Construction cranes in hurricane's path cause unique concerns for Miami

As Hurricane Irma barrels closer to a likely landfall in South Florida, officials are bracing for the potential of unique structural damage in downtown Miami where two dozen large construction cranes are currently helping to build high-rise buildings.

Terri Norton, associate professor of construction engineering at the University of Nebraska-Lincoln, whose research includes disaster debris management and structural dynamics and control, said the dangers posed by these construction sites is unique.

“It’s not something you commonly see in a natural disaster,” Norton said. “Not only do you have to deal with the storm surge, but also the wind and the vulnerability of the structures because of the construction cranes.”

Most of the cranes are positioned either atop or alongside buildings. Their long arms are designed to withstand hurricane-force winds of up to 145 miles per hour and spin like the rotors of a helicopter.

As of early Friday, Sept. 8, Irma had been downgraded to a Category 4 hurricane moving westward along the north coast of Cuba with measured wind speeds of slightly more than 150 miles per hour.

Forecast models used by the National Oceanographic and Atmospheric Administration (NOAA) late Friday morning projected Irma to begin a northward track Saturday night (Sept. 9)with a landfall in South Florida by mid-morning Sunday (Sept. 10). Those models still project Irma’s winds to range from 130 to 155 miles per hour.

Even if the winds from the hurricane don’t cause significant damage, there is still the potential of tornadoes being spun off from the storm.

For Norton, whose research has included studying the debris zones following the earthquake and tsunami that hit Japan in 2011, the biggest danger the cranes pose would be the 30,000-pound counterbalances falling and damaging adjacent buildings.

Norton said there is also the potential threat of the storm surge turning a collapsed crane or its components into wind-borne or water-borne debris that will cause it to behave like a missile to surrounding structures. Similar incidents happened both during both the tsunami in Japan and Hurricane Andrew, a less massive but stronger (Category 5) storm that devastated South Florida in 1992.

“For the winds to be higher than the rated wind speed for the cranes, in addition to them being elevated, if they were to fall they would not only cause direct damage to the buildings that they’re on, but significant damage to other structures and whatever they will land on,” Norton said.

Taking down the giant cranes before Irma hits would seem to be a logical solution to the problem, but construction experts have said it can take up to two weeks to safely disassemble and remove a crane.

Norton also said there is another potential concern – structural failure caused by the combination of Category 4 wind speeds with the load of a heavy crane atop a building.

“Because it has a super-mass on the top, that also affects how the building itself will behave as a structural system,” Norton said. “When we think of vibrations and you have a larger load on the top, that becomes detrimental in terms of how the building itself performs.”

Norton has been keeping a watchful eye on the recent spate of natural disasters that have struck the Caribbean and North America – not only Irma, but Hurricane Harvey hitting the east coast of Texas two weeks ago, the 8.1 earthquake in Mexico on Friday and Hurricane Jose strengthening to a Category 4 and hitting some of the islands that Irma struck early this week.

“I’ve been following them, but I am especially interested in Irma because I have some cousins and friends who live in Miami and my parents live in Tampa,” Norton said. “My parents are just outside the evacuation zone and they say they’re going to stay. They have everything they need from fuel to water.

“When this is all done, I may be down there doing work and checking in on my family and friends.”