Wednesday, December 8, 2010

Unknown Date

China. A worker was pinned and appears to have been killed during a climbing operation in China. The story only comes from a video that I found, but the scenario appears to be quite unique. The counter-jib appears to have structurally collapsed in the middle and come down simply to crush the man. Warning: This video may be beyond the normal tastes of North American's.

The crane is short with a climber on it. Since I don't see any tall buildings nearby, I'm presuming that the crane has just been erected and was going to be climbed to it's operating position. From the low height, I would even assume that the crane had not even gotten one tower section in yet. The excavator used to get the fire fighters into position seems to easily reach the lower climber platforms.

While climbers do get an ugly and uncomfortable bounce from time to time, could it really cause this? I suppose that you can't rule it out, but a more likely scenario is that the climber wasn't set properly and it slipped or structurally failed causing the crane to crash down hard on the tower. Let me draw it out a bit.

Normally the crane's counter jib has stresses that run down the pendants on the black arrows. From there, the pendants prevent the weight from dropping in the back which translates to force pushing in against the turntable or tower top depending on the design. If you drop the entire crane on to the tower itself during a climb operation, somewhere between the red arrow and the yellow arrow you are going to see significant downward deflection. At the same time the stresses on the pendants are going to increase multi-fold which also increases those inward stresses on the beams, but now on a structure that has deflected. In this case yield was found and the whole counter jib came down like a battering ram killing the man. 

This isn't unique in the sense that we've seen structural failure in the middle of counter jibs. It's happened here, and here. You need to be looking there too for cracks in splicing plate welds and in the horizontal diagonal lacings. I've found cracks in those lacings before.  

Again, the Video shows a body and the man trapped. 


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