In the seven years since Hurricane Wilma swept through South Florida,unexpectedly shattering thousands of glass windows in the gleaming high-rises in Miami and Fort Lauderdale,a new generation of towers have reshaped the skyline.
The new towers,built to tougher standards than the older ones that bore the brunt of the damage,have yet to face a real test of the turbulent wind,water and flying debris of a major storm.
Many residents of the trendy urban oases never have experienced a hurricane either.
While South Florida has dodged major damage this season,Sandy's Caribbean pass is a reminder the region's odds-defying lucky streak could end abruptly.
"We have been lulled to sleep as a community.When we do get the next Cat[egory]5 or anything above a Cat 3 or 4,that will be the first test,''said Dan Whiteman,president of Miami-based Coastal Construction Group,a large builder of condominiums,hotels and office towers in South Florida.He predicts the new buildings–typically designed with wind-tunnel testing–should perform well in all but the most severe conditions."I don't think there are going to be any significant problems with the buildings built since 2005.''
Which is not to say the residents of the new towers who opt to stay during a big storm would get off easy.
"If you're on the 49th floor of a 52-story building,the wind could be whipping pretty severely up there,''said Assistant Fire Chief Joseph Zahralban,emergency manager for the City of Miami,who urges residents to respect evacuation orders.
As a rule of thumb,those who somehow end up in a tower during a strong windstorm should take shelter in the fire-escape stairwell,he said.Staying below the 10th floor is another standard guideline.
Tamara Oyarzabal and her husband,however,found themselves caught in their 30th-floor condo at Brickell Bay Club in Miami during Hurricane Katrina.Despite storm shutters,the pounding rain seeped through flimsy sliding-glass doors flooding the apartment.The electric went out.Ankle-deep in water amid howling winds,they bailed buckets of water into their bathtub."The windows bent to a degree I would not have thought glass could bend,''said Oyarzabal,who responded to an inquiry for The Miami Herald's Public Insight Network."We will never again stay in our high-rise if an actual hurricane comes our way.''
Rob Vango,who holed up on the 10th floor of a Hollywood Beach condo during Hurricane Wilma,said the older building"swayed like you wouldn't believe''even as the ocean seemed eerily flat.During a gust of wind,the building moved so much,Vango said he was knocked off the toilet in the pitch black of a power outage.Water was flying out of the fish tank,Vango recalled in Public Insight Network comments.
Wind speeds typically are higher at the higher floor levels,and buildings may sway to and fro.As with the fish tank,water dutifully collected in high-rise bathtubs per emergency checklists will slosh around as the building moves.
"There is nothing wrong with the building moving.The only trouble is if people in the building decide to ride it out.They're going to feel the building swaying backward and forward a foot or two.That's to be expected,''said Peter A.Irwin,who is Wall of Wind professor of practice in the civil and environmental engineering department at Florida International University."People can feel the motion,and if you're in a very flexible building,it may not take much to make it move.People who bought in one of those buildings may not be very happy.''
With the benefit of wind-tunnel testing,a high-rise design that"crosses the comfort threshold''for people inside by swaying too much can be revamped to change the aerodynamics,especially at the corners,said Arindam Gan Chowdhury,director of the Laboratory for Wind Engineering Research at the International Hurricane Research Center at FIU.
Another fix is to add dampers that counterbalance a building's vibration.The Taipei 101 skyscraper in Taiwan,where both typhoons and earthquakes are a threat,has a 730-metric-ton,gold-colored mass tuned damper,suspended in the center between upper floors to cancel out the building's vibrations.
"The natural frequency of the damper is tuned with the natural frequency of the building,''Chowdhury said.
During a hurricane,conditions at the street level around skyscrapers can get pretty dicey,too.Wind caught by a tall building tends to run down the face and find its way around corners in a downwashing action.As it goes around corners,there is high suction,pulling windows outward.Wind channeled between two buildings intensifies,too,augmenting the force and wreaking havoc along the concrete canyons at the street level.Even a strong building's fate depends in good part on how neighboring structures are holding up.
