logybogy
November 20th, 2005, 05:28 PM
Posted on Sun, Nov. 20, 2005
Tougher requirements called for on skyscraper windows
It wasn't just Hurricane Wilma's winds that shattered the gleaming glass facades of so many South Florida skyscrapers
BY CURTIS MORGAN, ANDRES VIGLUCCI AND ERIKA BOLSTAD
cmorgan@herald.com
It wasn't just Hurricane Wilma's winds that shattered the gleaming glass facades of so many South Florida skyscrapers. It was the junk flying around in that wind 40 stories off the ground -- jagged shards of glass and fist-size chunks of stucco.
That's something South Florida's high-velocity wind code doesn't address.
High-rise windows are designed to survive ''small missile'' impacts from debris like roof gravel, a standard tested by shooting pea-size metal balls at glass.
But after three weeks of surveying hundreds of blown-out high-rise windows in Miami and Fort Lauderdale, building inspectors, engineers and glass consultants believe that the bulk of the damage was caused by high-flying debris -- bigger, heavier and more destructive than anything the highly touted code contemplates.
Underscoring concerns is the fact that many windows failed in Wilma, a storm whose sustained winds were generally no more forceful than 100 mph -- well below the threshold that high-rise buildings are supposed to meet.
Now, some experts are calling for strengthening the code. The current test ''gives you some misleading confidence in the system, which obviously has failed,'' said A.A. ''Sak'' Sakhnovsky, whose Construction Research Laboratory in Miami pioneered the science of testing high-rises to withstand forces of nature. ``The whole concept of the glass construction has to be rethought.''
Most of the towers that sustained heavy damage were erected under old, pre-Hurricane Andrew codes that allowed relatively weak glass. Even some newer buildings that were hit hard -- such as the Espírito Santo Plaza and the Four Seasons tower -- were approved before high-rise glass standards were beefed up again in 2000.
Some experts, including building officials, say there may be less reason to worry about buildings approved under those newer standards; most use stronger laminated glass, and they escaped serious damage.
Valerie Bohlander, director of Fort Lauderdale's building department, called the explanation a ``no-brainer.''
''The older buildings, the windows blew out,'' she said. ``The newer buildings, they didn't. I don't think there's really a lot of mystery to it.''
WINDOWS IN QUESTION
LATEST VERSIONS NOT
TESTED FOR BIG DEBRIS
Still, some experts caution that even the latest windows may not be all they are cracked up to be, because they have not been tested for the kind of large debris that Wilma propelled along Brickell Avenue and through downtown Fort Lauderdale.
The failure of about two dozen building ''envelopes'' during Wilma resulted in tens of millions of dollars in damage and lost work, putting some offices, hotels and residences out of commission for weeks or more.
City and county inspectors, as well as engineering consultants and glass contractors and makers who examined the damage, said it may be months before they can fully explain what happened. They say a combination of factors, including a ''wind-tunnel'' effect that may have exacerbated Wilma's power, likely played some role in the window loss.
But experts have largely ruled out tornadoes, excessive building flexing or widespread failures of window framing or sealants. No one found twisted metal frames or windows sucked out whole.
Most windows simply shattered in their frames -- compelling evidence that debris played a leading role.
Something as small as gravel, layered on rooftops to protect waterproofing material, may have caused the first cracks, experts say. Gravel did extensive damage during Hurricane Alicia in Houston in 1983 and blew out windows in Kendall and Homestead during Hurricane Andrew.
It's also the prime suspect in Fort Lauderdale.
Michael Shiff, an architect and developer who designed the annex to the Broward County courthouse, said one of his architects walked around two battered towers on Broward Boulevard and returned with a bag full of roofing gravel. Shiff found the same debris near the courthouse and school district building.
Although it's no longer widely used, gravel is still allowed under most building codes, and Shiff echoed others in calling for the state to ban it in hurricane zones.
Along Brickell, what first cracked glass is uncertain. But the problem quickly became far weightier than flying pebbles.
