Engineers Use Hurricanes to Study Houses
From: Psalm 110 (Gods_Fist_at_sbcglobal.net)
Date: 09/26/04
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Date: 26 Sep 2004 02:39:43 -0700
http://www.phillyburbs.com/pb-dyn/news/247-09262004-371986.html
Engineers Use Hurricanes to Study Houses
By MATT CRENSON
The Associated Press
GULF BREEZE, Fla. - When a hurricane makes landfall in the continental
United States, Dave Prevatt and his colleagues in the Florida Coastal
Monitoring Program are out evaluating which construction methods best
protect homes from the tremendous wind forces the storms deliver.
Established after Hurricane Andrew socked an unprepared South Florida
in 1992 with an estimated $26 billion in damage, the program sets up
portable instruments along stretches of coast where hurricanes are
poised to come ashore. Engineers erect weather towers that make
detailed measurements of the hurricane's winds. They learn about the
forces that houses are subjected to by putting pressure sensors on
their roofs.
Three days after Hurricane Ivan, some of the one-story ranch houses in
a Gulf Breeze neighborhood about a half-mile from the water have lost
patches of shingles. There are a few piles of broken branches dotting
the lanes and cul de sacs. But everybody's windows are intact, the
spindly trees are upright and kids are playing in their yards.
Then Mary Kort throws open her front door with a flourish, revealing a
scene that would render cable TV's perkiest home makeover queen
catatonic. Slabs of soggy ceiling sit in the middle of the living
room. Pink insulation coats everything. Overhead, slivers of sunlight
cut through gaps in the roof.
"I never knew this would happen," says Kort. "It's trashed."
Prevatt smiles ruefully and shakes his head. He's heard words like
those before.
A professor of civil engineering at Clemson University, Prevatt is
here to see which houses withstood the tremendous forces of Hurricane
Ivan, and how they differed from the ones that didn't.
Unlike many of her neighbors, Kort stayed put the night Ivan rolled
through. So she can tell Prevatt exactly how this happened.
With her husband and a friend, she watched Ivan's winds accelerate to
more than 100 mph. Around that time the screen enclosure on the back
of the house ceased to be.
"It was, like, scooped up," Kort says, her eyes wide. This is clearly
a woman who is thankful just to be alive.
Water started leaking through the windows and then the ceiling. Kort
and her companions started putting buckets and trash cans under the
leaks, but soon there were too many to keep up with.
When pieces of the ceiling started coming down they retreated to the
garage, where the former skydiver and self-described adrenaline junkie
says she began to pray.
"We thought, 'This is it,'" Kort tells Prevatt. "I'm thanking God on
my knees I am alive."
The program to study homes is sponsored by the Florida Department of
Community Development and staffed by scientists from Clemson, the
University of Florida, Florida International University and the
Institute for Business and Home Safety, an insurance industry research
center in Tampa.
"If we can define the wind forces, then as engineers we believe we can
design something that will withstand these forces," says Tim Reinhold,
vice president of engineering at the Institute for Business and Home
Safety.
For the research team, the hours before a hurricane are a frenzy of
driving and speedy construction. As Ivan's outermost fringes blustered
into Mobile, Ala., four researchers were scrambling to put up a
portable weather tower along the city's industrial waterfront.
After cruising around looking for a suitable spot, they settled on an
open area just outside the chain-link fence surrounding the Mobile
airport. For the next hour they were all socket wrenches and
screwdrivers. The men unhitched the trailer they'd been pulling behind
a pickup, then extended the four steel legs that can keep the
6,000-pound tower from toppling in 200-mph winds. They tested the
instruments and winched the tower up to its full 33-foot height.
Towers like this one can measure wind motions in much greater detail
than standard meteorological instruments, explained Kurt Gurley, a
professor of civil engineering at the University of Florida. When
meteorologists talk about a storm's maximum wind speeds, they're
citing readings of the air's horizontal flow taken in a single
location at one-minute intervals.
But in a hurricane, the most damaging winds are gusts that last a few
seconds or less. They don't come in a smooth horizontal flow, but in
localized updrafts, downdrafts or swirling eddies that might demolish
one house without touching the one next door.
