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.