'mttt':
Not as one would notice! As Mr. L'Hote suggests (some time back, I built some, more or less to his 'recipe' quite a while before reading his piece on them), I second this recommendation to build some yourself - they're cheap enough - and see.
Mine are of 2x8's @ 24"oc (4'-0"wide) x 18'-6", supported @ ends only. I built in a 3" crown, Faced with 1/2" ac/both sides, glue with 'Liquid Nails' (full bead) and 1-1/4" ss screws @ 8" oc all the way around. 'Have loaded them with a (sloppily distributed) array of less than daily use equipment totaling 850lbs. (+/-). within approx. 1 year, the crown was reduced to 2" (under load) and I have not bothered to unload it to see yet. All that equipment, all that work, mere curiosity...too much 'time' wasted! Well THAT'S MY STORY...and I'm stickin' to it!
Seriously, a (very) good idea. Use it!
I sincerely hope this helps you out...
Warmly, Griz
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"Torsion boxes resist twisting and warping - and are great for provding a "flat surface, I believe. But would they resist sagging better than a similar "sized and supported piece of MDF or Baltic Birch plywood? "