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We have 4000 lb jib cranes with a 8"I beam as the boom. It was
bolted to a mast and was supported at twelve feet. This beem had not
measurable flex anywhere along its lenght.just make sure the load is
lifted by tha bottom rail so not to roll

Which 8" I or WF (WideFlange) beam?
http://www.saginawpipe.com/steel_i_beams.htm