Wednesday, October 20, 2010

August 13th, 2005

Caracas Venezuela A tower crane erected on a cross base (cruciform) that appears to be the manufacturers design tipped over and crashed on to the job site. Two workers were injured can needed treatment. Link to story.

There were two potential causes noted. One was the crane fell out of a plumb condition. It would be odd, but not unheard off. You should always have your soils checked prior to erecting a crane even if it were being installed in the most reliable concrete footing. Through out the job you have to protect the integrity of the foundation as well. I've seen jobs that needed to be shut down due to a dirt contractor becoming over zealous and undermining the crane foundation. The big concrete block doesn't do you any good if it's sitting on air.

The other potential speaks loudly and is much more common. It was indicated that a large I beam may have been hoisted and overloaded the crane. This would be a significant overload of 30% or more. While it's true that some tower cranes don't have scales, an overload of this amount is clearly noticeable to anyone whom has load tested a tower crane. I've been on a crane that was approaching 117% or so and it was flat out uncomfortable. Sadly it took protesting loudly to point out that we were way past what the crane was rated for. This was a case of over reliance on the computers. We had no alarms going off, the display had noted the overloaded condition due to yet the technician wanted to continue on with the test. The next morning they came back and found the problem. That desire to get everything done rather than having to come back literally endangered my life.

Load tests on cranes must be carried out. You must test the limit switches to be sure that they are in good working order and that no overload can happen. Tower Crane operators trust their limits as a habit, good or bad, and we owe it to them to get the limits properly set. Don't scrimp and assume that the limits are still working since the time that the crane was erected.Test the limits properly. Adjust them as needed. Then we can all go home!

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