MIAMI – As Hurricane Irma threatens to pound Miami with winds of mind-boggling power, a heavyweight hazard looms over the city's skyline: two dozen enormous construction cranes.
And because those cranes weren't designed to withstand a storm of Irma's ferocity, city officials are telling people who live in the shadows of the giant lifting devices to leave.
Construction sites across Irma's potential path in Florida are being locked down to remove or secure building materials, tools and debris that could become flying missiles in hurricane winds.
The horizontal arms of the tall tower cranes, however, will remain loose despite the potential danger of collapse. According to city officials, they can't be tied down or moved.
Many are questioning why the cranes can't be moved, Miami officials said Tuesday in a tweet. "The answer — it's a slow process that can take about TWO weeks and there is NOT enough time," they said. "Consider that the counterbalances on tower cranes weigh about 20,000 to 30,000 pounds."
Tower cranes can rise hundreds of feet into the air on steel frameworks, and are used to lift steel, concrete, heavy construction equipment and other building materials.
They are designed to withstand winds up to 145 mph (233 kph), but not a Category 5 hurricane, when winds top 157 mph (252 kph), city officials said.
The deputy director of Miami's building department, Maurice Pons, advised anyone living in a building next to a construction crane to leave.
"We're telling people that if you live by a construction site you should evacuate," said Alyce Robertson, executive director of the Miami Downtown Development Authority. "The winds are so strong that it's not known what will happen."
If high-rise dwellers choose to stay, officials said the safest place in a high-rise building to ride out a hurricane is an interior, concrete-enclosed stairwell.
Dan Whiteman, vice chairman Coastal Construction, said he has 12 cranes in the Miami area. The tower cranes with a boom on top are designed to spin like weather vanes, so they should be stable if Irma strikes, Whiteman said.
"If we get a direct hit, the widespread damage is not going to be done by a crane failure," Whiteman said. "It will be done by wind-blown debris."
The major exception, Whiteman said, is if a tornado forms, which "virtually nothing" would be able to withstand.
Parts of the Miami area are under mandatory orders to evacuate. Miami-Dade and Broward counties issued those orders starting Thursday for barrier islands and low-lying mainland areas in the metro area of 6 million people, where forecasters predict the hurricane with winds of 180 mph (290 kph) could strike by early Sunday.
The cranes are evidence of a real estate boom that has doubled the greater downtown Miami population to 88,540 since 2000. The Miami housing market has cooled in recent years, but new luxury condo towers are still under construction as the downtown and Brickell neighborhoods have emerged as hot spots, particularly for younger people who have never dealt with a major hurricane.
Florida's building codes require structures in the Miami area to be able to withstand extreme hurricane wind speeds, but not all building elements perform the same way under storm stresses. In 2005, Hurricane Wilma's Category 2 winds blew out windows in new Miami high-rises that met the updated code.
Andy Castaldi, head of catastrophe perils at the reinsurance company Swiss Re, said the cranes added a terrifying element to the hurricane hazards that Miami potentially faces, including airborne debris and flooding exacerbated by rising sea levels.
"You could have a building built to code, but if the two next to it are knocked over by storm surge into your building, you could suffer loss as well," Castaldi said.
Boak reported from Washington, D.C.