducted to provide an estimate of overstrength and an approximation of the behaviourfactor. Incremental Dynamic Analysis is subsequently performed to obtain a refined representation of response throughout the desired range of seismic intensity. Besides the widely-adopted first-mode spectral acceleration, state-of-the-art intensity measures such as the so-called average spectral acceleration, are used to illustrate the severity of the ground motions considered. The dynamic analysis results for the considered modes of failure are conveniently summarised into fragility functions, which are further convoluted with the seismic hazard function in order to derive the associated mean annual frequency of exceedance. A comparison among the derived and the allowable mean annual frequencies determines the applicability of the behaviour factor for the structure examined, showing that EN1998-compatible factors may accurately be evaluated.
Vamvatsikos D., Bakalis K., Vayas I., Castiglioni C., Kanyilmaz A., Morelli F., Stratan A., D’ Aniello M., Calado L., Proenca J.M., Degee H., Hoffmeister B., Pinkawa M. (2017). The INNOSEIS approach on determining EN1998-compatible behavior factors for introducing new steel lateral load resisting systems. Proceedings of the 9th Hellenic National Conference on Steel Structures, Larisa, Greece
Abstract | A risk-consistent approach is proposed for the evaluation of behaviour factors that are compatible with Eurocode 8 using nonlinear static and dynamic analysis. The proposed process comprises seven discrete steps, involving hazard assessment and record selection at multiple sites, designing and modelling multiple archetype buildings and assessing their performance vis-à-vis target safety objectives. In all cases, uncertainty is incorporated and propagated to the final results whereby a flexible verification procedure is offered to account for the confidence of the investigator on the data available. The value added goes beyond the current state of art, offering a consistent risk basis for the seismic design of different systems that is compatible with current uniform hazard design spectra and future risk-targeted hazard maps.
Vamvatsikos D., Bakalis K., Pyrza S. (2017). Q-factor verification of a 6-storey concentrically braced frame via the INNOSEIS risk-based approach. Proceedings of the COMPDYN2017 Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Rhodes, Greece
Abstract | A case study example is presented to support a methodology that evaluates the design behaviour-factor on a risk-basis, using the code-compatible performance targets for life safety and global collapse. The case study employs a 6-storey concentrically braced frame with a detailed physics-based representation of braces. Nonlinear static analysis is conducted to provide an estimate of overstrength and an approximation of the behaviour-factor. Incremental Dynamic Analysis is subsequently performed to obtain a refined representation of response throughout the desired range of seismic intensity. Besides the widely-adopted first-mode spectral acceleration, state-of-the-art intensity measures such as the so-called average spectral acceleration, are used to illustrate the severity of the ground motions considered. The dynamic analysis results for the considered modes of failure are conveniently summarised into fragility functions, which are further convoluted with the seismic hazard function in order to derive the associated mean annual frequency of exceedance. A comparison among the derived and the allowable mean annual frequencies determines the applicability of the behaviour factor for the structure examined, showing that EN1998-compatible factors may accurately be evaluated.
Bakalis K., Vamvatsikos D. (2015). Direct performance-based seismic design for liquid storage tanks. Proceedings of the SECED 2015 Conference, Cambridge, UK
Abstract | A performance-based design methodology has been developed for liquid storage tanks based on a surrogate, yet robust beam-element model. Following the identification of failure modes through Incremental Dynamic Analysis, appropriate performance levels are defined based on an existing seismic assessment methodology. The concept of Response Frequency Spectra (RFS) is proposed in view of offering a unique representation of the entire solution space for structural performance. RFS find an excellent application for the case of liquid storage tanks by adopting design parameters such as the tank wall thickness and the anchorage ratio. Although the wall thickness changes the strength capacity for the well-known Elephant’s Foot Buckling failure mode, the corresponding probabilities of exceedance are not significantly modified. On the contrary, anchorage seems to be very important as the associated probabilities may be reduced even by 50% in some cases.
