Athanasia ΚΑΖΑΝΤΖΙ
Athanasia Kazantzi holds a degree in Civil Engineering (Democritus University of Thrace, Greece, 2002), an M.Sc with distinction in Structural Engineering (The University of Sheffield, UK, 2004), and a Ph.D. (University of Surrey, UK, 2008). Following the completion of her PhD studies, she worked as a graduate design engineer in a renowned engineering consulting firm in UK, joining the special projects and geotechnical groups. Upon her return to Greece, Athanasia was associated with the Institute of Steel Structures (The Lambda Lad group) as a fellow researcher. Her research interests mainly include earthquake engineering, performance-based design and assessment methodologies, reliability and loss estimation under multiple hazards, building class vulnerability, decision support systems and seismic performance/protection of nonstructural elements. She is an active researcher, having participated so far in various national and international research projects funded by the Applied Technology Council (ATC), the Global Earthquake Model (GEM) Foundation, the EU Researsch Executive Agency and the Hellenic General Secretariat for Research and Technology whereas she has published several papers in peer-reviewed journals and conferences. She is active as a freelance structural engineer (member of the Technical Chamber of Greece since 2003), while she has taught several undergraduate modules as an adjunct lecturer at the Aristotle University of Thessaloniki, the University of Thessaly and the Technological Educational Institute of Central Macedonia (now International Hellenic University).
JOURNAL PAPERS
Now - 2006
Kazantzi A.K., Karaferis N.D., Melissianos V.E., Vamvatsikos D. (2024). Acceleration-sensitive ancillary elements in industrial facilities: alternative seismic design approaches in the new Eurocode. Bulletin of Earthquake Engineering, 22: 109-132.
Abstract | The Eurocode 8—Part 4 approaches, per their December 2022 update, are presented for the design of acceleration-sensitive industrial ancillary components. The seismic performance of such nested and/or supported ancillary elements, namely mechanical and electrical equipment, machinery, vessels, etc. is critical for the safety and operability of an industrial facility in the aftermath of an earthquake. Of primary importance are the structural characteristics of the supporting structure and the supported component, pertaining to resonance, strength, and ductility, and whether these are known (and to what degree) during initial design and/or subsequent modifications and upgrades. Depending on the availability and reliability of information on the overall system, the Eurocode methods comprise (a) a detailed component/structure-specific design accounting for all pertinent component and building characteristics, equivalent to typical building design per Eurocode 8—Part 1–2, (b) a conservative approach where a blanket safety factor is applied when little or no such data is available, and (c) a ductile design founded on the novel concept of inserting a fuse of verified ductility and strength in the load path between the supporting structure and the ancillary element. All three methods are evaluated and compared on the basis of a case-study industrial structure, showing how an engineer can achieve economy without compromising safety under different levels of uncertainty.
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.
Kazantzi A.K., Lachanas C.G., Vamvatsikos D. (2022). Seismic response distribution expressions for rocking building contents under ordinary ground motions. Bulletin of Earthquake Engineering, 20: 6659–6682.
Abstract | Analytical expressions are proposed for predicting the rocking response of rigid free-standing building contents subjected to seismic-induced floor excitations. The study considers a wide range of rigid block geometries and seismic floor acceleration histories that were recorded during actual earthquakes in instrumented Californian buildings, so as to cover, in a fully probabilistic manner, the entire spectrum of potential pure rocking responses, i.e. from the initiation of rocking up to the block overturning. Contrary to past observations on anchored building contents (prior to any failure in their anchorage system that could alter their response and mode of failure), it is shown that the response of free-standing blocks is not influenced by the predominant period of the supporting structure. The proposed set of equations can be utilised for estimating the response statistics and consequently for undertaking an analytical seismic fragility assessment on rocking building contents.
[pre-print version]
Kazantzi A.K., Lachanas C.G., Vamvatsikos D. (2021). Seismic response distribution expressions for on-ground rigid rocking blocks under ordinary ground motions. Earthquake Engineering and Structural Dynamics, 50(12):3311-3331.
Abstract | Predictive relationships are offered for the response of on-ground 2D rigid blocks undergoing rocking. Among others, this is pertinent to (1) modern or classical antiquity structures that utilize rocking as a seismic protection mechanism and (2) freestanding contents (e.g., cabinets, bookcases, and museum artifacts) located on the ground or lower floors of stiff buildings. Blocks of varying dimensions were subjected to a full range assessment of seismic response under increasing intensity levels of ordinary (no-pulse and no-long-duration) ground motions, parameterized by peak ground acceleration or velocity. Both response and intensity were normalized, allowing the fitting of general-purpose parametric expressions to determine the mean and dispersion of response for an arbitrary block of interest. These can be utilized in the same way as conventional strength-ratio/ductility/period relationships of yielding oscillators, to enable the rapid assessment or design of simple rocking systems.
Kazantzi A. K., Vamvatsikos D. (2021). Practical performance-based design of friction pendulum bearings for a seismically isolated steel top story spanning two RC towers. Bulletin of Earthquake Engineering, 19: 1231-1248.
Abstract | A case study of performance-based design is presented for a seismically isolated steel structure that rests on top of two adjacent high-rise reinforced concrete towers, the latter separated by means of an expansion joint. The isolation system comprises Friction Pendulum Bearings (FPBs) that are designed to accommodate two salient characteristics of the system. First, the isolated top floor is subjected to narrow-band floor acceleration histories as the ground motion excitation is filtered by the dynamic response of the supporting towers. Second, the displacement demands imposed to the FPBs are affected by the in-phase or out-of-phase movement of the supporting structures, with the latter case potentially giving rise to higher displacement capacity requirements for the bearings. In a search for a solution beyond conventional design norms, the probability of bearing failure associated with a wide range of FPB displacement capacities was determined via an explicitly risk-consistent performance-based seismic design. Overall, the case-specific design approach is shown to be able to meet any desired performance objective, consistently determining the final compromise between safety, cost-efficiency and practicability.
Kazantzi A.K., Miranda E., Vamvatsikos D. (2020). Strength-reduction factors for the design of light nonstructural elements in buildings. Earthquake Engineering and Structural Dynamics, 49: 1329–1343
Summary | Strength‐reduction factors that reduce ordinates of floor spectra acceleration due to nonlinearity in the secondary system are investigated. In exchange for permitting some inelastic deformation to occur in the secondary system or its supports, these strength reduction factors allow to design the nonstructural elements or their supports for lateral forces that are smaller than those that would be required to maintain them elastically during earthquakes. This paper presents the results of a statistical analysis on component strength‐reduction factors that were computed considering floor motions recorded on instrumented buildings in California during various earthquakes. The effect of yielding in the component or its anchorage/bracing in offering protection against excessive component acceleration demands is investigated. It is shown that strength‐reduction factors computed from floor motions are significantly different from those computed from ground motions recorded on rock or on firm soils. In particular, they exhibit much larger reductions for periods tuned or nearly tuned to the dominant modal periods of the building response. This is due to the large differences in frequency content of ground motions and floor motions, with the former typically characterized by wide‐band spectra whereas the latter are characterized by narrow‐band spectra near periods of dominant modes in the response of the building. Finally, the study provides approximate equations to estimate component strength‐reduction factors computed through nonlinear regression analyses.
