Every spring and summer, tornadoes affect thousands of households and businesses across the country. According to the National Oceanic and Atmospheric Administration (NOAA), there is no specified U.S. tornado “season”; however, most tornadoes tend to occur in May and June. In the course of a year, the United States experiences approximately 1,253 tornadoes. Strong winds, large hail, and flying debris are the biggest dangers associated with tornadoes.
In addition, the Federal Emergency Management Agency (FEMA) has created the following map to illustrate the frequency of tornadoes in the United States:
To keep occupants safe, many businesses and commercial buildings across the nation utilize storm doors (colloquially known as tornado doors) on buildings designated as storm shelters. A tornado-certified door needs to follow particular specifications.
The door, frame, and hardware must be tested to prove they will hold strong against windstorm dangers. The testing standards for tornado doors are developed and enforced by third party certifiers, such as Underwriters Laboratories (UL). The testing process applies to “true to life” simulations, similar to the fire door testing process. The Missile Impact Load Test for tornado doors uses 2×4-foot pieces of wood, weighing 15 pounds/each, projected at 100 mph at the door in three different locations. An example of how tornado doors perform compared to hurricane and residential doors can be found in this video.
Also, during tornadoes negative pressure develops on the outside of buildings while positive pressure builds on the inside of buildings. To keep the building’s doors from being sucked out or blown in, possibly harming the building occupants, the door’s hardware is also tested to hold strong during the pressure changes.
Main attributes of a tornado door include:
- The door and frame need to be sold as a unit.
- Steel is the most common material used in tornado doors.
- The opening should withstand pressures of +296/-270 pounds per square foot.
- The steel gauge used for the door should be 16 or 14 gauge depending on core construction
- The steel gauge used for the frame is 14-gauge.
- The door’s thickness must be 1¾ inch thick
- Tornado doors are flush only. If a window is needed in a room, you can go with an alternative product, available through LaForce.
In addition to meeting rigorous specifications, a tornado door must also be carefully installed according to the manufacturer’s instructions, to maximize effectiveness. FEMA recently published updated storm shelter guidelines that address hardware and door swing requirements. FEMA P-361 is for 12+ people, while FEMA P-320 is for less than 12 people. Storm shelters must be constructed according to these guidelines. It is also important to note that even though they are similar, tornado and hurricane doors are constructed differently, and are held to different standards. We will write about hurricane doors in late summer.
Like all door and hardware products, each manufacturer’s certified tornado doors are slightly different. LaForce has access to a wide variety of manufacturers, to help building owners and contractors find the best match for their construction project.
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Special thanks to Larry Coenen, Hollow Metal Office Manager for LaForce, for his in-depth industry expertise.