Akrivi CHATZIDAKI
Akrivi was born in Athens, Greece in 1992. She holds a Diploma (5-year program, integrated master) in Civil Engineering with major in Structural Engineering from the National Technical University of Athens (NTUA, 2016) and an MSc in the Analysis and Design of Earthquake Resistant Structures from the same university (2018). She joined the Institute of Steel Structures at NTUA in 2018 as PhD Candidate, working in the multi-hazard resilience assessment of highway systems and infrastructure. Her research interest is mainly focused on integrating structural modelling, computational techniques and probabilistic concepts in a coherent framework suitable for the performance and risk assessment of road infrastructure. Her research is co-funded by the Eugenides Foundation and she is, also, participating in EU-funded research projects that deal with the multi-hazard assessment of urban areas and infrastructure. Akrivi is working as a Teaching Assistant at NTUA for the compulsory undergraduate course “Steel Structures I” and the graduate course “Seismic Hazard and Risk Assessment”. Finally, Akrivi is a licensed Civil Engineer since 2018.
JOURNAL PAPERS
Now - 2019
Ruggieri S., Chatzidaki A., Vamvatsikos D., Uva G. (2022). Reduced-order models for the seismic assessment of plan-irregular low-rise frame buildings. Earthquake Engineering and Structural Dynamics, 51(14): 3327-3346.
Abstract | A procedure is presented for deriving low-complexity structural models to predict the global response of asymmetric-plan low-rise frame buildings for purposes of class-level assessment. As a compromise between employing a full-scale multi-degree-of-freedom structural model versus an equivalent single-degree-of-freedom one, the challenge is to create an idealized 3D structure with few degrees-of-freedom that can match the inelastic response of a building for which full knowledge of geometrical and mechanical properties is available. Such a 3D reduced-order model can offset the computational cost related to performing multiple nonlinear dynamic analyses within the framework of Performance-Based Earthquake Engineering. To this goal, rules and equations are proposed for achieving equivalence among the linear and nonlinear properties (e.g. mass, stiffness, strength) of the building analysed and the related 3D reduced-order model. The procedure is applied on a sample of 15 existing reinforced-concrete frame school buildings, from the province of Foggia in Southern Italy, for which the full numerical models are available. Both calibrated and uncalibrated reduced-order models are created, exploring the limitations of the proposed order-reduction in a real-life case study.
Hernández-Montes E., Chatzidaki A., Gill-Martin L.M., Aschheim M., Vamvatsikos D. (2022). A seismic design procedure for different performance objectives for post-tensioned walls. Journal of Earthquake Engineering, 26(1): 475-492.
Abstract | A method is presented for the design of unbonded post-tensioned concrete walls for seismic loading to satisfy multiple performance objectives. It takes advantage of the fact that the initial stiffness of the wall is nearly independent of the amount of post-tensioning reinforcement, and thus the fundamental period of the building can be considered to be a stable parameter in design of walls of a given cross section, independent of the degree of post-tensioning. The design spectra used to follow the specification provided in Eurocode 8 considering the probability of exceedance of the seismic action. A detailed example is provided.
Chatzidaki A., Vamvatsikos D. (2021). Mixed probabilistic seismic demand models for fragility assessment. Bulletin of Earthquake Engineering, 19: 6397–6421.
Abstract | A mixture model approach is presented for combining the results of different models or analysis methods into a single probabilistic demand model for seismic assessment. In general, a structure can be represented using models of different type or different number of degrees of freedom, each offering a distinct compromise in computational load versus accuracy; it may also be analysed via methods of different complexity, most notably static versus dynamic nonlinear approaches. Employing the highest fidelity options is theoretically desirable but practically infeasible, at best limiting their use to calibrating or validating lower fidelity approaches. Instead, a large sample of low fidelity results can be selectively combined with sparse results from higher fidelity models or methods to simultaneously capitalize on the frugal nature of the former and the low bias of the latter to deliver fidelity at an acceptable cost. By employing a minimal 5 parameter power-law-based surrogate model we offer two options for forming mixed probabilistic seismic demand models that (i) can combine different models with varying degree of fidelity at different ranges of structural response, or (ii) nonlinear static and dynamic results into a single output suitable for fragility assessment.
