A new strategy for the vulnerability study of buildings exposed to open-airexplosions

En este artículo se describe una nueva metodología para evaluar el efecto de las explosiones a cielo abierto sobre estructuras equivalentes a las fachadas de los edificios. El daño sufrido por la estructura se define mediante superficies de vulnerabilidad que son función de la magnitud de la explosión, la distancia de esta a la estructura y del índice de daño por detonación desarrollado en este artículo. El índice propuesto considera la degradación de la capacidad de carga de la estructura, la fracturación y la perdida de material debido a la explosión. Para ello, la estructura se modela mediante elementos discretos (DEM) los cuales permiten representar adecuadamente estados de multifractura. La capacidad de carga de la estructura se cuantifica mediante un ensayo virtual sobre la estructura dañada. Las fuerzas provocadas sobre la estructura por la explosión se modelan utilizando una metodología semiempírica, lo que permite obviar el análisis con base en la dinámica de fluidos reduciendo el tiempo de cálculo.In this paper, a new methodology is described to evaluate the effect of open air explosions on equivalent structures to the facades of buildings. The structural damage is defined by vulnerability surfaces that are a function of the explosion magnitude, the distance to the structure and the detonation damage index developed in this article. The proposed index considers the structural load capacity degradation, the fracturing and the loss material due to the explosion. The structure is modeled by means of discrete elements (DEM) which allows describing the multifracturing state. The load capacity of the structure is quantified by a virtual compression test on the damaged structure. The forces on the structure caused by the explosion are modeled by a semi-empirical methodology, which avoids the fluid-dynamic analysis and reduces the computation time.Peer Reviewe

Una nueva estrategia para el estudio de la vulnerabilidad de edificios expuestos a explosiones a cielo abierto

ResumenEn este artículo se describe una nueva metodología para evaluar el efecto de las explosiones a cielo abierto sobre estructuras equivalentes a las fachadas de los edificios. El daño sufrido por la estructura se define mediante superficies de vulnerabilidad que son función de la magnitud de la explosión, la distancia de esta a la estructura y del índice de daño por detonación desarrollado en este artículo. El índice propuesto considera la degradación de la capacidad de carga de la estructura, la fracturación y la perdida de material debido a la explosión. Para ello, la estructura se modela mediante elementos discretos (DEM) los cuales permiten representar adecuadamente estados de multifractura. La capacidad de carga de la estructura se cuantifica mediante un ensayo virtual sobre la estructura dañada. Las fuerzas provocadas sobre la estructura por la explosión se modelan utilizando una metodología semiempírica, lo que permite obviar el análisis con base en la dinámica de fluidos reduciendo el tiempo de cálculo.AbstractIn this paper, a new methodology is described to evaluate the effect of open air explosions on equivalent structures to the facades of buildings. The structural damage is defined by vulnerability surfaces that are a function of the explosion magnitude, the distance to the structure and the detonation damage index developed in this article. The proposed index considers the structural load capacity degradation, the fracturing and the loss material due to the explosion. The structure is modeled by means of discrete elements (DEM) which allows describing the multifracturing state. The load capacity of the structure is quantified by a virtual compression test on the damaged structure. The forces on the structure caused by the explosion are modeled by a semi-empirical methodology, which avoids the fluid-dynamic analysis and reduces the computation time

The response of glass window systems to blast loadings: An overview

The failure of glass windows in terrorist bombing attacks and accidental explosion incidents has been cited as one of the major causes to the vast casualties. Many studies have been carried out to investigate the response and vulnerability of glass windows against blast loadings. These include laboratory and field tests that have been carried out to experimentally study glass window performance under explosion scenarios and development of analytical and numerical models to analyze and predict glass window responses. This article reviews literatures on the studies of the response of glass window systems to blast loadings. Over 100 papers and documents that are available in the open literature are reviewed. The background and history of the studies on the topic are also briefed. Understandings about the dynamic material properties of glass and available material models are summarized. Popularly used analysis methods and design standards for monolithic and laminated glass windows are outlined, and their accuracies are discussed. Recent studies including analytical solution, numerical simulation, and experimental investigations on glass window systems are summarized. Mitigation measures for blast-resistant windows are also briefly discussed