Kazantzi A.K., Vamvatsikos D. (2020). Seismic and vibration performance for an industrial steel building. ASCE Practice Periodical on Structural Design and Construction, 25(2): 05020001
Kazantzi A.K., Vamvatsikos D., Miranda E. (2020). Evaluation of seismic acceleration demands on building nonstructural elements. ASCE Journal of Structural Engineering, 146(7): 04020118
Abstract | As proven by several past earthquakes, seismic losses associated with nonstructural damage in modern buildings are likely to significantly exceed those associated with structural damage. Hence, to satisfactorily assess the overall seismic performance of a building and consequently the associated losses, it is paramount to properly account for the nonstructural damage through the adequate estimation of acceleration demands that are imposed on its acceleration-sensitive nonstructural elements in any one-floor level during an earthquake. Component acceleration amplification factors, ap, which measure how much the acceleration of a component is amplified relative to the peak floor acceleration are evaluated by floor motions recorded during earthquakes on instrumented buildings in the United States. The study shows that component amplification factors currently used in codes significantly underestimate acceleration demands for components whose periods are tuned or nearly tuned to modal periods of the supporting structure. Simplified equations are proposed to estimate component acceleration amplifications. The study also evaluates inelastic floor acceleration spectral ordinates. As a result of the filtering and amplification of the ground motion by the structure, the results show that even small levels of nonlinearity in the nonstructural element or its attachment to the structure lead to significant reductions in acceleration demands.
Kazantzi A.K., Vamvatsikos D., Miranda E. (2020). The effect of damping on floor spectral ordinates as inferred from instrumented buildings. Bulletin of Earthquake Engineering, 18: 2149–2164
Abstract | This study investigates the effect of damping on the seismic demands imposed on lightweight nonstructural components. The investigation was performed utilizing a total of 113 floor acceleration recordings obtained from instrumented buildings located in California. Results are presented as damping modification factors, which provide information on the seismic demands imposed on secondary systems with various levels of damping relative to 5% damped components. Evaluations of the results indicate a strong period dependence, with the effect of damping being much larger for components that are tuned or nearly tuned and much smaller for components with periods far from the modal periods of the supporting building. Therefore, a better characterization of the effect of damping is achieved if the damping modification factors are a function of the ratio of the period of the component to the modal periods of the supporting structure. As expected, record-to-record variability increases as the level of damping in the secondary component deviates from 5% damping, with an overall probability distribution that is approximately lognormal. Thus, a full probabilistic characterization of the influence of damping on component response is offered via a parametric, period- and damping-dependent model of the mean and lognormal standard deviation of the damping modification factor.
Papadopoulos A.N., Vamvatsikos D., Kazantzi A.K. (2019). Development and application of FEMA P-58 compatible story loss functions. Earthquake Spectra, 35(1): 95-112
Abstract | The quantification of seismic performance, using metrics meaningful to both engineers and stakeholders, has been a focal point of research in performance-based earthquake engineering. The prevalent paradigm is currently offered by the FEMA P-58 guidelines in the form of a component-by-component approach that provides detailed assessment capabilities at the cost of requiring a complete inventory of the structural, nonstructural, and content components. In an attempt for simplification, a fully compatible story-by-story approach is offered instead, where story loss functions are employed to directly relate monetary losses to engineering demand parameters given the story area. These functions can be adjusted for application to different situations, assuming the ratio of cost and quantity of each component category inventory remains relatively constant. As an example, they are generated for a standard inventory makeup, characteristic of low/mid-rise steel office buildings. They are shown to offer a favorable compromise of simplicity and accuracy that lies between the component-by-component and building-level approaches that are currently prevalent in building-specific and regional loss assessment, respectively.
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.
Kazantzi A.K., Vamvatsikos D. (2018). The hysteretic energy as a performance measure in analytical studies. Earthquake Spectra, 34(2): 719-739
Abstract | Hysteretic energy dissipation is often employed as a measure of performance for systems subjected to earthquake excitation. This mainly stems from quasi-static cyclic tests where fuller hysteresis loops (i.e., higher energy absorption) are taken to indicate better performance when comparing systems with similar strength under the same cyclic loading protocol. However, seismic loading offers a different proving ground, where energy absorption is strongly correlated with energy input, while the nonstationary loads imply that the beneficial hysteretic effects observed in a cyclic test may never be realized. Given the current state of art in models and methods of performance-based earthquake engineering, we ask whether earthquake records at a given seismic intensity will cause peak/residual displacements or accelerations that favor models having fuller hysteresis. Using incremental dynamic analysis on story-level oscillators with varying hysteretic characteristics, it is demonstrated that hysteretic energy dissipation does not consistently correlate with seismic performance.
Vamvatsikos D., Kazantzi A., Aschheim M.A (2016). Performance-based seismic design: Avant-garde and code-compatible approaches. ASCE-ASME Journal of Risk and Uncertainty in Engineering Systems, Part A: Civil Engineering, 2(2): C4015008
Abstract | Current force-based codes for the seismic design of structures use design spectra and system-specific behavior factors to satisfy one or two predefined structural limit states . In contrast, performance-based seismic design aims to design a structure to fulfill any number of target performance objectives, defined as user-prescribed levels of structural response, loss, or casualties to be exceeded at a mean annual frequency less than a given maximum. First, a review of recent advances in probabilistic performance assessment is offered. Second, the salient characteristics of methodologies that have been proposed to solve the inverse problem of design are discussed. Finally, an alternative approach is proposed that relies on a new format for visualizing seismic performance, termed yield frequency spectra (YFS). YFS offer a unique view of the entire solution space for structural performance of a surrogate single-degree-of-freedom oscillator, incorporating uncertainty and propagating it to the output response to enable rapid determination of a good preliminary design that satisfies any number of performance objectives.
Kazantzi A.K., Vamvatsikos D. (2015). Intensity measure selection for vulnerability studies of building classes. Earthquake Engineering and Structural Dynamics, 44(15): 2677-2694
Summary | The selection of a scalar Intensity Measure (IM) for performing analytical vulnerability (loss) assessment across a building class is addressed. We investigate the ability of several IM choices to downgrade the effect of seismological parameters (sufficiency) as well as reduce the record‐to‐record variability (efficiency) for both highrise and lowrise sets of ‘index’ buildings. These characteristics are explored in unprecedented detail, employing comparisons and statistical significance testing at given levels of local engineering demand parameters (story drift ratios and peak floor accelerations) that relate to losses, instead of global variables such as the maximum interstory drift. Thus, a detailed limit‐state‐specific view is offered for the suitability of different scalar IMs for loss assessment. As expected, typical single‐period spectral values are found to introduce unwanted bias at high levels of scaling, both for a single as well as a class of buildings. On the other hand, the geometric mean of the spectral acceleration values estimated at several periods between the class‐average second‐mode and an elongated class‐average first‐mode period offers a practical choice that significantly reduces the spectral‐shape bias without requiring the development of new ground motion prediction equations. Given that record selection remains a site‐ and building‐specific process, such an improved IM can help achieve reliable estimates for building portfolios, as well as single structures, at no additional cost. Copyright © 2015 John Wiley & Sons, Ltd.
