Monday, December 29, 2008

December 29th 2008

Sydney Australia 
A hotel in Sydney was evacuated after concern was raised about the stability of the crane. Upon review the crane was declared safe. I've heard of this coming up a few times in the last year. Cranes have back moment towards the counter jib side, no matter what direction the crane is swung over. Many maximum free standing cranes could lean off of center line plumb up to two feet due to the ductility of the steel. Cranes need to be erected in the US to a tolerance of 1 inch for every 40 feet of vertical, but the weight difference between the counter jib and ballast versus the jib's unloaded weight causes it to lean back. It's good that people are observant enought to notice this but rather than raising the red flag and running away it might be prudent to verify the deflection with a instrument such as a theodolite and the manufacturer's advice. Take a paper clip, straighten it out, hold it straight up and down and bend it a milimeter out of straight and let it go. It returns to vertical and that is an example of ductility. You'll find that you have to move it a quarter of an inch or more before you have permanent deformation and eventual buckling. We've had a good end to the year so hopefully crane safety is making strong strides or the economic slow down has pushed those not qualified out of the industry. 

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