Three days after the 25-story boom of a crane toppled onto the Tappan Zee Bridge, creating havoc and a huge traffic jam but killing no one, three separate investigations were proceeding on Friday into what caused the accident.
The sequence of events surrounding the collapse remain uncertain. Officials have interviewed the operator of the crane and have examined the equipment’s black box, which records data like the angle of the boom and the distribution of weight along the machinery. The agencies involved in the inquiries include the New York State Police, the State Labor Department and the federal Occupational Safety and Health Administration.
Jeff J. Loughlin, the business manager of International Union of Operating Engineers Local 137, said he had spoken to the crane operator on duty that day. Mr. Loughlin said the crane operator, a union member who has not been identified, had told him “he knows what caused the problem.”
“It’s absolutely not an error by him,” Mr. Loughlin said, adding that he had promised investigators he would not publicly reveal what the operator said. But Mr. Loughlin said he had his own theories, which involve the kind of crane being used.
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At about noon on Tuesday, with barely a breeze in the air, the operator of the 25-story tall crane and the operator of a remote-controlled piece of machinery known as a vibrating hammer were performing in tandem, driving a tubelike pile into the Hudson River’s muddy bottom. A 60-ton hammer dangles from a hook on the crane’s boom, with its jaws gripping the pile and pounding it into the riverbed.
The task had become routine as the four-year job of building a replacement for the Tappan Zee Bridge reached its halfway point. Roughly 1,000 piles — steel tubes up to six feet in diameter and up to 300 feet long — have been planted to form the cores of the concrete piers that will hold up the new twin-span replacement bridge. The pile in question was being installed just off the Rockland County shore of the river in a narrow stretch of water between the existing bridge and the southernmost of the new spans.
Something went wrong. The crane operator, positioned in a cab mounted on the deck of the new bridge, felt the 256-foot boom suddenly descend and then buckle, Mr. Loughlin said. It smashed into the existing bridge, blocking all seven lanes of traffic, forcing drivers to slam on their brakes, piercing a hole in the road deck of the southernmost lane of traffic big enough to expose the river below, and hacking off part of the waist-high guard rail.
One theory, Mr. Loughlin said, was that the pile had struck a very soft spot of river mud and had begun to sink rapidly, jolting the 60-ton hammer into a rapid drop and putting a particular strain on the boom, which buckled.
Mr. Loughlin, emphasizing that he could only speculate about what had happened until the investigations were complete, said that an operator on an old-fashioned crane with a foot-operated brake might have taken his foot off the brake and allowed the load of the hammer to go into a free fall, relieving the strain before the buckling occurred.
But the state-of-the-art Manitowoc MLC 300 lattice-boom crawler crane, like most newer cranes, has a hydraulic braking system that lowers the boom at a steady rate of about 800 feet per minute, said Mr. Loughlin, who has more than 40 years of experience operating cranes.
“If you’re in trouble, and you have to get a load down quickly you can’t do it with this model,” Mr. Loughlin said.
Nevertheless, he said, sometimes, “if the boom is going to buckle, it’s going to buckle no matter what.”
The accident caused gridlock on the bridge, the New York State Thruway and on feeder roads. Two drivers were hurt trying to avoid the crane, according to Mr. Loughlin. One worker injured his knee.
By 8 p.m. Tuesday, workers had cut up and removed pieces of the boom, allowing six of the bridge’s seven lanes to reopen. The seventh lane was still shut on Friday afternoon, slowing traffic. An average of 138,000 vehicles use the bridge each day.
In the hours after the crane collapsed, Gov. Andrew M. Cuomo, a Democrat, said there were 28 cranes working on the bridge the day of the accident, including a super-crane that can lift the equivalent of 12 Statues of Liberty. Many of the cranes are hydraulic, though the Manitowoc 300, which has been involved in the bridge project since April, may be the only one of its kind on the river, Mr. Loughlin said.
Ion Warner, vice president of marketing and investor relations for Manitowoc Cranes in Wisconsin, did not return several calls for comment. Officials of the New York State Thruway Authority, the agency overseeing the $4 billion project, and Tappan Zee Constructors, the consortium of bridge-building companies that was awarded the contract, said they could not discuss the accident’s cause until the investigations were complete. Tiffany Portzer, a spokeswoman for the Labor Department, also declined to comment.
A crane operator must be licensed by the Labor Department. Typically, a license requires three years of experience working in and around cranes, a written test and a practical test. The man who operated the crane on Tuesday had 30 years of experience manipulating cranes, Mr. Loughlin said.