Fragility Analysis Framework Research Articles

Probabilistic-based seismic fragility analysis of a ground-bridge structure system considering site liquefaction

Bridge structures are vulnerable to severe damage in previous earthquake events. Meanwhile, the earthquake-induced site liquefaction can significantly affect the seismic performance of bridge structures. Seismic fragility analysis is considered as an effective approach for evaluating seismic performance of bridge structural systems exposed to seismic hazards. This study aims to assess seismic fragility of a ground-bridge structure system considering site liquefaction. Based on the probabilistic seismic demand model (PSDM) and joint probabilistic seismic demand model (JPSDM), a holistic framework for fragility analysis considering site liquefaction is proposed to derive the liquefaction fragility as well as the structural system fragility. The system fragility surfaces obtained by the proposed method can reflect the influence of site properties on the damage exceeding probability of bridge structures. The results demonstrate an increasing probability of undergoing high-level damage states for the whole system containing highly loose sand under strong earthquakes, which may provide significant basis for probabilistic-based aseismic design.

Seismic fragility analysis of slopes based on large-scale shaking table model tests

Seismic fragility analysis is one probabilistic method that represents the conditional probabilities of exceeding different predetermined damage states for various given earthquake hazard levels, which is the essential part of probabilistic seismic risk assessment. It has gained much popularity recently because much more comprehensive and richer results than traditional probabilistic methods can be provided. The classical seismic fragility analysis of slopes is always performed through analytical approaches. In the present study, a novel seismic fragility analysis framework combined with shaking table model test results is proposed for probabilistic seismic performance and risk assessment of slopes. The shaking table model tests of a rock slope reinforced by pile-anchor structures were firstly performed subjected to a suite of real earthquake records with different intensity levels. Subsequently, the seismic responses from shaking table model tests were then gathered for the optimal analysis to determine the optimal engineering demand parameter (EDP) and earthquake intensity measure (IM) for seismic fragility analysis of slopes. At last, the identified most appropriate IM and EDP were selected to construct the scalar-valued fragility curves, vector-valued fragility surfaces and fragility surfaces considering acceleration elevation amplification effect by the proposed framework. The results indicate that using single IM to construct scalar-valued fragility functions may result in different failure probabilities depending on the chosen earthquake IM. The vector-valued fragility surfaces provide more information than ordinary fragility curves, and lead to more proper and realistic evaluations of seismic hazard of slopes. The methods and results presented in this paper also provide the basis for the probabilistic seismic risk and loss assessment of earthquake-induced slope geological disaster.

Fragility Analysis Framework for Bridges Subjected to Successive Natural Hazards

Bridges are key parts of transportation networks and hence, their post-hazard condition is vital for the recovery of the regions affected by natural hazards. However, the fact that they are often subjected to more than one destructive natural hazard during their life span resulting in accumulated damage that may influence their capacity has attracted relatively little research interest, while almost no design standards take it into consideration. The multiple hazard events may be successive single events, separated by considerable time intervals (e.g. an earthquake followed by a flood event), combined or cascading multiple-hazard events (e.g, landslide after an earthquake), or even simultaneous events. The main objective of this study is to propose an effective framework for the development of multiple-hazard fragility curves of bridges considering the effect of subsequent events and taking into account different possible damage scenarios of the bridge critical components, i.e. piers, bearings and the abutment-backfill systems. In this framework, the effect of the damage level of the critical components and their interdependence on the fragility assessment for multiple events is examined and compared with the bridge fragility for single events. A pilot case study is also included in the present work, aiming to highlight the effect of the initial damage state on the final fragility estimate of a typical riverine bridge.

Seismic fragility assessment for highway bridges incorporating multi-level shape memory alloy cable dampers

An innovative multi-level shape memory alloy (SMA) cable damper is proposed for bridges in near-fault seismic regions. This study commences with the introduction of a constitutive material model for simulating SMA cables. The proposed model not only considers the self-centering hysteretic property but also accounts for the strength degradation and residual strain accumulation effects. Subsequently, the layout and working mechanism of the proposed multi-level SMA cable damper is presented. A seismic fragility analysis framework involving Box-Cox transformation and Bayesian inference is subsequently presented. The Box-Cox transformation is utilized to establish the nonlinear probabilistic seismic demand models (PSDMs), and the Bayesian-based logistic regression is employed to formulate fragility functions. Finally, a three-span continuous reinforced concrete (RC) girder bridge incorporating the multi-level SMA dampers is selected as an example to demonstrate the proposed framework. A total of 160 near-fault ground motions, containing pulse and non-pulse motions, are applied along the longitudinal direction of the bridge. To investigate the influences of different parameter values of the SMA cable damper on fragility functions, various parameters are generated with respect to a central composite design philosophy. Based on the seismic fragility analysis, it is found that the SMA isolation damper reduces the damage probabilities of the bearing and pier for all four damage states. In addition, the failure probability of the bearing decreases with the increase of the axial stiffness of the SMA damper, which is opposite to the pier. Moreover, the optimum designed SMA damper can significantly reduce the damage probability of the case study bridge, and the parameter values of the damper should be carefully selected to achieve the desired seismic performance.

