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I have a steel "I" Beam in my back yard that I want to get rid of. It would be perfect for your backyard set up. The top probably needs to be cleaned up but I have an angle grinder and can probably help you out with that.
The beam is about 3 inches wide, 4 inches tall and about 18 FEET long!! It has to metal plates welded to the ends so you can more easily make some feet for it. It is long but I do have a long bed truck that I can help deliver for an extra fee. I would like to get $50 for the beam!! I live in Prior Lake, so just south of the twin cities.