Challenging storms

Building advances minimize damage

Construction

Hurricanes' Path Of Destruction

September 19, 2004|By Michael Hill | Michael Hill,SUN STAFF

When hurricanes start stalking America's Southeast coast, David Prevatt hopes that they hit one of his houses.

Not one that the Clemson University engineer lives in, but one of the many he has wired from South Carolina to the panhandle of Florida to measure the stresses they face from extraordinarily high winds.

Prevatt said that he takes no joy in the destruction and discomfort that these storms cause, but he cannot hide his pleasure when he records direct hits, as he did when Frances struck Florida on Sept. 5.

The wired houses are part of an extensive engineering research effort that is producing buildings better able to resist the forces of hurricanes -- saving property worth many millions of dollars and, judging from the toll of previous storms, probably scores of lives.

"Yes, we can, in fact, build better buildings to survive hurricanes," Prevatt said. "After all, we can send a man to the moon, so anything is possible from an engineering standpoint."

The results of such work have been evident in the flurry of storms that recently hammered Florida.

While Ivan -- one of the most powerful storms ever to hit the mainland United States -- caused significant damage to structures in Pensacola where it roared ashore, a trip to a trailer park was often necessary to find evidence of residential devastation from Frances and Charley in southern Florida.

Experts believe much more can be done to protect lives and property as their hurricane research continues.

"We have to be really happy and excited about Hurricane Frances," Prevatt said of the direct hit on some of his wired houses. "We have great full-scale data from several houses that were fully instrumented."

Prevatt uses data from the houses to design better experiments in his wind tunnel. Benjamin Shafer, an engineer at the Johns Hopkins University, which has had a house wired on North Carolina's Outer Banks for several years, said the houses make the leap from theory to reality.

"They help us learn what real hurricane wind is on structures in terms of stress and strain," he said. "Wind tunnel science is great, but it is using model buildings. ... It is not real wind on real structures."

The real wind of Hurricane Andrew in 1992 reminded people of the necessity of this work.

That storm left behind $20 billion in damage, turning a large corridor of southern Florida that had been filled with subdivisions and cul-de-sacs into a no man's land of devastation.

Andrew, which hit after a period of relatively few storms and rampant development, revealed two major problems with the state's building code -- it was neither stringent enough nor well enforced.

"The big lesson, I think, was from Andrew," said Ronald F. Zollo, a professor of civil and architectural engineering at the University of Miami. "Since that time, a lot has been done."

Zollo said Andrew revealed -- quite literally, as it tore the roofs off countless homes -- huge problems in housing construction. "When it hit, Florida had probably the highest wind-resistance standards in the country, but it still caused significant devastation.

"That wasn't just because its winds were Category 4 -- actually it was a Category 5 storm -- it was because of defective conditions at the time," he said. The problems were often as simple as nails that missed their mark, or staples being used to hold on roof shingles instead of nails to cut costs.

Building codes

Zollo said that after Andrew, Florida abandoned building code standards that were in use throughout the country and opted for its own. "But even that did not come up to the standards that, in the opinion of people in the engineering community, that it should. So, rather than just accept the code, south Florida won the right to have an even higher standard code."

Along the coast, Zollo said, this called for resisting a 148 mph wind, a standard that decreased farther inland.

It turns out that, unlike going to the moon, building storm-resistant buildings is not rocket science. It is testing window and wall systems before approving them for use. It is requiring that an approved roofing system be used uniformly rather than being mixed with another system. It means specifying the length of nails, the number used.

"Imagine if you built a house out of playing cards," said Prevatt. "Put four cards standing up vertically and one on the roof. You hardly even have to blow it down, it just falls down. Now if you took the same cards and put a bit of Scotch tape on at the corners where they meet, it would take more wind to blow it down."

The improvements to the Florida housing stock were much sturdier versions of that. The idea is to make the house a solid unit, to connect securely the roof to the trusses, the trusses to the walls, the walls to the foundation.

"We know that the three most important things in getting a building to survive a hurricane are connection, connection and connection," Prevatt said.

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