CONFERENCE PAPERS
Now - 2019
Chatzidaki A., Ntaifoti A., Bilionis D.., Gantes C., Vamvatsikos D. (2023). Fatigue damage assessment of the Egnatia Odos sign-support structures under spatially-correlated time-varying wind fields. Proceedings of the 10th Hellenic National Conference on Steel Structures, Athens, Greece. (in greek)
Abstract | A methodology is proposed for assessing the long-term wind-induced fatigue damage accumulation of steel sign-supporting structures of highways, as it applies to Egnatia Odos. These structures are usually configured as portal, L-shaped or T-shaped frames, consisting of one or two columns and a horizontal beam that supports the sign. During their lifetime, they are exposed to environmental conditions thus being susceptive to corrosion, while the critical load is the wind that subjects them to fatigue due to its dynamic nature. To this scope, the cumulative fatigue damage is computed for the critical connections of the sign-supporting structures on a site-agnostic basis for 10min loading intervals, both for corroded and for non-corroded structures. This data is combined with 10min wind time-series that are compatible with the long-term weather hazard at the location of the structures and are computed based on alternative climate change scenarios. The aim is to assess the damage accumulation due to fatigue for the critical connections of the structures during their lifetime. The proposed methodology allows assessing the long-term risk due to fatigue for an ensemble of spatially distributed sign-support structures and is incorporated into a tool that aims to help road operators rapidly assess the state of the highway assets and help them efficiently manage their inspections.
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Ntaifoti A., Chatzidaki A., Gantes C., Vamvatsikos D. (2023). Corrosion influence on fatigue resistance of highway sign bridges. Proceedings of the 10th Hellenic National Conference on Steel Structures, Athens, Greece. (in greek)
Abstract | Road infrastructure (RI) resilience is significant for the prosperity and development of modern societies. Lightweight steel structures, such as sign bridges, constitute an integral part of modern highways and their proper design ensures the safety of the RI users. Sign support structures provide direction instructions and/or traffic information to the drivers and their spans become constantly longer due to lane number increase. A common characteristic of such structures is the large dimensions of signpost plates, compared to the surfaces of the structural members, resulting in a critical role of wind loading in structural design. Moreover, such structures are exposed to harsh environmental conditions, thus being prone to corrosion, attacking the metal surface, creating a rust film, progressively reducing the material thickness, consequently producing a decrease in the structural performance in terms of strength, stiffness, and ductility. Moreover, corrosion accelerates fatigue effects and reduces the remaining lifetime of the connections and hence of the entire structure. In the present work the effect of corrosion on the fatigue resistance of typical sign bridges is quantified. Towards that objective, a methodology to assess the remaining lifetime is developed and applied to typical sign bridge structures of contemporary motorways with different shapes.
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Chatzidaki A., Giannelos C., Gerontati A., Vamvatsikos D., Loli M., Tsatsis A. (2022). Seismic risk and resilience assessment for the Metsovo-Panagia segment of Egnatia Odos. Proceedings of the 5th Panhellenic Conference on Earthquake Engineering and Engineering Seismology, Athens, Greece (in greek).
Abstract |
Chatzidaki A., Gerontati A., Vamvatsikos D. (2022). Seismic damage and implied traffic delay assessment for a highway bridge of Egnatia Odos Greece. Proceedings of the 3rd European Conference on Earthquake Engineering and Seismology (3ECEES), Bucharest, Romania.
Abstract | The seismic damage and the implied traffic delays are assessed for two structurally independent twin bridges, one per travel direction, which form the G7 bridge of the Egnatia highway in Greece. They are reinforced concrete structures with a monolithic pier-to-deck connection that were built using the cantilever method of construction. To enhance the seismic assessment resolution, a component-based approach is followed that allows evaluating damage scenarios for individual critical bridge components and propagating them to assess the performance of the entire system. This necessitates linking the component damages to the actions that the road operator would take in order effect repairs, i.e., by reducing the speed limit in any of the lanes and/or closing any of them until repairs are finished. These interventions typically lead to traffic delays for the entire highway that are computed on an event basis via event-based probabilistic seismic hazard analysis by considering the component-to-asset and asset-to-system interdependencies. The aim is to develop a decision support tool for pre-event risk assessment and rapid post-event inspection of critical road infrastructure by combining hazard, vulnerability and sensor
information to predict the resulting consequences both on the asset and the system level at every step during an asset’s recovery back to full functionality.
