"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.
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.