Monday, November 8, 2010

April 30th, 2010

Magnitogorsk Russsia A self erecting tower crane was being dismantled when the truck crane being used to assist was apparently overloaded and tipped. No one was reported as being injured in the accident.

The crane appears to have been climbed down  at which point many of the older style self-erectors would lower the jib tip on to the ground. From here the pendants would have been disconnected, the truck crane connected to the top chord of the jib, and when sufficient load is pulled off by the truck crane, the lower chord pins at the mast would be pulled and the jib lowered to the ground.

Looking at the picture of the truck crane, it doesn't appear that there is a soils issue as the ground is quite rocky. The outriggers swing out and lock into place so it wasn't as if the outriggers were only at the mid extend point. My suspicion would be that the crane was scaled out to be able to pick the load of the jib, but someone forgot that either the truck crane would have to boom down to clear the tower, or boom up to to clear the tower. In reality, sitting farther away and booming up would be the way to go if you think that you are close to a stability or structural limit. By being farther away, you can have the truck crane hoist up to about the proper load and read the pins (see if the pin is sitting down hard in the hole or riding up) and even pull one pin to see how the jib hangs and the truck crane is performing. At this point, if you have a problem you can re-make the pin and get a bigger crane. By doing this, when you cut the jib free, it would be booming up towards yourself which would eliminate the stability problem. If you are going to boom down and away, you better be sure that you have the right weight and radius figured out.

If you are close on weight, it would also be better to be closer to the butt of the jib near the mast. It's marginal, but you want to keep as much weight on the jib tip as possible.

The tower crane may have only received minimal damage to a few lacings or potentially dents in the box chords. Easy repairs. The truck crane would really need to be torn town and fully inspected, even to the bearing. I don't know if that's worthwhile in a crane that old, so it may be a loss. Looking at this accident and thinking about how it likely happened, I'm reminded of a saying that my first iron working boss used to repeat often, "Know your escape route." You have to plan for the worst in this business and by doing so, you'll likely avoid it. Link to story

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