Sunday, July 19, 2009

July 17th, 2009

Oetwil, Switzerland A self-erecting tower crane that appears to have been well set up had tipped over while being used in a concrete pouring operation. !5 workers were on site but fortunately no one was injured.

The immediate thought given the clear stability of outriggers and number of counterweights is the limits. Is it that the limits were never tested or set up? Usually self-erectors limits are contained and once set up for the crane, they won’t need to be re-set unless the configuration of the crane is different. In Washington State the cranes are treated as tower cranes and the limits must be retested during each crane erection. Under the not yet released ASME B30.29 (new self-erector standard) the cranes also must be tested after each crane erection. Of course this is the US standard and even that is voluntary… at least until you have an attorney in a deposition asking you why you didn’t follow it as he stares at your wallet. Then it doesn’t feel so voluntary.

Self-erectors are a great tool, but they need to be operated by competent people. Running it rough and not catching loads near the limit can lead to problems. Imagine trying to run fast on an older crane. You are trolleying out and not paying attention to how heavy the load is or the load chart. From top trolley speed the over-turning moment limit goes off and stops the trolley instantly. The load has 60 feet of line out and now it just stopped at 100% of its capacity. The load swings out 20 more feet before stopping which is now at the toppling capacity due to dynamic loading. I personally think that this is the type of thing that causes most self-erectors to topple. Just an opinion.

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