EXPERIMENTAL COMPARISON STUDY OF THE RESPONSE OF POLYCARBONATE AND LAMINATED GLASS BLAST RESISTANT GLAZING SYSTEMS TO BLAST LOADING

This thesis recounts the experimental study of the dynamic response of polycarbonate blast resistant glazing systems to explosive loading through the use of triaxial load cells, pressure sensors, and a laser displacement gauge. This instrumentation captured the response of the glazing systems to blast loading over three phases of testing. The first phase of testing characterizes the load distribution around the perimeter and the second phase examines the repeatability of the results. The final phase of testing pushes the samples to failure. The results are then compared to HazL, a commonly used blast resistant glazing system analysis software tool. The experimental data is also compared to data available characterizing the response of laminated glass

Security risks and probabilistic risk assessment of glazing subject to explosive blast loading

A probabilistic risk assessment (PRA) procedure is developed which can predict risks of explosive blast damage to built infrastructure. The present paper focuses on window glazing since this is a load-capacity system which, when subject to blast loading, has caused significant damage and injury to building occupants. Structural reliability techniques are used to derive fragility and blast reliability curves (BRCs) for annealed and toughened glazing subjected to explosive blast, for a variety of threat scenarios. The probabilistic analyses include the uncertainties associated with blast modelling, glazing response and glazing failure criteria. Damage risks are calculated for an individual window and for windows in the facade of a multi-storey commercial building. If threat probabilities can be estimated then the paper shows illustrative examples of how this information, when combined with risk-based decision-making criteria, can be used to optimise risk mitigation strategies

Assessment of risk of disproportionate collapse of steel building structures exposed to multiple hazards

Vulnerability of buildings to disproportionate (or progressive) collapse has become an increasingly important performance issue following the collapses of the Alfred P. Murrah Federal Building in Oklahoma City in 1995 and the World Trade Center in 2001. Although considerable research has been conducted on this topic, there are still numerous unresolved research issues. This dissertation is aimed at developing structural models and analysis procedures for robustness assessment of steel building structures typical of construction practices in the United States, and assessing the performance of these typical structures. Beam-column connections are usually the most vulnerable elements in steel buildings structures suffering local damage. Models of three typical frame connections for use in robustness assessment have been developed with different techniques, depending on the experimental data available to support such models. A probabilistic model of a pre-Northridge moment-resisting connection was developed through finite element simulations, in which the uncertainties in the initial flaw size, beam yield strength and fracture toughness of the weld were considered. A macro-model for a bolted T-stub connections was developed by considering the behavior of each connection element individually (i.e. T-stub, shear tab and panel zone) and assembling the elements to form a complete connection model, which was subsequently calibrated to experimental data. For modeling riveted connections in older steel buildings that might be candidates for rehabilitation, a new method was proposed to take advantage of available experimental data from tests of earthquake-resistant connections and to take into account the effects of the unequal compressive and tensile stiffnesses of top and bottom parts in a connection and catenary action. These connection models were integrated into nonlinear finite element models of structural systems to allow the effect of catenary and other large-deformation action on the behavior of the frames and their connections following initial local structural damage to be assessed. The performance of pre-Northridge moment-resisting frames was assessed with both mean-centered deterministic and probabilistic assessment procedures; the significance of uncertainties in collapse assessment was examined by comparing the results from both procedures. A deterministic assessment of frames with full and partial-strength bolted T-stub connections was conducted considering three typical beam spans in both directions. The vulnerability of an older steel building with riveted connections was also analyzed deterministically. The contributions from unreinforced masonry infill panels and reinforced concrete slabs on the behavior of the building were investigated. To meet the need for a relatively simple procedure for preliminary vulnerability assessment, an energy-based nonlinear static pushdown analysis procedure was developed. This procedure provides an alternative method of static analysis of disproportionate collapse vulnerability that can be used as an assessment tool for regular building frames subjected to local damage. Through modal analysis, dominant vibration modes of a damaged frame were first identified. The structure was divided into two parts, each of which had different vibration characteristics and was modeled by a single degree-of-freedom (SDOF) system separately. The predictions were found to be sufficiently close to the results of a nonlinear dynamic time history analysis (NTHA) that the method would be useful for collapse-resistant design of buildings with regular steel framing systems.Ph.D.Committee Chair: Ellingwood, Bruce; Committee Member: Kardomateas, George; Committee Member: White, Donald; Committee Member: Will, Kenneth; Committee Member: Zureick, Abdul-Hami

