The 3x3x.250 square tube would rank highest, regardless of how you orient the axis. Followed by the 3x2x.250 rectangular tube on the strong axis and the 3" OD .250 wall round coming in last.

If your 3" steel pipe happens to be 3" ID and 3.5" OD then it would be the strongest choice. All the above is assuming that your end connections are roughly equivilent.

As you probably guessed, the anticipated load on the bars isn't as important as how high you dump from and how quickly you dump the material. None of the tubes you have listed come close to the strength of a 1x4 bar laid-up on the strong axis. My guess is you already knew that and you intend to build something scaled to your needs.

Good luck with your project.

 

For any given diameter vs. cross section, with the same wall thickness, square tubing is far stronger. It also weighs more and costs more.

 

I agree with the above Square box is far stronger than tubing

 

rock and gravel would be slowly dumped onto the bars

Unless you're the one going to be running the loader 100% of the time don't believe this for 1-minute!:nono: I built a grizzly for a rock yard about 25 to 30-years ago, seems as I remember using 8 or 10-inch WF-beams for the frame, and 1” x 6” flatbar for the screen supported in 3 or 4 places. The crew were running 988 CAT loaders and in a big a$$ hurry at all times! :laugh:

 

A second thing to take into concideration is abrasion it will not take long at all for rock to wear thru 1/4" wall tube

 

Wet sand and gravel is very abrasive and will wear through 1/4" material very quickly as mentioned. I would build it out of min 1"x4" flat bar on edge with a solid well gusseted frame made from I-beam.

 

Most grizzly bars are 1" thickness minimum, AR material if you can afford it.

 

most gravel pits around here use W-beams to scalp of the large rock, and they still get all bent to heck.

i don't think you can make it too strong.

build the best one you can afford and budget it's replacement accordingly.

 

Bob...

There was a thread here a few months back that addressed building one of these. The options presented were extensive and from some guys who worked at quarries, etc. where their experience speaks mountains. Might want to do a search for it.

 

He knows about that thread since he was the one who started it. We've already tried to convince him on this before.

http://weldingweb.com/showthread.php?t=190971

If you don't like the price of 1x4" stock then how about 1x3"? Shouldn't be that much more than the tubing.

Also consider putting some supports across under the bars one or two would help tremendously considering the span you're doing. You can see one support on the one I made in the pics on the thread below.

http://weldingweb.com/showthread.php?t=125511

I got to look at it the other day while replacing a chute on the same machine last week. It's still in good shape except for some minor bulging on the 1/4" plate wings (which I knew would happen).