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Chatzidaki A., Vamvatsikos D., Auvinen M., Hellsten A., Barmpas F., Lehtonen I. (2022). A baseline approach to downscaling Euro-CORDEX data for wind hazard assessment of the Egnatia Odos highway. Proceedings of the 3rd International Conference on Natural Hazards & Infrastructure ICONHIC 2022, Athens, Greece.
Abstract | The wind hazard is assessed for the Egnatia Odos highway in Greece by considering Climate Change effects via the Euro-CORDEX future climatic projections. The aim is to derive spatially correlated region-wide wind fields for a stochastic event set of thousands of storm realizations that are suitable for risk and resilience assessment of the entire highway network. The coarse spatial and temporal resolutions of the Euro-CORDEX wind projections prohibit their use as a direct input in weather-related risk and resilience assessment of highway structures that may measure down to a few meters in size and require at most 10-min average wind speeds. To improve the temporal scale resolution, we leverage machine learning tools and continuous measurements from National weather stations to generate composite “Frankenstein” days comprising 144 jigsaw pieces of actually measured 10-min wind time-histories that are scaled and matched together to form a continuous daily record. These point-estimates, valid only at the locations of the weather stations, are expanded spatially by employing high-fidelity Computational Fluid Dynamic simulations that take into account the topographic complexity of the site to simulate turbulent wind flows, thus generating spatially correlated wind fields of 10-min average wind speeds. These allow estimating load distribution and risk on (i) an event-by-event basis and (ii) in the long-term for an ensemble of spatially-distributed highway assets that are vulnerable to wind actions, such as signpost bridges and power network pylons.
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Vamvatsikos D., Chatzidaki A. (2022). The HAPI sensor-aware framework for infrastructure risk and resilience assessment. Proceedings of the 3rd International Conference on Natural Hazards & Infrastructure ICONHIC 2022, Athens, Greece.
Abstract | The (new) 20’s have allowed us to dream big on protecting our infrastructure from natural hazards. Powerful computers, machine learning, terrestrial and airborne sensors are at our disposal to help us quantify the consequences of potential hazardous events that may come in the future, are already unfolding, or have already happened. Owing to its origins in four European projects, namely HYPERION, ARCHYTAS, PANOPTIS and INFRASTRESS, the HAPI framework has been formulated to perform pre/trans/post-event risk and resilience assessment of diverse infrastructure, comprising different layers of networked, loosely-connected or autonomous assets within a city, region or country. Building upon the well-worn basis of hazard-exposure-vulnerability that underpins practically all insurance risk estimates, HAPI enables assessment of cascading (e.g., mudflow/landslide after earthquake) and cotemporaneous (e.g., extreme precipitation, temperature, ice and wind scenario) hazards, while it offers sensor integration with near-real-time updating of predictions based on hazard/asset/consequence information input. Both “static” memoryless hazards (e.g., earthquake), as well as “dynamic” time-dependent hazards (e.g., climate projections) are incorporated in tandem with static/dynamic vulnerabilities, allowing the tracking of complex phenomena, such as climate change, and their effect on the aging/corrosion/fatigue of a diverse set of assets, including buildings, bridges, piping, powerlines, highways and cultural heritage monuments. At the very basis lies a vast database of hazard and asset realization scenarios, employing Total Probability Discrete Event Simulation to explicitly track network interdependencies and propagate uncertainty from our source information to the projected integrated-system functionality and eventual recovery.
