Precision design is a hallmark of any high-quality specialized door product. But what happens when a product designed primarily for one particular purpose is unintentionally put to the test of another? Here’s the story of how an unexpected mishap lead to an illuminating discovery.

It begins when a customer requested a set of extremely large acoustical doors for a research facility. Acoustical doors, of course, are engineered to prevent a specific amount of sound from passing through the door. They are commonly used in high-noise environments such as manufacturing plants and music studios. In this case, the doors were being installed in a highly specialized research and testing facility. The pair of hybrid doors, each leaf of which measured 8 feet wide (16 feet total width of both doors) by 20 feet high by 3 inches thick, had a Sound Transmission Class (STC) rating of 50. The STC rating signifies a product’s ability to block sound at a given frequency, or the number of decibels that sound of a given frequency is reduced in passing through. The higher the STC rating, the better the product performance. The single STC rating system allows a designer to ensure that all architectural elements (doors, walls, windows, etc.) match up in terms of acoustical properties.

In addition to the acoustical properties, the oversized hybrid doors also offered a certain level of pressure resistance of 41.39 PSI (pounds per square inch) peak reflective pressure and 98.49 PSI MSEC (duration of the incident) added to the oversized STC-50 construction. The hybrid steel-stiffened doors were fabricated with continuously welded, seamless edges, steel top and bottom closure channels, steel stiffeners, and custom acoustical core of non-combustible U.L.-approved insulating materials. They were constructed around a custom-engineered, steel rib core as is customary for pressure- and blast-resistant doors. The full assembly included the doors, frame for jamb, Oversized Cam Lift hinges (3 per door) and multipoint latching devices, Krieger’s oversized seal system and embed sill. The adjacent man-size hybrid doors with the same specifications utilized builder hardware of a multi-point exit device with outside trim, surface heavy-duty closers, seal system with head and jamb gasketing and integral door bottom, and aluminum threshold.

Pressure-resistant or blast-resistant doors are designed to protect people and property in the event of an explosion. Blast doors are used in a variety of environments such as government buildings, laboratories, refineries, chemical storage, and more. Pressure or blast ratings are specified by certified engineers expertly trained in calculating trajectories, distances, and blast forces expected from various types of hazards. Typically, a dynamic blast pressure is given in PSI or PSF (pounds per square foot) with an impulse and or duration of the incident. This pressure ratings determine the construction and design of a blast assembly. After the door assembly was installed and the customer began using their new facility for research and testing, an incident occurred. A large stainless steel capped tube projectile measuring 10 inches in diameter by 10 feet in length exploded and hit the doors with tremendous force. The adiabatic condition was more representative of an instantaneous burst event in which tube fragments rapidly achieved terminal velocity. Table 1 reports fragment momentum rather than velocity to best capture worst-case fragment strike velocity and force for fragments that are 25%, 50%, and 75% of the total vessel mass. The analysis results conservatively assumed no reduction in projectile momentum across its trajectory

What’s remarkable about the doors in this case is that although the acoustical properties were also designed to withstand an overall pressure blast, they were not engineered to withstand a spot fragmentation such as what occurred. Despite the massive force of the tube projectile, the doors remained completely intact and operable. The fact that the doors did not get blown off their hinges and become flying projectiles themselves, is nothing short of amazing. The blast 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 blast engineer’s specifications.

When it comes to blast door testing and certification, several industry and government bodies are working towards standardization, but nothing concrete currently exists. “Shock tube testing,” a process that utilizes airbursts at controlled levels, can be used to test low and medium PSI ratings without the negative phase of a typical real-life explosion. High-range blast assemblies can undergo “destructive testing,” but these tests are typically costly and produce the same results as mathematically expected. Hiring a “blast consultant” with expertise in structural dynamics and experience with the governing criteria documents is another way to test and can be a valuable resource for any blast or pressure door application. It remains a testament to the integrity and quality of the Krieger acoustical door assembly that a pair of doors on this scale, which were not engineered to withstand the magnitude of force they were subjected to, remained intact and fully operable. The customer did not experience any additional losses due to the unanticipated event. Of course, it’s standard in the industry to perform testing to ensure a product performs according to its specifications as intended—but here’s a case where a custom product got tested in the field to reveal results that exceeded specifications and expectations. Continued operations without any business disruption was the real performance of these doors—and a demonstration of how Krieger is “advancing the possibilities.”