Friday, February 17, 2012
February 16th, 2012
New York City, NY Imagine a empty dump truck falling from the sky at near terminal velocity. It virtually happened. One of the Favelle Favco tower cranes at the World Trade Center suffered a load line failure while hoisting 3 girders weighing up to 40,000 lbs around 40 stories. The load came crashing back down on to the truck it had been hoisted from. No one was reported as injured.
For me, this is a mind blowing accident. Favco's can hoist 40,000 lbs on a single line pull, depending on the model. Not only that, but at a rate of 190 meters per minute. I'm on a pretty short crane right now because we are on a two crane site, but I'm feeling pretty certain that even with a empty hook, it doesn't move at 190 meters per minute, let alone with it's maximum load. The point being, it's a horse. Both a Clydesdale and a Quarter horse in one. So what could the cause be?
NYDailyNews.com is reporting that the crane had a rope rubbing a bolt in October of 2011. Simple rubbing is not a cause for replacement. Flattening of individual wires to 2/3 their original diameter is cause for counting the wire as broken. Crown breaks would be another cause to count them as broken and then the rejection criteria according to manufacturer of the crane and the rope would begin to be sorted out. So did the company replacing the cable not deal with the rub problem root cause? Maybe, but not likely. The crane would have been inspected since then, and likely beyond the daily operator inspection. Not that this rules out error.
Another cause that occurs to me is the potential for the wrong rope to be installed. Tower crane load lines are to generally designed at a 5:1 ratio. So if you have a 40,000lb capacity on a single line pull, you need a 200,000 capacity rope. What strikes me is the potential for a incorrect rope installation. While this would be stunning, would it be more stunning than not properly addressing a bolt rub? Think about it, if you have a non-rotating rope, you have a swivel at the attachment point. If you have a rotating rope, you have a fixed attachment point. You can also have right hand or left hand lays depending on the drum rotation and attachment side that can affect the design and installation. Were one of these detail missed in the October rope replacement? I was talking with a friend about it this morning and he wondered about a rope defect. After some consideration, I thought, If you have a 37x7 rope, you have 259 individual wires. What are the odds that you would have multiple failures within one rope? While it can't be ruled out, it doesn't seem likely. I have to lean toward the wrong rope being installed. You can't have left a potential known rub point as a problem... right?
The lesson for us is daily inspection. Don't show up early and take a morning nap in the cab. Go through the crane. Look at the rope on the drum. It's not complete, but it's better than putting your feet up and setting the alarm. Make sure that we are going through our ropes against the horizon regularly and monthly have the bell man wrap it with a towel as we hoist up or down as appropriate. I've seen 9 month old ropes birdcaging off the drum with the hook at the lowest point. If I hadn't simply walked back to the drum, who knows what would have happened. Look at all of the rope. Make it a habit. Think of the ride this operator must have went through losing 40k and the fear that he might have just had a death under his hook Take that thought with you. It's what should be motivating you daily because you never know unless you inspect.
Posted by Gaytor Rasmussen at 8:37 PM