I need to design a straddle carriage to be able to lift a maximum of 1 ton. I'm getting quotes from different guys but because they are using different plans I'm not sure which one makes the most sense. The unit is to be 5' x 5' at the base & and 7' 3" high. The options are: 3 x 3 x 3/16 square tubing around the perimeter with 1/4" plate as the floor 3 x 3 x 3/16 square tubing for uprights & Header. The uprights are braced on the two sides with a diagonal from corner of base to halfway up the upright. 2 x 2 x 1/4 square tubing around the perimeter and a cross brace underneath a 3/16" plate as the floor 2 x 2 x 1/4 square tubing on the uprights & a 2 x 3 x 1/4 Header. The uprights are braced on the two sides with a diagonal 2 x 2 x 1/4 from corner of base to 3/4 up the upright. 2 x 3 x 3/16 rectangular tubing around the perimeter and double 2 x 3 x 3/16 bracing in both directions underneath. A 1/8" plate or extruded matting on the floor 2 x 3 x 3/16 rectangular tubing on the uprights & a 3 x 3 x 1/4 Header. The uprights are braced on the two sides with a diagonal 2 x 3 x 3/16 from corner of base to halfway up the upright. __________________ "Nothing in life is so hard that you can't make it easier by the way you take it."
To me, the last one there makes the most sense, the strength is in the vertical part of the tubing. If it was me though I would probably make an "A" frame on each side going up to the header, with the feet of the "A" placed roughly 15 inches in from each end. I always gotta think out of the box.
Not sure about this but was wondering if adding a short "A" across the top with the "eye" to hook to at the top of the "A". Looking at the sketch the header looks like it would tend to bend, thinking that the short "A" on top of it would put the header under a compression load. Then again I didn't stay at a Holiday Inn Express last night so take it for what it's worth! Hope that made some sense.
What are you lifting it with and what are the attachment points. That would be needed To help figure out what design makes the most sense. Is it going to be loaded even at all times, or is there a possibility of all the load on one side or end? Makes a different on hoisting attachment in my mind. If picking by a cable device the most stable is to pick from all 4 corners.
Unless there is some important reason to reduce the sectional size of the tubing, I'd use larger, thinner wall tube. You'll get more stiffness for the same weight, and the welded joints will have greater bending moment strength. Or do an A-Frame on each side, like lantraxco says; triangulation is stronger than cantilever.