Kazantzi A.K., Vamvatsikos D., Lignos D.G. (2014). Seismic performance of a steel moment-resisting frame subject to strength and ductility uncertainty. Engineering Structures, 78: 69-77
Abstract | The reliable estimation of the seismic performance of structures requires quantifying the aleatory and epistemic uncertainties of the system parameters. This is efficiently achieved for a case study of a four-story steel moment-resisting frame through several important advances. First, a state-of-the-art numerical model is formed with full spatial parameterization of its strength and plastic deformation properties. Empirical relationships derived from experimental data are used to model the cyclic behavior of steel sections using probabilistically distributed parameters that include intra- and inter-component correlation. Finally, incremental dynamic analysis and Monte Carlo simulation are employed to accurately assess the seismic performance of the model under the influence of uncertainties. Of interest is the extent to which model parameter uncertainties may trigger negative demand-capacity correlation in structural fragility evaluation, where, for example, a lower ductility capacity for a component may decrease the threshold for local failure while at the same time raising the local demand estimate from an uncertainty-aware model. With respect to the examined steel moment-resisting frame and considering three construction quality levels (i.e. very good, average, low) as per FEMA P-58, it is shown that, despite the good agreement of the evaluated structural demands obtained with and without consideration of the model parameter uncertainties for well-designed modern buildings, the potential demand-capacity correlation is likely to give rise to unconservative estimates of fragility for local damage-states, especially in cases where substandard quality control is exercised during construction.
A.K. Kazantzi, T.D. Righiniotis, M.K. Chryssanthopoulos. (2011) A simplified fragility methodology for regular steel MRFs, Journal of Earthquake Engineering; 15(3), 390-403
Abstract | During the last decade, significant progress has been made toward the development of probabilistic methods for the seismic assessment of structures. However, the use of existing analytical fragility methodologies for practical applications related to structures of moderate to low importance remains prohibitive, in part due to substantial computational demands. This paper explores, through a case study, the robustness of a simplified methodology for fragility assessment of regular steel Moment Resisting Frames (MRFs). Through comparisons with a more accurate but computationally demanding methodology, it was confirmed that the “2000 SAC/FEMA” procedure gives robust fragility estimates. Moreover, it was found that a closed-form fragility assessment, where the structural response is evaluated by means of an Equivalent Single Degree Of Freedom (ESDOF) oscillator, can yield sufficiently accurate results for regular steel MRF structures, providing acceptable construction quality has been achieved in the connections.
Kazantzi A.K., Righiniotis T.D., Chryssanthopoulos M.K. (2008). The effect of joint ductility on the seismic fragility of a regular moment resisting steel frame designed to EC8 provisions. Journal of Constructional Steel Research, 64: 987-996
Abstract | In order to evaluate the seismic reliability of moment resisting frames at performance levels associated with highly nonlinear structural responses, structural modelling of the rotational capacity of the joints needs to address potential failure modes. This paper begins by reviewing European experimental studies on the cyclic behaviour of steel joints. A linear regression equation, which relates the joint plastic rotation capacity to the beam depth, is proposed based on an analysis of the test data. Thus, the effect of joint ductility and failure on structural response is quantified, within a probabilistic context, through seismic fragility analysis of a mid-rise steel frame designed to Eurocode 8. The variability in structural demand is estimated at increasing ground shaking intensity levels, and fragility curves, conditional on a given ground motion record, are derived for two different performance levels. The inherent randomness in ground motion is taken into account by using an ensemble of recorded accelerograms. The effect of joint rotation capacity is noticeable in mean fragility curves corresponding to high seismic demand and response levels.
Kazantzi A.K., Righiniotis T.D., Chryssanthopoulos M.K. (2008). Fragility and hazard analysis of a welded steel moment resisting frame. Journal of Earthquake Engineering, 12(4): 596-615
Abstract | With the move towards performance and consequence-based design and assessment of structures under seismic loading, the engineering community is becoming increasingly convinced that design practices need to be developed and checked using probabilistic methods. In this article, a methodology for the probabilistic assessment of low-rise steel buildings is presented and applied to a welded Moment Resisting Frame (MRF). In light of recent field experience for this form of construction, emphasis is given to the modeling of connections, particularly with respect to fracture characteristics. The seismic behavior of the building is assessed by means of nonlinear dynamic time history analyses, using a set of ground motions scaled according to spectral acceleration. Randomness related both to structural properties and earthquake excitation is explicitly taken into account. Fragility curves are generated using the Monte Carlo (MC) simulation method coupled with the Latin Hypercube Sampling (LHS) technique, and the failure probabilities are presented in terms of drift angles at different performance levels. Furthermore, evaluation of the seismic risk through a hazard analysis is presented in order to compare present results with previous pertinent studies. The study reveals that structures experiencing brittle connection fractures undergo large deformations, resulting in a low reliability in terms of achieving code-related performance requirements.
CONFERENCE PAPERS
Now - 2006
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.
[paper]
Lachanas C.G., Vamvatsikos D., Kazantzi A.K. (2023). Intensity measures for assessing the rocking response of server racks in steel buildings. Proceedings of the 10th Hellenic National Conference on Steel Structures, Athens, Greece. (in greek)
Abstract | Alternative seismic intensity measures (IMs) are examined for the case of non-structural rocking building contents and in particular for the case of server racks standing freely on the higher floors of steel buildings. Observations following recent earthquakes in developed countries have revealed that most of the damages after a seismic event consider non-structural building contents of high value. On the other hand, the selection of a robust
IM is a basic requirement in the context of the performance-based earthquake engineering framework for assessing the seismic risk and the consequential loss/damage of engineering structures or non-structural contents with high fidelity. Hence, a bunch of alternative IMs are tested in terms of efficiency and sufficiency as potential IMs for rocking vulnerability studies. The simple planar rocking block model is employed for running nonlinear dynamic analysis with a set of 34 floor motions that were recorded during past earthquakes on the roof of instrumented steel buildings. Rocking blocks of various shapes and sizes are analyzed that resemble two-dimensional analogues of server racks. After analysis, efficiency and sufficiency of the examined IMs are compared aiming to propose optimal IMs for the case of rocking contents in the different stages of rocking response from rocking uplift to overturning.
[paper]
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.
