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Force needed to bend 2 inch square tube

  1. Nov 27, 2013 #1
    My question is:
    Researching two sizes of square steel tubing to use for a project.
    material 2 inch square tubing with 0.25 thick wall and 3 inch square tubing with 0.25 thick wall.
    The square steel tubing will be welded on to a frame at an angle so the corners point up and down , or another way of saying it, the corners will be where the ends of the letter "t" or an addition sign "+" are.
    For each tube size what force would be needed to bend the tube?
    Want to build a gravel grizzly to screen out rock from sand in gravel pit.
    The wheel loader bucket of gravel; would be slowing dumped onto the square steel tubing.
    Maximum weight that would be dumped is 7500 pounds. A full loader bucket of gravel.
    Surface area that gravel would be dumped onto is 8 feet wide and 4 feet long with a steel support 1 foot in from edge of the 4 feet long area.
    Length of the incline (slope) of steel tubing top to bottom 8 feet long.
    Slope angle is 40 degrees.
    From top of the steel tubing where the slope begins go down slope 2.5 feet, this will be the location of the steel brace under each steel tube and the remaining 5.5 feet of length no support under it, until the end of the slope where it will be welded to the frame.
    The gravel will have mostly screened out through the steel bars at the 4 to 5 foot mark from the top.
    Only over size rocks that did not go through the bar spacing of 1 inch, would travel the rest of the way over the bars to the edge of the grizzly and off the deck
    Largest rock/boulder I think going on the steel bars would be 18 inch round rock weighting 1,700 pounds.
    Attached are two photos of what a grizzly screen for gravels. Similar to what I want to build.
    Photos worth a 1,000 words.
    18 steel bars over a 10 foot width, the bucket is 8 feet wide.
    Steel bar then 1 inch opening, then another steel bar, another 1 inch opening; etc. over the 10 foot wide area. So lot of steel bar surface in contact with bucket load of gravel. But I need to know if the 2 inch square steel would bend under the weight of the gravel or large rock, given the area that is not supported be bracing under the steel bars.
    Each steel bar would be braced a the 2.5 foot mark from the top to the slope.
    So it's the distance from the 2.5 foot mark to the 4 to 5 foot mark where most of the gravel would have gone though the bar openings that I am wondering if the bars would bend with in this 3 foot area.
    Bucket reaches over the top of the grizzly and dumps the gravel onto the deck of steel bars at about 1 to 2 feet in from top, with most gravel at the 2.5 feet to 4 feet from top area.
    The four foot impact area is 1 foot then the steel brace then the 3 feet, by then most gravel through the bars only rocks on top to slide off the bars and over end of sloping deck.
    Any input would be greatly appreciated.
    These small gravel grizzly plants cost around $10,000 with freight and taxes, That's not cost effective (it can not pay for itself with my small operation) so I have to make one myself.

    Attached Files:

  2. jcsd
  3. Nov 27, 2013 #2


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    If you are really concerned that the grizzly screen could be damaged from the rock drop, why not start with a plywood chute leading to the grizzly. You can replace sheets of plywood as needed if it gets too battered and the grizzly will do the sorting without getting squashed by falling rocks.
    Separately, based on your description, I think the 2 inch tubing would be less vulnerable than the 3 inch.
    Frankly, for a home brew, I'd recommend using some scrap rails instead. They are bullet proof.
  4. Nov 27, 2013 #3


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    By scrap rails, you mean to just use metal flats maybe quarter inch by 3-inch each spaced inch apart, with some bracing every foot or so to avoid twist.. Could be cheaper and there would be more area for the gravel to fall through. Middle section(s) could be made removable for replacement when they become too battered.
  5. Nov 28, 2013 #4
    256 bits and others
    Flat rails collect rocks between them, expecialy when the spacing is narrow say one inch. that's why the industry has moved away from them.
    What about the force needed to bend the square tubing 2 and 3 inch square?
    Does anyone have any input?
  6. Nov 29, 2013 #5


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    OK that makes sense. Less lateral movement with square tubing.
  7. Dec 1, 2013 #6


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    lakesidebob: I made a plethora of assumptions, too numerous to name. To make a long story short, when I checked your 50.8 mm square tubing, it had a bending stress level of 172 % of the allowable bending stress, which exceeds 100 %, which therefore indicates the 50.8 mm square tubing was overstressed.

    When I checked your 76.2 mm square tubing, it supposedly had a bending stress level of 70.0 %, which supposedly indicates it was not overstressed. But any results depend on the assumptions. And my results have not been checked, at all.
    Last edited: Dec 1, 2013
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