|
code
watch
Learning an Important Lesson
An open and shut case, except for 'the human factor.'
by David Bowman, P.E.
As engineers and designers we cannot overlook the importance of the human factor. For example, during the development of a new International Code Council (ICC) and National Storm Shelter Association (NSSA) consensus standard for storm shelters, the committee came upon a serious design problem: people don’t always keep the doors closed, even during very high winds.
When doors and windows are left open, high winds can create internal pressure in the building. The more openings, the greater the internal pressure that must be accounted for in the shelter design. In addition, the higher the wind speeds, the greater the pressure. With the high wind speeds this design standard requires, there would be a significant amount of internal pressure in the building just from opening a normal 36- by 80-inch door.
This internal pressure can have a significant impact on the design of the structure, and that translates into a significant increase in costs. For instance, roof systems would have to be designed to withstand this internal pressure. The roof connections, size of the deck, and even the main roof support members might need to be increased.
You may be thinking, “So what? Just keep the door closed.”
But there lies the problem. Engineers have witnessed human behavior during recent hurricanes. These observations of human behavior in storm shelters have shown that people cannot be counted on to keep the doors closed and stay inside, even during the period of the highest winds of the storm.
The point is that people without a structural engineering background don’t understand that they can lose the whole building because they’re standing there with the door open. It’s just not intuitively obvious to people.
DRAFT STANDARD
People who have to stay inside the shelter for 24 hours can complain of feeling claustrophobic, even the ones that aren’t clinically diagnosed as claustrophobic to begin with, and may feel a desperate need to open the door. Late arrivals might show up during high winds and need to enter the shelter for protection. Some people open the doors just because they’re curious to see what is going on outside. At any rate, the ICC/NSSA storm shelter standard committee decided it would have to account for such a possibility of internal pressure. It would be unwise not to consider it and risk losing the roof of the shelter, or the entire shelter, because an occupant innocently decided to open a door.
Internal pressure was just one of a number of challenges the committee tackled to develop this standard. The current draft standard addresses requirements for the design and construction of shelters to protect people from the violent winds of hurricanes and tornadoes.
Some of the features of the storm shelter standard include:
1. Wind loads. The storm shelter standard will require the shelter to be designed to withstand wind pressures associated with wind speeds based upon a 10,000-year mean return period. This design ensures a hurricane storm shelter can resist the winds anticipated in a Category 5 hurricane. For example, one of the contour maps the committee referenced shows 200 mph wind speeds on the coast of Florida and the Carolinas and wind speeds higher than 200 mph in some locations.
2. Flooding. The storm shelter standard will require the lowest floor of the shelter to be located above flood levels from a storm surge associated with a Category 5 hurricane.
3. Essential features. The storm shelter standard also will require the shelter to be equipped with emergency lighting, mechanical and natural ventilation, flame resistant interior furnishings, sanitation facilities, potable water and fire extinguishers.
In the fall of 2006, ICC/NSSA storm shelter standard committee completed the long-awaited first draft and posted it on the ICC Web site for public comment. We received almost 150 public comments, which the committee discussed at a meeting in Orlando, FL, earlier this year.
You are invited to view what the committee has done so far and read the draft standard. Visit the International Code Council Web site at www.iccsafe.org, and stay tuned. The results of the committee’s hearings upon receipt of public comments will be posted on the ICC Web site sometime this spring.
David Bowman, P.E. is Manager of Codes at the International Code Council. The International Code Council, a membership association dedicated to building safety and fire prevention, develops the codes used to construct residential and commercial buildings, including homes and schools. Most U.S. cities, counties and states that adopt codes choose the International Codes developed by the ICC.
The National Storm Shelter Association is a nonprofit organization dedicated to developing performance standards, product certification, and education programs for the design, construction, and performance of storm shelters. |