[paper]
Kazantzi A.K., Elkady A., Vamvatsikos D., Lignos D., Miranda E. (2023). The use of ductile steel fuses for the seismic protection of acceleration sensitive non-structural components: Numerical and Experimental verification. Proceedings of the 10th Hellenic National Conference on Steel Structures, Athens, Greece.
Abstract | Recent seismic events have showcased the vulnerability of non-structural components to even low- or moderate-intensity earthquakes that occur far more frequently than design-basis ones. Thus, community-critical buildings, such as hospitals, telecommunication facilities, or fire stations, often face lengthy functionality disruptions despite having suffered little structural damage during an earthquake. This paper summarises the numerical, and corroborating experimental, studies that were undertaken as part of the NSFUSE project at the University of Bristol’s shake-table facility. The primary focus was to investigate the concept validity of using ductile steel fuses for protecting acceleration-sensitive non-structural components in the aftermath of earthquakes. The objective was to offer a reliable and inexpensive solution, via replaceable sacrificial elements, for the protection of such components. The experimental program involved a series of planar shake table experiments. These were conducted using narrow-band floor acceleration input signals that were recorded in instrumented buildings through the California Strong Motion Instrumentation Program during three different earthquake events. By changing the mass of the carriage-like test specimen, as well as the fuse height and its cross section, different component-to-building period ratios (tuned and slightly detuned cases) along with yield strength levels were investigated. For each test, the input signals were incrementally scaled, if needed, to induce different ductility demands. The tests provided insight into the seismic performance of non-structural components that are mounted on a structure and the benefits of allowing controlled yielding to occur in the attachments of non-structural components that are tuned or nearly tuned to one of the primary modal periods of the supporting structure.
[paper]
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., Karaferis N., Melissianos V.E., Vamvatsikos D. (2023). Design of acceleration-sensitive ancillary elements under uncertainties in the new eurocode. Proceedings of the SECED 2023 Conference, Cambridge, UK.
Abstract | The overall seismic safety and operability of industrial building-type structures located in critical infrastructure facilities, largely depend on the seismic performance of their nested and/or supported ancillary elements, namely mechanical and electrical equipment, machinery, vessels, etc. Hence, on account that (a) no or minimal direct structural damages are anticipated in the equipment-supporting structures per se during moderate or even strong earthquake events since such structures are typically overdesigned and (b) the sustained structural damages are mostly due to the inferior seismic performance of the nested ancillary elements that could trigger a series of adverse cascading incidents (e.g., uncontrolled fires, explosions), significant effort has been
invested towards developing a design framework that could deliver safe designs for the latter. Despite the significant advancements in the relevant field, the development of a robust design framework is often undermined by several uncertainties that come into play in the evaluation of the capacity and demand of such nonstructural components. In particular, critical information that is needed for the design of the ancillary elements, such as the dynamic characteristics of the component and the supporting structure, are often abstract and/or require substantial effort for being retrieved with certain confidence. On that basis, the new Eurocode 8, offers three distinct design options that allow for adjustments in the conservatism that is induced in the design of the acceleration-sensitive ancillary elements according to the availability and reliability of information on the overall system. This study investigates, by means of a case study industrial structure, the extent to which the seismic reliability of an otherwise code-compatible component designed to comply with each one of the three alternative Eurocode design routes, is likely to be undermined for small discrepancies of the assumed properties from their actual values.
[paper]
Elkady A., Vamvatsikos D., Lignos D., Kazantzi A.K., Miranda E. (2022). Experimental study to validate an improved approach to design acceleration-sensitive nonstructural components. Proceedings of the 5th International Workshop on the Seismic Performance of Non-Structural Elements (SPONSE2022), Stanford, CA.
Abstract |
Kazantzi A.K., Miranda E., Vamvatsikos D., Elkady A., Lignos D., (2022). Analytical studies in support of an improved approach to the design of acceleration-sensitive nonstructural elements. Proceedings of the 5th International Workshop on the Seismic Performance of Non-Structural Elements (SPONSE2022), Stanford, CA.
Abstract |
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.
[paper]
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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Kazantzi A.K., Lachanas C.G., Vamvatsikos D. (2022). Normalized response distribution expressions for ground-supported rigid rocking bodies. Proceedings of the 3rd International Conference on Natural Hazards & Infrastructure ICONHIC 2022, Athens, Greece.
Abstract | Estimating the seismic response of ground-supported rocking rigid blocks, is a topic that has attracted significant research interest in the past few decades, since it concerns, among others: (a) several modern structures or ancient monolithic columns that utilize rocking as a seismic protection mechanism and (b) numerous free-standing contents (e.g. museum artefacts) located on the ground floor or lower floors of stiff buildings. In the present research work, by means of a parametric study, utilizing two-dimensional rectangular blocks of varying sizes and ordinary earthquake records, the rocking response at increasing intensity levels was assessed through Incremental Dynamic Analyses. Following the demand evaluation and in order to allow for an easier utilization of the findings in practical applications, simplified approximate equations have been obtained via nonlinear regression analysis. The proposed equations provide an estimate of the peak rocking response distribution, expressed in terms of the normalized, to the dimensionless slenderness angle , peak rocking angle, at increasing ground motion intensity levels.
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Kazantzi A.K., Vamvatsikos D. (2021). Attribute-driven fragility curves through class disaggregation. Proceedings of the 17th World Conference on Earthquake Engineering (17WCEE), Sendai, Japan
Abstract | Fragility curves are an important ingredient in the seismic loss assessment process. For a regional scale loss estimation, to reduce to reasonable levels the computational burden associated with determining the seismic demands for individual buildings, analytical seismic fragilities are instead evaluated on a broad building class basis. The latter process essentially involves representing a population of buildings having similar characteristics with a set of characteristic “index” buildings to avoid analyzing every single building within this population. For the definition and modeling of index buildings, two main options are currently available, these being (a) defining a limited number of index buildings to represent the class and modeling them with relatively complex, yet more accurate, MDOF systems, and (b) defining numerous index buildings to represent the class and modeling them with simplified approximate SDOF systems. Apparently, the dilemma of defining the optimal way to sample the index buildings comes down to the use of few MDOFs or many SDOFs. Despite the fact that the use of many SDOFs is a rather attractive option, given that they are an easy and computationally inexpensive choice in terms of both modeling and analysis, they are often a bad approximation of the actual problem. This is the case, for example, of tall or irregular buildings, where non-negligible higher modes render the SDOF approximation ineffective. Then, the more expensive and accurate MDOF option has to be employed. However, using a limited number of MDOFs to represent the class of interest inherently offers very little flexibility towards capturing individual buildings that might belong to that class yet their salient features do not necessarily match those of the “average” index building. Aggregating the results of all index buildings into a single class fragility means that one cannot provide a more accurate answer than the mean class fragility plus some dispersion, even if the building in question actually closely matches one of the underlying index ones. This may not matter for estimating long-term average losses over a region, but it becomes increasingly important as the size of the portfolio is reduced and individual structures stand out. To resolve the aforementioned issue, we propose here a method for adding substance back to the class fragility and consequently obtaining fine-grained attribute-driven fragility estimates. The term attribute-driven is key in our approach, since it implies that the process explicitly accounts for the specific characteristics of the building in question. It is essentially a meso-scale approach that stands between the building-specific FEMA P-58 style approach (micro-scale) and the building-class approach (macro-scale). Our testbed is a population of modern high-rise reinforced concrete buildings, represented by seven index buildings, for which we have evaluated fragility functions. With this information at hand, our proposed approach employs statistical methodologies for effectively disaggregating the index building fragility functions, to provide attribute-aware response and collapse fragility spot estimates for individual sample buildings, other than the index ones, that belong to the same class.