Reliability-Based Progressive Collapse And Redundancy Analysis Of Bridge Systems

Highway bridges like most structural systems are usually designed on a member by member basis and little consideration is provided to the effect of a local failure on system safety. There are concerns that some systems optimized to meet code-specified member design criteria may not provide sufficient levels of structural redundancy to withstand a possible local failure. In fact, a local failure of one structural element may result in the failure of another element creating a chain reaction that might progress throughout the whole structure or a major portion of it leading to a catastrophic collapse. Several recent catastrophic structural collapses have alerted the structural engineering community to the importance of designing structures with sufficient levels of structural redundancy and robustness to make them capable of withstanding local failures and retaining some level of limited functionality. This has led several agencies to develop criteria for evaluating the robustness of structural systems. However, in a departure from LRFD-based code developments, these recently proposed criteria, which are based on deterministic concepts, do not properly account for the random material properties, the variations in the strengths of the members, or the uncertainties associated with modeling the response of structural systems. Furthermore, it is not clear if the existing criteria which were developed for office buildings are applicable to highway bridges subjected to highly stochastic live loads or whether these criteria will lead to similar safety levels for different types of structures. The object of this Dissertation is to propose a methodology to evaluate the redundancy of highway bridge systems and verify their ability to withstand progressive collapse should a local failure take place. In keeping with current code development approaches, the proposed methodology must be calibrated to provide an acceptable and consistent level of reliability for different types of structures accounting for the uncertainties in estimating the bridge behavior and material properties. A first step for achieving the objectives of this study is to define non-subjective reliability-based criteria for evaluating the performance of originally intact bridge systems, those that have been subjected to local damage, and assessing the ability of the system to survive the sudden occurrence of local damage. The development of such reliability-based criteria requires the availability of probabilistic analysis algorithms capable of handling complex structural systems with low probability of failure. The review of existing structural system reliability methods shows that a Markov-Chain simulation known as the Subset Simulation method offers many advantages over other available methods for evaluating the reliability of complex structural systems with high numbers of failure modes and low probabilities of failure. To further improve the existing subset simulation algorithm, a hybrid Markov chain Monte Carlo method referred to as RASS is proposed. The proposed improvements include: a) a more efficient advanced Markov Chain sample generation algorithm; b) a Delayed Rejection process that allows partial local adaptation of the generated candidate samples at each time step of the Markov chain; c) an Adaptive Algorithm that uses the history of the chain to update the variances of the intermediate proposal probability distribution function; d) a Regeneration process to help in reducing the correlation between the generated samples; and e) a componentwise generation of samples is used to reduce the computational effort associated with multivariate input. This study demonstrates that the proposed simulation approach is robust to dimension size and is efficient in computing small probabilities of failure for complex structural systems. In addition, this approach can be used to obtain approximate expressions for the limit state equations for the pertinent failure modes. The applicability of the proposed reliability algorithm in analyzing the system performance of bridge structures and evaluating their levels of redundancy as well as their ability to resist dynamic progressive collapse is demonstrated through several examples for typical I-girder bridges, steel box-girder bridges, and truss systems. Since involved reliability analyses are beyond the day-to-day practice of bridge engineers, this study proposes an approach to develop a deterministic progressive collapse analysis method for bridges. Following current practice in the development of structural design codes, the deterministic analysis and associated criteria are calibrated to provide adequate and consistent levels of structural reliability for different bridge topologies. The validity of the proposed approach for calibrating progressive collapse analysis criteria is illustrated using two different bridge configurations subjected to different local damage scenarios

