Blast protection doors have been around since the invention of gunpowder. Commercially manufactured blast doors, however, were only introduced to the market in the 1950s. It was the Cold War era and for obvious reasons the scientific communities took a real interest in the physics of explosions.

Since then, the study of blast hazards and the means to mitigate them has expanded rapidly. So fast, in fact, it has spawned an entire engineering field dedicated to developing protection against blasts and explosions. How do blast doors work? While the physics explanation would overwhelm even the most intellectual minds, a workable understanding can be achieved by examining three factors:

1. Pressure ratings and basic mathematics
2. The components and construction of a blast door assembly
3. The standards that guide the industry

Pressure ratings are specified by certified engineers expertly trained in calculating the anticipated trajectories, distances, and blast forces expected from various types of hazards. Typically given in PSI (pounds per square inch) or PSF (pounds per square foot), pressure ratings determine the construction and design of a blast assembly. Blast doors with PSI ratings of 6 or higher are referred to as high-range blast door assemblies. Medium-range blast assemblies offer PSI ratings of 4-5, and low-range blast assemblies are those that offer PSI ratings of 1-3.

To illustrate how PSI ratings translate to the total pressure a door is expected to withstand, let’s examine an assembly specified 36 x 84 inches in size with a PSI rating of 3. In this example, we will consider the pressure “static,” meaning the pressure is continuous and uniformly applied to the entire door.

Because the total pressure is determined by multiplying the square-inch surface area of the door by the PSI rating, this assembly would have to be constructed to withstand 9,072 pounds of pressure (3,024 sq in x 3). If the dimensions of the door increase just slightly to 48 x 96 inches with the same specification of 3 PSI, the door would now have to be constructed to withstand 13,824 pounds of pressure. The door, frame, and hardware all work together to achieve a designated PSI rating. To ensure a solid working unit, the exterior of a blast door is manufactured out of cold-rolled steel sheets, while the interior support systems are comprised of either “hat sections” or “structural tubing,” constructed out of carbon steel. Hat sections are in the shape of a hat and structural tubing is in the shape of a square or a rectangle. Determining which interior structural shape to use depends on the size of the door, the PSI required, and the engineering specifications.

Frames are also manufactured using steel and their configurations are determined by door thickness and the specific wall anchors utilized. Because frames must transfer blast pressure to the walls, low-to-medium blast-rated frames are engineered to be anchored to typical wall materials such as a tube stud, block, or concrete. High blast ratings, on the other hand, will likely require frames to be anchored to embedded steel channels or installed within the wall during the concrete pour with anchor bars of various diameters.

The choice of blast door hardware depends on whether the pressure will seat the door into the frame or unseat it against the hardware, and whether latching will be required to hold all or a portion of the pressure. This is determined by considering the rebound scenarios set forth in the specifications.

Typically, the rebound resistance of the hardware is half the peak pressure the door is expected to withstand. High blast ratings typically require custom-designed hardware, but “off-the-shelf” hardware can be modified for use with low and medium ratings. Locksets can then be specified as either manual or power operated depending on the anticipated blast hazard.

When it comes to visual design, blast doors could theoretically be made in any shape because the pressure dynamics remain the same. Even concave and convex designs are possible but are very expensive. Singles, pairs, sliders, vertical lift, and swinging configurations are also possible.

In addition, blast doors can be veneered to be aesthetically appealing when needed for office and retail settings such as banks, embassies, consulates, courthouses, etc. Even blast resistant vision lights can be incorporated as long as the glass has the appropriate PSI rating.

The only component a blast door assembly typically avoids is the threshold because a bump in the road is the last thing you want when operating in the presence of explosive materials and other blast hazards. Mathematics and design aside, the remaining intriguing element of blast door assemblies are the limited standards, requirements, and regulations that dictate the use of blast doors. In 1991, the U.S. General Services Administration (GSA) developed some initial “security criteria” for the construction of U.S. government buildings. Other nations may issue similar requirements.

The Interagency Security Committee (ISC) adapted GSA’s criteria in May 2001 and released its own “Security Design Criteria for New Federal Office Buildings and Major Modernization Projects.” The GSA-ISC combined guidelines now apply to all federal buildings except those under the jurisdiction of the Department of Defense (DOD), which has its own guidelines.

DOD building requirements are published under Unified Facilities Criteria (UFC) 4-010-01 and titled, “Minimum Antiterrorism Standards for Buildings.: More stringent than the GSA-ISC Security Criteria, UFC 4-010-01 outlines 23 prescriptive standards in four major categories including site planning, architectural, structural, and electrical-mechanical. For practical reasons, security and engineering consultants currently use both documents to help architectural teams develop designs that lower the risk of loss of life and damage to property exposed to blast hazards.

As for blast door testing and certification, several industry and government bodies are working towards standardization, but nothing specific currently exists. “Shock tube testing,” a process that delivers airbursts at controlled levels, can be used to test low and medium PSI ratings. High-range blast assemblies can undergo “destructive testing,” but these tests are typically costly and produce the same results as mathematically expected.

In most cases it is safe to assume that if the calculations done by the blast engineer are correct, then the doors should perform to the level required. To achieve the desired blast protection you need to work with an experienced, certified blast engineer and be sure to ask them to double-check the math.