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Zafeiropoulos C., Protopapadakis E., Chatzidaki A., Doulamis A., Vamvatsikos D., Zotos N., Bogdos G., Kostaridis A., Schmidt F., Ientile S., Sevilla I., Tilon S., Rallis I. (2022). A holistic monitoring scheme for road infrastructures. Proceedings of the 15th International Conference on Pervasive Technologies Related to Assistive Environments (PETRA ’22), 622–627. DOI: 10.1145/3529190.3534745
Abstract | This monitoring system aims at increasing the resilience of the road infrastructures and ensuring reliable network availability under unfavourable conditions, such as extreme weather, landslides, and earthquakes. The main target is to combine downscaled climate change scenarios (applied to road infrastructures) with simulation tools (structural/geotechnical) and actual data (from existing and novel sensors), so as to provide the operators with an integrated tool able to support more effective management of their infrastructures at planning, maintenance and operation level. Towards this, the proposed framework aims to use high resolution modelling data for the determination and the assessment of the climatic risk of the selected, transport infrastructures and associated expected damages, use existing SHM data (from accelerometers, strain gauges etc.) with new types of sensor-generated data (computer vision) to feed the structural/geotechnical simulator, utilize tailored weather forecasts (combining seamlessly all available data sources) for specific hot-spots, providing early warnings with corresponding impact assessment in real time; develop improved multi-temporal, multi-sensor UAV, computer vision and machine learning-based damage diagnostic for diverse transport infrastructures; design and implement a Holistic Resilience Assessment Platform environment as an innovative planning tool that will permit a quantitative resilience assessment through an end-to-end simulation environment, running “what-if” impact/risk/resilience assessment scenarios. The effects of adaptation measures can be investigated by changing the hazard, exposure and vulnerability input parameters; design and implement a Common Operational Picture, including an enhanced visualisation interface and an Incident Management System. The integrated platform (and its sub-modules) will be validated in two real case studies in Spain and in Greece.
Vamvatsikos D., Chatzidaki A. (2021). Fragility and Loss assessment via mixed probabilistic models of seismic demand. Proceedings of the 17th World Conference on Earthquake Engineering (17WCEE), Sendai, Japan
Abstract | A mixture model is presented for combining the results of different models or analysis approaches into a single probabilistic seismic demand model that is suitable for fragility assessment. A structure can be represented using different model types or even levels of resolution for the same type, while it may also be analyzed via methods of different complexity, most notably static versus dynamic nonlinear approaches. Combining the results from different sources can be beneficial as it allows updating the results of a simpler approach or combining the strengths of two different models. For example, as the static pushover analysis offers inexpensive yet low-fidelity demand assessment at any level of intensity, its results may be locally or globally updated by adding stripes of (computationally expensive) response history analysis. On the modelling side, different model types may offer accuracy advantages in complementary response regions. This is the case of distributed-plasticity fiber models that offer higher fidelity for reinforced concrete frames at low (pre-capping) deformations, while lumped-plasticity models are more reliable for larger (post-capping) deformations closer to collapse. Through the combination of the results of multiple models of differing fidelity we can potentially better capture the performance of a structure at all levels of seismic intensity. By employing a minimal 5 parameter power-law-based model we offer viable options for forming mixed probabilistic seismic demand models that can combine both different models and different analysis methods into a single output suitable for fragility and loss assessment.
Loli M., Chatzidaki A., Vamvatsikos D., Gazetas G. (2021). Seismic vulnerability of motorway bridge on active landside. Proceedings of the 17th World Conference on Earthquake Engineering (17WCEE), Sendai, Japan
Abstract | The western section of Egnatia Odos motorway in Greece runs through a challenging mountainous terrain where geohazards are exacerbated by moderate seismicity. Located a few kilometers east of the town of Metsovo, the twin girder bridges of the Panagia interchange have their central piers founded on an active landslide. Being part of a research project that aims to develop a tool for rapid inspection and assessment of the motorway, this study presents a thorough seismic vulnerability analysis of the hybrid caisson–pile group foundation system that supports the most critical piers of the interchange. This foundation system has been designated as a slope stabilizing measure, in addition to carrying the loads transmitted by the bridge superstructure. Numerical modelling with nonlinear 3D finite elements has been employed, together with site-specific hazard-consistent selection of ground motion records. A hybrid numerical approach has been developed using segregated models of varying refinement with rational approximation of interactions between the nonlinear response of soil, the movement of the slope, the kinematic distress imposed upon the foundation and the inertial loads from the vibration of the superstructure. The method estimates pile performance under large-scale, dynamic landslide action with sufficient engineering accuracy for cases where slope actions dominate the response. The softening behavior of the sliding surface is calibrated versus monitored slope displacements. The effect of groundwater recharge after heavy rainfall is incorporated in the vulnerability analysis in a simplified manner, using different scenarios for water table elevation. Focusing on the performance of the foundations, damage is described in terms of their permanent displacements and curvatures. Results indicate that a range of excitations with an exceedance
probability of 2% in 50 years are capable of inflicting substantial permanent pile damage, even complete failure, if combined with a fully saturated soil condition. The computed residual foundation displacements can serve as input for a detailed structural model simulating the performance of the superstructure for estimating the bridge vulnerability.