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.
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Kazantzi A.K., Vamvatsikos D. (2019). Performance-based design of friction pendulum bearings for a steel top story spanning two RC towers. Proceedings of the 7th International Conference on Computational Methods in Structural Dynamics and Earthquake Engineering (COMPDYN 2019), Crete, Greece
Abstract | The collapse performance of code-designed base-isolated structures has received considerable criticism, having been found to be deficient vis-à-vis conventional buildings in several situations. As a remedy, prescriptive minima have been recommended in the literature for the bearing deformation capacity. These are independent of structure or site characteristics, yet they are already finding use in design. We put this concept to the test by means of a case study of a seismically isolated steel structure that rests on the roof of two adjacent high-rise reinforced concrete towers. To seismically isolate the steel structure Friction Pendulum Bearings (FPBs) are used, and their displacement capacity is determined to comply with a 1% probability of collapse in 50 years performance objective.
The case study possesses two salient features that distinguish it from pertinent past investigations. The first is that the isolated steel structure rests on top of two others and consequently it is subjected to narrow-band roof acceleration time histories, shaped by the filtering of the ground motion excitation through the supporting buildings. The second is that the two supporting towers have different modal characteristics, thus displacement demands imposed to the FPBs are mainly affected by their in-phase or out-of-phase movement. Overall, a case-specific performance-based design approach is shown to achieve the desired safety while requiring 1.5 times lower displacement capacities for the bearings, when compared to prescriptive “performance-based” approaches.
Kazantzi A.K., Vamvatsikos D., Miranda E (2019). Damping influence on the seismic demands of non-structural components and building contents. Proceedings of the 4th Panhellenic Conference on Earthquake Engineering and Engineering Seismology, Athens, Greece (in greek)
Abstract | In most seismic code provisions, the design of nonstructural elements is based on the evaluation of the (absolute) acceleration demands at the floor levels, usually assuming a critical damping of 5% for those elements. However, the actual critical damping for the nonstructural components is well known to be an unknown parameter, that could well deviate from the abovementioned value, whereas its influence remains by large an unexplored field.
To study the effect of damping on the seismic demands of nonstructural elements 113 actual seismic records obtained from instrumented buildings in the USA were selected. The study concluded that: (a) the use of damping modification factors evaluated based on ground level excitations are not suitable for correcting the nonstructural component spectral accelerations demands and (b) the component damping effect on the imposed to the nonstructural elements floor spectral demands is highly dependent on the proximity of their natural period to that of the building. On account of the above and a detailed statistical analysis two equations are proposed for estimating the mean and coefficient of variation of the component damping modification factors.
Kazantzi A.K., Vamvatsikos D. (2019). Prescriptive approaches in performance-based design? A case-study on base isolation. Proceedings of the 13th International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP13, Seoul, South Korea
Abstract | The collapse performance of code-designed base-isolated structures has recently received considerable criticism, having been found to be deficient vis-à-vis conventional buildings in several situations. As a remedy, prescriptive minima with a tenuous probabilistic justification have been recommended in the literature for the bearing deformation capacity. These are independent of structure or site characteristics, yet they are already finding use in design. We put this concept to the test by means of a case study of a seismically isolated steel structure that rests on the roof of two adjacent high-rise reinforced concrete towers. To seismically isolate the steel structure, Friction Pendulum Bearings (FPBs) are used, and their displacement capacity is determined to comply with a performance objective of 1% probability of collapse in 50 years. The case study possesses two salient features that distinguish it from pertinent past investigations. The first is that the isolated steel structure rests on top of two others and consequently it is subjected to narrow-band roof acceleration time histories, shaped by the filtering of the ground motion excitation through the supporting buildings. The second is that the two supporting towers have different modal characteristics, thus displacement demands imposed to the FPBs are mainly affected by their in-phase or out-of-phase movement. Overall, a case-specific true performance-based design is shown to achieve the desired safety while requiring 1.5 times lower displacement capacities for the bearings, when compared to prescriptive “performance-based” approaches.
Kazantzi, A.K., Vamvatsikos D., Miranda E. (2018). Effect of yielding on the seismic demands of nonstructural elements. Proceedings of the 16th European Conference on Earthquake Engineering, Thessaloniki, Greece
Abstract | As proven by several past earthquakes, the seismic losses associated with nonstructural damage in contemporary buildings are likely to exceed those associated with structural damage by several orders of magnitude. Hence, for assessing satisfactorily the overall seismic performance of a building and consequently the associated losses, it is paramount to properly account for the nonstructural damage via estimating the acceleration and deformation demands that are imposed to its nonstructural elements and contents in any one floor level during an earthquake. Furthermore, being able to better understand the behavior of nonstructural components during a seismic event could have a direct impact on the pertinent design methodologies. To address such issues, we assess the validity of the dominant code/design approaches with reference to the evaluation of the component amplification factor, ap, which is essentially a measure of how much the acceleration of a component is amplified compared to the peak floor acceleration. The state-of-practice in the evaluation of ap is to account only for the component flexibility and period, an approach that essentially caps ap to a maximum value of 2.5. Instead, we investigate the extent to which this factor may be further amplified if we consider the vibration characteristics of the building that contains the component under consideration by employing actual recorded floor motions from instrumented buildings in the United States. At the same time, we evaluate the effect of yielding in the component or its anchorage in offering protection against such amplified acceleration demands.
Miranda E., Kazantzi, A.K., Vamvatsikos D. (2018). New approach to the design of acceleration-sensitive non-structural elements in buildings. Proceedings of the 16th European Conference on Earthquake Engineering, Thessaloniki, Greece.
Abstract | Nonstructural elements are typically at the end of a long chain of uncertainties making them particularly challenging to design or to estimate their seismic performance. Seismic design of nonstructural elements is typically based on over simplistic equations to estimate equivalent static forces that not only neglect the dynamic properties of the nonstructural element and of the building in which they are mounted on, but they also use large component force reduction factors that have no rational basis. Floor motions are characterized by being narrowband motions that produce extremely large accelerations on nonstructural elements whose frequencies of vibration coincide with those of the building in which they are mounted. For many years now, seismic design of structures relies on the identification of predetermined locations where non-linearities are expected to occur in the event of moderate or severe earthquake ground motions and other components and connections are designed to remain elastic. At present time, there is no equivalent approach in the design of nonstructural elements. The purpose of this paper is to introduce a new approach in which a new type of bracing elements of nonstructural elements are designed and detailed to work as fuses that limit forces acting not only in the nonstructural elements but also in the attachments to the structure and in the attachment(s) to the nonstructural element. It is shown that the proposed approach, which accounts for the narrow-band characteristics of floor motions, not only results in reductions in design forces but can also result in important reductions in deformation demands, especially for components that are tuned to modal frequencies of the building in which they are mounted on.
