I’ve been watching this building go up from my office window for the last year or so:
http://www.calgaryherald.com/Cranes+start+coming+tower/4531068/story.html
and it’s had three cranes on the top until last week. I’ve been curious as to how they get the cranes off the building once they’re done.
I assume that the cranes on the building lower the components of the disassembled ones, but what about the last one?
You leave it in the basement as the building’s boiler and hire the crane operator as a janitor.
I heart you.
That only works with steam shovels. The crane becomes a TV/radio tower.
You disassemble it and helicopter it off the roof in pieces.
I can’t tell what kind of crane it is from the picture, but for most tower cranes, you don’t take it down from top to bottom. There’s a special part with jacks in it that lets you insert and take out pieces of the crane. So, if the crane were a tower of blocks all of the same shape stacked on top of each other, you take the crane down by taking blocks out closer to the bottom, shortening it that way, and not from the top down. The last little bits usually come down using a crane on some sort of vehicle.
Now, if there are several cranes, they sometimes use one of them to erect and take down the others, but the last one usually goes the way I’ve just said.
They normally use a portable crane to help set up a tower crane. I guess they could use it to take one down too.
I’ve been watching that building to see how it might be done. Last week there were 3 cranes, then a thick fog covered the city for days. The fog cleared - TWO cranes left! I have to believe that the construction company is messing with people like me who give this WAY too much thought.
RadicalPi, I’m trying to envision how this might work. Is the base of the tower kind of jacked up, and sections taken out?
Ah, others that remember Mike Mulligan.
Here’s a nice overview I just found:
I’m notoriously bad at these sorts of verbal explanations, but I will do my best. Going back to my blocks stacked on top of each other metaphor, imagine that the block at the bottom is bigger than the others, but that it is also cut out so that a block can fit inside the bigger block, and also, there is a cut out in the bigger block so that you can stick a block in from the side. At the bottom of this bigger block, outside where the block would go, are hydraulic jacks. So, first, you take the stack of blocks with the last block inside the big block. Then you jack up the whole thing. Then, through, the side you insert the next block. Then you lower the jack, and repeat.
Now, turn this whole thing upside down, except for the arm of the crane, which remains on the top, and use the crane to lift each block to where the jacks are, and that’s how it’s done. Doing it this way keeps you from having too much weight to jack up, and allow you to attack the crane to building itself, but sometimes, they do it from the bottom, I’m pretty sure. Let me know if this doesn’t make sense.
To take down the crane, do all this in reverse. Remove a block, lower the hydraulic jacks, and repeat.
I don’t think this applies to the crane in that picture, but I once watched a building being erected, not too big, less than 10 stories, and the tower for the crane was located in the center where the elevator shafts were. I heard it said by several people that the tower actually became the structure for the elevator shaft, but I have no way to confirm that. I did see them remove the horizontal part of the crane, disassemble it into a few large pieces that were lowered to the ground using a small crane (basically a winch on a short beam attached to the roof somehow). There was no sign of the tower being removed, but it could have been disassembled internally out of view.
The tower cranes that everyone is referring to I don’t think come into play here. They can dismantle themselves, sure, but they would still be on the roof. I always figured they either brought in a bigger crane and lifted it off, or they broke it down into small enough pieces that they could bring it down the temporary elevator that they have built into the side of the building. If neither of those options work, they could use a helicopter.
Don’t forget, you see that one crane, but there’s going to be other heavy equipment up there as well that they have to deal with. Cement mixers, compressors, generators etc…
Here’s a 30 sec video that get’s the point across pretty clearly. It leaves details out, but it will help make the jump from “How, wait, what?” to “Ohhhhh, got it”
If you want to learn more, just keep your eyes open for Modern Marvels or a similar show on Cranes or Sky Scrapers or something like that, they usually have a section on how tower cranes build themselves.
Somewhat-related note - we live on the north end of town, and you can see that frigging tower from EVERYWHERE. Man, that thing is big.
I’ll ask my husband if he wants to come in here - he watches cranes go up and down on his job on occasion (he works with a construction management company).
Part of me wants to say “Unscrew the bolts holding it onto the roof and give it a good shove.”
How do they disassemble the cross-arm? Do you take out one section, then a chunk of counterweight? and re-jig those guy wires?
I can see where it would be real simple to hook up a temporary small crane arm on the roof that just lowered the big crane pieces over the side…
I can dismantle itself all the way down to the ground and from there you should be able to do it with ground equipment (small skid mounted cranes etc). As for balancing, don’t forget, once everything is assembled, mounted and the counterweights are in, it’s stable enough to pick up a load and move it all the way in and out without tipping, so I assume you could dismantle it (and take out the counterweights as you go) without too much trouble.
ETA, or you can cheat and when you get low enough to the ground use a regular crane to pull the top off.
Cat Whisperer’s hubby in to make some comments from the perspective of someone who has worked on several projects involving tower crane erection and dismantle…
The piece on tower erection is a reasonably accurate description of “centre climbing” the mast. As also noted by RadicalPi, basically, you separate the mast and insert a new section to grow it taller and then pull those sections out as you take the crane down. This is fairly common and routine for erection and disassembly of a crane that has been placed outside the footprint of the structure it is building. If, however, the crane is placed inside the footprint of the building, this isn’t possible during the teardown, since the upper sections of the crane will no longer fit into the building as the crane gets lowered. So another solution is needed.
But let me start at the start. Pretty much without exception, the crane is initially assembled using a mobile crane which stacks the mast and jib to as high as the project initially requires it to be. Mobile cranes can reach quite a staggeringly high distance in some cases, so you can build quite a tall tower crane right from the ground. If there is never a need to extend the height of the tower crane, of course this will also be the way the crane comes back down once it is no longer needed. Here’s a pic from one of my former jobs, back in 2007, of a mobile crane putting up a mast for one of the tower cranes that was used on the project. The video in Joey P’s post above also shows the dismantle process beautifully.
In cases where it is decided to build the crane inside the footprint of the building, the typical way to raise the crane is by jacking it up. Once the building’s height has caught up to the minimum operating height of the crane, a heavy duty jacking system mechanically raises the whole kit ‘n’ kaboodle a few stories, then resets it at that that new height. This is what has been done with the cranes on The Bow tower in the OP. In the case of The Bow, when it comes time to take down a tower, the tallest crane will be used to dismantle and lower the jib pieces and then extract the pieces of the mast bit by bit from the building itself. When only the final tower crane is left, there are two options to take it apart: either use a helicopter to pick the pieces and bring them down as suggested by dolphinboy, which is very risky but sometimes makes sense, or build a temporary davit arm on the roof of the building tall enough to pick the jib and then the mast pieces. As the davit arm is typically assembled with the help of the tower crane it just helped take down, another smaller davit arm must then be put up to take the first one down, and so on until all the construction parts are gone.
Putting up or taking down a tower crane is a surprisingly quick operation, typically requiring anywhere from a half day to two days, so they do seem to appear out of thin air and disappear just as quickly. But I have to say, I never get bored watching the process happen – it’s really cool! 
How will they get this one down? I don’t think the explanations above cover it (not in my mind anyway):