Sakhnovsky, whose lab originally tested the glass systems on both the Four Seasons and Espírito Santo as well as international landmarks like the Twin Towers in New York, found the telltale clues everywhere -- in shards he picked up between the two buildings.
He weighed a few and found them 15 to 20 times heavier than the gravel his lab had fired at glass from both buildings. The chunks were also far heavier than steel ball bearings now used under the upgraded 2000 code.
Those first shards were likely swept up as Wilma's wind whirled around the buildings, Sakhnovsky and others agree, triggering a cascade of damage that scored and shattered adjacent windows and shotgunned into surrounding buildings.
Sakhnovsky theorizes that the Four Seasons, the tallest building in Florida, may have been Building Zero because it sat at the upwind corner of the heaviest cluster of damage.
''Some of the shards weighed over one ounce, quite large,'' he said. ``When they hit the Espírito glass, out it went. Of course, the Espírito then generated some of its own missiles.''
Stucco chunks as big as a fist were found inside the Espírito Santo. Nearby high-rise buildings lost sections of concrete and metal roofs, even roof exhaust fans the size of steel drums. All potentially could have careened into towers at high speed.
William Ross, president of Estoril Inc., which manages the Espírito Plaza, said consultants found damage suggesting that the cluster of buildings may have generated a wind-tunneling effect that accelerated debris flung by Wilma, a storm with estimated gusts topping out at less than 100 mph in both cities.
''They've got a piece of metal up there where the glass is driven into the metal, glass pulverized like powder,'' he said.
1994 BUILDING CODE
'LARGE MISSILE' TEST
ONLY ON LOWER FLOORS
The building code that Broward and Miami-Dade counties adopted in 1994 was designed to protect homes and buildings from flying debris. Only lower-floor doors and windows were required to survive the well-known ''large missile'' test -- a nine-pound, two-by-four piece of lumber hurled at 34 mph.
Above 30 feet, the assumption was that only ''small missiles,'' nothing much larger or heavier than roof gravel, would be flying around. So South Florida's original small-missile test -- the one applied to the Four Seasons and Espírito Santo -- specified using small rocks in tests, fired at 55 mph, followed by a cycle of wind-pressure tests.
But even before the small-missile code was adopted, some engineers were questioning whether it was tough enough -- chiefly because there was no standard for how hard the rocks should be.
As a result, ''soft rock'' that pulverized on impact was sometimes used, undermining some test results. By June 2000, Miami-Dade and Broward joined other places in adopting a stiffer and more uniform test: ball bearings fired at 88 mph.
In the interim, at least 20 high-rises were approved in the city of Miami alone under a small-missile code that many engineers considered too weak.
As a result, those buildings were generally permitted to use systems with double panes of tempered and heat-strengthened glass -- but not the laminated glass now generally installed under the 2000 code. Laminated glass, which consists of a puncture-resistant plastic sheet sandwiched between two panes, is stronger and less likely to shed shards should the outer pane crack.
`A WINDOW OF TIME'
GLASS WAS TEMPERED,
BUT NOT YET LAMINATED
''You have a window of time where a lot of tempered glass went in when it probably should not have,'' said Joseph Minor, a consulting engineer who helped Miami-Dade fashion the first post-Andrew missile code.
Jaime Gascon, acting chief for product control for Miami-Dade's Building Code Compliance Office, said the county moved as quickly as it could to add the tougher impact test. Code changes can take a year or more because they are scrutinized closely and challenged by builders and suppliers.
Ross, president of the Espírito Santo's management company, said developers went with what was considered state of the art at they applied for a permit. The building's 18 types of windows cost about $25 million and passed the gravel test.
Ross believes that the Espírito Santo's unlaminated windows -- two panes of glass separated by only a pocket of air -- did perform better than those in older adjacent buildings. Although the Espírito tower lost hundreds of outer panes, only about 60 windows lost the inner one as well. At the Four Seasons, where numerous outer panes were damaged, there were only about a dozen blow-throughs, managers said.
