The tornados that struck Moore, Okla., earlier this week brought to the forefront a myth that involves glass. According to Valerie Block, senior marketing specialist with DuPont Glass Laminating Solutions, some people believe you should open the windows if you hear a tornado because it will equalize air pressure, potentially saving a roof or home from destruction. This is not true, she explains. Just as in the event of a hurricane, open windows increase internal pressure.
Block explains that in hurricane-prone areas one of the main reasons they have gone to impact-resistant glass or protective coverings over the glass is “because you want to avoid having windows/doors break, allowing for the build up of internal pressure in the home,” which can lead to the destruction/collapse of the house.
This myth, however, is not the only comparison that can be made about the two types of storms.
Block says a tornado usually has a very distinct path; one that is very narrow and briefly touches down in many cases. This is compared to a hurricane, which can be very wide.
Just as hurricanes, most common in South Florida, can occur anywhere in the U.S., tornados, most common in the Midwest in an area known as “tornado alley,” can also occur anywhere.
“They can and do occur anywhere and any time of year,” she says.
Another difference is that tornados offer little warning, unlike with a hurricanes where there is a lot of advanced storm advisories and warnings.
“Tornadoes spring up quickly and reaction time is critical to survival,” she says, noting that while hurricanes in themselves cause damage, tornadoes often occur during a hurricane event. This can be due to factors such as thermal instability and vertical shield.
“Almost all hurricanes spawn at least one tornado,” says Block, who pointed out that Hurricane Ivan spawned 117 tornadoes over a three-day period and these affected many states other than Florida, going as far north as Pennsylvania.
When it comes to rating tornados, this is done on what’s called the Fujita scale, which goes from F0 to F5. Likewise, hurricanes are ranked on the Saffir-Simpson scale, which also encompasses five levels.
But what about building codes? The country saw building codes for hurricane-prone regions strengthen as a result of Hurricane Andrew. Experts say, however, it’s a different case when it comes to tornados.
As Thom Zaremba, a code consultant for the Glazing Industry Code Committee, explains, “No building code in use today would result in a structure capable of surviving an F4 tornado. Building codes are generally minimum codes and the criteria used in the development of such codes do not include the forces found in an F4 tornado.”
But there has been some standard development activity in recent years. Though voluntary unless adopted by a state or local jurisdiction, there are options. For example, Block says the International Code Council (ICC) and the National Storm Shelter Association (NSSA), together developed the ICC/NSSA 500 Standard for the Design, Construction and Performance of Storm Shelters, which provides guidelines on what is needed in terms of tornado protection. Block stresses, though, that goal with codes and standards is first is to maximize safety, keeping people safe and then structural damage.
“Life safety is always first,” she says.
She adds though, “Building codes, which only represent the minimal standards, do not really require storm shelters even in high-risk areas.”
Other documents that do address storm shelters include FEMA 320 and FEMA 361. The 361 document, for example, includes a section on tested glazing material. Glass-clad polycarbonate and multi-ply laminated glass with a 90-mil PVB interlayer were tested.
The test panels were impacted by a 15-pound 2-by-4 travelling at 100 MPH. Results found that “glass shards propelled at great distances and at speeds considered dangerous to the occupants.” Its recommendation was that “unless glazing can pass the missile impact test, windows are not desirable in tornado safe rooms.”
However, there is still a place for glazing products in these tornado-prone areas. As Block noted, a tornado leveled 95 percent of Greensburg, Kan., in 2007, and rebuilding efforts have included the use of glazing. In fact, she says architects placed no limits on the use of glass–expect a recommendation that it be high-performance.