Risk Assessment of Offshore Wind Turbines Suction Bucket Foundation Subject to Multi-Hazard Events

For the offshore wind turbines (OWTs) located in a seismically active region, the occurrence of earthquakes combined with scour is a highly possible multi-hazard event. This study developed an alternative fragility analysis framework to assess the seismic performance of suction bucket-supported OWTs under the action of scour. First, the probabilistic approach was applied to calculate the occurrence probability of scour depth (SD) and earthquake events. Then, the possible combinations of these two events were considered in the analysis model to establish the fragility surface of the suction bucket foundation. Finally, by integrating the product of scour and earthquake hazard, as well as fragility curves, the suction bucket foundation failure probability was obtained. The developed framework provides a reliable approach to risk assessment for OWT-supporting structures in extreme event situations and can be applied to other complex natural hazards.

Seismic Fragility Assessment of RC Columns Exposed to the Freeze-Thaw Damage

Freeze–thaw damage is one of the primary causes deteriorating the seismic resistance of reinforced concrete (RC) structures. This paper proposed a freeze–thaw damage deterioration model for C30 concrete, and it can be employed to study the time-varying seismic performance of aging RC columns. Next, this study developed a seismic fragility analysis framework for deteriorating RC columns considering the effect of freeze–thaw damage. Considering the geometric parameters of the case-study bridge, the deterioration characteristics of material, and the uncertainties involved in structural modeling and ground motions, a probabilistic seismic fragility analysis on aging RC columns was conducted. The results indicate that the influence of freeze–thaw damage cannot be ignored in studying the seismic performance of aging RC structures. The seismic fragilities of deteriorating RC columns shown a nonlinear increase trend as the increased of freeze–thaw cycles and severity of the damage state. In the early stage of freeze–thaw cycles, the seismic fragilities of RC columns increased slowly. However, the closer to the later stage of freeze–thaw cycles, the more significant of the increase in the seismic fragilities of the columns.

Seismic Fragility Analysis of the Aging RC Columns under the Combined Action of Freeze–Thaw Cycles and Chloride-Induced Corrosion

The combined action of freeze–thaw cycles and chloride-induced corrosion are generally recognized as one of the main causes of the degradation of the mechanical properties and seismic performance of reinforced concrete (RC) structures in the northern frozen coastal regions. To investigate the degradation mechanisms of the seismic performance of RC columns subjected to the combined action of freeze–thaw cycles and chloride-induced corrosion, the impact of freeze–thaw cycles on the chloride diffusion coefficient of concrete was studied through concrete deterioration tests and theoretical analysis. This paper proposed a time-dependent deterioration model for RC columns, which is suitable to consider the combined action of freeze–thaw cycles and chloride-induced deterioration. The proposed deterioration model could be applied to the investigations of time-dependent seismic performance and the seismic fragility of RC columns. Based on the established deterioration model, this paper proposed a time-dependent seismic fragility analysis framework for the aging RC columns, considering the combined action of freeze–thaw cycles and chloride-induced corrosion. In addition, a representative three-span RC continuous T-shaped girder bridge that is located in the high-latitude northern frozen coastal regions of China was taken as the case study, and the time-dependent seismic fragility analysis of RC columns was conducted considering the involved uncertainties in geometric parameters, the deterioration mechanisms of the materials, and ground motions. The time-dependent seismic fragility curves of RC columns were obtained at different service time points. The results indicated that the combined action of freeze–thaw cycles and chloride-induced deterioration had a significant influence on the time-dependent seismic responses of the deteriorating RC columns. Under the combined action of freeze–thaw cycles and chloride-induced corrosion, when the RC bridge was in service for 75 years, the stirrup strength decreased by 3.88% and the cross-sectional area decreased by 30.03%. The peak stress of the confined concrete decreased by 52.1% and its peak strain increased by 12.2 times, respectively. Moreover, the time-dependent seismic fragilities of the aging RC columns under different damage states exhibited a nonlinear increase as the service life increased.

