Crane Collapse In N.Y.C.

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Mechanical principles

Crane at skyscraper construction in Barcelona, SpainThere are two major considerations that are taken into account in the design of cranes. The first is that the crane must be able to lift a load of a specified weight and the second is that the crane must remain stable and not topple over when the load is lifted and moved to another location.


Lifting capacity
Cranes illustrate the use of one or more simple machines to create mechanical advantage.

The lever. A balance crane contains a horizontal beam (the lever) pivoted about a point called the fulcrum. The principle of the lever allows a heavy load attached to the shorter end of the beam to be lifted by a smaller force applied in the opposite direction to the longer end of the beam. The ratio of the load's weight to the applied force is equal to the ratio of the lengths of the longer arm and the shorter arm, and is called the mechanical advantage.
The pulley. A jib crane contains a tilted strut (the jib) that supports a fixed pulley block. Cables are wrapped multiple times round the fixed block and round another block attached to the load. When the free end of the cable is pulled by hand or by a winding machine, the pulley system delivers a force to the load that is equal to the applied force multiplied by the number of lengths of cable passing between the two blocks. This number is the mechanical advantage.
The hydraulic cylinder. This can be used directly to lift the load or indirectly to move the jib or beam that carries another lifting device.
Cranes, like all machines, obey the principle of conservation of energy. This means that the energy delivered to the load cannot exceed the energy put into the machine. For example, if a pulley system multiplies the applied force by ten, then the load moves only one tenth as far as the applied force. Since energy is proportional to force multiplied by distance, the output energy is kept roughly equal to the input energy (in practice slightly less, because some energy is lost to friction and other inefficiencies).


Stability of crane
In order for a crane to be stable, the sum of all moments about any point such as the base of the crane must equate to zero. In practice, the magnitude of load that is permitted to be lifted (called the "rated load" in the US) is some value less than the load that will cause the crane to tip.

Under US standards for mobile cranes, the stability-limited rated load for a crawler crane is 75% of the tipping load. The stability-limited rated load for a mobile crane supported on outriggers is 85% of the tipping load.
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For instance, I finally found out how tower cranes get onto the rooftops of skyscrapers.
They jump up there.

Bottom climber

There are many types of tower cranes, and the size, model and their positions on the site are all strategically selected long before the job begins. Freeman's crane, which is stout and strong ("because we're going to be climbing high, in the high wind"), is known as a bottom climber, and it is essentially on the inside of the very building it is helping to construct. It seems to be getting shorter, because the growing building obscures the tower, but soon a hydraulic pump located at the base will jack the entire crane up and place it onto a new base eight floors up. It will "jump" about 14 times in all, until the crane sits atop the Trump Tower's 92nd floor -- about twice as high as the IBM Building.

"Of course, the higher the building goes the more you're working blind," Freeman said. But he's always quick to point out that he's not alone up there. In addition to his oiler, there's constant communication from signalmen on speaker boxes throughout the site. "They tell us `come down slow,' or `trolley in a little bit,' or `swing one, swing two.' One means to the operator's right, two is to the operator's left."

And he has computers, which, like safety harnesses, are a modern convenience he did without in his first decade on the job. "So say I'm pouring concrete, and there's a truck out there, and I can't see it. When I set the bucket the first time, I can look at my computer and note the radius is at 51 feet. The next time I drop I know to set the bucket at 51 feet. That helps. We used to paint marks on the jib -- just walk out there and paint a little mark," he said.
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