Research Topic: Blast & Penetration Protection

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1 Research Topic: Blast & Penetration Protection Problem Statement Balancing aesthetics, practicality and security for government and pubic projects is an engineering challenge. For new construction and renovations, a plethora of options exist to harden a building. There is much difficulty in filtering through these options to choose what is best for a particular project. By researching the topic, the author hopes to gain a basic understanding of the strategies and methods to later benefit future projects. Background While the DCYC does incorporate detention rated assemblies out of necessity, the author took interest in what entails taking those assemblies one step further and making them blast rated assemblies. This is a pertinent issue in today s market as many government buildings are in need of blast retrofit and new buildings must comply with elevated standards. There exist several strategies to protect buildings and each strategy has different methods. Research Goal The goal of the research was to develop a basic understanding of the different methods and materials used to harden and protect buildings (and occupants) from blasts. It is intended to identify said options and to use the knowledge to benefit future projects. 37

2 Research Findings Defining Threats & Protection When a building is attacked by an explosion, there are several phenomena that pose a threat to the inhabitants. Those threats are listed and explained below. Over Pressure When an explosion occurs, potential energy is rapidly released and transfers energy to its immediate surroundings, typically the earth and atmosphere. This results in a blast wave (Figure 15) which creates high pressure. This pressure is what causes building materials to fail and sends projectiles through the air. The pressure can also rupture ear drums, collapse lungs and impale. Flying Debris Figure 15 - Blast Wave Pressure vs. Time (AMPTIAC Quarterly) When the overpressure hits building materials, it creates a load they may not have been designed to withstand. Materials fail when they break shatter or crumble and when they are disconnected from the structure. Once a material fails, it becomes a projectile. The over pressure will also turn interior items into projectiles, from things as small as pens to whole desks. This is the main cause of injury for those closest to the blast. 38

3 Building Collapse Building collapse occurs because structural members are disabled. Strategies to prevent localized collapse entail hardening the structural members and connections to withstand blast loads. What causes the high fatalities is progressive collapse of the structure in which a domino effect of structural failures occurs. Levels of Protection Figure 16 - Levels of Structural Protection (AMPTIAC Quarterly) In summary, Level 1 protection prevents vandalism and criminal acts, Level 2 protects building occupants from blast related injuries and Level 3 protection means the building is designed to continue operations following an attack. Each level of protection includes and incorporates lower levels of protection to balance its security. 39

4 Protection Strategies Level 1 - Stand-Off Distance and Physical Security The most cost-effective solution, if land is available, is allowing a stand-off distance. (AMPTIAC article Protecting personnel at risk) Stand-off distance is simply employing measures to ensure bombs cannot detonate within certain distances of a building. The pressure from a bomb s blast drops exponentially with distance as shown in the figure below. Figure 17 Blast Pressure vs. Range Graph (WBDG.org) By creating the standoff distance, much damage can be prevented or reduced, at what would be minimal cost (estimated 1.5-2% additional cost of construction). The methods of providing Level 1 security is to create layers of defense that would detect or slow a threat including perimeter barriers, check points, and electronic security. Barriers may be a simple fence, concrete walls or an earth berm. Barriers primarily serve control access points to the building and more significant barriers will provide necessary stand-off distances or even deflect blast waves. Initial protection strategies were primarily Level 1 because of the low costs to build perimeter barriers and availability of personnel to provide security. Figure 18 - Cost vs. Stand-Off Distance (FEMA.gov) 40

5 Focus has shifted from stand-off distance because ineffectiveness of barriers against all potential threats. Fences and concrete barriers installed served primarily to mitigate the threat of suicide bombers using cars or explosive vests. Level 1 protection methods are not entirely effective against aerial attacks, projectiles or infiltrated attacks. Level 2 Antiterrorism Protection The main purpose of Level 2 protection is to protect assets (usually people). Level 2 protection incorporates elements of a hardened structure to prevent excessive deaths from a blast. Main methods of Level 2 protection are limiting and containing fragmentation of building materials and preventing progressive collapse. Many existing government buildings have been, or need to be, upgraded in order to protect occupants from fragmentation. A number of solutions have been tested and implemented for windows and walls. Existing windows can be given a laminated film application which acts to prevent glass shards from becoming projectiles by making the pane a whole cohesive unit. When the window fails with the laminated film, it becomes a blanket much like some car windshields. Defragmentizing existing walls involves adding a layer material on the inside of the wall which will act to catch or limit movement of debris rather than adding strength to the wall to out right resist the blast pressure. These treatments are, for the most part, non-structural and are usually applied to nonstructural walls. The first method is installing a fabric net material (like that used for sandbags) directly the back of the wall (concrete or masonry) with glue and anchoring the top and bottom into the ceiling and floor, respectively (see figure below). For a finish, drywall is glued directly to the net. Figure 19 - Fabric Net for Wall Defragmentation 41

6 Steel sheets can be installed in a very similar fashion to the fabric net shown in Figure 19. Steel can provide more strength and ductility to the wall and contractors are more familiar with its installation. It is also common practice to build another separate interior wall. There may or may not be a fabric net or steel sheet as described above installed in between the two walls. The interior wall would be built of steel studs, each one anchored to the floor and ceiling and a bearing strip would be installed on the drywall to prevent the drywall from pulling out over the screw head. Installing a new wall allows for the possibility of installing blast rated windows properly anchored to the structure. Figure 20 - Anchored Stud Wall with Screw Bearing Strips The last method to be discussed is the application of an elastomeric polymer coating. The material is similar to what one would use as a truck bed liner. The coating is sprayed directly to the inside of wall and several inches are also applied to the floor, ceiling and any supports to provide a good bond. The elastomeric polymer absorbs energy of the blast and debris through deformation. Because the material is very ductile, fragmented material does not penetrate it. This is the least labor intensive process of those discussed. Polymer foams can also be placed inside wall cavities to absorb blast energy but this is not always a practical approach. Progressive collapse causes turns localized failures into massive ones and is responsible for unproportionately high casualties relative to the initial blast damage. The direct design approach involves finding an alternate load path to handle the newly unsupported load. This must be addressed in the design of the structure. The indirect design approach uses redundancy and can be done as a retrofit. Redundancy applies to the use of moment frame connections and continuity along joints. The structure may also be designed or retrofitted for reverse loading to the effects of overpressure. 42