Time-varying seismic fragility analysis of bridges under the condition of the scour and freeze–thaw cycles

This article proposes a seismic fragility analysis framework of bridges, which considers the freeze–thaw cycles and scour comprehensively. The time-varying seismic fragility analysis method is used to analyse the bridge system based on the degradation law of mechanical properties of concrete under the freeze–thaw cycles, combined with the uncertainty of earthquake, scour depth and freeze–thaw cycles. Under the condition of the scour, with the growth of service time, the seismic fragility of piers and bearings increased and the exceeding probability of bearing is higher than the pier, which makes the bearing more easily damaged than the pier in the case of an earthquake, thus reduces the seismic damage of the pier. The plastic hinge of the bridge pier transfers from the immersion zone to the freeze–thaw zone in freeze–thaw cycles. In this process, the curvature in the freeze–thaw zone grows up, dissipates seismic energy and leads the pier’s fragility to decrease before the plastic hinge transfer. When combined with the scours and freeze–thaw cycles, the failure probability of the pier is slightly lower than that of only the scour action due to the phenomenon of plastic hinge transfer.

Fragility analysis of a pipeline under slope failure-induced displacements occurring parallel to its axis

Providing urbanized centers with hydrocarbon supplies from remote production centers, transmission pipelines run for thousands of kilometers, inevitably traversing mountainous and rugged terrain, where slope failure is the dominant source of risk. Strategies to manage these risks heavily depend on the quantification of uncertainty. To this end, this paper presents a fragility analysis framework to quantify the effect of uncertainties, that makes use of a Random Forests algorithm as a surrogate model to thoroughly explore the uncertain parameter space. The Random Forests algorithm is trained and tested against a small, yet statistically significant sample of Sliding Slope-Pipeline Interaction (SSPI) cases that are meticulously analyzed using a rigorous 3D continuum-based numerical methodology. The fragility analysis framework is applied to a 36 in. pipeline descending a slope, with possible soil movement occurring parallel to its axis. Uncertainty stems from the variability in soil properties (friction angle, cohesion and density), in pipe steel material properties (yield strength), in the magnitude of operational loads (pressure and temperature) and in the pipeline placement depth around their expected values, as well as from the lack of knowledge of the geological conditions (the depth of the eroded surficial layer) that ultimately control the shape of the failure surface. Τhe significance of each uncertain variable on the response of the pipeline is investigated, concluding that the depth and shape of the failure surface (correlated with the depth of the surficial layer) is be the most influential parameter, governing the pipeline performance and ultimately the safety thresholds.

Fragility analysis for performance-based blast design of FRP-strengthened RC columns using artificial neural network

In this paper, fragility analysis for performance-based blast design of FRP-strengthened RC columns is carried out. The blast intensity levels, performance levels and performance objectives of the RC columns are defined. A simplified probabilistic risk assessment framework incorporating the performance-based design concept and fragility analysis is established. Since fragility analysis is the most important but time-consuming process of probabilistic assessment risk, an artificial neural network (ANN) based fragility analysis framework is proposed to improve its computational efficiency. Based on the rapid fragility analysis method, fragility curves of several typical RC columns with or without FRP strengthening are calculated to analyze their damage probabilities. This study provides avenues for engineers to estimate the failure probabilities of RC columns with or without FRP strengthening under blast loads and make decisions quickly.

Life‐cycle seismic fragility of a cable‐stayed bridge considering chloride‐induced corrosion

AbstractThe coastal or sea‐crossing bridges located in seismic regions are experiencing long‐term chloride‐induced corrosion in their life cycles and have a high risk of suffering from strong earthquakes. Thus, the fragilities of these bridges must be properly evaluated under the multiple hazards of corrosion and earthquake. Conventional fragility assessment requires expensive computational efforts despite extensive applications in previous research. To this end, this paper proposes a life‐cycle fragility analysis framework based on the endurance time method to investigate the deterioration impact on the seismic fragility of bridges. An example sea‐crossing cable‐stayed bridge is utilized as a case study and modeled by OpenSees considering different service years. The life‐cycle fragility curves of the example bridge are generated using the artificial endurance time series. Fragility analysis results show that the proposed method is capable of assessing the life‐cycle seismic fragility of the bridge with high efficiency. The bearing shows a higher damage probability than the pylon and pier. As the service time increases, the structural deterioration has a marginal impact on the system fragility of the bridge, while the component fragility varies across different components. The structural deterioration results in beneficial effects on the pylon and the bearing at the pylon but exerts adverse effects on the pier and the bearing at the pier.