Bakalis K., Fragiadakis M., Vamvatsikos D. (2015). Seismic fragility assessment of steel liquid storage tanks. Proceedings of the ASME 2015 Pressure Vessels & Piping Conference PVP2015, Boston, MA
Abstract | A seismic fragility assessment procedure is developed for atmospheric steel liquid storage tanks. Appropriate system and component-level damage states are defined by identifying the failure modes that may occur during a strong ground motion. Special attention is paid to the elephant’s foot buckling failure mode, where the estimation of the associated capacity and demand requires thorough consideration within a probabilistic framework. A novel damage state is introduced to existing procedures with respect to the uncontrollable loss of containment scenario. Fragility curves are estimated by introducing both aleatory and epistemic sources of uncertainty, thus providing a comprehensive methodology for the seismic risk assessment of liquid storage tanks. The importance of dynamic buckling is acknowledged and the issue of non-sequential damage states is finally revealed.
Bakalis K., Fragiadakis M., Vamvatsikos D. (2015). Surrogate modelling of liquid storage tanks for seismic performance design and assessment. Proceedings of the COMPDYN2015 Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Crete, Greece
Abstract | The finite element method (FEM) is often employed to create detailed models for the seismic assessment and design of liquid storage tanks. This comprehensive approach offers accuracy, which is counter-balanced however by its computational inefficiency, especially for the case of large-scale engineering problems. Regardless of the continuous evolution of computer technology, surrogate models are necessary when such problems of engineering practice are encountered. This study, attempts to develop an appropriate surrogate modelling approach, tailored for the design and seismic risk assessment of liquid storage tanks. A formulation that disregards fluid-structure-interaction is employed in view of providing a reasonable compromise between modelling complexity and error. At the same time, a simplified methodology based on nonlinear static procedures is proposed for the assessment of atmospheric tanks. The comparison with Incremental Dynamic Analysis reveals a reasonable, yet conservative in some cases, match for the damage states considered, thus offering an alternative methodology that may easily be incorporated within code-based provisions.
Bakalis K., Vamvatsikos D., Fragiadakis M. (2014). Surrogate modelling and sensitivity analysis of steel liquid storage tanks. Proceedings of the 8th Hellenic National Conference on Steel Structures, Tripoli, Greece
Abstract | Large-capacity atmospheric tanks are widely used to store liquids, such as oil or liquefied natural gas. The seismic risk of such industrial facilities is considerably higher compared to ordinary structures, since even some minor damage induced by a ground motion may have uncontrollable consequences, not only on the tank but also on the environment. Recent earthquakes have shown that heavy damage on tanks may lead to temporary loss of function, usually followed by leakage and/or fire. Therefore, a Performance-Based Earthquake Engineering (PBEE) framework should be employed for the seismic design and performance assessment of such critical infrastructure. Current design codes and guidelines have not fully adopted the performance-based concepts, while their application to industrial facilities is still under research. The proposed PBEE framework consists of a series of nonlinear response history analyses based on a simplified modelling of the tank. Our aim is to improve upon the existing body of work by offering a surrogate model that can be implemented with minimum effort for both anchored and unanchored tanks, for application within a PBEE framework using either static or dynamic analysis methods. In that sense, a robust sensitivity analysis is presented to acknowledge the uncertainties involved in the model presented.
Bakalis K., Vamvatsikos D., Fragiadakis M. (2014). Seismic reliability assessment of liquid storage tanks. Proceedings of the 2nd European Conference on Earthquake Engineering and Seismology (2ECEES), Istanbul, Turkey
Abstract | Large-capacity atmospheric tanks are widely used to store liquids, such as oil or liquefied natural gas. The seismic risk of such industrial facilities is considerably higher compared to ordinary structures, since even some minor damage induced by a ground motion may have uncontrollable consequences, not only on the tank but also on the environment. Recent earthquakes have shown that heavy damage on tanks may lead to temporary loss of essential service, usually followed by leakage and/or fire. Therefore, a Performance-Based Earthquake Engineering (PBEE) framework should be employed for the seismic performance assessment of such critical infrastructure. Current design codes and guidelines have not fully adopted the PBEE concept, while its application to industrial facilities is still at the academic level. This study provides an insight on the seismic risk assessment of liquid storage tanks using a simplified performance-based oriented modelling approach. Appropriate system and component-level damage states are defined by identifying the failure modes that may occur during a strong ground motion. Fragility curves are estimated by introducing both aleatory and epistemic sources of uncertainty, thus providing a comprehensive methodology for the seismic risk assessment of liquid storage tanks.