strength of 2x2x.25 vs 3x3x.25 tubing

[burntside bob]

I am in the final stage of planing to build a simple gravel screen.
I need input from you on the strength of comparison between two sizes of steel tubing.
size 2"x2"x.25" vs. 3"x3"x.25"

The steel tubing will be for the frame of the box. and sloping screen deck.
A three sided box will be welded together. The open end is for the loader bucket to pick up the screened fines from under the screen deck, after the loader dumps its bucket of aggregate onto the screen deck.
On two of the sides there will be three steel tubes rising from the bottom of the box to the top of the screen deck. One in the middle and one on each end. Highest tube will be 8'6" long, middle tube will be 5' long and bottom tube 3' long which will be incorporated in to a 3' high box frame on the three sides.
The 5' tube will have a cross tube connecting each 5' tube to support the center of the screen deck.The top and bottom tubes will also have cross tubes. Will 2'x2"x.25" cross tubes be strong enough?
The gravel will be dumped slowing on to the screen deck not all at once, so as the gravel is sorted between the aggregate that falls below the screen and the over sized that falls off the screen surface, the weight of the screen surface deck is 1900 pounds.
I estimated that at most one yard or 2800 pounds will be on the screen deck surface at one time. So 2800 pounds of gravel + 1900 pounds steel screen deck = 4700 pounds.
The screen surface is planned to be 12 feet wide, cross tubes will be the 12 feet width, by 8 feet long. the slope will be the 8 feet long run. slope is 40 degrees, so the gravel slides off the screen fast.
What I am wondering is the 2"x2"x.25" steel tubing to small??
Will it support the 4700 pound load?
Will it last for a couple of thousand yards of gravel screened on it.
I do not plan on using it that much, if it screens 5000 yards over its life that would be a lot.
I do not have much knowledge of steel structure strength so I am asking these questions before I build.
The price between 2x2 vs 3x3 tubing is right at $1,000 more for 3x3 tubing.
That's why I am considering 2x2x.25 steel tubing.
What ever input you give will be greatly appreciated.
Thank you.

[DSW]

Is this getting used as a beam or a column?

Taller structures are almost always stronger pound for pound than shorter structures. If you are using this as a beam horizontally taller thinner tube will usually beat shorter thicker tube in bending. The down side to this is the walls are easier to collapse if you ding thinner tube. 2" wide x 3" tall tube might be a good compromise or drop to say 3/16" wall 2" x 4" tube.

The same applies when talking about tube used as columns. Larger sections are more resistant to buckling under loads than smaller sections of the same length.


I can't find my steel book right now that shows beam and tube tables so I can't help with more than general theories.

[FODFA]

I work in a quarry and it sounds like what you are describing is a Grizzly screen, at least thats what we call them here in Australia. Without knowing too much about your exact design 2x2 seems a bit on the light side to me,

Being in Australia the weights in pounds are a bit confusing but a yard is a bit under a metre so I am picturing dropping a bucket from our 920 cat loader onto a screen made from 2x2 shs, I would be looking at heavier.

Cheers Andrew

[fortyonethirty]

This might help: http://www.engineersedge.com/calculators/section_square_case_10.htm

Roughly translated, moment of inertia = stiffness. These numbers will be good for comparison of different size tubes, but using them to calculate the actual strength of your design will require a lot more calculations.

[burntside bob]

Thank you for the three reply's so far, it is information to think about before I purchase steel.
Yes the screen can be called a grizzly screen. No vibration devise to shake the aggregate just a steep pitched slope and close together grizzly bars, as close as one inch.
I was wondering about the strength of the 2x2x.25 tubing for the cross tubing from one side to the other.
I can understand how the wider say 3x2 vs 2x2 tubing would reset bending compared to the 2x2 tube.
Good comments so far; thank you for your input and suggestions.
Any one else have some input???

[Farmerboy]

this may help you,
http://www.steeltubeinstitute.org/pdf/brochures/LRFD_Beam_Load.pdf

http://www.steeltubeinstitute.org/pdf/brochures/beam_load_table_bro.pdf

[Joshfromsaltlake]

2x2x0.25 may be sufficient for the vertical columns if you use something to tie the legs together on the bottom, on three sides. In other words so you have a both-ends fixed configuration. In terms of buckling that will actually double the load capacity.

It's equally important to have a certain amount contact area for the footings, to spread out the load on the bottom of the stricture. Otherwise you'll gradually hammer the thing into the dirt.

Something I'd suggest is using a three pieces of 4"x 4.5# channel to run along the bottom. Use channel for the horizontal members since channel it's a hell of a lot cheaper than RSW square tubing. Horizontal members only need to support load in one direction.

Something you might consider doing instead of putting a ton of x-braces, just weld some 10 gauge plate on three of the sides. (note that you'll want to use something like 2-on-8 stitch welds to connect the plate to the outside of the members. Continuous seam welds will weaken the columns.)

this will improve the stiffness drastically and make buckling of the legs unlikely. Save you a hell of a lot of labor too. This will also have the benefit of keeping the screened material contained for easy pickup.

You're planning on having 7 vertical columns all together? You probably don't need. You could get away with 4 if the sides are covered in plate. If you want to add cross members to improve stiffness you could stitch weld some light angle on top of the plate.

Don't use expanded metal for the screen. that tends to get clogged. Use a series of parallel flat bars, so it looks like adjustable blinds.

[Joshfromsaltlake]

Is this getting used as a beam or a column?

Taller structures are almost always stronger pound for pound than shorter structures. If you are using this as a beam horizontally taller thinner tube will usually beat shorter thicker tube in bending.

I think i understand what you're trying to say here, DSW. However this is really poorly worded. It's easy to get the wrong impression.

Do you mean "Wider" instead of "taller"

A wider tube with thinner walls is more resistant to buckling than a more slender tube with thick walls.

[burntside bob]

Thank you Joshfromsaltlake, appreciate the input.

I understand the idea of welding the thin plate to the 2 inch tubing on the three sides to stiffen it up and as a benefit to retain the screened gravel inside the box under the screen.
Welding 2 inch tubing at say a 45 degree angle from the upright tube to the bottom tube on the ground would stiffen it up also, both would be better.
I understand the angle iron 4 inch wide welded on the bottom of the box where it contacts the ground would help to prevent the screen from settling into the ground.

Question; what are your thoughts on the cross brase steel connecting the four upright 2 inch tubes at the corners, I was thinking of using a 2 inch wide by say 4 inch high tube. As I have the 12 foot wide space with no upright 2 inch tube under it for the loader bucket to get under the screen to remove the screened aggregate, the other three sides would have 2 inch tubes spaced along the length of the sides. would the 2" x 4" x 12 foot long steel tube have the strength to hold up to the screen weight and the gravel on the screen is my question.
Thank you for your thoughts.