Hamburger adds that the current engineering standard is based on wind loads that have a 1,700-year mean return period. The 2015 ASCE 7 standard will be based on a 3,000-year mean return. Compared to 100-year flood designs or 2,000-year earthquake designs, the new standard will be very rigorous, he says.

There are six tornado-strength categories. An EF-0 twister causes light structural damage at wind velocities of 86 mph or greater. An EF-5 tornado, with winds greater than 200 mph, causes major structural damage.

Thanks to a dearth of products tested by tornado damage, owners seeking to harden construction against tornadoes elect to use components and products that are hurricane tested. But beyond wind speed, there are differences between the two storm types.

"Hurricane winds are primarily horizontal," says Tim Reinhold, senior vice president of research and chief engineer at the Institute for Business and Home Safety (IBHS), an insurance sector-funded group that tests high-wind impacts on buildings.

"The vortex in a tornado has strong updrafts, so uplift could be stronger, but that's not well defined at this point," adds Reinhold.

Consequently, the windborne debris impact standard for tornado shelters is stricter than for hurricanes, which generally are not as intense, says Ernst W. Kiesling, executive director of the National Storm Shelter Association (NSSA) and a professor of civil engineering at Texas Tech University, Lubbock. Hurricane projectile tests at Texas Tech require a 9-lb 2x4 traveling at nine-tenths the location's design wind-speed map.