Nope, no braces. No ground anchors, either; it's completely freestanding. The columns do indeed look rather skinny, but are surprisingly strong. the tubing is 6061 T651, 2.5 inch x .25 inch wall. A 24 foot long column of this stuff has a critical buckling load of about 330 lbs. [this is gotten by assuming a worst-case configuration in the rig of the top bar rotating about the axis of applied load, and considering any individual leg to be in a one-end-pinned, one-end-guided configuration. k = 2.0. I = 1.132 in^4, E = 10000 ksi, L = 288 in. [a both-sides-pinned configuration would give you about 1300lbs! really!]). Not that this number is really useable; this column is not ideally axially loaded, plus there are the sleeves and their slop-around-the-sleeve-fit to reckon with that causes an initial out-of-true starting point. In any case we know the column isn't going to remain perfectly straight. It will "buckle" This is OK. Buckling != failure.
What i actually did was to take an assembled column and axially load it at various tensions, up to an equivalent of hanging a 2000 lb weight on the rig, and measure column deflection and end-slope conditions. from here, compute the induced bending moment at the weakest point (the center sleeve joint). Compare to the bending moment required to cause the sleeve to yield via bending stress. It's plenty strong enough! [x-sectional area of column is 1.767in^2. 6061 T651 is 37ksi stuff; at the loads the rig will see, we can completely ignore the compressive stress the tube (but not the sleeve) sees.]
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Date: 2008-09-16 04:40 am (UTC)A 24 foot long column of this stuff has a critical buckling load of about 330 lbs. [this is gotten by assuming a worst-case configuration in the rig of the top bar rotating about the axis of applied load, and considering any individual leg to be in a one-end-pinned, one-end-guided configuration. k = 2.0. I = 1.132 in^4, E = 10000 ksi, L = 288 in. [a both-sides-pinned configuration would give you about 1300lbs! really!]). Not that this number is really useable; this column is not ideally axially loaded, plus there are the sleeves and their slop-around-the-sleeve-fit to reckon with that causes an initial out-of-true starting point. In any case we know the column isn't going to remain perfectly straight. It will "buckle" This is OK. Buckling != failure.
What i actually did was to take an assembled column and axially load it at various tensions, up to an equivalent of hanging a 2000 lb weight on the rig, and measure column deflection and end-slope conditions. from here, compute the induced bending moment at the weakest point (the center sleeve joint). Compare to the bending moment required to cause the sleeve to yield via bending stress. It's plenty strong enough! [x-sectional area of column is 1.767in^2. 6061 T651 is 37ksi stuff; at the loads the rig will see, we can completely ignore the compressive stress the tube (but not the sleeve) sees.]