Blast Mitigation Demands Careful Attention
Well-planned blast mitigation protects buildings and saves lives.
Donnie Hunter, Kawneer North America
Threats to our national security are ever-present in today’s global society. Within seconds, buildings can be irreparably damaged and landscapes can change. Increasing the safety of building occupants and preserving human life is the primary goal of blast protection. The demand for blast-mitigation products is increasing in new and retrofit building applications, giving architects a unique opportunity to safeguard human lives.
Determining which fenestration solutions and systems to specify for blast mitigation is a formidable responsibility. Making the right decision lies in understanding how the products are produced and what they’re designed to do. Developing high-performing blast-mitigation solutions involves several interdependent steps to ensure the effectiveness of the products.
The Sarasota Police Headquarters is an example of a whole-building solution. It features an impact-resistant curtain wall and entrances, including the 1600 Wall System1 curtain wall and 350 IR medium-stile entrances from Kawneer North America. Photo by Greg Wilson Group.
Step One: Understanding Blasts. According to the Oxford Dictionary, a blast is “a destructive wave of highly compressed air spreading outward from an explosion.” Hazards resulting from a blast include flying glass shards, building components, shrapnel, dirt, rocks, and debris, leading to possible structural damage and/or building collapse. Fenestration products cannot provide complete protection from an explosion, but in the unlikely event of a blast in or near a building, products designed to address blast hazards can help reduce the damage.
When looking at the nature of a blast there are two elements to understand: the charge weight and the standoff distance. The charge weight is the amount of explosives used (TNT equivalent), while the standoff distance is the distance of the point of detonation from the target. Ultimately, the greater the standoff distance, the slighter the explosive threat to the building or target.
Step Two: Assessment and Testing. According to the American Architectural Manufacturers Association (AAMA), Schaumburg, IL, the conditions for blast-mitigating fenestration systems include limiting flying shards, glass preservation, frame support, and wall integrity. These criteria are all taken into consideration during product evaluation.
The first step to any blast-mitigation project is a thorough threat assessment by a third-party consultant to determine the estimated size and nature of the potential explosive device, as well as the standoff distance. The consultant will evaluate issues such as the likelihood of an attack and how close a vehicle or carrier can get to a building. Risk-analysis software and site surveys help blast consultants determine the answers to those questions and more. Whatever the method, this step is critical and the results must go to the design team as the foundation for the project design.
After the initial threat assessment, several test methods that simulate the effects of an explosion can measure blast resistance. These methods include:
- Static testing, an economical method that employs the conventional static-testing methods used to test fenestration products.
- Shock-tube testing, a moderate-cost method that uses a compressed-gas charge or a live explosive device to achieve the positive pressure of an explosion. However, this kind of test usually lacks negative-phase effects.
- Open-air arena testing in which an actual explosive charge is positioned at the projected standoff distance. The most realistic and costly method, it produces positive- and negative-phase blast effects.
The blink of an eye
The blast duration of a large explosive device is typically in the range of 28 to 42 milliseconds, which is roughly an eighth of the time it takes for an eye to blink. Destruction can vary greatly based on distance and the size of the explosive, so thorough testing can provide vital insight.
The shock-tube and arena-test methods use a number-zoned “witness chamber” to help measure the effects of debris entering the room as a result of the blast. The performance of the glazing system is determined and given a performance condition or hazard rating relative to the testing standards.
Building blast-resistance requirements, known as blast mitigation standards (BMS), are driven by the size of the explosive device and distance to the target. The BMS is typically expressed in pounds/square-inch•milliseconds (psi-msec) of reflected pressure and psi-msec of impulse. Without these elements, the BMS is incomplete and the building is likely to be inadequately protected.
During this process, testers evaluate the building and wall assessment/reinforcement, addressing design and product options as well as the blast resistance of the products and attachment methods. The evaluation and assessment must look at the products as a system and examine how the various elements work together. Key factors to consider include:
- The glass can break but cannot send flying shards into the room.
- The glass lite must stay in the frame, along with all interior window components.
- The frame must remain anchored to the wall.
- The wall must resist the load and retain the frame.
Depending on building and construction methods, materials, and construction, several wall-anchoring options are offered:
- Basic trim and clip, usually suitable for low-blast-load applications
- Expansion and adhesive anchors
- High-performance grout-filled anchors, particularly effective in retrofit applications where support for high blast loads requires tying together unrelated construction elements or deep anchoring.
Highly skilled glazing contractors and window installers are essential to the overall process and critical to assuring proper anchorage and craftsmanship. These teams work closely with project designers, consultants, and engineers to create safe spaces that help protect building occupants and anyone nearby.
Kawneer’s 350 IR medium-stile entrances and IR 500/501 framing system, shown on the Tarpon Springs, FL, public-safety building, can offer protection from explosions. Photo by Gordon Schenck, Jr.
