Quick steel strength question??

[Adz_4]

Hi All,

A quick question, what would be stronger / more rigid, 1 piece of 2"x2" steel or 2 pieces of 1"x2" steel bolted on top of each other using u-bolts (see diagram below)................



Cheers,

Adam

[Ageless]

The bolted unit. The bolts would resist the flexing.

[starleen2]

Thank goodness you didn't ask which one is lighter!

[aggie79]

Assuming the same wall thickness, the bolted would if you rotated it 90 degrees. As it is shown, it is sort of like a leaf spring. The bolts wouldn't keep the pieces aligned, at it would flex along the joint.

[eamarquardt]

I assume you are dealing with tubing.

I don't think mere ubolts would prevent the tubes from slipping against one another so you're really dealing with simply adding the strenght of two independant tubes so doubling the number of tubes would double the strenght.

The strenght of a rectangular tube increases with the cube of the distance between the height of the upper and lower surfaces. So a 2" high tube is eight (2X2X2) times stronger than a 1" high tube.

So a 2" high tube is 4 times stronger than two 1" tubes stacked together. If you were to weld the two tubes together so they couldn't possibly slip, it would be a new ballgame and the two tubes would be stronger than a single 2" high tube.

Hope this helps.

Cheers,

Gus

[Adz_4]

Thanks guys, so from what I gather, a single 2"x2" steel tube is stronger but would bolting 2 pieces of 1"x2" steel tube on top of each other be stronger/more rigid than a single 1"x2" on its own?

Cheers,

Adam

[eamarquardt]

Two tubes stacked one upon the other would be twice as strong assuming that they are free to slip against one another to some degree. If you weld them together they will be MUCH stronger (4 or more times stronger) together than if allowed to slip against each other.

Cheers,

Gus

[Carter]

My 2 cents worth (and I am no expert)

The two 1x2's stacked would be not as strong as a 2x2 if they are allowed to slip. Welded together they may be slightly stronger. Two 1x2 pieces stacked vertically along side each other would be much stronger vertically than a single 2x2 (4 vertical wall thicknesses instead of 2)

Jim

[asianflava]

A single tube 2X2X3/16 would be ok on a 4X8 or even a 4X10. A larger trailer will need something like a 2X3X3/16 or 3X3. Don't ask how I know.

[eamarquardt]

The discussion continues. I built my frame out of 2X3X1/8 wall but used a 3X3X3/16 piece for the tongue. Simply put, the whole thing is overkill. I used the 3/16 wall for the tongue cause I had it "in stock". Also you can really tighten the coupler on it w/o fear of crushing the tubing as might be the case with 1/8 wall but there is a way around that by putting spacers inside the tube that the coupler bolts pass through.

What exactly are you plannig? Are you getting the 1X2 tubing for free? I agree that if you put the two 1X2s on end they will be stronger than if flat. But if you must use 1X2 welding them together (I don't think the weld would have to be continuous) so the don't slip in the horizontal orientation would be the strongest.

Let us know what you're up to.

Cheers,

Gus[/list]

[challengmymopar]

two welded, keep them from moving back and forth

[kennyrayandersen]

Thanks guys, so from what I gather, a single 2"x2" steel tube is stronger but would bolting 2 pieces of 1"x2" steel tube on top of each other be stronger/more rigid than a single 1"x2" on its own?

Cheers,
Adam

This is a different question, but a good one. This isn't quite as straight-forward as it might first seem. These are hollow sections for one thing, so some of the ratios are off.

For a solid section the equation for the moment of inertia is 1/12bh^3 so the solid section standing vertically has 4 times the moment of inertia as the one laying down. But for a hollow section that's not exactly the case.

The moment of inertia for a 14 gage (t= .0781) 1X2 laying down is:

I = 1/12[(bout*hout^3)-(bin*hin^3)]
I = 1/12[(2*1^3)-(1.8438*.8438^3)] =.8923 in^4

If you laid them on top of each other you'd get, as was pointed out only 2 times the strength (the moment of inertia per unit remains the same).