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.
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.
Chatzidaki A., Vamvatsikos D. (2019). Mixed probabilistic seismic demand models for fragility assessment. Proceedings of the SECED 2019 Conference, Greenwich, UK
Abstract | A mixture model is presented for combining the results of different models or analysis approaches into a single probabilistic seismic demand model that is suitable for fragility assessment. A structure can be represented using different model types or even levels of resolution for the same type, while it may also be analysed via methods of different complexity, most notably static versus dynamic nonlinear approaches. Combining the results from different models or analysis methods can be beneficial as it allows updating the results of a simpler approach or combining the strengths of two different models. For example, different model types may offer accuracy advantages in complementary response regions. This is the case of distributed-plasticity fiber models that offer higher fidelity for reinforced concrete frames at low (pre-capping) deformations, while lumped-plasticity models are more reliable for larger (postcapping) deformations closer to collapse. Through the combination of the results of both models we can potentially better capture the performance of the frame at all levels of seismic intensity. By employing a minimal 5 parameter power-law-based model we offer viable options for forming mixed probabilistic seismic demand models that can combine both different models and different analysis methods into a single output suitable for fragility assessment.
Chatzidaki A., Lyritsakis C., Vamvatsikos D., Aschheim M., Hernández-Montes E. (2019). Seismic assessment of a 4-story RC building designed on an intensity versus a performance basis. Proceedings of the 4th Panhellenic Conference on Earthquake Engineering and Engineering Seismology, Athens, Greece
Περίληψη | Τρεις μεθοδολογίες σχεδιασμού συγκρίνονται ως προς τα αποτελέσματά τους για ένα τετραώροφο κτήριο από οπλισμένο σκυρόδεμα με περιμετρικά πλαίσια ανάληψης οριζοντίων δυνάμεων. Οι δύο είναι μεθοδολογίες σχεδιασμού βάσει επιτελεστικότητας και βασίζονται στη χρήση των Φασμάτων Συχνότητας Διαρροής για να επιτύχουν με αξιοπιστία αυξημένους στόχους επιτελεστικότητας, λαμβάνοντας πλήρως υπόψη τις αβεβαιότητες και τη σεισμική επικινδυνότητα. Η τρίτη χρησιμοποιεί το Φάσμα Σημείου Διαρροής για να επιτύχει ταχεία σύγκλιση σε μια ικανοποιητική σχεδίαση που είναι συμβατή με τις σύγχρονες κανονιστικές διατάξεις. Και στις τρεις περιπτώσεις δημιουργήθηκαν τα μη γραμμικά προσομοίωματα των κτηρίων και πραγματοποιήθηκε Ικανοτική Δυναμική Ανάλυση για την εκ των υστέρων αποτίμηση της επιτυγχανόμενης σεισμικής επιτελεστικότητας. Τα αποτελέσματα δείχνουν ότι όλες οι παραπάνω μέθοδοι ικανοποιούν τις απαιτήσεις του κανονισμού, δηλαδή στόχους επιτελεστικότητας που σχετίζονται με την Προστασία Ζωής.
Ωστόσο, μόνο οι δύο μέθοδοι σχεδιασμού βάσει επιτελεστικότητας επιτρέπουν την ακριβή επίτευξη στόχων που σχετίζονται π.χ. με την προστασία έναντι κατάρρευσης ή στόχους άλλους από την Προστασία Ζωής, που ορίζονται εκ των προτέρων από το μελετητή και πηγαίνουν πέρα από τις διατάξεις του κανονισμού.