Tornado simulation reveals need for sturdier homes

Ed Sutt and colleagues are now evaluating damage from six-inch diameter pine trees by using a 20-foot-long metal column weighing a quarter of a ton

April 7, 1999
Web posted at: 4:15 PM EDT

On average, 42 Americans are killed by tornadoes each year. Most of these deaths occur when buildings collapse.

In an effort to prevent some of these deaths, researchers are dropping tree-like poles on houses to help develop practical, cost-effective tornado "safe rooms" in homes.

The project, being conducted at Clemson University with a $73,000 grant from the Federal Emergency Management Agency, will specifically investigate ways in which interior rooms such as bathrooms and bedrooms can be reinforced to prevent damage by wind-blown debris and falling trees.

To test what structural components work best against falling trees, researchers engineered what they call the "Drop Zone," a 24-foot-tall truck-mounted device that can winch columns weighing up to 1,500 pounds into place and then drop them, ax-like, onto the structure being tested.

Researchers are now evaluating damage from six-inch diameter pine trees by using a 20-foot-long metal column weighing a quarter of a ton. Poles approximating the effect of a larger tree will also be tested.

According to early tests, the six-inch diameter pole crunches half-way through a typical 6-by-8 wall panel made of wood studs and fiberboard sheathing. The same column barely takes a bite out of a metal-reinforced panel. For a person asleep in a bed, that three-foot differential could mean the difference between life and death.

Roofs and other components will, of course, give additional levels of protection and will be factored into the final recommendations.

"But if the tree were to fall between the roof structural members and impact the plywood roof sheathing, the roof wouldn't make much of a difference anyway," warned Ed Sutt, a Ph.D. student who is helping with the research. "And keep in mind, we're only seeing the effects of a six-inch pine at this point. Larger trees could have far more devastating consequences."

To simulate wind-borne debris, researchers use an air cannon to fire 2x4s and other materials against wall sections at speeds up to and exceeding 100 mph.

The research has a focus on low-cost fixes, but the results will also be used to develop technical guidance for contractors and practical application material for homeowners.

Low-tech fixes under study include installation of wood-reinforced wall panels, addition of a layer of metal decking under the wall panel, different combinations of plywood or common insulation foams.

"This won't lead to code changes, but we will come up with inexpensive suggestions that we think homeowners will want to include, particularly, in new construction," said Sutt.

"We need to break the cycle of thinking that says tornadoes and hurricanes are acts of God and that nothing can be done," said Ben Sill, Alumni Distinguished Professor of Civil Engineering at Clemson and one of the founders of Clemson's wind research program.

"Although some tornadoes or hurricanes will be so strong that extensive damage would be expected, it is not unrealistic to expect that most buildings should successfully withstand severe storms -- very frequently, though, that's not the case."

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