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Will Miami’s Skyscrapers Withstand Irma?

As Irma continues to drive its way through the Atlantic Ocean, bringing with it 150+ mph winds, a lot of comparisons are being drawn to Hurricane Andrew, which hit South Florida 25 years ago. That makes sense – it was the last Category 5 hurricane to hit the United States (Irma is currently forecast to be at least a Category 4 with at least 150 mph winds at landfall), and Andrew made U.S. landfall in South Florida, the way Irma is expected to. Andrew was so bad that its name was retired, and it helped inspire evacuation and emergency management protocols around the country. It’s a living and painful memory for many in South Florida.

But even if Irma turns out to be like Andrew, South Florida is not the place it was 25 years ago. The coastline has seen rapid development since then, especially after the 2008 recession. While that development includes suburban communities springing up from the destruction left by Andrew, encroaching on what was then farmland, there’s also a near-wall of high-rise buildings that dot the area’s coastline. And while construction completed since Andrew arrived in 1992 must adhere to more strict building codes, there’s a lot that’s unknown about how tall buildings, and the materials from which they are constructed, will respond to the high winds currently expected from Irma.

The skyscrapers’ most vulnerable points may be their windows. While glass can usually withstand the intense pressure within a storm, high winds can turn even pebbles into window-shattering missiles. New requirements put in place after Andrew required tall buildings to use special, impact-resistant glass in the first 30 feet of construction, where debris is most likely to fly. Engineers test the strength of that glass in laboratory experiments by firing projectiles at high speeds into it to see whether it will shatter. But scientists don’t know the exact size, speed and force of projectiles flying in a hurricane. And despite the use of stronger materials, Hurricane Wilma (a Category 3 storm when it hit the U.S. in 2005) destroyed windows throughout Miami’s downtown high-rises in upper floors. Wilma spurred builders to cover more of their towers in impact-resistant glass, but Wilma only generated peak wind speeds of around 100 mph in Palm Beach County — much less than Irma is expected to bring to South Florida. (New forecasts show Irma drifting west, which may mean weaker winds in Miami. There’s still time for the forecast to shift again before it makes landfall, though.)

Even with lower wind speeds, Andrew and Wilma did much of their damage by hurling gravel from roofs into the windows of nearby buildings. “We all know that you can have the best glass in the world, but you start throwing rocks at it and it changes things,” said Timothy Marshall, an engineer and meteorologist with the firm Haag Engineering who specializes in wind. That interaction between buildings doesn’t get enough attention, Marshall said. Having many high-rises in close proximity, as happens in cities, can create wind tunnels — low-pressure areas where wind travels particularly fast, increasing the load on nearby buildings.

There were high-rises in Miami when Andrew hit; about 51 percent of Miami’s 10+ story high-rises were built prior to that year, according to crowdsourced data from SkyscraperPage.com.1 But we don’t know how many installed impact-resistant windows. And after Wilma, some buildings successfully petitioned to rebuild up to the codes required when they were first constructed, not stricter codes that had been established in 2000. Design standards shifted again in 2012, and SkyscraperPage.com suggests that about 7 percent of Miami high-rises went up since then. The end result of the shifting building codes is a string of high-rises along South Florida’s waterfront that may or may not be up for Irma’s fierce winds.

It’s not just that it’s unclear how these buildings will react, but also that the wind force is even higher on upper stories than it is lower down. A 2003 paper by James L. Franklin and colleagues from the National Oceanic and Atmospheric Administration found that above three stories, wind moves faster than it does below. At 10 stories, it’s 108 percent of the surface wind speed. At 25 stories, it’s 117 percent. That means that 150 mph winds at the surface could equal 175 mph winds on the upper (or middle) floors of a skyscraper.

Marshall himself is headed to Miami to see what impact Irma has on the city. “Testing in a lab is one thing, but testing with Mother Nature could be a little different. Not all debris is two-by-fours,” he said.

There’s also the question of the high-rises that have yet to be completed. The City of Miami warned residents of towers in the downtown area that the 20 to 25 giant construction cranes located in the area are designed to withstand up to 145 mph winds, approximately a Category 4 hurricane. If Irma hits as a Category 4 (with winds up to 156 mph), they could collapse. There are many more cranes looming in other parts of the Miami-Broward-Palm Beach metropolitan area, as well as other construction materials that require securing.

And of course, with the majority of the high-rises being built along the waterfront, they are also likely to bear the worst brunt of storm surges that are expected to top nine feet.

That flooding can shut down the elevators and electricity of skyscrapers, since generators and fuel stores are often built on the first floor. The combination of storm surge and high waves is the deadliest aspect of most storms. Even in Andrew, known for its high winds, the storm surge was more than 16 feet in some places.

Many of the towers are in mandatory evacuation zones, so most residents should have left by the time the storm hits. For those who stay behind, officials recommend descending to lower floors and riding out the storm in interior stairwells.

Footnotes

  1. We don’t know of any official national database of high-rises or skyscrapers. As with any crowdsourced site, the data may be incomplete. A cross-reference of one Miami neighborhood suggests it is fairly comprehensive.

Rob Arthur is FiveThirtyEight’s baseball columnist and also writes about crime.

Anna Maria Barry-Jester reports on public health, food and culture for FiveThirtyEight.

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