My city is having a building boom like never before. All over the place there are cranes in the sky, perched atop these slender pipe-frame towers. They are casually moving what seem like enormous loads over the heads of workers and pedestrians while all around the worksite busy urban life goes on.
I have no formal knowledge about how these machines work, but I assume that when an airborne crane picks up a load, some counterwieght mechanism is tugging on the other side of it, keeping the center of gravity of the whole package squarely on top of the tower. (If this is wrong, please correct me and I’ll be quiet and go on my merry way.)
So, what would happen if the hoist cable (or one of the chains or straps around the load*) snapped? Suddenly there would be no counterforce to balance the crane’s lopsided weight. Are those framework towers anchored sufficiently to withstand such a “shove”?
Thanks all, in advance.
An accident like this happened here in 2007 when the strap around a load bundle snapped. There was an injury or death from the falling material. The crane itself, I believe, was not affected, but then again, I don’t think the load was very heavy in crane-lifting terms.
That’s an interesting question. I thought that the crane might shift the counterweights back and forth to keep things balanced, but after looking at a few web pages, it appears that they are fixed. So if it can safely handle “no load” in a static condition, would the structure flex enough to store an appreciable amount of energy when fully loaded?
Oh yes,lots of flexing,and not just from workloads.The cranes in the OP move around in the wind too,and if they’re picking something that has a large surface area that intensifies.
The operators must have gimbal mounted brains and stomachs.
There was a similar accident in a St. Louis-area community yesterday. These were mobile cranes, but the problem was caused when the load became unbalanced. From the linked article:
In addition, in a TV interview, one of the construction foremen made the point that the crane could handle the weight, but the counterweights weren’t positioned for the direction of the load after the clamp failed. Nobody was injured in this incident; the operator (in a ground-level cab, not a tower like the OP asks about) just crawled out the window.
This accident occurred not while picking up a load, but rather while the crane was lifting itself up to install more tower to make itself taller. I have seen this done, and it is kind of like picking yourself up by your own boot straps.
The tower cranes I have worked with have had stationary counterweights. If a load binder were to break, the load would probably fall, but the crane would stay up. Also the operator had a warning buzzer in his cabin if he tried to pick up too heavy a load, or trolly out to far with too heavy a load. If this occurred he would signal us that the load was too heavy set it down and we would have to lighten it.
BTW before the tower got set up, we had a truck crane tip over into the hole at one hospital job. It was lowering a load of lumber and as it got all that weight out over the hole, it became over balanced and tipped over into the hole. No one was hurt, thank OG, but if you want to see big, you should see the crane they bring into pick up a crane that fell over. It is so damn big, it has to have a little crane come with it just to pick up the boom sections off of the two semi trucks to assemble it.
This thread reminds me of the one posted in August of 2001 speculating whether or not there was room to land all of the airplanes in North America. Of course, we found out the answer on September 11 of that year.
I’d link to it, but apparently we’ve lost access to anything earlier than May, 2002. Which is a shame, because there are some threads with some historical value.
The crane in Manhattan apparently collapsed because the load struck the supports after the cable had snapped. Had it missed the structure, New Yorkers would only have had to deal with the falling load itself.