Tower cranes are a common fixture at any major construction site. They're pretty hard to miss -- they often rise hundreds of feet into the air, and can reach out just as far. The construction crew uses the tower crane to lift steel , concrete, large tools like acetylene torches and generators, and a wide variety of other building materials.




Tower cranes can rise 150 feet in the air and lift up to 19 tons.

When you look at one of these cranes, what it can do seems nearly impossible: Why doesn't it tip over? How can such a long boom lift so much weigh­t? How is it able to grow taller as the building grows taller? If you have ever wondered about how tower cranes work, then this article is for you. In this article, you'll find out the answers to all of these questions and more!

Parts of a Tower Crane

• The base is bolted to a large concrete pad that supports the crane.
• The base connects to the mast (or tower), which gives the tower crane its height.
• Attached to the top of the mast is the slewing unit -- the gear and motor -- that allows the crane to rotate:

On top of the slewing unit are three parts:

• The long horizontal jib (or working arm), which is the portion of the crane that carries the load. A trolley runs along the jib to move the load in and out from the crane's center:

• The shorter horizontal machinery arm, which contains the crane's motors and electronics as well as the large concrete counter weights:

So these cranes are essentially bolted to the ground to ensure their stability. In the next section, you'll learn how tower cranes "grow."

How Do They Grow?

Tower cranes arrive at the construction site on 10 to 12 tractor-trailer rigs. The crew uses a mobile crane to assemble the jib and the machinery section, and places these horizontal members on a 40-foot (12-m) mast that consists of two mast sections. The mobile crane then adds the counterweights.
The mast rises from this firm foundation. The mast is a large, triangulated lattice structure, typically 10 feet (3.2 meters) square. The triangulated structure gives the mast the strength to remain upright.

1. The crew hangs a weight on the jib to balance the counterweight.
2. The crew detaches the slewing unit from the top of the mast. Large hydraulic rams in the top climber push the slewing unit up 20 feet (6 m).
3. The crane operator uses the crane to lift another 20-foot mast section into the gap opened by the climbing frame. Once bolted in place, the crane is 20 feet taller!

Once the building is finished and it is time for the crane to come down, the process is reversed -- the crane disassembles its own mast and then smaller cranes disassemble the rest.