Miranda E., Kazantzi A.K., Vamvatsikos D. (2018). Towards a new approach to design acceleration-sensitive non-structural components. Proceedings of the 11th National Conference on Earthquake Engineering, Los Angeles, CA
Abstract | Nonstructural components often represent the largest portion of the investment in new commercial buildings, typically accounting for 70 to 85% of the total construction cost. Furthermore, damage to nonstructural components can lead not only to serious injuries and even casualties, but can lead to partial or total temporary loss of use of the building and to large economic losses. Nonstructural components are often subjected to seismic motions that are completely different both in amplitude and in frequency content than ground motions. Floor motions are characterized by being narrowband motions that produce extremely large accelerations on nonstructural elements whose frequencies of vibration coincide with those of one of the modal frequencies of the building in which they are mounted. The purpose of this paper is to propose a new approach in which bracing elements of nonstructural elements are designed and detailed to work as fuses that limit forces acting not only in the nonstructural elements but also those acting on the attachments to the structure and on the attachment(s) to the nonstructural element. It is shown that the proposed approach not only results in reductions in design forces but can also result in important reductions in deformation demands, especially for components that are tuned to modal periods of the building in which they are mounted on.
Kazantzi A.K., Vamvatsikos D. (2017). Vibration control of an industrial composite slab subjected to impact loads and improvement of the overall building seismic performance using toggle-brace dampers. Proceedings of the 9th Hellenic National Conference on Steel Structures, Larisa, Greece
Abstract | This paper presents the preliminary findings of a case study, undertaken for enhancing the vibration performance of an industrial two-way composite slab subjected to impact loads as well as for the seismic retrofitting of the entire building. Struts as well as toggle-bracedamper configurations have been effectively combined to achieve the aforementioned twofold goal. With respect to the vibration control of the slab, provided that the availability of the field measurements was not enough to undertake a full forensic engineering study, the proposed strengthening scheme in both directions was selected so as the resulting system fundamental frequency (i.e. beam and girder combined mode) would lie well above the minimum recommended values. Furthermore, in the girder direction, these struts were aligned in a toggle-brace-damper configuration in order to: (a) Employ shallow trusses, due to operation-related restrictions, while (b) magnifying the vertical slab displacements associated with the impact load and (c) the horizontal deformations imparted by seismic loads to this stiff building so that the dampers experience increased velocities and maximize their effectiveness at both the serviceability and the ultimate limit-state.
Papadopoulos A., Vamvatsikos D., Kazantzi A. (2017). Development of FEMA P-58 compatible story loss functions: steel office buildings in high seismicity regions. Proceedings of the COMPDYN2017 Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Rhodes, Greece
Abstract | Since their release in 2012, the FEMA P-58 guidelines for seismic performance assessment of buildings have been regarded as the state-of-the-art paradigm for buildingspecific risk assessment and loss estimation. The latter is carried out through a rigorous component-by-component procedure that requires a complete component inventory, along with fragility and repair cost information. A fully compatible story-based approach is investigated herein as a simplified alternative that can potentially reduce the required input data with only a minor drop in accuracy. As an example, a set of story loss functions relating story repair cost with story-level engineering demand parameters are derived for standard inventory makeups of low/midrise steel office buildings. Preliminary results for a 4-story steel building show a promising balance between accuracy and simplicity.
Kazantzi A.K., Vamvatsikos D. (2015). A next generation scalar intensity measure for analytical vulnerability studies. Proceedings of the COMPDYN2015 Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Crete, Greece
Abstract | To assess the seismic performance of a structural system within an analytical context, we need, among others, to specify a ground motion Intensity Measure (IM). The wary IM selection is undoubtedly an important step towards the successful implementation of a risk assessment, since insufficient and/or inefficient IMs can induce unwanted bias and variance in the vulnerability estimates. Supplementary issues related to practicality, necessitate the use of IMs for which ground motion prediction relationships exist, such as the elastic response spectral values (i.e. acceleration, velocity and displacement). Several past studies suggested as an improvement the use of IMs defined as the geometric mean of spectral acceleration values computed over a period range. The latter range may span between periods that are below, at or above the fundamental one. Some of these choices were proven to significantly improve both efficiency and sufficiency of the IM compared to more commonly used counterparts.
This study investigates the efficiency and sufficiency of a newly developed scalar IM that combines the geometric mean IM concept with the significant duration of the ground motions. Improving the geometric mean IMs via including the significant duration of the ground motions, was driven by recent findings suggesting there is a strong tie between the collapse capacity of a structure and the ground motion duration. Hence, the performance of the proposed next generation IM is addressed in detail by means of comparisons and statistical significance tests. The testing is performed at specific levels of local engineering demand parameters that are closely related to losses, using a testbed capacity-designed steel moment-resisting frame. It was demonstrated that ground motion duration is closely related to the collapse capacity whereas its effect at lower demand levels is insignificant. Hence, the proposed IM may be employed to improve the estimates in collapse assessment studies. Nevertheless, at least for steel moment-resisting frame buildings that exhibit moderate cyclic degradation rates and sustain most losses prior to the global collapse state, the significant duration is anticipated to only minimally affect the evaluated vulnerability and consequently may be disregarded.
Kazantzi A.K., Vamvatsikos D., Porter K. (2015). Analytical seismic vulnerability assessment for a class of modern low-rise steel MRFs. Proceedings of the 12th International Conference on Applications of Statistics and Probability in Civil Engineering, ICASP12, Vancouver, Canada
Abstract | A set of guidelines was developed for the Global Earthquake Model (GEM), aiming to offer a practical, yet sufficiently accurate, analytical method for assessing the relationship between the ground shaking and the repair cost for a building class. The present work illustrates the methodology for a class of modern low-rise steel moment-resisting frames (SMRFs). The structural analysis is performed using Incremental Dynamic Analysis (IDA). The selection of a single Intensity Measure (IM) to parameterize IDA results and, eventually, vulnerability curves is being tackled through an extended IM comparison study across the entire structural response range considering both interstory drifts and peak floor accelerations. It is demonstrated that scalar IMs, defined as the geometric mean of spectral accelerations values Sagm(Ti) estimated at several periods Ti can have an overall satisfactory performance. Once the uncertain structural response is determined, the methodology proceeds to the vulnerability estimation and consequently to loss assessment that is built upon a simplified componentbased FEMA P-58 style methodology. The end product of this study is a high-quality set of vulnerability curves whose weighted moments are taken as the uncertain vulnerability function of the investigated building class.
