when a skyscraper is done being built, how does the crane come down?

Answer 1

Skyscraper cranes (for construction) are usually dismantled into small sections and pieces after final stage of construction is done. Some of the smaller pieces are transported down by construction evaluators, while bigger pieces are lifted down by series of pulleys or cranes mounted at the side of the buildings edge.

Answer 2

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. 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!

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:
The machinery arm contains the motor that lifts the load, along with the control electronics that drive it and the cable drum, as shown here:

The motors that drive the slewing unit are located above the unit's large gear:

Now let's find out how much weight this equipment can handle.

A typical tower crane has the following specifications:
Maximum unsupported height - 265 feet (80 meters)
The crane can have a total height much greater than 265 feet if it is tied into the building as the building rises around the crane.

Maximum reach - 230 feet (70 meters)
Maximum lifting power - 19.8 tons (18 metric tons), 300 tonne-meters (metric ton = tonne)
Counterweights - 20 tons (16.3 metric tons)
The maximum load that the crane can lift is 18 metric tons (39,690 pounds), but the crane cannot lift that much weight if the load is positioned at the end of the jib. The closer the load is positioned to the mast, the more weight the crane can lift safely. The 300 tonne-meter rating tells you the relationship. For example, if the operator positions the load 30 meters (100 feet) from the mast, the crane can lift a maximum of 10.1 tonnes.
The crane uses two limit switches to make sure that the operator does not overload the crane:

The maximum load switch monitors the pull on the cable and makes sure that the load does not exceed 18 tonnes.
The load moment switch makes sure that the operator does not exceed the tonne-meter rating of the crane as the load moves out on the jib. A cat head assembly in the slewing unit can measure the amount of collapse in the jib and sense when an overload condition occurs.
Now, it would be a pretty big problem if one of these things fell over on a job site. Let's find out what keeps these massive structures standing upright.

When you look at a tall tower crane, the whole thing seems outrageous -- why don't these structures fall over, especially since they have no support wires of any kind?
The first element of the tower crane's stability is a large concrete pad that the construction company pours several weeks before the crane arrives. This pad typically measures 30 feet by 30 feet by 4 feet (10 x 10 x 1.3 meters) and weighs 400,000 pounds (182,000 kg) -- these are the pad measurements for the crane shown here. Large anchor bolts embedded deep into this pad support the base of the crane:

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.

To rise to its maximum height, the crane grows itself one mast section at a time! The crew uses a top climber or climbing frame that fits between the slewing unit and the top of the mast. Here's the process:

The crew hangs a weight on the jib to balance the counterweight.
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).
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.


Most construction companies rent their tower cranes from a company like Heede Southeast. Heede ships the crane to the site, assembles it and charges a monthly fee while the crane is on the site.
The typical fee for installation and disassembly runs around $60,000. This price includes shipping the crane to the site, renting the mobile crane used to assemble the tower crane, the cost of the crew that handles the assembly, etc. A typical monthly fee for a 150-foot-tall tower crane is approximately $15,000, with an additional charge to rent the climbing frame and extra mast sections.

Answer 3

Most probably you are referring to the Tower Crane.

Interesting questions should have followed, like:

How is it constructed while the construction is in progress?
How is the placement of the Tower Crane determined?
What happens to the space occupied by the Tower Crane
after work completion?
How far should the 'boom' (the arm that has a moving pulley)
go?
Anyway, your question is a good one, because when I had my first construction work, the Tower Crane was already there and I was thinking of how to dismantle it afterwards.

The dismantling process, is done part by part, set by set starting from the top going down.

Another interesting question is:

How would that excavating truck or bulldozer be able to get out of that deep matt excavation after all excavation works have been accomplished?

It is interesting to note that an earth ramp is created while excavation progress, until the machine is down deep in the excavation and seems that it is impossible for it to get out after excavation work is done?

Answer 4

In pieces, section by section, and then the base is taken out by Helicopter. Or at least that is how I would do it.