NYC, New York a report is out as of yesterday officially blaming the first tower crane collapse (May 30th 2008) on improper rigging of the tie - in collar. The report is 339 pages and I certainaly have not read it. I'll give you a synopsis from a dozen different stories over the last year.
The collar is installed in halves. One side is brought in and the rigging is transferred from the crane to come alongs attached to the tower itself. The the same process is done to the other side. The crane manufacturer requires that four attachments per half are used, In the case of the accident only two were allegedly used per half. This would be very common for a multitude of reasons such as only having to adjust one comalong per corner to adjust the elevation and level of the collar, less equipment, less time wasted, etc. Four points is how most manufacturers specify the work be accomplished. however, most crane collars do not weight 11,000 pounds either. Most are in the neighborhood of 4 to 6,000 lbs.
Another claim made is that the rigging used was old, flat straps (Nylon Webbing) both UV infiltrated and having suffered nicks in the edge of the eyes and noted prior to use. With flat straps it's important to not used them twisted or bunched up. The eyes failed and a common cause of this is that the eyes are used with the same size hooks and shackles wearing over and over in the same points. A single nick is important to evaluate and realize that 50% of the breaking strength may be gone. I have flat straps in my personal truck for pulling that are nicked, but would never used them for hoisting.
The attorney for the rigger who has been charged in this case claims that the reports claim that one broken strap would not cause the failure of the otehr three is either lawyering or plain ignorant. As the collar is adjusted to the proper elevation and level, the stresses are virtually never equal. As we all know, when rigging something off of four points, you should only calculate the rigging for three points as at any given moment one leg is seeing minimal load. Add in adjusting one corner at a time and often a corner or two comes slack. If you had a rigging failure at this point, you will not only see shock loads doubling and quadrupleing intended loads, you would likely see a zipper effect as the load transferes in milliseconds from corner to corner until the final failure. It would not be a sudden drop evenly onto all three straps in which case the attorney would be correct.
The cures are... Use the eight pick points specified by Favco when installing their massive collars. Currently many companies still aren't doing this. Use the proper rigging for the job. When bunching up or breaking over a sharp edge, do not use flat straps. you can use wire rope, endless round nylons, or grade 80 or 100 chains designed for rigging.
It's unfortunate to hear that people died and the ignorant union leadership hasn't ensured that their membership is kept safe and are using the best practices possible. Maybe that's not fair... where is the DOB on this, general contractors, crane owners... pull your heads out boys and learn from the mistakes. Somebody lead the way.
No comments:
Post a Comment