Kazantzi A., Vamvatsikos D., Porter K. (2014). Analytical seismic vulnerability assessment of lowrise steel MRFs. Proceedings of the 8th Hellenic National Conference on Steel Structures, Tripoli, Greece
Abstract | The Global Earthquake Model (GEM; http://www.globalquakemodel.org/) is a grand effort to proffer a comprehensive open source tool for large scale loss assessment studies. For this to be accomplished, an analytical seismic vulnerability assessment methodology needs to be developed that links ground shaking with repair cost for a building class. The test bed for the present study is a set of low/mid-rise steel moment-resisting frames (SMRFs) designed for high seismicity US regions and selected appropriately so as to represent all important aspects within their class. The structural analysis was performed using Incremental Dynamic Analysis (IDA). On that premise, the selection of a single Intensity Measure (IM) to parameterize IDA results and, eventually, vulnerability curves needs to be tackled. It was demonstrated that scalar IMs can have an overall satisfactory performance. Once the uncertain structural response is defined in terms of interstory drifts and floor accelerations, across a wide range of intensities, the methodology proceeds to the vulnerability estimation and consequently to loss assessment. The end product of this study is a high-quality set of vulnerability curves whose weighted moments are taken as the uncertain vulnerability function of the investigated building class.
Vamvatsikos D., Aschheim M.A., Kazantzi A.K. (2014). Direct performance-based seismic design: Avant-garde and code-compatible approaches. Proceedings of the 9th European Conference on Structural Dynamics (EURODYN 2014), Porto, Portugal
Abstract | Current force-based seismic design codes use design spectra and system-specific behavior factors to satisfy two pre-defined structural limit-states: Serviceability and Life Safety. Instead, performance-based seismic design aims to design a structure to fulfill any number of target performance objectives, defined as user-prescribed levels of structural response, loss or casualties to be exceeded at a maximum rate less than a given mean annual frequency (MAF). Even at its simplest structural response basis, the inverse probabilistic nature of the requirements has not yet allowed a satisfying solution without cumbersome cycles of re-design and re-analysis. An alternative approach is proposed, relying on a new format for visualizing seismic performance, termed Yield Frequency Spectra (YFS). YFS offer a unique view of the entire solution space for structural performance, as indexed by the MAF of exceeding arbitrary ductility (or displacement) thresholds, versus the base shear strength of a structural system with given yield displacement and backbone capacity curve. YFS can be instantly computed for any system that is satisfactorily approximated by a single-degree-of-freedom oscillator, as in any nonlinear static procedure. Thus, stated performance objectives are directly related to the strength and stiffness of the structure while fully incorporating aleatory and epistemic sources of uncertainty, as needed to achieve any required level of confidence. The combination of ductility (or displacement) demand and its exceedance MAF is readily determined, allowing a satisfactory initial design to be realized in a single step. Using a simple safety factor approach given the period and the hazard curve slope, the benefits can also apply to contemporary seismic codes, essentially introducing true performance-based capabilities in a traditional format.
Kazantzi A.K., Vamvatsikos D., Porter K., Cho I. (2014). Analytical vulnerability assessment of modern highrise RC moment-resisting frame buildings in the Western USA for the Global Earthquake Model. Proceedings of the 2nd European Conference on Earthquake Engineering and Seismology (2ECEES), Istanbul, Turkey
Abstract | The Global Earthquake Model (GEM) has commissioned the preparation of analytical seismic vulnerability guidelines for loss assessment of highrise buildings. The guidelines attempt to provide a practical method to assess the relationship between ground shaking and repair cost for a class of buildings, in face of the time, skill and computational challenges posed by a class rather than a single structure. An example is presented that reflects a class of modern (post-1980) highrise reinforcedconcrete moment-frame (RCMRF) office buildings in the western United States. Seven characteristic “index” buildings were selected through moment-matching, so as to represent all important aspects of the class. Each is modeled and analyzed through Incremental Dynamic Analysis (IDA). To remove issues of sufficiency without performing record selection, an extensive comparative study of intensity measures (IMs) was undertaken. Scalar IMs, defined as the geometric mean of spectral acceleration values at adjacent periods, were found to be satisfactory. Then, for each of the seven index buildings, poor, typical, and superior-quality variants were considered to establish the seismic loss and its ratio over replacement cost new in each case. The end product is a high quality set of vulnerability curves, whose weighted moments are taken as the uncertain vulnerability function of the class.
Vamvatsikos D., Aschheim M.A., Kazantzi A.K. (2013). Direct performance-based seismic design using yield frequency spectra. Proceedings of the Vienna Congress on Recent Advances in Earthquake Engineering and Structural Dynamics (VEESD 2013), Vienna, Austria
Abstract | Yield Frequency Spectra (YFS) are employed to enable the direct design of a structure subject to a set of performance objectives. YFS offer a unique view of the entire solution space for structural performance. This is measured in terms of the mean annual frequency (MAF) of exceeding arbitrary ductility (or displacement) thresholds, versus the base shear strength of a structural system with given yield displacement and backbone capacity curve. Using publicly available software tools or closed-form solutions, YFS can be nearly instantaneously computed for any system that can be satisfactorily approximated by a single-degree-of-freedom oscillator, as in any nonlinear static procedure application. Thus, stated performance objectives can be directly related to the strength and stiffness of the structure. The combination of ductility (or displacement) demand and its mean annual frequency of exceedance that governs the design is readily determined, allowing a satisfactory design to be realized in a single step.
Kazantzi A.K., Vamvatsikos D., Lignos D.G. (2013). Model parameter uncertainty effects on the seismic performance of a 4-story steel moment-resisting frame. Proceedings of the 10th International Conference on Structural Safety and Reliability (ICOSSAR), New York
Abstract | The reliable estimation of the seismic performance of structures requires quantifying the effect of aleatory and epistemic uncertainties of the system parameters. This is efficiently achieved for a case study of a steel moment-resisting frame through several important advances. First, a state-of-the-art numerical model is formed with full parameterization of its strength properties. Empirical relationships derived from experimental data are used to model the cyclic behavior of steel sections using probabilistically distributed parameters that include intra- and inter-member correlation. Incremental dynamic analysis is employed to accurately assess the seismic performance of the model. Finally, an efficient Monte Carlo simulation algorithm is used, based on record-wise incremental Latin Hypercube Sampling to propagate the uncertainties from the numerous parameters to the actual system demand and capacity. Consequently, this modern steel frame is shown to be only mildly sensitive to parameter uncertainties when quality is tightly controlled.
