# Round vs. Square Tube Strength

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#### lake_harley

##### Well-Known Member
When considering the resistance to bending and the compression strength of square tube vs round tube, which is "stronger". I'll qualify that with an example; 3/4" X .065" square 6061-T6 aluminum vs. 3/4" X .065" round of the same material. How would they compair in bending and compression loads? Considering the round is +/-75% of the weight of the square, is the resistance to bending and strength in compression compairable, higher or less? Is the load comparison somewhat similar to the difference in weight? Tension I would expect to be reduced by the reduction in cross section area.

A second way to ask the question would be how do they compair on a Lb. for Lb. basis? Which would offer more bending resistance and handle a higher compression load, a round tube with thicker wall or a square tube, but with the same O.D., ie. 3/4" Sq vs. 3/4" Round, but weighing the same in terms of #/Ft.?

I was always under the understanding that # for #, round was the stronger shape compaired to square. Maybe the answer is different for torsional strength vs. bending or compression?

Square is certainly easier to use in fabrication since no "fishmouthing" has to be done to intersecting tubes. I'm more interested in getting the biggest bang/Lb.

Lynn

#### PTAirco

##### Well-Known Member
The round tube wins on a pound for pound basis, usually. Take as an example a freely supported column of 3/4" x 065 tubing , 36" long, and the 3/4" x 065 round, also 36" long"

Used as a strut the square tube will support about 1066 lbs before buckling, the round 625 Lbs.

Support the same tubes at the ends and the square one will support 73 lbs hung form the center before failing and the round will support about 42 lbs (in reality of course other factors come in such as local buckling etc, I'll ignore these for this example).

In shear, the sqaure will take a load of 4800 lbs and the round 3775 although tubes like this are of course rarely used in pure shear.

Torsionally the square is about 30% stronger , but it also about 30% heavier.

So in column loading and in bending the square will take 1.7 times the load of the round tubing, for this example.

You have to look at it case by case, but for a typical fuselage truss square tubing will be heavier than round if both are optimised. Also long struts such a lift struts will be lighter as round tubing.

That is not to say the advantage of ease of manufacturing of the square stuff are always outweighed by the weight increase. A lot of old British aircraft used high strength square steel tubing in their fusleages, for example. Simple to put together (since you couldn't weld the stuff) with flat gussets and rivets/bolts.

I bet that didn't help a lot, did it....?

#### lake_harley

##### Well-Known Member
So in column loading and in bending the square will take 1.7 times the load of the round tubing, for this example.

You have to look at it case by case, but for a typical fuselage truss square tubing will be heavier than round if both are optimised. Also long struts such a lift struts will be lighter as round tubing. quote]

Everything was sounding like on a pound for pound basis the round would be the preferred material until I read and re-read these 2 paragraphs. If the round is +/-75% of the weight of the square for a given dimension, but the square offers 1.7 times the load capacity in compression and bending based on O.D and wall thickness, it seems that round might have to be up-sized in diameter and/or wall thickness to be a "like" substitute to the degree that it could actually end up heavier in weight. Possibly not that much increase in the round's diameter/weight would be needed to give the same results? Maybe I'm reading something wrong? I should have stayed in school:dis:!

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Evan's Light Plane Handbook was the document I grabbed for this. They list the section coefficients and weights for round, square, and streamline tubing. Useful book. It also has charts of the maximum loads for each of the round tubes in column loading.

IF you are looking at bending strength, a square tube makes a more efficient beam than a round tube of the same size. Figures of merit are I and I/y=Z, which relate to bending strength, and wt/length. Now if you are building a fuselage truss, many of the tubes will see compression loading, and the strength in compression when the tube is long enough for column buckling to matter is also related to the I/y of the section. So, you could theoretically build a lighter truss of the same strength at lower weight using square tubing. Somehow I suspect that most of our fuselage trusses do not use long enough sections between welds to get far into column buckling, and so you are back into the tensile and compressive strengths of the tubes, which basically changes directly with the area of the tube...

I will bow out on the rest of the discussion, because I have never gotten into designing and building a fuselage truss. I am certain that the round tubes have advantages of some sort, or they would not be the dominant component in these structures.

Billski

#### lr27

##### Well-Known Member
If you bend a square tube across the corners instead of across the flats, I think you will not like the results. As I recall, it starts to fail in the corners because the stress gets high there without supplying a lot of moment to take the load, because there isn't much material out there in the corner. So, if you know the orientation of the load, square is probably better. If not, or if there are many possible loads, the round is better.

#### lake_harley

##### Well-Known Member
Thanks for the replies.

wsimpso1, I think you hit on the factor that applies, that most of the lengths of individual tubes in a truss aircraft structure are not very long, most 24" and less. I remember a documentary on PBS which regarded the building of a bridge (different animal I realize) at Alton IL, just north of St.Louis, and the statement that was made is "anyone can build a structure to support a load, but it is a challenge to do it without over-building it". I'm paraphrasing of course, but the idea is the same.

