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Toward Guidance for Identifying and Measuring Structural Damage Following a Blast Event
Structural identification continues to develop an expanding role within performance-based civil engineering by offering a means to construct high-fidelity analytical models of in-service structures calibrated to experimental field measurements. Although continued advances and case studies are needed to foster the transition of this technique from exploration to practice, potential applications are diverse and range from design validation, construction quality control, assessment of retrofit effectiveness, damage detection, and lifecycle assessment for long-term performance evaluation and structural health monitoring systems. Existing case studies have been primarily focused on large civil structures, specifically bridges, large buildings, and towers, and the limited studies exploring application to damaged structures have been primarily associated with seismic events or other conventional hazards. The current paper produces the first experimental application of structural identification to a component of a full-scale building structure with structural deterioration resulting from an internal blast load. Experimental modal analysis, nondestructive testing, and visual documentation of the structure was performed both prior to and after the internal blast, while a suite of blast overpressure transducers and shock accelerometers captured applied loads and structural response during the blast event. This paper presents an overview of the field testing and observed structural response followed by extensive treatment of the experimental characterization of structural damage in a masonry infill wall. Combined stochastic-deterministic system identification is applied to the acquired input-output data from the vibration testing to estimate the modal parameters of the infill wall for both the in-service state and in the postblast condition with damage characterized by interfacial cracking and permanent set deformation. Structural identification by global optimization of a modal parameter-based objective function using genetic algorithm is employed over two stages to produce calibrated finite-element models of the wall in the preblast and postblast conditions. Damage characterization is explored through changes in the structural properties of the calibrated models. Plausibility of the results are supported by observed cracking and spall documented in the experimental program and further reinforced through nonlinear applied element simulation of the response of the wall. (C) 2014 American Society of Civil Engineers.
Assessment of damage to industrial buildings due to accidental explosions in air is considered. It is suggested to formulate the result of the damage assessment in the form of risk. The expression of risk embraces probabilities of foreseeable damage events (damage probabilities) and outcomes (consequences) of suffering these events. The situation is examined when blast loading imposed by an accidental explosion is predicted by a series of experiments. They yield a small-size sample of blast loading characteristics. It is suggested to idealise the formation of explosive damage to industrial buildings by means of event trees diagrams. A quantitative analysis of these diagrams can be carried out by developing fragility functions for their branching points. Each branching point is used to represent a structural failure contributing to the final explosive damage. The fragility functions are applied to expressing the conditional probabilities of explosive damage. With these probabilities, a technique of frequentist (Fisherian) inference is applied to assessing the explosive damage. This technique is called statistical resampling (Efrons bootstrap) and applied as a practical, albeit not equivalent alternative to the Bayesian approaches. It is shown that statistical resampling is capable to yield confidence intervals of damage probabilities and can be applied almost automatically. It operates without using cumbersome meth-ods of statistical inference developed in the classical statistics. The bootstrap confidence intervals do not contain any subjective information except the degree of confidence for which these intervals are computed. The degree of confi-dence must be chosen by the engineer. The bootstrap confidence intervals are applied to estimating damage probabilities on the basis of the small-size sample of blast loading characteristics. An estimate of the risk of explosive damage is expressed as a set of bootstrap confidence intervals computed for damage probabilities and related outcomes of this damage.
In the design of protective structures, concrete walls are often used to provide effective protection against blast from incidental events. With a reasonable configuration and proper reinforcement, the protective structure could sustain a specified level of blast without global failure. However, the concrete wall may generate spallation on the back side of the wall, posing threats to the personnel and equipment inside the structure. For this concern, it is important to establish appropriate concrete spallation criteria in the protective design. Earlier analytical studies have been based on simplified one-dimensional wave theory, which does not consider the complex three-dimensional stress conditions in the case of close-in explosion and the structural effects. This paper presents a numerical simulation study on the concrete spallation under various blast loading and structural conditions. A sophisticated concrete material model is employed, taking into account the strain rate effect. The erosion technique is adopted to model the spallation process. Based on the numerical results, the spallation criteria are established for different levels of spallation. Comparison of the analytical results with experimental data shows a favorable agreement. It is also shown that the structural effects can become significant for relatively large charge weight and longer distance scenarios.
In 1985, the Technical Council on Forensic Engineering (TCFE) of the American Society of Civil Engineers (ASCE) tasked the Forensic Engineering Practice Committee to create the set of Guidelines for Forensic Engineering Practice. The overall purpose of the set of guidelines is to commit to writing the current state of Forensic Engineering Practice. The set of guidelines is organized into general areas of interest, namely, Qualifications, Investigations, Ethics, Legal Forum and Business Considerations.
