Working At Height & Wind Speed

Skyscrapers like One World Trade Center, now the tallest building in the United States, are not the place to be during hurricanes.

Sandy spared New York’s skyscrapers, but high-rises carry high risk during hurricanes: On Oct. 28, 2012, the day before Superstorm Sandy’s center came ashore over southern New Jersey, the National Weather Service’s New York City office cautioned that one of its computer models was projecting a “possibility of skyscraper window damage” and warned residents to stay away from windows if they lived on the 10th floor or higher.

Fortunately Sandy’s strongest winds had weakened to 70 mph when it came ashore near Brigantine, N.J., the next day. The strongest wind recorded at the NWS Central Park station that day was 38 mph with gusts to 62 mph. In Atlantic City, N.J., however, where the highest reported surface wind was 51 mph with gusts to 64 mph, Sandy did cause minor damage to high-rise building roofs and higher floors. “The problem I saw in New York, especially lower Manhattan, was when the power went out and back-up generators were in flooded basements,” says Tim P. Marshall, a meteorologist and engineer who’s been studying building wind damage for Haag Engineering in Dallas since 1983. “If you’re on the 40th floor, that’s a long way to walk down the stairs.”

Even if you think trudging up and down several stories in a high-rise without elevators is a good way to get in shape, you should evacuate a high-rise condo, apartment, hotel or time share that’s threatened by a hurricane because it’s a potentially deadly place to be. At the very least you should take the following advice from the New York City Office of Emergency Management (and other emergency managers):

On or below the 10th floor: Close and lock all windows and securely cover them to reduce damage and injury caused by flying debris. Above the 10th floor: Be prepared to take shelter on or below the 10th floor.  If you have a balcony or rooftop, remove all items that cannot be securely tied down.

Mean hurricane eyewall wind variation with elevation. To calculate the wind speed at a particular height, put a decimal point after the first ’1′ in the percentage and multiply the surface wind by that number. For example, if the wind at the surface is 80 mph, for the speed at the 20th floor multiply 80 by 1.15.

In the article they say: “The top of a 25-story coastal high-rise in the hurricane eyewall will experience a mean wind that is about 17% higher (or one Saffir–Simpson hurricane-scale category higher) than the surface or advisory value. For this reason, residents who must take refuge in coastal high-rises should generally do so at the lowest levels necessary to avoid storm surge.”

Their study was based in part on data from GPS dropwindsondes that hurricane hunter airplanes had been using since 1998. These are small packages of instruments that measure and radio back atmospheric pressure, temperature, humidity, and wind speed and direction as they fall to the ocean under small parachutes.

Marshall says that buildings “up north, seem to be better built” than many he’s examined across the South. Windows are only one of the problems. Once debris breaks a window the flying glass can break other windows, as he and his colleagues found after Hurricane Alicia hit Galveston and Houston on Aug. 18, 1982 (less than three weeks after he started with Haag Engineering) showering downtown Houston with flying glass.

“The problem is with EIFS clad (Exterior Insulation Finishing System) buildings in the South. I have seen this type of façade get into trouble many times from many hurricanes. There also have been problems with keeping windows intact. Part of the problem is flying debris and the cascade or domino effect when one window breaks and so on.” All in all, an upper floor in a high-rise is no place to be in a hurricane.