Steel in the News
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Posted by Ted Sheppard on April 27, 2011 at 9:14 AM.
When you are dismantling girders or trusses, you sometimes have drawings and a lot of information. But sometimes you just have the member itself. It is, of course, up in the air, and it may not be easy to inspect its condition. You may do all the calculations and see that it is stable for picking with a single point hookup. This is where you may fall into a trap.
Deterioration of a truss or girder is quite probable if exposed to the elements or in a place where maintenance has been poor. Thicknesses and widths may not be what the drawing says they are. If you can, inspect the member before making any rigging decisions. It may be prudent to use a lifting beam or two cranes to handle the dismantling.
When we did new construction calculations, we always looked for a 1.67 safety factor for allowable over actual stress. When we were dismantling, we raised this to 1.85, and we frequently also used a lifting beam, anticipating that the truss or girder may have been flimsy.
Rust can mislead you. I once wanted to kick the rust from a girder stiffener, and instead my foot went right through it. It had not been designated for replacement, but the owner got a brand new stiffener courtesy of Ted Sheppard.
On one project I had to dismantle an ore bridge in a steel mill. That was quite difficult. Everything was very heavy, and it required more than one crane plus falsework. When you dismantle any structure or any single member, always be cautious. Always take one more step than you think you need. It will pay off in the end. As one of the superintendents for whom I once worked put it, “When you over rig no one knows it, but when you under rig, everyone knows it.”