To capture all that chaotic motion, the engineers mount two sets of
gauges on their towers at heights of 16 and 33 feet that measure wind
speed and direction in all three dimensions. Three of the program's
four portable weather stations also have two smaller towers that are
set 50 and 100 feet from the main one in order to determine how much
territory sudden gusts cover.
The monitoring program has also wired up more than 30 homes along
Florida's coast so they can be quickly outfitted with rooftop pressure
sensors that measure the forces exerted on the buildings during the
storm. In return for their hospitality the homeowners get free
hurricane protection retrofits, including storm shutters and sometimes
even new roofs.
For Ivan, the program's engineers mounted pressure sensors on 10 homes
along the Gulf Coast from Seaside, Fla., to the Alabama line. Then
they sped back east on Interstate 10 in a mad dash to get out of
harm's way.
"With all this storm chasing we've been doing, we've just about had
it," Gurley says.
This year is by far the busiest hurricane season in the Florida
Coastal Monitoring Program's seven-year history. In past years the
engineers have been lucky to see one hurricane-strength storm hit the
U.S. mainland, but so far in 2004 there have been three.
First Charley, a compact 145-mph whirlwind that hit Florida's west
coast communities of Punta Gorda and Port Charlotte with enough force
to damage 12,000 homes. After that storm the researchers were relieved
to see that homes constructed since Florida's most recent building
code revision suffered little or no damage. The 2001 code requires
nailing instead of stapling roof shingles, reinforced connections
between a home's roof and walls, and features such as hurricane
shutters or shatter-proof windows to defend against flying debris.
"The new Florida building codes have really helped increase the safety
and decrease the amount of damage overall," says Jason Smart, a
project engineer with the Institute for Business and Home Safety.
After Charley, he adds, "we saw a lot of failures of old structures -
and by old I mean 15 years - that wouldn't have failed if they'd used
nails" to attach the shingles.
Frances, a less powerful but wetter storm than Charley, came next,
causing at least $4 billion in damage as it swept across the state
from east to west. Engineers found the same pattern as Charley - homes
built in the last few years fared far better than even slightly older
ones.
Then Ivan, a storm that combined Charley's ferocity with Frances'
deluge. The storm barreled ashore with winds of 130 mph and sustained
much of its power even miles inland. Because the program started after
Andrew, this was the first time the researchers have been able to
study the impact of such a powerful storm in such detail.
And this weekend, another hurricane, Jeanne, was threatening the
beleaguered state's east coast.
"We'll be there," Reinhold says.
All of this season's information - the damage surveys, the rooftop
pressure readings and the detailed meteorological observations - will
prove useful in a number of ways, Prevatt says. It will help improve
the accuracy of equations that engineers use to calculate how the
forces exerted on a house grow as wind speeds increase. Right now
those calculations assume smooth air flow without accounting for the
turbulence of hurricane winds.
The data can also be used to improve wind tunnel tests, said Prevatt.
At his Clemson laboratory, he is trying to build models of entire
neighborhoods to see how the distance between houses and other factors
influence wind damage. The real-world information he's collecting in
Florida will help improve the accuracy of those models.
This research will also help engineers find the most cost-effective
ways to prevent houses from failing during a hurricane. If they find,
for example, that lost shingles are the main cause of damage, there's
no point in spending thousands of dollars making a house's walls
stiffer.
The cost of protecting houses from hurricane-force winds has been a
big issue in Florida in recent years. Building code changes inspired
by Hurricane Andrew have provoked complaints from some builders that
they add too much to construction costs.
Smart says the codes "have really helped to increase the safety and
decrease the amount of damage overall," but he acknowledges cost is a
concern.
So the Florida Coastal Monitoring Program aims to identify inexpensive
building methods that can make a big difference.
As Prevatt walks through Kort's soggy living room, he sees that
rainwater poured into spots where shingles blew off, collected in the
crawl space, then brought the ceiling down. Although he finds no
damage to the roof structure itself, he advises her to pay close
attention to how the roofer does the rebuilding job.
Because in a hurricane every little nail counts.
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