For what it's worth, I'm not designing a airframe from the ground up, just wanting to understand the reasoning for the choice of material and sizing in a particular design. I do fabrication of various commercial/industrial weldments, mostly from mild steel and stainless steel, and even though I have a tendence to overbuild things of my design not to the extent of some of the drawings done by others that we build from. This does relate to a airframe design I'm considering that seems a bit overkill in tube sizing, but don't consider myself wise enough to change the design without guidance. Potential weight savings would only amount to 3-5# in the entire airframe, not enough to go it alone without consulting an engineer.

Thanks again for the replies. I always welcome more discussion and input.

Lynn

#### PTAirco

##### Well-Known Member
Also, it's not fair to compare 3/4 size round and 3/4 square tubing - For the same weight as a 3/4 x 0.065 tube, you could use a 1" x 0.058 (still a fraction lighter) and now the same 36" strut could support 1445 lbs, about 40% more, for the same weight. That is why round usually beats square, but you have to have the freedom to use larger sizes.

#### lake_harley

##### Well-Known Member
Also, it's not fair to compare 3/4 size round and 3/4 square tubing - For the same weight as a 3/4 x 0.065 tube, you could use a 1" x 0.058 (still a fraction lighter) and now the same 36" strut could support 1445 lbs, about 40% more, for the same weight. That is why round usually beats square, but you have to have the freedom to use larger sizes.
Your example of up-sizing the round to handle the same or higher load with similar or less weight is what I was interested in knowing. I suppose that it's within what I had always understood to be true, that pound for pound the round cross section offers more "strength". Maybe strength isn't actually the correct engineering term but for the scope of our discussion and my understanding of compairing square vs round tube I'll call it good.

Thanks for your 3/4" X .065" sq. vs 1" X .058" round example. That is exactly the type of comparison that makes it more understandable. I didn't want to ask anyone to do the math to do such an example but thank you for it! You've come through, in spades!

Lynn

#### PTAirco

##### Well-Known Member
Figuring out the strength of coloums (struts) is really pretty simple; it's just a simple formula. If you know anyone who has a copy of Bruhn's "Analysis if Flight Vehicle Structures" you can find simple graphs for finding out the strengths of common tubing sizes. Not that that is the kind of book you find on everyone's coffee table.

The thing to rember is length - strength goes down rapidly with an increase in length ; by the square of the length in fact. For example that 36" tube would support four times the load at 18" or only a quarter of the load at 72".

#### Horserich

##### New Member
Regarding PTAirco's
"For the same weight as a 3/4 x 0.065 [rect.] tube, you could use a [round] 1" x 0.058 (still a fraction lighter) and now the same 36" strut could support 1445 lbs, about 40% more"
as well as
"The thing to remember is length - strength goes down rapidly with an increase in length ; by the square of the length in fact. For example that 36" tube would support four times the load at 18" or only a quarter of the load at 72". "

Does the same general conclusion that round tubing of the same weight per foot (hence, bigger) is substantially stronger than rectangular, apply equally to bending loads (as in your initial answer you gave the example of the tube supported at both ends with a weight applied in the middle)?

If yes, is it by similar large increases (such as "40%", "four times", etc.)? Or is it primarily in compression that round has the advantage?

Same question for elongation (pulling at the end of the tube)?
(am I ccrrect to assume your words "Torsionally the square is about 30% stronger , but it also about 30% heavier" were speaking of a twisting force, as in unscrewing a lid from a jar, as opposed to trying to pull the two ends of the tube apart?)

Also, since your first answer said "usually" the round tube wins on a pound for pound comparison, I wondered under what use does the sq tube win?

#### Bart

##### Well-Known Member
This is why I always wondered why Bede in his BD4 and BD5 planes used round alu. tubing for the spars. Seemed to me, square alu. tubing would have been much better, since centroids of the flanges would have put more material further apart, for exponential increase in stiffness.

So, Bede could have used thinner wall square tubing for the same strength as round, for weight savings yet still with equivalent or greater stiffness.

Thoughts?

And, previous poster (whose input I much appreciate, btw) said square tubing is significantly more resistant to torsional (twisting) loads. That is interesting, as I'd have thought round better in torsion. Any elaboration on this topic also much appreciated.

##### Well-Known Member
This is why I always wondered why Bede in his BD4 and BD5 planes used round alu. tubing for the spars. Seemed to me, square alu. tubing would have been much better, since centroids of the flanges would have put more material further apart, for exponential increase in stiffness.

So, Bede could have used thinner wall square tubing for the same strength as round, for weight savings yet still with equivalent or greater stiffness.