An endurance time method-based fragility analysis framework for cable-stayed bridge systems under scour and earthquake

Seismic fragility assessment using structural dynamic analysis remains challenging for complex systems due to the high computational demand, such as the sea-crossing cable-stayed bridges. The paper aims to overcome this difficulty by developing an efficient seismic fragility analysis framework based on endurance time method (ETM). To demonstrate the framework for developing fragility curves, an example sea-crossing cable-stayed bridge is used as a case study. The ETM framework is validated by comparing the calculated fragility curves with those calculated by IDA framework. It is shown that the ETM is capable of significantly reducing computational effort without compromising the accuracy of fragility analysis results. The validated framework is then used to discuss the seismic fragility of sea-crossing cable-stayed bridges under scour and earthquakes. The influence of the time duration of endurance time acceleration functions (ETAFs) on the structural fragility is discussed. The influence of the time duration of ETAFs on the component fragility varies across different components, but the system fragility increases with the increasing time duration on the whole. Scour can produce either beneficial or adverse effects on the component and system seismic fragilities depending on the scour depth.

Typhoon risk and climate-change impact assessment for cultural heritage asset roofs

Recent catastrophic events in Southeast Asia have emphasized that roofs made of wood/steel frames and lightweight metal roofing sheets are the most vulnerable component in the building envelope when subjected to typhoon-induced wind uplift. This also applies to aging cultural heritage (CH) assets, which deserve special consideration because of their intangible value for local communities, and their essential role for inclusive and sustainable socio-economic development through cultural tourism.This paper introduces a simulation-based framework for fragility analysis and typhoon risk assessment of CH-asset roofs. Fastener pullout and roof-panel pullover are explicitly considered in the proposed framework to model the progressive failure of the roof system. A simplified roof geometry is assumed, requiring limited information about the structure under investigation and low computational resources. Such a low computational burden allows modeling wind-induced demands and component capacities probabilistically as well as considering the effects of load redistributions due to fastener failure and fastener/roof-panel corrosion. Variance-based sensitivity analysis (i.e., Sobol’ indices) based on polynomial chaos expansions of the limit state function is also performed, highlighting the parameters most affecting typhoon-fragility variance and then requiring special attention during data collection. Climate-change impact on the typhoon risk estimates is finally investigated through the use of various scenarios and a time-dependent function modifying the wind hazard profile of the site where the assets of interest are located.The proposed framework is applied to 25 CH assets in Iloilo City, Philippines. The required input data was collected through rapid visual surveying combined with new technologies, such as drones. It is shown that the proposed framework can be adopted in practice for both risk prioritization at a building-portfolio level and simplified risk assessment at a building-specific level.

Multi-hazard vulnerability of buildings to debris flows

Landslide hazards often occur in succession with multiple hazardous processes. The elements at risk such as buildings are therefore exposed to a multi-hazard environment. Multi-vulnerability assessment is a critical component of multi-hazard risk analysis. In this paper, effect of hazard interactions on the physics of building damage is formulated from the perspective of triggering and temporal relations. A fragility analysis framework for generating physics-based vulnerability models for sequentially and concurrently occurring hazards is proposed. Monte Carlo simulation is used in the framework to include the uncertainties from both hazard intensity and building properties. The framework is illustrated by evaluating the failure probability of a typical reinforced concrete building impacted by multiple surges of debris flows. Considering specific debris flow-building interaction mechanisms, nonlinear finite-element pushover analysis is adopted to obtain the building response under debris flow impact. By quantifying the physical damage caused by the primary debris flow, the cumulative damage effect of the sequentially occurring debris flows is formulated. The amplified damage effect of the concurrently occurring debris flows is however case specific. The proposed methodology shows potential to address more complex multi-hazard scenarios and enhance the multi-vulnerability assessment, filling the research gap on multi-vulnerability assessment for landslide hazards.