"One that has gone through wind-tunnel testing is not likely to be hurt,''said FIU's Irwin."but if it's hit by flying debris off another building,that's different.''
Other key challenges for high-rises:Many are perched on the oceanfront,vulnerable to storm surge and flooding on lower floors.Back-up generators intended to keep emergency elevators and power running are standard.But not everyone is sure such generators will be reliable if a storm surge brings flooding.
"Generators are typically on the first floor.Fuel tanks are typically on the first floor,''said Assistant Fire Chief Zahralban of Miami."Many high-rises are built in a way that all the command and control[facilities]are on the on the first floor.All those issues cause concern.''
Julie Magaldi,director of high-rise operations assessments for Hollywood,Fla.-based Continental Group,which manages more than 400 high-rise condominiums in Miami-Dade,Broward and Palm Beach counties,has witnessed numerous elevators in seaside high-rises knocked out of service by flooding.
"On a pretty regular basis,the elevator pits flood.They're below the water table,''said Magaldi."It could take several days before elevator service can be resumed.''
And tower parking garages tucked below ground level are on the wrong side of gravity when a storm surge roars inland,she added.
Many of the condo towers along Collins Avenue have underground garages where walls back up against the ocean or the bay."I love my beach,but I sometimes say:'Building concrete structures on the sand.What were they thinking?'''said Magaldi,who often makes presentations on best practices to condominium associations.
A crucial factor for any building in a windstorm is protecting the"envelop''that seals the structure from the elements.
In Wilma,most high-rises that were hit hard were built under older building codes that allowed relatively weak glass.The newer buildings that lost a lot of glass in 2005,like the Espírito Santo Plaza and the Four Seasons tower on Brickell Avenue,were approved before high-rise glass standards were strengthened further in 2000.
"After Wilma,everyone looked and wondered why all that glass fell over Biscayne[Boulevard]and Collins Avenue,''said Coastal president Whiteman."They found out a lot of roofs have gravel and a lot of roof debris became flying missiles.''
When the older buildings were repaired after Wilma,some owners fought building officials and won the right to rebuild to the old standards instead of the costlier new ones,said Charles Danger,Miami-Dade county's building chief.Others opted to go with the new,tougher ones that took effect after Hurricane Andrew and were upgraded several times since.
"Basically some of these buildings were put back the same way and they are going to fail the same way.Some owners didn't do anything different.They followed the old code because they were allowed to,''said Danger."Some used the old material,but attached it better.Others put it according to the new code.If we ever get hit,we'll see who was smart and who wasn't.
"We're better off now than we were before,but I'm still concerned about buildings that were revamped not to meet the new code,''Danger said.
At least one Miami high-rise,1450 Brickell,completed in 2010,was built to even tougher standards than the latest code requires.
Impact-resistant glass that is strong enough to endure a 9-pound,six-foot-long,2x4 piece of lumber fired end on at 34 miles an hour is required for windows up to 30-feet above ground.The idea is that most flying debris originates from ground level.
Above that,builders can use small-missile protection–impact glass that can survive test strikes by steel ball bearings traveling 50 miles per hour followed by wind tests.
The building at 1450 Brickell uses the stronger glass bottom to top.
"We have the whole building cladded with what is required to the first 30 feet,''said Alan Ojeda,president and CEO of Rilea Group,the developer of tower,who also maintains his office there.
Another extra:In addition to the required back-up generator to provide electricity for elevators and emergency lighting,the building has a second generator to keep air conditioning and full power if the regular service is cut off.
"We have a second generator with 80 tons of diesel that could run full time for about a week,''said Ojeda.
"The stronger glass prevents an implosion of the glass,''he said."We have tenants who are in 24-hour mode,dealing with Hong Kong,Europe,the West Coast.They cannot afford to be without power.''
