Somebody asked me this yesterday, and while I was willing to join him in enthusiastic speculation, I didn’t have an answer.
Are they single-piece units (or at least, made of two separate pieces, the column and the arm)? How are they transported? Lifted into place via helicopter?
Or are they modular? Assembled in place by some means?
I think the latter is far more likely, but I don’t know how something that big and spindly would be put together in such a way that it could safely manipulate the loads that it does. Also, I don’t know how the control booth would be put in place, let alone the huge counterweight.
I’m more familiar with truck mounted cranes, which often times use a forklift to attach and detach extra boom, which the truck carries around parallel to the main boom when it’s traveling between job sites.
I’ve seen use of a truck crane (300 ton capacity) in which the boom and counterweight were so big that another truck crane had to be brought in to assemble it. (A truck crane was used rather than a tower or jib because the crane had to roll away from the launch pad but be able to return on short notice to pick the motors in case of a destack scenario like a sudden hurricane or unsolvable pre-launch payload failure.)
The design and operation of large cranes is a delicate balance between weight and strength, and because large cranes (especially tower and jib) can easily be misassembled or operated in negative margin condition they have to be proof-tested after loading (by picking up a load at a specific overunity percentage of operating capability) and run by a licensed operator. Bad things happen when an operator gets outside of his load chart.
On the topic of using helicopters for heavy lifting, while this is occasionally done for commercial applications (especially in remote oil fields) this is an extremely dangerous operation, not only because of the suspended load but also the downdraft from the rotors is huge, enough to push an unbraced person to the ground, and the amount of foreign object debris (FOD) flying around is tremendous. The amount of static electricity than can be built up by the rotating blades is also not an insignificant hazard, especially if you are working around combustables or ordnance. And this is obviously something you would only want to do in no more than light wind. In general, it’s a lot easier to be stuck to the ground.
That’s pretty much what I was looking for, as far as the tower is concerned, but what about the arm? It’s also pieced together, I presume? And the counterweight attached on the ground? How many pieces?
Sorry for the insistence on details, but when the guy I talked to last night found out I was “smart” (I know, I know – I answered a couple of his random questions and showed him how to take the fuses out of the plug on a string of Christmas lights), he started grilling me with all of his burning curiosities. This was the only one I couldn’t help him on, and I wound up promising to look into it. Plus, now I’m curious myself.
That I don’t know specifically for tower cranes. I assume the boom is assembled on the ground and then a truck crane lifts it onto the first tower section.
Yes, that’s pretty much how they did it at the building site next to my office. One crane stacked the pieces of the tower, and then a truck crane lifted the boom into place. What was most interesting to me was that the tower sections for that model of crane were held together by only two bolts in each corner, one on the interior and one on the exterior. When the crane was at full height (approx. 10 stories in this case) the truck crane lifted the completed boom assembly, counterweights and all, to the top of the tower. The assembled tower crane had I believe an 8 ton capacity. The truck crane could have lifted just about anything it wanted to.