Flood-fragility analysis of instream bridges – consideration of flow hydraulics, geotechnical uncertainties, and variable scour depth

Floods, bridge scour, and flood-associated loads have caused over sixty percent of bridge failures in the U.S. Current practices for the vulnerability assessment of instream bridges under the effect of such flood largely rely on qualitative methods, such as visual inspection, without considering uncertainties associated with structural behaviors and flood loads. Recently, numerical methods have been investigated to quantitatively consider such uncertainty effects by adapting fragility analysis concept that has been well established in the earthquake engineering area. However, river hydraulics, geotechnical uncertainties of foundation, variable scour-depth effects, and their significance in structural fragility of bridges have rarely been systematically investigated. This study proposes a comprehensive fragility analysis framework that can effectively incorporate both flow hydraulics and geotechnical uncertainties, in addition to commonly considered components in flood-fragility analysis of bridges. The significance of flow hydraulics and geotechnical uncertainties has been demonstrated through a real-bridge case study. Conventional fragility curves with maximum scour depth may not represent actual vulnerability during floods, as the scour may not reach to the maximum in many cases. Therefore, fragility surface with two intensity measures, i.e. flow discharges and scour depths, is introduced for real-time vulnerability assessment during floods in this study.

Application of endurance time method to seismic fragility evaluation of highway bridges considering scour effect

Earthquake and flood scour are two main causes of failures to highway bridges. This study developed an alternative fragility analysis framework of bridges based on endurance time method (ETM). A two-span reinforced concrete highway bridge supported by three types of substructures, e.g. shaft foundation, two-column bent foundation and pile group foundation, was taken as the example bridge. The developed framework was used to obtain the fragility curves of the example bridges. The influences of scour depth, substructure design, ground motion direction and scour countermeasure on the seismic fragility of bridges were investigated. The developed seismic fragility analysis framework based on endurance time method yields accurate and efficient calculation compared with IDA method. Scour reduces the bearing capacity of the foundation and affects the seismic fragility of bridges. However, the effect of scour on the seismic fragility varies with the bridge substructure design, the ground motion direction and the application of the scour countermeasure.

Seismic fragility analysis of AP1000 SB considering fluid-structure interaction effects

AP1000 nuclear power plant has an advanced passive cooling system with a vast water tank located above the shield building, which may affect the seismic performance of the structure. This paper conducts a fragility assessment of shield building and considers the fluid-structure interaction (FSI) effect on the damage of the shield building. Arbitrary Lagrangian Eulerian algorithm is applied to calculate the FSI and eight cases of water levels are considered. Three different fragility analysis methods, namely linear regression, quadratic regression and truncated maximum likelihood estimation, are adopted for assessing the seismic vulnerability of shield building. It is indicated from the results that the regression and the truncated maximum likelihood estimation are close to each other. The maximum likelihood estimation not only can effectively save the computational cost, but also can assure the reasonable calculated results. It is observed that different water levels have the individual probabilities of failure, and case 6 has the smallest probability of failure with a more considerable seismic reduction, whereas the case 1 and case 2 have the higher failure probabilities. The fragility analysis framework can thus be applied for evaluating the vulnerability of shield building under earthquakes considering the FSI effect.

Computationally efficient seismic fragility analysis of geostructures

Seismic fragility analysis is considered nowadays as a very efficient computational tool for determining the structural behaviour over a range of seismic intensity levels. There are two approaches for developing fragility curves, either based on the assumption that the structural response follows the lognormal distribution or using reliability analysis techniques for calculating the probability of exceedance for various damage states for a variety of seismic hazard levels. The Monte Carlo simulation (MCS) technique is regarded as the most consistent reliability analysis method having no limitations regarding its applicability range. However, the required computational effort is the only limitation which increases substantially when implemented for calculating lower probabilities. Incorporating artificial neural networks (ANN) into the fragility analysis framework enhances the computational efficiency of MCS, since ANN require a fraction of time compared to the conventional procedure. In this work two types of ANN are implemented into a MCS-based vulnerability analysis framework of geostructures, where the randomness of material properties, geometry and of the pseudostatically imposed seismic loading is considered.

The Role of Fragility Assessment in Consequence-Based Engineering

Concepts of uncertainty modeling and fragility assessment in earthquake engineering and their applications in consequence-based engineering (CBE) of buildings in a region of low-to-moderate seismicity are presented. Efficient treatment of uncertainties in components and systems that are major contributors to building vulnerability is an essential ingredient of risk assessment and decision making. Current methodologies are reviewed first. A framework for fragility analysis then is demonstrated using an unreinforced masonry building as an example. The importance of fragility analysis in various stages of risk assessment, loss estimation, and decision making in CBE to achieve the desirable long-term objective in reduction of loss and consequence with the most efficient intervention measures is clearly indicated. Current research issues related to vulnerability assessment are identified.