Dangling Crane Was Weather-Vaned, Contractor Says
The Favelle Favco-brand tower crane-whose luffing jib dramatically flipped backwards as Superstorm Sandy blew into Midtown Manhattan-was properly prepared for wind, says the construction manager in charge of the project.
Before the crane was called in to help build the roughly 1,000-ft-tall residential tower, engineers developed procedures for how to dress the crane, including weather-vaning it, prior to storms, according to one official for Lend Lease, the construction manager for the One57 tower.
"It is our understanding that the operator followed the procedures on those drawings, as done many times prior to this event (every thunderstorm, Hurricane Irene, etc.)," says a statement that Mary Costello, Lend Lease spokesperson, e-mailed to ENR on Oct. 31.
"The crane was left in the 'free slewing' position as is evident that it is now in a different position than when the boom folded over," the statement adds. The crane, which is owned and operated by Pinnacle Industries, is secure, city officials say.
Probable Causes?
Wind is a known risk for people operating cranes of any type, including towers. According to Aviad Shapira, a civil engineering professor at Technion in Haifa, Israel, and textbook author, wind is the third-largest risk around tower cranes behind operator proficiency and site-level safety management. Crane operators weather-vane a crane when it is not in use to mitigate this risk.
"Tower cranes are built to sustain winds up to intensities specified by manufacturers and regulations. However, operating in winds is still dangerous," says Shapira in a paper appearing in the November 2012 edition of the Journal of Construction Engineering and Management. "Sudden wind gusts are especially dangerous."
Weather-vaning a tower crane involves releasing the unit's swing brake so the jib can move freely in the wind. Then, as wind pushes against the jib, the jib rotates until it is pointing downwind. The counterweight points upwind, helping to balance the vertical tower structure and provide gust resistance.
Although early speculation pointed to the lack of weather-vaning the crane as a possible cause, tower crane experts now tell ENR that the partial collapse could have been due to unexpected wind forces acting in combination on the building and the rig.
One crane expert and engineer, who asked not to be named, says that wind gusts moving up the side of the building could have created a vortex that lifted and flipped the boom over backwards.
"Rather than having all the wind pushing the way it is supposed to make [the crane] vane, it comes from underneath and pushes on the tip. That would be my first guess." The engineer estimates that the wind at that height was moving at 120 to 130 mph.
So why didn't the crane fall into the streets below? Many cranes contain boom stops that, when contacted, push against a crumple zone in the lattice framework of the boom, says the expert engineer. The effect is like stepping on a dented soda can.
"They create localized buckling, and the boom would just fold over the back of the crane," the expert explains. "That's exactly what happened here." Tower crane jibs typically are set at an angle of no more than 60 for storm preparation, the expert adds, noting that higher angles make it easier for wind to flip a jib.
Although Lend Lease asserts that the crane was properly weather-vaned and the jib angled correctly, Terry McGettigan, a crane operator and inspector in Seattle, says that operators are sometimes complacent in doing so.
"Just because you press a button or pull a lever doesn't mean the brakes are released," he says. "There is a false sense of security."
Best practice dictates that operators release the swing brake, engage the swing gear and then turn off the crane, McGettigan notes.
"The crane should just gently swing," he says.
Next Steps?
With the crane not in danger of falling down, the streets below blocked off and nearby buildings evacuated, the task at hand is to investigate damage to the crane and remove damaged parts from the site so contractors can get back to work on the building.
In the afternoon of Oct. 30, the city gained access to the building and began assessing the damage. Contractors were permitted to enter the scene the following morning. They started by installing safety netting and inspecting the crane wreckage, including the unit's steel tie-ins to the building, says Lend Lease.
"There was enormous torque when this fell over, and the fact that those connectors withstood the pressure just testifies to how well they were put it [sic] and how stable that tower is," adds Mayor Michael Bloomberg in an ">Oct. 31 statement.
Contractors will have several options to remove the damaged rig, experts say. One would be to carry a portable derrick to the roof, which would then be used to build a larger derrick hoisted up from the ground.
The larger derrick would then help dismantle the damaged jib, which weighs between 40,000 and 50,000 pounds, estimates the crane expert. The structure would be dismantled piece by piece, rather than jacked down to a lower level, as is typically done.
Another option would be to use gin poles, or lattice booms tied to guy wires mounted to the roof, to help disassemble the rig and gently lower it to the ground. Either option would be a delicate engineering project of its own.
"They'll have to put their heads to it," says McGettigan. "It's not going to be simple."
According to the city, contractors are more likely to use a third option: erecting another crane next to the damaged one. The replacement crane would be used to dismantle the Favco and then stay on to finish the building.
"That can take weeks, but once the boom is secured we should be able to minimize any disruption on the street below," says Bloomberg, who adds that contractors will soon be "tying the boom to the building so that you can then work on top of it" in an effort to starting "shrinking the zone around the frozen area" in the crane's footprint.
What About Costs?
Once the broken jib is out of the picture, removing the old crane and replacing it with a new one could cost up to $3 million and take up to one month, crane experts tell ENR.
"The labor alone to bring it down and the equipment...will run $250,000 to $500,000," says the crane engineer. "And that's assuming not a lot of problems." The cost to erect a tall tower crane in New York City runs around $1 million, the source adds.
The owner and developer, Extell, or the builder's risk insurance policy would likely pay for the turnaround, the expert source notes.
"Certainly, it's a wind event," the expert says. Lend Lease did not respond to ENR's questions about insurance.
This story was updated on Oct. 31 at 8:46 P.M.
