I’ve seen more than a few films of the World Trade Center being built and most of the views show large cranes on top of the buildings in construction, hundreds of feet in the air.
How on earth do they get these cranes down from the tops of completed or almost completed skyscrapers?
Well, when they put them UP, they tend to raise them by crane A lifting B into position, then B lifts A to ITS new position, and so on…
Taking down might be trickier. My GUESS is that they dismantle them in place, and take the parts down by heavy-lift helicopter. Or maybe the reverse the going-up process (pretty much impossible on the WTC)
Taking down the cranes is done by having the large crane at the top lift a smaller crane to the top. The large crane is then dismantled and the smaller crane then lowers the pieces of the first crane. This is repeated at least one more time until the crane at the top is small enough to have its pieces removed in the buildings feight elevator, or I suppose by a heavy helicopter.
I don’t think so. Assuming we are talking about those cranes that sit on a spindly steel tower and extend horizontally far out then you’ll have a problem with this. The crane can’t lift higher than itself so it couldn’t raise another crane up higher to do the back-and-forth thing suggested. Besides…most buildings only have one of these cranes going.
I have actually been meaning to ask this question for awhile so I’m glad to see it. I have discovered the answer to the going up part myself from observation but the coming down part is still a big question in my mind.
Again assuming we are talking about the type of crane I described above the crane actually jacks itself up the tower. The steel tower has a steel cage built around it maybe 2/3 of the way up. This cage has pneumatic jacks that lift the crane several feet. Steel tubing is then installed in the gaps. Rinse and repeat as necessary.
However, coming down seems more tricky. Often, the building is built around the crane’s steel tower (you can see this on a new building being built in Chicago on the corner of Wacker and Lake). Once the building is done you could lower the crane only as far as the top floor but I don’t know what they do then. The crane is longer (horizontally) than the building so it’d be sticking out on both sides. How you dismantle it without the ends falling I don’t know. In addition, the crane usually has huge cement counter-weights at the back end. What’s done with those? Bringing-up another crane seems problematical. First-off these long ‘tube cranes’ (sorry…I don’t know what they are properly called) don’t have a very heavy lifting capacity. I seriously doubt it could lift a crane big enough to pick itself up. Assuming you could lift a big crane in pieces and assemble it at the top might work but how does that crane get down once assembled? The big crane lifting successively smaller cranes might work but that seems like a huge amount of work. If that’s what you’ve got to do then that’s what you’ve got to do I guess but geez…what pain.
SIDE NOTE: I talked with a guy who operated one of these ‘tube-cranes’ at a bar (unfortunately I neglected to ask him how one is brought down). He was telling me that when he puts the cart at the far end of the crane and lifts close to the crane’s maximum capacity the whole crane leans forward several feet (IIRC he said ten feet). Eventually whatever is being lifted comes off the ground and the whole crane snaps back upright and wobbles a bit before settling down. Sounded pretty scary to me and I said so and that I hoped he got paid alot for his work. He said he loved his job but it paid pretty badly actually…one of the lowest paying jobs on the constructions site.
They push them over the side when no one is looking. Really.
Here’s a link that talks about how they dismantled the big crane from the Las Vegas Stratosphere.
How Stuff Works is your friend.
BTW, they’re called “Tower Cranes”, and dismantling is the exact opposite of assembling.
Thanks for their proper name.
Unfortunately the “How Stuff Works” link doesn’t give the whole story.
I understood how they grew and I could certainly understand how they could do that in reverse. Unfortunately, some of these cranes have the building built literally around them (I mentioned before that those who live or work in downtown Chicago can see this on the corner of Lake and Wacker Dr…NW corner of the Loop). They can only lower themselves as far as the roof. now you have tons of crane lying across a roof and overhanging the building…including several tons of counterweights.
How does this crane get down?
Guess I didn’t read the OP carefully enough. Why A Duck’s link appears to give a good explanation of dismantling a roof-mounted crane; I suspect that the type with a tower going through the building could be done similarly.
This is straight out of a book I’m currently reading, Twin Towers: The Life of New York City’s World Trade Center by Angus Kress Gillespie.
They used cranes from Australia, the Favco Standard 2700 Crane, nicknamed “kangaroo cranes” for their origin. There were four cranes, all having a 120ft high steel base that fit inside each one of the four main elevator cores.
“When the steel for the 3 floors (above) was lifted and placed, the cranes could lift themselves for the next 3 floors…a series of hydraulic lifts installed at the base of each crane lifted the entire crane ‘like a grease rack in a gas station’.” (p.93)
After completion, each crane was dismantled and lowered to the ground until there was one left. That crane then lifted a guy derrick to the roof, which was used to lower the parts of the last crane. The guy derrick was dismantled and taken through a roof hatch, then brought down by freight elevator. (p.118)
That guy Derrick must be awfully strong.