Metropolitan horizons are dotted with cranes, constant fixtures in downtowns with buildings rising higher and higher. Without construction cranes, building skyscrapers would be impossible, but with cranes, we’re able to build towering structures with relative ease. Cranes are capable of moving extremely heavy objects into place, enabling methods of construction that were unthinkable just a couple hundred years ago.
Although most of us frequently see cranes, many people don’t know exactly how cranes work. How do tower cranes reach such incredible heights? How do they assist in constructing a building? What does the operator do while controlling a crane?
In this article, we’ll answer all of those questions. Although there are many types of cranes, we’ll focus specifically on tower cranes, which are the ones you see most often on large commercial construction sites. These cranes are extremely sophisticated, and they can also be expensive, sometimes costing more than $15,000 a month to rent.
You can jump straight to our infographic, or read more about:
The various parts of a crane
How tower cranes are stabilized and assembled
How tower cranes perform lifts
How an operator controls a crane
Parts of a Crane
To understand a crane’s movement, it’s helpful to have a good grasp of the various parts that make up a construction crane. All of these parts work together to make the incredible lifting power of a crane possible.
When looking at a crane, you can identify all of the parts easily by starting at the base of the crane and working your way up, then running your eyes along the length of the crane from the long end to the short end.
Concrete foundation: A tower crane always sits on a concrete foundation, where anchors are placed to secure the crane to the ground.
Tower or mast: From its base, the crane rises with the tower, also called the mast, which consists of lattice sections stacked on top of one another.
Turntable: At the top of the tower, the crane has a turntable that enables it to rotate 360 degrees.
Operator’s cab: Near the turntable, the operator’s cab gives the crane operator a place to control the crane with an unobstructed view.
Jib: Stretching out forward from the cab is the jib, the long horizontal section of a crane.
Trolley and hook block: Along the jib, a trolley with a hook block runs back and forth, enabling loads to be moved along the length of the crane.
Counter jib and counterweights: Behind the cab is the counter jib, where counterweights are placed to stabilize the crane at rest and during movement.
Main winch and motors: At the back of the counter jib sits the main winch and motors, which enables the long rope to be lowered or raised to hoist heavy loads.
Tower peak or apex: Rising above the cab is the towering peak, also known as the apex, where the pendants extend out to support the jib and counter jib.
Remember that there are many varieties of tower cranes, and some will have parts that are not mentioned here. For example, a luffing jib crane has an arm that can raise up and down, and a flat top crane does not have the apex and pendant system shown in the crane above.
Some tower cranes also have self-climbing hydraulic systems, which makes it possible for them to climb internally as a skyscraper is constructed around them.
Overall, though, the basic parts of tower cranes are fairly similar, and this list will give you a good understanding of the parts that make up the cranes that you see in your everyday life.
How Cranes Are Built?
Before cranes can perform the essential lifting operations that make them valuable on job sites, they must be stabilized and assembled.
A tower crane is always built in three distinct phases:
A concrete foundation is prepared so that the crane will be safely anchored to the ground and stable during lifting.
A mobile crane assembles the tower crane, lifting several segments of the tower into place along with core components.
The tower crane builds the rest of its own structure as it rises to its ultimate height.
The concrete foundation is essential for the safe operation of the crane. Complex calculations take into account all of the forces that will act on the crane as it performs lifts, rotations and trolley movements. When the foundation is prepared, special consideration is taken to ensure that underground utilities are not affected by the placement of the crane. Once the foundation is ready, the base of the crane is anchored on and the rest of the crane is ready for assembly.
How do cranes get to the top of skyscrapers?
Perched above New York City’s skyline, two tower cranes are piecing together the new One World Trade Center, which just became the city’s tallest skyscraper. How do those cranes get up there?
By using their own hoisting power to make themselves taller. While that’s the most common method, there are actually three ways: 1) the external climbing method, in which the crane hire expands upward along the outside of the building, 2) the internal climbing method, in which the crane builds a few floors at a time from the inside and then “jumps” to a higher spot, and 3) the “sky crane” method of airlifting in a crane on a helicopter. (Engineers at 1 WTC are using both of the first two methods.)
In the external climbing method, the base of the crane is fixed in a concrete slab in the ground, and the crane tower is erected beside the building using smaller, mobile cranes. Once the building is about 180 feet tall (some 15 stories), the crane is fastened to the building with steel collars, and new segments are inserted into the crane’s backbone. To add segments to itself, the crane has a special climbing section—a big metal sheath that scales the outside of the crane tower. This sheath raises the arm above the last vertebra and temporarily supports it, then takes in a new tower segment from the arm. The sheath holds that segment in place while workers bolt it in. They repeat this process every 180 feet or so: The crane constructs the building, then it’s fastened to the building, then it receives new vertebrae to grow taller.
The second way is the internal climbing method. The crane stands inside the centre of the building, in a kind of makeshift courtyard, where it constructs the skyscraper around itself about a hundred feet at a time. A hydraulic cylinder at the crane’s base elevates it through the hollow middle of the building to a higher floor. Then workers slide steel beams underneath the crane to give it a sturdy new footing, and the crane begins building again.
The third method is to have a heavy-lift helicopter (or “sky crane”) fly the crane to the top of the construction site. This must be done piece by piece—just a single segment of a crane’s tower can weigh between 3,000 and 20,000 pounds. However, because of the cost, and because flying a load-bearing helicopter over a populated area is logistically very difficult, this method is the rarest, used only a few times a year nationwide.
How do cranes get back down? Often by constructing the very gallows that will destroy them. To disassemble themselves, tower cranes construct derricks on the rooftop of the finished project. (Derricks are tower cranes’ simpler great-granddaddy.) These derricks then help dismantle the tower cranes, and—in the case of internal climbers—lower their parts one by one to the ground using extremely long cables. Once the parts reach the ground, they’re taken back to the rental service on flatbed trucks, then the workers take apart the derricks. Most of the derricks’ parts can just take the construction elevator.