Still, Ross said he was surprised and disappointed after Wilma.
''We expected some to break, but we did not expect to come in and see the building looking like a patchwork,'' he said.
Buildings that used laminated glass typically escaped such serious damage.
Nelson Navarro, South Florida representative for DuPont, which makes the plastic laminate used by many window manufacturers, pointed to Fort Lauderdale's new and unscathed Summit Las Olas condo, fitted with laminated windows that met the post-2000 codes.
Next door, dozens of nonlaminated windows shattered in the older Broward Financial Center. A similar contrast between new and older structures is visible along Brickell Avenue.
''It's a big change in the performance of these buildings versus the ones that didn't have laminated glass,'' Navarro said. ``We can see the difference. Even a Category 1 storm, close to 2, I would say it was a good test.''
Still, a number of engineers, building experts, hurricane forecasters and even some laminated-glass makers believe that the rain of debris unleashed aloft by Wilma shows that the testing standard for high-rise glass needs a harder look.
For example, even laminated windows can still drop outer shards, creating a hazard to surrounding structures. Laminating outer panes, a more expensive alternative already used in airports for noise reduction, may be a potential solution.
Another concern centers on the dramatically stronger wind speeds experienced at the top of skyscrapers. The current code steps up wind-pressure tests for higher floors, but not the speed of the metal balls fired at the glass. That standard is designed with roughly Category 3 winds in mind -- but at ground level.
In the last decade, scientists have discovered that winds at higher altitudes are often far more powerful, said Stanley Goldenberg, a hurricane researcher with the National Oceanic and Atmospheric Administration in Miami.
While ''it's not a rule,'' he said, wind at 30, 40 or 50 stories up can often be one category, even as much as two categories, stronger.
''This is what we've been concerned with for years,'' he said.
``With this tremendous increase in hurricane activity, and this tremendous increase in building high-rises, it's only a matter of time before some major structures start getting hit with major storms.
``The ramifications are scary.''
Tougher requirements called for on skyscraper windows
It wasn't just Hurricane Wilma's winds that shattered the gleaming glass facades of so many South Florida skyscrapers
BY CURTIS MORGAN, ANDRES VIGLUCCI AND ERIKA BOLSTAD
cmorgan@herald.com
It wasn't just Hurricane Wilma's winds that shattered the gleaming glass facades of so many South Florida skyscrapers. It was the junk flying around in that wind 40 stories off the ground -- jagged shards of glass and fist-size chunks of stucco.
That's something South Florida's high-velocity wind code doesn't address.
High-rise windows are designed to survive ''small missile'' impacts from debris like roof gravel, a standard tested by shooting pea-size metal balls at glass.
But after three weeks of surveying hundreds of blown-out high-rise windows in Miami and Fort Lauderdale, building inspectors, engineers and glass consultants believe that the bulk of the damage was caused by high-flying debris -- bigger, heavier and more destructive than anything the highly touted code contemplates.
Underscoring concerns is the fact that many windows failed in Wilma, a storm whose sustained winds were generally no more forceful than 100 mph -- well below the threshold that high-rise buildings are supposed to meet.
Now, some experts are calling for strengthening the code. The current test ''gives you some misleading confidence in the system, which obviously has failed,'' said A.A. ''Sak'' Sakhnovsky, whose Construction Research Laboratory in Miami pioneered the science of testing high-rises to withstand forces of nature. ``The whole concept of the glass construction has to be rethought.''
Most of the towers that sustained heavy damage were erected under old, pre-Hurricane Andrew codes that allowed relatively weak glass. Even some newer buildings that were hit hard -- such as the Espírito Santo Plaza and the Four Seasons tower -- were approved before high-rise glass standards were beefed up again in 2000.
Some experts, including building officials, say there may be less reason to worry about buildings approved under those newer standards; most use stronger laminated glass, and they escaped serious damage.