7 Adding structural strength in a retrofit situation to concrete structures involves the use of atypical materials. Polymer and carbon materials have been bonded to or embedded in concrete and show that they can significantly increase shear and flexural capacity of concrete by adding tensile strength. The upgrade can be done to repair or strengthen concrete for blast and reverse loading but is expensive compared to new construction. While the upgrades can enhance column, beam and slab strengths, not much can be done to strengthen the connections. This is why this retrofit method is only Level 2 protection, because it can prevent progressive collapse but not a localized structural failure necessarily. Level 3 - Building Hardening Hardening a building has much higher initial costs and is done only if the building must remain intact, after an attack, to resume operations. In this strategy structural and non-structural elements are designed with more strength. The methods of this strategy have mostly to do with material selection and the strength of said material and its connections. The most common metal used in building also happens to be the most effective in withstanding blast and penetration. Other metals are used to resist blast and penetration but usually for products other than buildings and will not be discussed in this report. In designing with steel, a trade off must be made between hardness and ductility. Ductility allows steel to absorb energy and remain intact to continue to provide structural support. Typical steel members fit into this category. Hardness provides protection from ballistic projectiles but one who is familiar with steel s properties knows increased hardness decreases its use a structural material because of the brittle failure. Concrete can be used to fill hollow columns or embed columns to harden structural members. Connections are also important for steel construction. Full welds and moment connections are often necessary to provide continuity of support and effectively transfer the load to other structural members. Steel design for blast loads require detailing similar to those used for seismic design. This is to make loads well distributed and provide redundant support to prevent progressive collapse. Concrete also makes a good blast resistant material because it is massive and hard. Many of its weaknesses can be countered through additional materials. Brittle failures can be prevented with steel reinforcing, fragmentation can be reduced with fiber reinforcement and fragmentation can be contained with layers of other materials like those discussed for Level 2 protection. Concrete s effectiveness against blasts can be further enhanced by increasing its strength. Very high strength concrete can range from 6,000 psi compressive strength up to 115,000 psi under controlled conditions. Several ways to enhance concrete s strength are to use the optimal ratio of products, increase compaction and control its environment during curing. The optimum water to cement ratio is much lower than what is typically used, resulting in a very unworkable paste. Water-reducing admixtures then become necessary to make this practical. Selecting the correct ratio of cement and aggregates is also critical since aggregates affect the concrete strength. Sand is the preferred aggregate as it is the smallest available. Size of the different components also matters to first, to make the 43

8 concrete more homogeneous and increase its compaction to eliminate voids which weaken concrete. Vibrators are used during placing to help eliminate voids. With the methods mentioned above, concrete can achieve 25,000 psi strength curing in ambient temperature. To achieve higher strength (30,000+ psi), it is necessary to control the temperature and/or humidity levels from 3 days to several weeks depending on the desired strength. Conclusion Solutions to blast and penetration protection are very project specific and there cannot be a general solution. For all buildings, it is important to asses the surrounding to identify where threats may come from and the building owner must decide the level of protection to provide. This should correspond to the use of the building and must assume a certain level of risk. 44

9 Appendix F References Bradshaw, J. C. (2003). Protecting Personnel at Risk: DOD Writes Anti-Terrorism Standards to Protect People in Buildings. AMPTIAC Quaterly, Cargile, J., O'Neil, E. F., & Neely, B. D. (2003). Very-High-Strength Concretes for Use in Blast-and- Penetration-Resistant Structures. APMTIAC Quaterly, Coltharp, D., & Hall, R. (2003). Blast Retrofit Research and Development: Protection for Walls and Windows. AMPTIAC Quaterly, Federal Emergency Management Agency. (2007, October 19). FEMA E155- Building Design for Homeland Security Course. Retrieved February 2009, from FEMA : Federal Emergency Management Agency. (2003). Primer for Design of Commercial Buildings to Mitigate Terrorist Attacks. District of Columbia, Washigton: FEMA. Hinman, E. (2009, Febuary 2). Blast Safety of the Building Envelope. Retrieved March 2009, from Whole Building Design Guide: Lane, R., & Craig, B. (2003). Material Ease - Materials for Blast and Penetration Resistance. AMPTIAC Quaterly, Lindsey, P. (20003). DOD Protective Desgn Manuals Have Wide Application. AMPTIAC Quaterly, Odello, R. (2003). Polymer Composite Retrofits Strengthen Concrete Structures. AMPTIAC Quaterly, Porter, J., Dinan, R., Hammons, M., & Knox, K. (2003). Polymer Coatings Increase Blast Resistance of Existing and Temporary Structures. AMPTIAC Quaterly, Smilowitz, R. (2008, May 12). Designing Buildings to Resist Explosive Threats. Retrieved February 2009, from Whole Building Design Guide: Walton, B. (2003). Designing Blast Hardened Structure for Military and Civilian Use. AMPTIAC Quaterly, Warner, S. M. (2005). The Effective Use of New Materials and Methodologies for Blast Mitigation in New and Renovated Facilites by the Armed Forces. Fort Leavenworth, KA: US Army Command and General Staff College. 71