Article From Coast Crane Comapny
Written By: Nancy Bruton
We often take for granted how these sometimes monumentally-sized machines shape the world around us. From a distance, or from the ground, it can be pretty hard to imagine how the crane accomplishes what it does without tipping over. It’s difficult to even fathom how something so tall is constructed without something equally tall next to it to facilitate its construction.
How are these cranes constructed? And how exactly do cranes and their operators lift concrete, steel beams and other building materials from the ground up to what can sometimes be over hundred-plus stories above the earth?
Very carefully is one answer.
But there’s a lot more to it than just an abundance of caution – there are some jaw-dropping physics and machinery involved.
THE PARTS OF A TOWER CRANE
There are several important parts that make tower crane operations possible.
In order to even construct a tower crane, a large, exceedingly heavy concrete pad has to be laid where the base will be. The actual base of the crane is then secured onto this concrete support pad with massive bolts.
The concrete pad is a very important component for tower crane construction and stability. The pad has to be poured weeks in advance. For large cranes the pad may be approximately 30 feet by 30 feet and four feet deep, weighing approximately 400,000 pounds.
The large tower that reaches way up into the air, sometimes as much as 265 plus feet or more, is referred to in the industry as the mast, much like a ship’s mast.
Atop the mast is a slewing unit, which is the mechanical component that enables the crane to rotate.
The long arm that extends out from the crane is referred to among crane professionals as the jib. This also has its roots in nautical terminology, as the jib is the sail that connects from the front of a ship, or bow, to the mainmast at the top.
The shorter arm, which extends out the opposite way of the jib, is known as the machinery arm. It contains the massive concrete counterweights that allow for balance when extending out heavy loads along the jib.
And, of course, on top of all that, sits the all-important operator’s cab, where the crane professional choreographs the delicate dance of moving construction materials high into the air.
WHAT CAN TOWER CRANES DO?
Generally, when anchored to the ground, a tower crane can’t be much more than 265 feet tall. They can be much taller if they are secured to a building as the building rises. The maximum jib reach is approximately 230 feet, and the maximum weight it can lift is 19.8 tons, or 18 metric tons. Because of the laws of physics, cranes can’t extend a maximum weight load to the very end of a 250-foot jib.
Cranes come equipped with “limit switches” which prevent the crane from being overloaded. One switch monitors the load to ensure it doesn’t exceed the 18-metric ton limit. A second switch makes sure the load, depending on its weight, doesn’t extend to an unsafe distance along the jib. The closer the weight gets to the 18-metric ton maximum the shorter the distance it is able to extend along the jib.
CONSTRUCTION OF THE CRANE
It can take approximately 10 to 12 semi-trucks to haul in all the components for a tower crane. Mobile cranes are used to assemble the jib first, including the slewing unit and counterweights on the machinery arm.
Growing the crane from the base up is a complicated procedure which uses a “climber.” In essence, the climber lifts the topmost part of the crane with the jib, slewing unit and machinery arm enough for a section of the mast’s latticework to slide in, which is done with the jib by the tower crane itself. Once the lattice is in place it is secured and the process with the climber is repeated. This process can take more than a week depending on the crane’s height.