Valerie Bohlander, director of Fort Lauderdale's building department, called the explanation a ``no-brainer.''
''The older buildings, the windows blew out,'' she said. ``The newer buildings, they didn't. I don't think there's really a lot of mystery to it.''
WINDOWS IN QUESTION
LATEST VERSIONS NOT
TESTED FOR BIG DEBRIS
Still, some experts caution that even the latest windows may not be all they are cracked up to be, because they have not been tested for the kind of large debris that Wilma propelled along Brickell Avenue and through downtown Fort Lauderdale.
The failure of about two dozen building ''envelopes'' during Wilma resulted in tens of millions of dollars in damage and lost work, putting some offices, hotels and residences out of commission for weeks or more.
City and county inspectors, as well as engineering consultants and glass contractors and makers who examined the damage, said it may be months before they can fully explain what happened. They say a combination of factors, including a ''wind-tunnel'' effect that may have exacerbated Wilma's power, likely played some role in the window loss.
But experts have largely ruled out tornadoes, excessive building flexing or widespread failures of window framing or sealants. No one found twisted metal frames or windows sucked out whole.
Most windows simply shattered in their frames -- compelling evidence that debris played a leading role.
Something as small as gravel, layered on rooftops to protect waterproofing material, may have caused the first cracks, experts say. Gravel did extensive damage during Hurricane Alicia in Houston in 1983 and blew out windows in Kendall and Homestead during Hurricane Andrew.
It's also the prime suspect in Fort Lauderdale.
Michael Shiff, an architect and developer who designed the annex to the Broward County courthouse, said one of his architects walked around two battered towers on Broward Boulevard and returned with a bag full of roofing gravel. Shiff found the same debris near the courthouse and school district building.
Although it's no longer widely used, gravel is still allowed under most building codes, and Shiff echoed others in calling for the state to ban it in hurricane zones.
Along Brickell, what first cracked glass is uncertain. But the problem quickly became far weightier than flying pebbles.
Sakhnovsky, whose lab originally tested the glass systems on both the Four Seasons and Espírito Santo as well as international landmarks like the Twin Towers in New York, found the telltale clues everywhere -- in shards he picked up between the two buildings.
He weighed a few and found them 15 to 20 times heavier than the gravel his lab had fired at glass from both buildings. The chunks were also far heavier than steel ball bearings now used under the upgraded 2000 code.
Those first shards were likely swept up as Wilma's wind whirled around the buildings, Sakhnovsky and others agree, triggering a cascade of damage that scored and shattered adjacent windows and shotgunned into surrounding buildings.
Sakhnovsky theorizes that the Four Seasons, the tallest building in Florida, may have been Building Zero because it sat at the upwind corner of the heaviest cluster of damage.
''Some of the shards weighed over one ounce, quite large,'' he said. ``When they hit the Espírito glass, out it went. Of course, the Espírito then generated some of its own missiles.''
Stucco chunks as big as a fist were found inside the Espírito Santo. Nearby high-rise buildings lost sections of concrete and metal roofs, even roof exhaust fans the size of steel drums. All potentially could have careened into towers at high speed.
William Ross, president of Estoril Inc., which manages the Espírito Plaza, said consultants found damage suggesting that the cluster of buildings may have generated a wind-tunneling effect that accelerated debris flung by Wilma, a storm with estimated gusts topping out at less than 100 mph in both cities.
''They've got a piece of metal up there where the glass is driven into the metal, glass pulverized like powder,'' he said.
1994 BUILDING CODE
'LARGE MISSILE' TEST
ONLY ON LOWER FLOORS
The building code that Broward and Miami-Dade counties adopted in 1994 was designed to protect homes and buildings from flying debris. Only lower-floor doors and windows were required to survive the well-known ''large missile'' test -- a nine-pound, two-by-four piece of lumber hurled at 34 mph.
