Explosion and Damage Assessment: Computer Simulation with HExDAM

Transcription

1 Explosion and Damage Assessment: Computer Simulation with HExDAM Prepared By: Thomas G. Grosch BREEZE SOFTWARE Merit Drive Suite 900 Dallas, TX (972) breeze-software.com Modeling Software for EH&S Professionals

2 INTRODUCTION Architectural and structural engineering firms, as well as the Department of Defense, other federal agencies, and private industry are increasingly challenged with the task of mitigating the threats of destructive blast waves associated with accidental or intentional explosions. Physical security and safety requirements continue to be reviewed and adapted to address these threats, with the intent to prevent mass casualties and protect property. Conventional explosives can easily be transported and pose a severe loading condition on most commonly used structural systems. Structural engineers designing government and other public buildings must often address the overpressures resulting from events involving conventional explosives. Simulation software that is specially designed to predict overpressure and resulting structural damage can help engineers to better design structures. The software can also help emergency managers run what if scenarios for planning purposes and for developing emergency response plans. This article presents the results of simulation software (BREEZE HEXDAM) that models the effects of blast waves from a hypothetical terrorist bombing. Four scenarios are modeled with HEXDAM. Since standoff distances are important to emergency managers and engineers, this article also uses HEXDAM to examine the effects of varying standoff distances on blast damage to structures and personnel; and the potential for secondary explosions resulting from weakening of structures. BLAST SIMULATION AND DAMAGE ASSESSMENT The High Explosive Damage Assessment Model (HEXDAM) is specialized explosion simulation software that allows engineers and emergency managers to analyze explosion scenarios and account for blast overpressures and more. Recent developments have enhanced HEXDAM s ability to predict the effects of blast waves on personnel and structures. The model also includes the ability to vary the composition of a structure. HEXDAM s fragmentation counterpart can also model secondary fragmentation from shattering glass and other frangible materials. The model s unique ability to account for structural shielding and secondary explosions provides architects, engineers, and emergency managers a tool that helps them determine structural and personnel vulnerabilities to blast effects. Figure 1 Damage results to ships, pier, and building from primary and secondary explosions with standoff distance of zero. Structural subdivisions enhance damage resolution. Red indicates severe damage while the grey areas indicate no damage. HEXDAM HISTORY HEXDAM evolved from the Department of Defense s (DoD) development of the Nuclear Damage Assessment Model (NDAM) in the early 1980s. NDAM was used to assess blast damage from nuclear weapons over areas, such as cities or military installations. In, 1986, the U.S. Army Strategic Defense Command requested that enhancements be made to the NDAM model, and these changes resulted in the Enhanced Nuclear Damage Assessment Model (ENDAM). The Huntsville Division Corps of Engineers (CEHND) received funding through the Facility Army System Safety (FASS) program to implement enhancements that made the model applicable to conventional high explosive weapons. The resulting application was called HEXDAM. The theoretical portion of the code as it is today is in large part due to the research and development efforts of Dr. Frank Tatom of Engineering Analysis, Inc. Today, the Software and Data Services division of Trinity Consultants, Inc maintains and continues to enhance HEXDAM. HEXDAM was designed to predict the effects of blast waves on a variety of structural types, with each type