Lattice Based Elastic Metamaterials with Ultra-wide Stopbands at Low Frequency

Lattice-based elastic metamaterials (EMMs) with periodically engineered cells exhibit unprecedented properties. One exotic attribute of EMMs is the bandgap, in which elastic waves， within specific frequency ranges are prohibited from transmitting. While putting EMMs into use in real-life applications, nonetheless, their applicability remains limited due to the deficiency in the well-established design methods for EMMs, especially three-dimensional (3D) EMMs (including both infinite and finite EMMs) to achieve low frequency and ultra-wide stopbands. Moreover, the inevitable system uncertainties stemming from heterogeneous sources in practical EMMs can lead to significant fluctuations in structural performance, and worse still, catastrophic structural failure may occur. Consequently, to facilitate the wide and safe application of these advanced materials, it is requisite to develop a comprehensive framework to analyze and design 3D EMMs with ultra-wide wave attenuation bands at low frequencies and implement reliability analysis for them. In this research, a systematic framework is developed for 3D latticed EMMs in order to improve their applicability across multiple engineering disciplines. Within the framework, the key components are the novel modal-based approaches, proposed for elaborating wave attenuation mechanisms, guiding the structural modifications, and manipulating geometrical parameters of 3D EMMs aiming at achieving low-frequency and ultra-wide stopbands. Besides, another core in the framework is the new virtual model-aided approach, which is introduced for estimating statistical information, including means, standard deviations, probability density functions (PDFs), and cumulative distribution functions (CDFs), and failure probabilities of the random bandgap characteristics for 3D EMMs involving material and geometrical uncertainties. Based on the numerical investigations, the wave attenuation mechanisms in latticed 3D EMMs are elaborated. Moreover, the effectiveness of the developed modal-based approaches to design 3D EMMs and the computational performance, such as robustness, efficiency, and accuracy of the virtual model-aided framework are demonstrated. Convincedly, the developed framework facilitates the wide and safe applications of EMMs, significantly benefiting their applicability in real-life scenarios across diverse fields

Critical Asset and Portfolio Risk Analysis for Homeland Security

Providing a defensible basis for allocating resources for critical infrastructure and key resource protection is an important and challenging problem. Investments can be made in countermeasures that improve the security and hardness of a potential target exposed to a security hazard, deterrence measures to decrease the likeliness of a security event, and capabilities to mitigate human, economic, and other types of losses following an incident. Multiple threat types must be considered, spanning everything from natural hazards, industrial accidents, and human-caused security threats. In addition, investment decisions can be made at multiple levels of abstraction and leadership, from tactical decisions for real-time protection of assets to operational and strategic decisions affecting individual assets and assets comprising a regions or sector. The objective of this research is to develop a probabilistic risk analysis methodology for critical asset protection, called Critical Asset and Portfolio Risk Analysis, or CAPRA, that supports operational and strategic resource allocation decisions at any level of leadership or system abstraction. The CAPRA methodology consists of six analysis phases: scenario identification, consequence and severity assessment, overall vulnerability assessment, threat probability assessment, actionable risk assessment, and benefit-cost analysis. The results from the first four phases of CAPRA combine in the fifth phase to produce actionable risk information that informs decision makers on where to focus attention for cost-effective risk reduction. If the risk is determined to be unacceptable and potentially mitigable, the sixth phase offers methods for conducting a probabilistic benefit-cost analysis of alternative risk mitigation strategies. Several case studies are provided to demonstrate the methodology, including an asset-level analysis that leverages systems reliability analysis techniques and a regional-level portfolio analysis that leverages techniques from approximate reasoning. The main achievements of this research are three-fold. First, this research develops methods for security risk analysis that specifically accommodates the dynamic behavior of intelligent adversaries, to include their tendency to shift attention toward attractive targets and to seek opportunities to exploit defender ignorance of plausible targets and attack modes to achieve surprise. Second, this research develops and employs an expanded definition of vulnerability that takes into account all system weaknesses from initiating event to consequence. That is, this research formally extends the meaning of vulnerability beyond security weaknesses to include target fragility, the intrinsic resistance to loss of the systems comprising the asset, and weaknesses in response and recovery capabilities. Third, this research demonstrates that useful actionable risk information can be produced even with limited information supporting precise estimates of model parameters

Advanced assessment methods for elderly bridges. State-of-the-art and justification based on LCA

