Tower Cranes: Efficient, Versatile -- but How Safe?
Monday, June 16, 2008
Tower cranes are the ubiquitous one-legged dinosaurs of the urban landscape -- gigantic, powerful, small-headed and not a little scary.
Over the past two decades, they have proliferated in the construction industry. It is possible to stand on many a city street and see more than one at work, their booms rotating on turntables as they reach over rooftops to lift loads hundreds of feet in the air and carry them equally far.
Developed in Europe (and in most cases built there), the cranes are the perfect solution to working in crowded spaces. They are built of modular units and are designed for the site where they will stand and the job they will do. They can grow taller as the structure they are helping to build rises next to -- or sometimes around -- them.
They also represent the application by engineering of the "just in time" strategy that has transformed manufacturing. When the job is done, they are taken apart like an Erector Set, the parts destined for another project in a different place.
The usefulness of tower cranes is not in doubt. Several recent accidents, however, have led some to wonder about their safety.
A tower crane collapsed in New York on March 15, killing seven people. On May 30, the working arm of another crane on a site 45 streets away became detached from the tower, falling and killing two people.
Those accidents, which have led to enhanced inspections in New York and the arrest of the city's chief inspector of the structures for allegedly taking bribes, are not the only recent ones. On March 25, a seven-ton section fell 30 stories while a crane was being extended in Miami. Two people were killed and five injured.
Despite their seeming precariousness -- and the catastrophic nature of the occasional accident -- tower cranes are extremely stable and have good safety records overall. Nevertheless, the people who operate them are skittish about saying even that on the record.
"Tower cranes are very safe. There are all kinds of safety devices written into the crane," said an executive at a Baltimore-Washington area company who insisted on anonymity because his insurer told him not to talk to the media. "The accidents that happen are due to human error, not due to the problems with the tower."
The tall and spindly vertical part of the crane is not where the chief hazard lies, experts say. Keeping it upright is easier than it appears.
"It's pretty elementary; it's just a matter of balancing forces," said Henry Petroski, a professor of civil engineering at Duke University and author of a dozen popular books on mechanics and engineering. "The geometry can get a little complicated, but it's not anything that's beyond a first- or second-year engineering student."
Construction engineers prefer to erect the tower outside the building it is helping to put up. When that is not possible because of the constraints of the lot, the building rises around the tower. The shaft it leaves becomes an atrium or is later filled in with flooring at each level.