3 having a unique vulnerability profile that varies with the strength of the explosion. It can be used as a damage/injury assessment tool to determine the amount of blast damage, or injury, to individual structures or persons. The blasts can result from explosions at ground level or aloft (airburst detonations). HEXDAM s unique shielding algorithms make the model an ideal tool for evaluating the effectiveness of a shielding wall or blast deflector. Additional information regarding the algorithms in HEXDAM is given in reference 1. THE EXPLOSION SCENARIO Four scenarios were modeled with HEXDAM. The scenarios involve an explosive device that is placed at a varying distance from a ship in a harbor. See Figure 2 for a display of the harbor. The objective of the scenarios was to determine the minimum standoff distance from the main ship of interest (Ship 1) required to prevent a secondary explosion from explosive material in the vicinity of the primary explosion; and prevent damage to the ship of interest. The charge weights of the primary and secondary explosives are not disclosed, as they are not critical to this article. A secondary explosion potential (charge weight is 7% that of the primary explosion) comes from within a truck parked alongside a single-story building on a pier adjacent to the ships of primary interest (Ships 1 and 2). The building is constructed of six-inch reinforced masonry walls and contains a single window and a door. Nine people are in or near the building. Other merchant ships and smaller vessels are also included in the analysis. Each of the structural components that make up the building, ships, piers, and personnel were selected from HEXDAM s database of material properties containing vulnerability parameters. Over 120 elements or structures are contained within the database. User-defined structure types and vulnerability parameters can be generated using HEXDAM s companion model, VASDIP (Vulnerability Assessment of Structurally Damaging Impulses and Pressures.) [2] Each scenario modeled is identical, with the exception of the location of the initial, primary explosion. The primary explosion in each scenario is detonated at a height of eight feet above water level from the bow section a 75- foot long boat. The standoff distances analyzed are in relation to the aft, port side of a 500 foot-long merchant ship (Ship 1) and increase in distance away from the ship and pier. The standoff distances are for Scenario 1) directly alongside ship, Scenario 2) 163 feet, Scenario 3) 203 feet, and Scenario 4) 244 feet. Structural Shielding An important feature of HEXDAM is its ability to perform analyses with or without the effects of structural shielding. By including shielding effects, the analyst can examine the blast deflection and shielding effects of each structural component, including people, on other structures and can identify specific vulnerabilities of individual structural components or entire facilities. Structural damage and personnel injury are based on the magnitude of the overpressure and the ability of a structure (or part of a person s body) to withstand the overpressure. In HEXDAM s default list of structural types, each ship was classified as being a Merchant Ship while the building and truck were created using various materials (e.g., glass, reinforced masonry, aluminum panels). The VASDIP model allows a user to create virtually an unlimited number of elements or structures that can be modeled by HEXDAM. VASDIP generates pressureimpulse diagrams, damage/injury levels for specific values of pressure and impulse, and vulnerability parameters by accounting for user-defined properties of 24 different basic inanimate structural components, as well as 28 human body components. [2] Structural Subdivision In each scenario, the building on the pier and adjacent merchant ships are of special interest -- the building, because nine people are located within and around it, and the ships because they are high profile targets. Because of their special interest, these structures have been automatically subdivided by HEXDAM to provide greater damage resolution. Structural damage and personnel injury are graphically presented in HEXDAM using color-coding. Overpressures, dynamic pressures and percentage of damage are generated as textural and graphical output for each subdivided component. The effect of this subdivision can be seen in Figure 1.

4 Secondary Explosions HEXDAM allows the analyst to create structures, destruction of which would pose the risk of collateral, or sympathetic, explosions. These are called secondary explosions. If the structure containing a potential secondary explosion is not sufficiently shielded from the primary explosion s blast wave, the structure could be damaged, causing subsequent secondary explosions and increased damage. In Figure 1, a grid was placed over the bow of Ship 2 so that the distribution of damage from a secondary explosion originating from the truck alongside the building could more closely be analyzed. A threedimensional plot of blast overpressures shows overpressures ranging from 1 to 100 pounds per square inch (psi). Damage to Ship 2 from the secondary explosion is apparent in the inset image of Figure 1. Similar grids could have been placed at any location within the modeling domain. Personnel Damage Assessment Blast damage to humans was also analyzed in these scenarios. HEXDAM allows for the creation of individuals that can be placed in 44 different postures. For each person modeled, the effects of the blast waves are assessed on 28 different body components (e.g., ear drums, larynx, lungs, feet). By default, people are modeled as individuals standing six feet tall weighing 180 pounds with a body height-to-width ratio of 5.4. Figure 3 shows the position of the six people within the building and damage to individual body components and building subdivisions. Results are from Scenario 3 (no secondary explosion from the adjacent truck). The person entering the door was not shielded by the building from the primary explosion to the extent that the other individuals were. The person standing in front of the window was also exposed to higher pressures. As a result, vulnerable body components suffered more damage. In Figure 3, blue represents essentially no damage. Injuries are produced by the blasts and do not include injuries from flying glass fragments of shattered windows. An associated model, HEXFRAG, can compute glass and other frangible material fragment injuries. Figure 3 - Personnel damage from the primary explosion only. Blue represents no damage; red, severe; orange, moderate; yellow, slight. Scenario 1 No standoff distance. Primary explosion detonates alongside Ship 1. Primary explosion - Causes severe damage to aft section of Ship 1 as well as a small section of pier. Slight to moderate damage occur along 1/3 the length of Ship 1. The upper portion of the aft superstructure shows no damage although this structure is closer than portions of the main weather deck to the primary explosion. The structural shielding effects of the aft portion of the aft superstructure on Ship 1 prevented damage to the forward section of the aft superstructure (see Table 1). Figure 4 shows a cross-sectional analysis of the overpressures (as they pass over the aft superstructure of Ship 1). The blast wave can be seen impacting the aft portion of the ship and rise over the superstructure while the effects of the blast penetrate the ship below the main deck, resulting in higher pressure levels at and below the main deck. Results HEXDAM was used to determine the approximate minimum standoff distance necessary that would result in no secondary explosion and no damage to Ship 1. The following briefly describes the results for each scenario while Table 1 graphically shows the damage to the ships and surrounding structures. Figure 4