In recent years, industrial concerns for safety and protection against high-explosive loadings have increased. Industries that manufacture explosive devices or use processes with the potential for explosions use concrete barricades to protect workers from exposure to accidental blast and fragmentation. Other industries use concrete barricades as protection against terrorist attacks that might include blast or fragmentation. Reinforced concrete barricades can provide effective protection against high-explosive devices; however, the high-intensity loadings from close-in explosions or fragment impact can create large magnitude, transient stress waves in concrete barricades that result in the generation of concrete spall on the back side of the barricade, even though the barricade itself does not suffer general failure. Concrete fragments generated by concrete spallation can be hazardous to personnel and equipment, depending on the size and velocity of the fragments. In this study, a numerical model was developed to predict spall damage to concrete walls from close-in explosions in air for cased and uncased munitions. The model was used to develop guidelines for designing concrete walls to prevent spallation.
On Friday, February 26, 1993, a van loaded with explosives entered the underground parking garage of the World Trade Center. The van was parked on the B2 underground level, next to the south wall of Tower A. At 12:18 p.m., the van exploded, instantly destroying a large portion of the garage under the Vista Hotel, blasting out a 38-mm (1.5-in.) thick steel double-tee tower cross brace, and sending 44 x 103 kN (5,000 tons) of debris crashing onto the heating and refrigeration plant for the World Trade Center complex. Evacuation of the complex commenced, with some office workers making their way down the smoke-filled stairs while others were trapped in elevators until rescued. Structural damage assessment, emergency repairs, and construction of temporary supports began soon after the explosion.
Accidental explosions have resulted from numerous sources, including fuel/air mixtures, reactive chemicals, combustible dusts, bursting pressure vessels, and high explosives. These impulsive events call for specialized forensic analysis methodologies because the damaging loads exist for only short durations (see Figure 1), making conventional static analysis inappropriate for backing out possible root causes. An important part of an explosion forensic analysis is to use damage indicators from the surroundings to determine the strength of the blast wave. These damage indicators, such as deflections in metal panels, deformed structural members, debris throw, or broken windows are analyzed using methods that range from semi-empirical pressure-impulse based damage correlations, to single-degree-of-freedom analysis, to dynamic nonlinear finite element analysis. The final result from analyzing many indicators is a magnitude for the explosion source energy, which, in turn, often leads to an understanding of what caused the explosion.
Many trials have been made to numerically simulate the Alfred P. Murrah Building bombing event using Finite Element Method. These trials were based on removal of one of the main supporting columns to investigate the generated internal forces and the possible deformations in other elements. However, these trials could not simulate real bomb explosion and automatic detection of the failed columns and floor slabs. Furthermore, the finite element analysis could not continue to model separation of failed elements, collision between structural elements till complete collapse. In this paper, a new technique, Applied Element Method (AEM), is used to simulate the collapse process of the Murrah Building. The bomb weight and location are considered in the simulation. Free-Field blast wave was assumed. The building dimensions, reinforcement and material properties were taken into account. The simulation shows real time analysis of the building performance since the blast occurs, failure of one of the supporting columns, and the failure of the supporting transfer girder till partial collapse of the structure. Two more cases were studied; the bomb was moved to the corner of the building and increasing reinforcement of the transfer girder to check building performance during these events. Results indicate that design firms, engineers, and insurance companies now can judge the safety of existing structures when subjected to extreme loads and to study the safety of proposed structures prior to their construction.
The main focus of this study has been the assessment of blast waves negative phase effects on glass panels. An approximate numerical model for the dynamic response simulation of glass panels subjected to blast loading has been developed, including stochastic considerations of the glass flaw characteristics. A parametric study was conducted and the results showed that glass panels would exhibit different responses at different scaled ranges, and for different charge sizes.
In this paper a methodology for assessing the performance of structural elements subject to explosive loading is presented. The method combines basic section analyses, equivalent single degree of freedom (SDOF) modeling, and a static finite element pushover analysis to calculate the blast resistance of an existing shear wall subject to an external explosion. The method optimizes the breadth of results while minimizing the calculation complexity required. A static pushover analysis is conducted to identify regions of vulnerability. The second floor wall is identified as the location of failure and is modeled as a system, which includes the stiffness contributions of adjoining wall sections, and also as a component with fixed ends. Pressure-impulse curves are developed to quantify the blast resistance of the wall relative to various levels of damage. The component model is found to underpredict the blast resistance by 7% when compared to the system model for impulsive demands. Simplified energy methods are also shown to bound results of the SDOF resistance curves.
The design or assessment of wall panels subject to blast loading requires the formulation of a dynamic flexural resistance-deflection function for the element. With the exception of situations in which the explosion occurs at very close proximity to the panel, the resistance to blast loading may be based upon a quasistatic resistance function that subsequently must be modified for dynamic effects. This paper describes a proposed method for the derivation of quasi-static elastic/plastic resistance functions for reinforced concrete wall panels with door and window openings based upon finite element analysis and yield-line theory. This approach is compared with the results of tests on model wall panels. It is shown to produce a conservative value of ultimate load but may underestimate the deflection at the limit of elastic behavior.
