A crane collapsed on Manhattan's Upper East Side Friday morning, damaging a high-rise apartment building and killing its operator and another construction worker. The collapse follows fatal crane accidents earlier this spring in New York City, Miami, and Annapolis, Md. Why do cranes fall over?
Because of oversize loads or mishaps during assembly. Between 1997 and 2006, there was an average of about 82 crane-related fatalities a year, according to the most recent data (PDF) from the Bureau of Labor Statistics. But most of these had nothing to do with a crane tipping over: One analysis (PDF) determined that roughly 60 percent of all these deaths occur when someone is either struck by a load or electrocuted. The cranes that do fall over tend to be of the smaller, mobile variety—as opposed to gigantic, fixed-tower cranes like those involved in both New York City accidents. In general, mobile cranes tend to fall over because of overly heavy loads, while tower cranes usually collapse in the course of being assembled, taken apart, or extended.
The deadly collapse of a tower crane in March, for example, occurred as workers in midtown Manhattan were preparing to "jump" the crane—i.e., to attach additional pieces to make it taller. Investigators believe that a steel collar used to connect the crane's tower to the neighboring apartments broke through the nylon slings that held it during assembly and slipped down into a pair of supports farther below. In the Miami accident, too, a 20-foot-long piece fell off during a bungled crane jump. (As of this writing, the cause of Friday's collapse has not been determined.)
Mobile cranes, on the other hand, are more likely to tip while they are being operated—particularly if they are carrying a load that is too big. Crane operators use a load chart specifying the maximum weight that a given crane can hold, given the length of the boom and the angle at which it is extended; accidents often occur when an operator either doesn't follow the load chart or is given an erroneous reading of the load's weight. According to one estimate, the rate of crane "upsets" is about one in every 10,000 hours of use.
In some cases, a crane can't handle a heavy load because its outriggers—external supports that anchor it to the ground—are faulty or aren't secured to firm terrain. Wind can also complicate matters: An enormous crane known as "Big Blue" fell in 1999 during the construction of Milwaukee's Miller Park as it attempted to raise a heavy roof panel in gusts of more than 30 mph.
Got a question about today's news? Ask the Explainer.
Explainer thanks James Beavers of the University of Tennessee, Graham Brent of the National Commission for the Certification of Crane Operators, Michael McCann of CPWR-The Center for Construction Research and Training, and Bernard Ross of Exponent.