If the 1X2 beam is laid on end the moment of inertia is:
I = 1/12[(1*2^3)-(.8438*1.8438^3)] = 2.7109 in^4

The stress due to bending equation is stress = M*c/I
where I is the moment of inertia from above, c is the distance from the center of the section to the outer fiber (upper or lower surface in this case). So, in the case of the vertically placed beam the distance is double, so the stress is double... Whoa Nellie!

So that actual stress each beam sees is not as different as it would first appear

In the single flat cast stress = (1/2)*M/.8923 = .56*M
for 2 flat together it would be .28*M

For the single vertical case:
stress = M(1)/2.7109 = .36*M, which is in-between the other two solutions

So, the 2 flat beams will actually carry more total load. I was actually a little surprised myself, but there you have it.

The deflection (stiffness) is different though as the deflection is purely a function of I (moment of inertia)

deflection for a simple beam with a load in the middle:
def = (48*L^3)/(48*E*I)

All of the terms are the same excepting I, so the vertical 1X2 is 2.7109/(.8923) = 3.04 or 304% stiffer (less deflection) than the one laying flat and about 152% stiffer than the 2 1X2s laying flat together.

It's obviously better to have 2 1X2s laying vertical together if you have to have 2.

Now, having said all of that, reality is that the tear box, if it's built worth its salt and attached to the frame, will be stiffer yet and will actually react most of the loads despite the steel frame underneath it and this is actually the BIG secret. Everybody thinks that the frame is adding the strength, but actually the walls of the trailer are so tall, and the inertia is a cube function of the height, so the wall is the stiffest and that's where the load is going.

[sagebrush]

WOW!!!! That made my head hurt. Will

[doug hodder]

Don't let the dog chew on it! sorry it was just there, someone had to say it. Doug

[Corwin C]

When I read kennyrayandersen's calculations, the moments of inertia seemed awfully high. He used the right equation, but made a couple of calculator errors ... (could happen to anybody)

The moment of inertia for a 14 gage (t= .0781) 1X2 laying down is:

I = 1/12[(bout*hout^3)-(bin*hin^3)]
I = 1/12[(2*1^3)-(1.8438*.8438^3)] =.8923 in^4 should be 0.0744 in^4 (forgot to divide by 12)

If you laid them on top of each other you'd get, as was pointed out only 2 times the strength (the moment of inertia per unit remains the same).

If the 1X2 beam is laid on end the moment of inertia is:
I = 1/12[(1*2^3)-(.8438*1.8438^3)] = 2.7109 in^4 should be 0.2259 in^4 (forgot to divide by 12)

Unfortunately, this also messed up his subsequent calculations.

The question was, however, between stacked 1x2's and a 2x2.

Using the same equation for a 2x2:

I = 1/12[(bout*hout^3)-(bin*hin^3)]
I = 1/12[(2*2^3)-(1.8438*1.8438^3)] = 0.3702 in^4

Here's my final comparison.
2"x2" = 0.3702 in^4
1"x2" vertical = 0.2259 in^4 or 61% of the 2x2. Two stacked side by side would be 122% of the 2x2.
1"x2" horizontal = 0.0744 in^4 or 20% of the 2x2. Two stacked on top of each other would be 40% of the 2x2.

Now ... if the stacks are attached rigidly (i.e. welded) it creates a whole new monster that would require modifying the equation a little (I'm willing if anyone wants it, right now I feel lazy.) I believe, however, that the 1x2's stacked on top of each other and rigidly welded would be very slightly higher than the 2x2 (I'm guessing less than 102% of the 2x2), and the 1x2's stacked side by side would be essentially the same as they are without being welded because there is essentially no shear between the parts (122% of the 2x2).

How's that for a brain fart ...

These calculations are for 14 gage tubing ... as light as I would even consider in my opinion. As the thickness increases, the differences between the different configurations will change. (i.e. these values aren't necessarily true for 1/8" or 1/4" thickness tubing.)

Now, having said all of that, reality is that the tear box, if it's built worth its salt and attached to the frame, will be stiffer yet and will actually react most of the loads despite the steel frame underneath it and this is actually the BIG secret. Everybody thinks that the frame is adding the strength, but actually the walls of the trailer are so tall, and the inertia is a cube function of the height, so the wall is the stiffest and that's where the load is going.

I agree with this statement. Most issues with trailer frames will be in the strength of the tongue, not the members which are attached to the floor.