Vamvatsikos D., Kazantzi A.K. (2013). Seismic fragility and vulnerability assessment using simplified methods for the Global Earthquake Model. Proceedings of the COMPDYN2013 Conference on Computational Methods in Structural Dynamics and Earthquake Engineering, Kos, Greece
Abstract | The Global Earthquake Model (GEM) has commissioned the preparation of analytical vulnerability guidelines for general use. Within this framework, a distinct modeling and analysis method hierarchy has been proposed, whereby both detailed and reduced-order models can be analyzed using nonlinear static or dynamic methods. Each subsequent reduction in complexity increases the speed of application, yet generates additional error that needs to be considered in the form of epistemic uncertainty. The available choices represent different levels of compromise between the accuracy achieved and the associated effort needed, meant to suit users having different levels of expertise and resource availability. Our particular focus will be on the middle path that is expected to become the most popular choice, combining (a) a simplified stick model of the structure with (b) a static pushover analysis with accurate record-to-record dispersion information. The entire procedure is cast within an appropriate probabilistic framework that can effortlessly incorporate all the epistemic and aleatory uncertainty sources to become a viable path for evaluating structural fragility for a building class.
Kazantzi A.K., Vamvatsikos D. (2012). A study on the correlation between dissipated hysteretic energy and seismic performance. Proceedings of the 15th World Conference on Earthquake Engineering, Lisbon, Portugal
Abstract | The hysteretic energy dissipated by systems undergoing quasi-static or dynamic loading is often thought to represent a useful measure of their performance when subjected to earthquake excitation. In general, fuller hysteresis loops mean higher seismic energy removal from the structure, which is logically taken to imply better performance when comparing systems with similar strength. However, such observations are typically based on quasi-static loading tests. Dynamic loading conditions differ as energy input and energy dissipation are intimately related with the details of the system’s hysteresis, in ways that often defy current intuition. Using incremental dynamic analysis on story-level oscillators with varying hysteresis characteristics, we can map this connection in detail. Structural response, as measured in terms of maximum or residual deformation, is shown to have little connection to the energy absorption. Therefore, hysteretic energy dissipation cannot quantitatively measure seismic performance but perhaps only serve as a general indicator.
Kazantzi A.K., Righiniotis T.D. (2009). Seismic reliability of a steel MRF designed to EC8. Proceedings of the 6th Hellenic National Conference of Steel Structures, Ioannina, Greece (in greek)
Abstract | In recognition of the large capacity and demand uncertainties involved in the seismic response of buildings, the engineering community has increasingly become, over the years, convinced that the problem may be treated more efficiently using probabilistic tools coupled with appropriate performance-related criteria. So far, the majority of the work carried out on the reliability of steel Moment Resisting Frames (MRFs) has focused on buildings typical of US design and construction practice, which are, in principle, different from their European counterparts. Thus, this study presents the results of a full probabilistic assessment performed on an EC8-designed, 5-storey steel MRF. In order to obtain fragility curves, the random structural responses were evaluated through a large number of non-linear time history analyses using an ensemble of European ground records. The limit state functions necessary to define fragility were expressed in terms of the previously mentioned structural responses as well as the probabilistically-defined FEMA 356 performance levels. Using these fragilities, annual failure probabilities were obtained for the city of Reggio Calabria in Italy. The aforementioned fragilities were then compared against those obtained from an equivalent SDOF coupled with a closed-form lognormal approximation. The study has revealed that (a) the influence of the ground record characteristics on fragility is significant, (b) the fragility variance increases with the level of induced non-linearity and (c) the fragility methodology based on the equivalent SDOF yields reasonable results.
Kazantzi A.K., Righiniotis T.D. (2008). Seismic reliability of an EC8-designed steel moment resisting frame. Proceedings of the 4th ASRANet International Colloquium, Athens, Greece
Abstract | So far, the majority of the work carried out on the reliability of steel Moment Resisting Frames (MRFs) has focused on buildings typical of US design and construction practice, which are, in principle, different from their European counterparts. Thus, this study presents the results of a full probabilistic assessment performed on an EC8-designed, 5-storey steel MRF. In order to obtain fragility curves, the random structural responses were evaluated through a large number of non-linear time history analyses using an ensemble of European ground records. The limit state functions necessary to define fragility were expressed in terms of the previously mentioned structural responses as well as the probabilistically-defined FEMA 356 performance levels. Using these fragilities, annual failure probabilities were obtained for the city of Reggio Calabria in Italy. The aforementioned fragilities where then compared against those obtained from an equivalent SDOF oscillator coupled with a closed-form lognormal approximation.
Kazantzi A.K., Righiniotis T.D., Chryssanthopoulos M.K. (2008). Probabilistic seismic assessment of a regular steel MRF designed to Eurocode 8 provisions. Proceedings of the 14th World Conference on Earthquake Engineering (14WCEE), Beijing, China
Abstract | The studies undertaken following the Northridge and Kobe earthquakes in 1994 and 1995, during which many steel buildings were hampered by unsatisfactory connection behaviour, also exposed limitations of the deterministic approaches in assessing the performance of new and existing buildings, thus leading to a renewed interest in probabilistic methods for earthquake engineering applications. The majority of the research has so far aimed at combining experimental and analytical efforts into seismic reliability methodologies for steel buildings typical of US practice. Corresponding studies on European steel Moment Resisting Frames (MRFs) are still to be developed and validated. Thus, the current study presents a probabilistic assessment carried out on a Eurocode 8-designed steel MRF, exploring explicitly the effect of joint ductility on its seismic reliability. Fragility curves are generated at different performance levels using the Monte Carlo simulation technique and performing time history analyses on the sample buildings subjected to a suite of ground motion records. Fragilities are presented for building realisations both with and without considering a limit in the total plastic rotational capacity of the beam-to-column joints. The rotational capacities are estimated using an empirical equation, derived from cyclic loading tests on European steel joints. A hazard study on a European site is combined with the structural fragility, thus evaluating the annual seismic risk. The results are used to quantify the notional reliability levels of a Eurocode 8-designed steel MRF.
Kazantzi A.K., Righiniotis T.D., Chryssanthopoulos M.K. (2006). Seismic fragility curves for a welded steel-moment resisting frame. Proceedings of the 3rd ASRANet International Colloquium, Glasgow, UK
Abstract | This paper presents the probabilistic assessment of a three-storey steel structure, designed as a benchmark case of the SAC steel project for the Los Angeles area. The seismic behaviour of the structure is here assessed using 2-D dynamic inelastic time-history analyses, accounting for global P-∆ effects, contribution of the interior gravity frames and P-M interaction. The building is subjected to a suite of ground records, which form part of the common seismic scenario for the applications of the LessLoss Sub-Project 9. Ground motions are scaled according to the spectral acceleration at the building’s elastic fundamental period. Randomness associated with material variability as well as the effect of connection fractures are included in the model. Fragility curves are generated using the Monte Carlo simulation method in conjunction with the Latin Hypercube sampling technique in order to reduce the variance in the probability estimates. The failure probabilities are presented in terms of interstorey drift limits at different performance levels. The results of the study show that the ground motion characteristics have a significant effect on the fragility curves, especially at high levels of spectral acceleration and hence nonlinearity.