Above 30 feet, the assumption was that only ''small missiles,'' nothing much larger or heavier than roof gravel, would be flying around. So South Florida's original small-missile test -- the one applied to the Four Seasons and Espírito Santo -- specified using small rocks in tests, fired at 55 mph, followed by a cycle of wind-pressure tests.
But even before the small-missile code was adopted, some engineers were questioning whether it was tough enough -- chiefly because there was no standard for how hard the rocks should be.
As a result, ''soft rock'' that pulverized on impact was sometimes used, undermining some test results. By June 2000, Miami-Dade and Broward joined other places in adopting a stiffer and more uniform test: ball bearings fired at 88 mph.
In the interim, at least 20 high-rises were approved in the city of Miami alone under a small-missile code that many engineers considered too weak.
As a result, those buildings were generally permitted to use systems with double panes of tempered and heat-strengthened glass -- but not the laminated glass now generally installed under the 2000 code. Laminated glass, which consists of a puncture-resistant plastic sheet sandwiched between two panes, is stronger and less likely to shed shards should the outer pane crack.
`A WINDOW OF TIME'
GLASS WAS TEMPERED,
BUT NOT YET LAMINATED
''You have a window of time where a lot of tempered glass went in when it probably should not have,'' said Joseph Minor, a consulting engineer who helped Miami-Dade fashion the first post-Andrew missile code.
Jaime Gascon, acting chief for product control for Miami-Dade's Building Code Compliance Office, said the county moved as quickly as it could to add the tougher impact test. Code changes can take a year or more because they are scrutinized closely and challenged by builders and suppliers.
Ross, president of the Espírito Santo's management company, said developers went with what was considered state of the art at they applied for a permit. The building's 18 types of windows cost about $25 million and passed the gravel test.
Ross believes that the Espírito Santo's unlaminated windows -- two panes of glass separated by only a pocket of air -- did perform better than those in older adjacent buildings. Although the Espírito tower lost hundreds of outer panes, only about 60 windows lost the inner one as well. At the Four Seasons, where numerous outer panes were damaged, there were only about a dozen blow-throughs, managers said.
Still, Ross said he was surprised and disappointed after Wilma.
''We expected some to break, but we did not expect to come in and see the building looking like a patchwork,'' he said.
Buildings that used laminated glass typically escaped such serious damage.
Nelson Navarro, South Florida representative for DuPont, which makes the plastic laminate used by many window manufacturers, pointed to Fort Lauderdale's new and unscathed Summit Las Olas condo, fitted with laminated windows that met the post-2000 codes.
Next door, dozens of nonlaminated windows shattered in the older Broward Financial Center. A similar contrast between new and older structures is visible along Brickell Avenue.
''It's a big change in the performance of these buildings versus the ones that didn't have laminated glass,'' Navarro said. ``We can see the difference. Even a Category 1 storm, close to 2, I would say it was a good test.''
Still, a number of engineers, building experts, hurricane forecasters and even some laminated-glass makers believe that the rain of debris unleashed aloft by Wilma shows that the testing standard for high-rise glass needs a harder look.
For example, even laminated windows can still drop outer shards, creating a hazard to surrounding structures. Laminating outer panes, a more expensive alternative already used in airports for noise reduction, may be a potential solution.
Another concern centers on the dramatically stronger wind speeds experienced at the top of skyscrapers. The current code steps up wind-pressure tests for higher floors, but not the speed of the metal balls fired at the glass. That standard is designed with roughly Category 3 winds in mind -- but at ground level.
In the last decade, scientists have discovered that winds at higher altitudes are often far more powerful, said Stanley Goldenberg, a hurricane researcher with the National Oceanic and Atmospheric Administration in Miami.
While ''it's not a rule,'' he said, wind at 30, 40 or 50 stories up can often be one category, even as much as two categories, stronger.
''This is what we've been concerned with for years,'' he said.
``With this tremendous increase in hurricane activity, and this tremendous increase in building high-rises, it's only a matter of time before some major structures start getting hit with major storms.
``The ramifications are scary.''