5 Secondary explosion The force from the primary explosion is sufficient to trigger the secondary explosion from within the truck on the pier. It causes severe damage to the pier, while the building and personnel are completely destroyed/killed. Ship 2 also sustains severe damage to the hull. Scenario 2 Standoff distance of 163 feet (49.6 meters). Primary explosion Causes slight damage to Ship 1 and pier. Ship 1 s hull integrity withstands the blast wave and prevents the damage to the main weather deck that was observed in Scenario 1. Note difference in damage coloring to Ship 1 in Table 1 as compared to Scenario 1. Secondary explosion Primary explosion is sufficient to trigger the secondary explosion from within the truck. Results are similar to those in Scenario 1. Scenario 3 Standoff distance of 203 feet (62.0 meters). Primary explosion Does not trigger secondary explosion. Causes a small section of Ship 1 to be slightly damaged. Portions of pier not shielded by Ship 1 are slightly damaged. Personnel inside building sustain slight damage to eardrums and severe damage to their feet. Foot damage is primarily a result of the blast wave propagating up through the floor of the building as it traveled under the pier, coupled with the fact that feet are highly susceptible to blast damage. Personnel outside of building sustain severe damage to the neck (larynx and cervical vertebrae), hands, and feet. Moderate damage to the shoulders and chest (lungs, thorax, and thoracic vertebrae) is observed, while slight damage occurs to the abdomen (GI tract, pelvis, and lumbar vertebrae). The figure in Table 1, Scenario 3 shows the building and personnel in it with resulting damage. Scenario 4 Standoff distance of 244 feet (74.4 meters) Primary explosion Does not trigger secondary explosion and does not cause damage to any ship. Portions of pier not shielded by Ship 1 are slightly damaged. The truck closest to the primary explosion sustains moderate damage to the aluminum paneling that makes up the cargo area (see Table 1) while the truck that contains the potential secondary explosive charge only sustains slight damage. The metallic cab portion of each truck sustains no damage. The figure in Table 1, Scenario 4 shows the west facing side of the building on the pier. A large portion of the right half of this side sustained no damage due to the shielding effects of the truck. SUMMARY OF RESULTS Results of these scenarios show the effects that blast waves on structures of varying composition as a function of standoff distance. The analyses predict different levels of damage to sub-divided building and ship components as well as personnel. Iterative analyses indicate that a standoff distance of approximately 200 feet prevented the secondary explosion potential within the truck from occurring. A distance of 244 feet resulted in no damage to any ship. Additional analyses could have identified the minimum standoff distance required to prevent any damage to Ship 1. HEXDAM is undergoing additional enhancements to incorporate the ability to perform parametric analyses that automate the identification of minimum standoff distances, charge weights, and structural properties for user-defined damage thresholds. This would allow an emergency manager or engineer to vary one parameter in HEXDAM and quickly re-run the model to see how the resulting damage and injuries would change. CONCLUSION Explosion modeling and damage assessment software can provide decis ion makers with information regarding the protection of structures and people. Shielding effects from structures or people are often an important consideration in evaluating explosion scenarios. By computing the damage and injury to structural components as a function of the magnitude of the overpressure and the ability to withstand the overpressure, coupled with shielding effects, HEXDAM gives decision makers a costeffective method of mitigating risk. By using simple data entry forms and graphical displays within BREEZE HEXDAM, the process of setting up and modeling explosion scenarios is streamlined and the presentation of the results is informative. Additional information and demonstrations of versions of HEXDAM can be obtained from the developers website at Thomas Grosch is an Atmospheric Scientist and Manager, Software and Data Services for Trinity Consultants, Inc, an environmental consulting firm in Dallas, Tx; tgrosch@trinityconsultants.com or (972)

6 Table 1 Damage results from modeled scenarios. Scenario 1 No standoff distance Scenario 2 Standoff distance =163 ft Scenario 3 Standoff distance = 203 ft Scenario 4 - Standoff distance =244 ft Damage Legend None - Grey Slight - Yellow Moderate - Orange Severe - Red REFERENCES 1. BREEZE HEXDAM and VEXDAM User s Guide, Version 7.0, Trinity Consultants, Inc., Dallas Texas, 2003, 2. BREEZE VASDIP User s Guide, Version 6.0, Trinity Consultants, Inc., Dallas Texas, 2002,