[ANGLÈS] Many of the existing bridges do not satisfy the structural requirements specified in design codes for new bridges. However, many of these bridges must remain in service and therefore decisions must be made in order to maintain their safety. In the design of new bridges, it is accepted “to be on the safe side” inherent in the standards; but for an assessment of an elderly bridge this procedure should be removed in order to have a more realistic understanding of the state of the structure. Otherwise, the decisions made, being too conservative, can result in unnecessary costs. The main problem is that many existing bridges near to the end of their live under conventional evaluation methods give results which imply or the replacement of the bridge or a high investment for repair it to bring it back to the performance level stipulated in the current standards. The existing advanced methods of structural assessment allow evaluating the actual state of the structure. It has been shown in many cases that better results can be obtained with advanced methods than with conventional methods because the advanced ones evaluate the structure decreasing as much as possible the existing uncertainties. This implies to move from a structure that initially seemed to require heavy investments or to be replaced, to a structure which would have acceptable conditions of behaviour at least for a certain period of time, with a much lower investment and an optimized repair and maintenance. Current methods of advanced evaluation are based on probabilistic methods of reliability through the updating of the variables that which contains uncertainty (traffic solicitations, etc.). This updating is carried out by site‐specific data taken with Structural Health Monitoring systems. Load tests also can be included within the advanced methodologies of evaluation. These evaluation methods have a great impact on the life‐cycle assessment of a bridge because apart from reducing the maintenance and repair costs allow, with a more accurate assessment, extend the lifespan of a structure while maintaining adequate levels of performance and safety. The present thesis aims to synthesize the state‐of‐the‐art of the mentioned advanced assessment methods used in bridges. It also highlights the involvement, influence and direct relationship of these methods with the different aspects which are currently considered in Life‐Cycle Assessment of existing bridges.[CASTELLÀ] Muchos de los puentes existentes no satisfacen los requerimientos estructurales especificados en los códigos de diseño para nuevos puentes. Sin embargo, muchos de estos puentes deben mantenerse en servicio y, por tanto, deben tomarse decisiones respecto a mantener su nivel de seguridad. Para el diseño de puentes nuevos, se acepta el “estar del lado seguro” inherente en las normativas; pero para una evaluación de un puente de avanzada edad dicho proceder debe eliminarse para poder tener un conocimiento más real del estado de la estructura. De lo contrario, las decisiones que se tomen, por demasiado conservadoras, pueden dar lugar a gastos innecesarios. El problema principal reside en que muchos puentes existentes cercanos al fin de su vida útil bajo métodos de evaluación convencional arrojan resultados que implicarían o la sustitución del puente o una inversión de reparación muy elevada para llevarlo de nuevo al nivel de comportamiento estipulado en las normativas actuales. Los métodos avanzados de evaluación estructural existentes, permiten evaluar el estado real de la estructura. Se ha demostrado en muchos casos que con métodos avanzados se obtienen mejores resultados que con los métodos convencionales ya que se evalúa la estructura disminuyendo al máximo posible las incertidumbres existentes. Ello conlleva pasar de una estructura que en un principio parecía requerir una inversión muy elevada o ser sustituida, a una estructura que volvería a estar en condiciones aceptables de comportamiento al menos durante un determinado periodo de tiempo, con una inversión mucho menor y optimizada de reparación y mantenimiento. Los métodos actuales de evaluación avanzada se basan en métodos probabilísticos de fiabilidad a través de la actualización de las variables que encierran incertidumbre (solicitación del tráfico, etc.). Dicha actualización se lleva a cabo con datos tomados in situ mediante sistemas “Structural Health Monitoring”. Las pruebas de carga también pueden englobarse dentro de las metodologías avanzadas de evaluación. Estos métodos de evaluación tienen un gran impacto en la evaluación del ciclo de vida de un puente puesto que aparte de reducir los costes de mantenimiento y reparación permiten, mediante una evaluación más precisa, alargar la vida útil de una estructura manteniendo unos niveles de comportamiento y seguridad adecuados. La presente tesis presente sintetizar el estado del arte de los métodos avanzados de evaluación estructural mencionados utilizados en puentes. También incide directamente en la implicación, influencia y relación directa de dichos métodos con los distintos aspectos que se consideran actualmente en la evaluación del Ciclo de Vida de los puentes existentes