This paper first describes the current state of analysis for the response of unreinforced concrete masonry walls subjected to lateral uniform pressure. The formulation is based on the initial elastic response, the subsequent initiation of cracks and the nonlinear rocking response, and the eventual large displacement and potential collapse. The necessary equations are developed for these phases in the form of a resistance function. The paper then incorporates membrane retrofit materials to strengthen the wall's resistance to lateral pressure, and develops the necessary resistance function equations. In blast tests, membrane retrofit unreinforced masonry walls have experienced severe cracking and large displacements without collapse. This is of high interest to the Department of Defense, the protection of diplomatic facilities, and the construction industry impacted by hurricanes and other high wind events. The paper concludes with examples that demonstrate application of membrane retrofits indeed increase the resistance of the wall to lateral pressure.
In May 1995, the Federal Emergency Management Agency deployed a building performance assessment team composed of ASCE and federal government engineers to investigate damage caused by the malevolent bombing of the Alfred P. Murrah Federal Building. This paper describes the investigation of damage caused by the blast, the failure mechanism for the building, and engineering details of the building.
The truck bombing of the Murrah Building caused significant damage to this structure. From the characteristics of the bomb crater, it was determined that the explosion yielded energy comparable to that from the detonation of 1,814 kg (4,000 lbs) of trinitrotoluene (TNT). The blast directly removed a principal exterior column, and the associated airblast caused the failure of two others. The airblast also destroyed some of the floor slabs in the immediate vicinity. This paper describes the blast loading and its effect on the building.
When blasts occur in urban areas, many injuries and sometimes deaths result when glass shards fly from windows fractured by airblast pressure. The use of blast-resistant glazing can mitigate the number and severity of glass-related injuries if blasts occur. In this paper, the writers present two methods to facilitate blast-resistant glazing design. One of these methods is primarily restricted to government facilities while the other exists in a consensus document for public use. Both of these methods rely on laminated glass as the blast-resistant glazing material. Both methods address all facets of blast-resistant glazing design, including attachment of the glazing to the framing members and an estimate of the forces necessary for designing framing members and connections.
Spall is defined as the ejection of fragments from the opposite side of a structural element from which it is impacted and/or impulsively imploded. This research was on the spall of reinforced concrete panels subjected to bomb fragment impacts and/or airblast loads from nearby bomb detonations. Theories of spall, tests involving spall, and current spall prediction methods were reviewed and evaluated. The current spall, and current spall prediction methods were reviewed and evaluated. The current spall prediction methods did not satisfactorily predict all of the previous test results found in the literature. Forty tests were conducted on reinforced concrete walls to investigate parameters which affect spall, to evaluate prediction methods further, and to provide data with which theory and prediction methods could be improved. Theoretical calculations were conducted for airblast loads under the compressive elastic limit.
The analysis of the structural failure of a reinforced concrete building caused by a blast load is presented in this paper. All the process from the detonation of the explosive charge to the complete demolition, including the propagation of the blast wave and its interaction with the structure is reproduced. The analysis was carried out with a hydrocode.The problem analysed corresponds to an actual building that has suffered a terrorist attack. The paper includes comparisons with photographs of the real damage produced by the explosive charge that validates all the simulation procedure.
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ASCE41Graduate Research Assistant E-mail: mwhelan3@uncc.edu3Professor Structural Identification and Damage Characterization of a Masonry Infill Wall in a Full-Scale Building Subjected to Internal Blast Load
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David C. Weggel, P.E., A.M.ASCE3; and Corey D. Rice, S.M.ASCE41Graduate Research Assistant, Dept. of Civil and Environmental Engineering, Univ. of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28223.2Assistant Professor, Dept. of Civil and Environmental Engineering, Univ. of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28223 (corresponding author). E-mail: mwhelan3@uncc.edu3Professor, Dept. of Civil and Environmental Engineering, Univ. of North Carolina at Charlotte, Charlotte, NC 28223.4Graduate Research Assistant, Dept. of Civil and Environmental Engineering, Univ. of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28223.. 2015. Structural Identification and Damage Characterization of a Masonry Infill Wall in a Full-Scale Building Subjected to Internal Blast Load. Journal of Structural Engineering 141:1.. [Abstract] [Full Text HTML] [PDF]
ASCE21Assistant Professor of Civil & Environmental Engineering, Idaho State Univ., Pocatello, ID 83209; formerly, Postdoctoral Research Scholar, Pennsylvania State Univ E-mail: soreandr@isu Utilization of Existing Blast Analysis Software Packages for the Back-Calculation of Blast Loads
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Andrew Sorensen, Ph.D.1; and William L. McGill, Ph.D., P.E., M.ASCE21Assistant Professor of Civil & Environmental Engineering, Idaho State Univ., Pocatello, ID 83209; formerly, Postdoctoral Research Scholar, Pennsylvania State Univ., University Park, PA 16802 (corresponding author). E-mail: soreandr@isu.edu2Assistant Professor of Information Sciences and Technology, Pennsylvania State Univ., Univ. Park, PA 16802. E-mail: wmcgill@ist.psu.edu. 2012. Utilization of Existing Blast Analysis Software Packages for the Back-Calculation of Blast Loads. Journal of Performance of Constructed Facilities 26:4, 544-546.