Step Three: Security Criteria and International Standards. Today the design process for most government and military structures must take blast demands into account. More and more, the education sector is also considering them. In addition to assessment and testing, architects are required to specify products that meet certain government and international standards. While no criteria or testing methods can promise total protection from explosions, government agencies designed these standards to consider the effects of explosive events on their facilities and offer steps to mitigate the extent or probability of damage.
Three common standards are:
- Security criteria by the Interagency Security Committee (ISC) and the General Services Administration (GSA), both based in Washington. Initially established to address glazing in all federal buildings after the bombing of the A.P. Murrah Federal Building in Oklahoma City, the GSA security criteria resulted in the increased use of blast-resistant fenestration products in federal courthouses and similar government buildings. The ISC took it further by developing its own security criteria, which were approved for use in all new GSA buildings and major renovation projects. These criteria, intended to reduce potential hazards, require that windows be designed to mitigate the risk from flying glass fragments in the case of an explosion. These measures were designed with the understanding that not all fenestration products will survive a blast.
- ASTM International Standards: Similar to the ISC/GSA performance, West Conshohocken, PA-based ASTM International’s ASTM F 1642-04 Standard Test Method for Glazing and Glazing Systems Subject to Airblast Loading establishes a testing method that evaluates levels of protection. These criteria require that glazed-window products meet performance levels that correspond to specific levels of protection.
- Department of Defense (DoD): The Department of Defense, Washington, developed a blast-mitigation standard to minimize hazards caused by glazing to personnel. Know as the Unified Facilities Criteria (UFC)-DoD Minimum Antiterrorism Standard for Buildings, the standard applies to windows, skylights, and glazed doors on new and existing inhabited buildings. In February 2012, the UFC 4-010-01 was revised with several significant changes. These include expanding the applicability of the minimum antiterrorism standards to the purchase of existing buildings, visitor centers, museums and visitor-control centers and expanding the requirements for design submittals, calculations, test reports, and narratives.
Additionally, the revised standard redefined conventional-construction standoff distance based on the type of construction materials and structural systems, which enables a project team to evaluate the resistance and strength of the structure with the standoff distances at a given site. Other modifications to the standard affect the requirements for unobstructed spaces and identifying required access control at building entrances.
Step Four: Whole-Building Solutions. Anyone evaluating products for blast mitigation must think about the building as a whole. Windows, curtain walls, storefront framing, and entrances are all tested for blast mitigation to ensure a safe building envelope. A comprehensive building solution is critical for optimum protection.
A great deal can be accomplished with today’s glass and systems technology. As products are evaluated for blast mitigation, it is important to consider what other safety, sustainability, and overall occupant comfort features they may also have.
When specifying products for schools, for example, daylighting and thermal performance are often considerations right along with safety. Many studies over the years indicate that natural daylight stimulates the brain more than artificial light. The same studies suggest that student performance increases where more direct daylighting is used. Natural ventilation and emergency egress are two more factors that architects have to consider when designing schools and educational facilities.
The combination of aesthetics, functionality, code requirements, and simple logic plays a major role in an architect’s desire to use glazed entrances, storefronts, curtain walls, and operable windows. However, some are suggesting that schools or government facilities should be windowless or have fewer glazed openings to be safe.
Plans put into practice
Government and federal facilities identify security as a top priority and often consider whole-building solutions. One such example is the Sarasota Police Headquarters in Sarasota, FL. When architects from Winter Park, FL-based Architect Design Group Inc. were developing the six-story, $38-million building they designed a facility that would achieve high energy performance and daylighting while delivering security against natural and manmade threats, balancing sustainability and security. The harsh Florida environment and hurricane-prone location was another major reason designers considered a comprehensive building solution.
The building contains more than 40,000 sq. ft. of exterior glass and 6,000 sq. ft. of interior glass. The exterior of the building features an impact-resistant curtain wall with insulating, laminated, low-e, impact-rated glass as well as impact-resistant entrances. A high-performance framing system used on the interior and several doors provides strength and durability. The design also allows ample daylight, which is important in public or government facilities where people often spend a great deal of time indoors.
The police headquarters achieved Leadership in Energy and Environmental Design (LEED) Silver certification from the U.S. Green Building Council (USGBC), Washington. By selecting products that achieved performance, safety, and aesthetic goals, the design team was able to provide occupants with a secure facility as well as energy and daylighting benefits, which contribute to overall occupant comfort.
The human element
Buildings can affect their occupants by enhancing moods, affecting health and productivity, and protecting them from natural or manmade disasters. Each product in a building plays a role in the overall influence on its occupants.
Incorporating any element into a building requires careful consideration, but when it comes to blast mitigation designed to safeguard human lives, thorough assessments, attention to regulations, and whole-building consideration are key elements in specifications. Protecting what’s inside is critical and selecting products with the highest levels of performance and defense help accomplish this objective. After all, it’s what–and who–is inside that matters most.
Donnie Hunter currently serves as the architectural manager for Kawneer North America, Norcross, GA.