Thoughts?
A couple issues come to mind.
Round doesn't have stress concentrations, at least not to the extent square or rectangular has them. (Spar cap shear and such)
Round has better torsional stiffness for the same weight. (Important for high-speed (torsional flutter) and low load (small aircraft)
Round is easier to adopt the wing incidence angle if you need to "fix" that. This is very important in flying stabilizers.
Round spars are easier to mount to a fuselage ("drill a big hole" vs a perfectly aligned rectangular hole)
Round is easier to make into a folding wing.

Not all of these apply to the BD's and I would also pick the rectangular over the round.

#### Dana

##### Super Moderator
Staff member
Did the BD-5 use the wing spars for fuel tanks as did the BD-1 (later American Yankee and still later Grumman)?

It's also very hard to find large size square tubing in the appropriate wall thicknesses. If you have to build it up, then you lose the labor savings of using a single tube, so round has the advantage there, too.

It's also easier to build washout on a wing with round spars. Otherwise, each rib has to be slightly different where it attaches to the spar.

-Dana

Friends come and go, but enemies accumulate in a pile outside.

#### Bart

##### Well-Known Member
A couple issues come to mind....

1.) Round doesn't have stress concentrations, at least not to the extent square or rectangular has them. (Spar cap shear and such)
2.) Round has better torsional stiffness for the same weight. (Important for high-speed (torsional flutter) and low load (small aircraft)
3.) Round is easier to adopt the wing incidence angle if you need to "fix" that. This is very important in flying stabilizers.
.....
Not all of these apply to the BD's and I would also pick the rectangular over the round.
More thoughts in order above, to stimulate more discussion:

1.) Square tube would have more stress concentrations (four) than round, but what about comparing square to an I-beam? Seems like square would be even less stress concentration than the traditional I-beam, albeit more than round, so would be good enough. Besides, wing skin adds torsional resistance unless fabric skin, so this might not be a big deal.

2.) That's what I always thought, too, until another poster above (post #2, paragraph 5) said square was more (~30%) torsion-resistant. Surely, compared to an I-beam, square would have greater torsion resistance, so would more easily pass the good-enough test. Again, I'd assumed for torsion resistance that round would be best, square next, I-beam last. So, I want to hear from the experts on this topic, which I am not.

3.) Per Bede slip-on ribs, I agree, although flat metal skins could be a problem to fit on a twisted set of ribs. But with hotwired foam cores glued to square tube spars, maybe not so much a problem. Either way, I'd like to learn more on this topic. If Bede had used square tube spars, surely he could have had matching square holes in his prefabricated aluminum ribs.

Then, in the case of a square or rectangular tube spar, is the question of making it lighter (where prudent) by drilling holes, such as in the webbing sides.

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#### HumanPoweredDesigner

##### Well-Known Member
The round tube is more efficient in compression and the square tube is more efficient in bending if the wall thickness and mass are the same for both. For compression, the minimum diameter matters most. For bending, the average diameter in the plane of bending matters most. But these apply only to simple materials on simple structures.

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#### HumanPoweredDesigner

##### Well-Known Member
2.) That's what I always thought, too, until another poster above (post #2, paragraph 5) said square was more (~30%) torsion-resistant. Surely, compared to an I-beam, square would have greater torsion resistance, so would more easily pass the good-enough test. Again, I'd assumed for torsion resistance that round would be best, square next, I-beam last. So, I want to hear from the experts on this topic, which I am not.
A square tube has more torsional strength if it is more massive (in the case of minimum diameter equal to the round tube) and it has those corners far out from the center. The outward mass gives leverage in its advantage, torsionally. But if they are the same mass and wall thickness, the round tube would be more torsionally stiff. It would also inclose more area.

Off topic:
Those links have viral properties. When I quote someone who uses them, my future posts are converted to links too.

#### Chuckdw

##### New Member
Also, it's not fair to compare 3/4 size round and 3/4 square tubing - For the same weight as a 3/4 x 0.065 tube, you could use a 1" x 0.058 (still a fraction lighter) and now the same 36" strut could support 1445 lbs, about 40% more, for the same weight. That is why round usually beats square, but you have to have the freedom to use larger sizes.
I looked for Bruhn's "Analysis if Flight Vehicle Structures" , not at library. $150+ online. Would somebody please tell me the compression strength of Al 2024-T3 1" x 0.035 round by 36" and also 48" long ? I assume that 0.035 is half of 0.065 , but I'm using 2024-t3. I didn't want to ask, but the book is outta my reach. I'm a tech, not an ENG & might not grasp it from the book. Thanks #### wsimpso1 ##### Super Moderator Staff member Log Member Twice a necropost! #### dougwanderson ##### Well-Known Member #### proppastie ##### Well-Known Member Log Member I looked for Bruhn's "Analysis if Flight Vehicle Structures" , not at library.$150+ online.
Would somebody please tell me the compression strength of Al 2024-T3 1" x 0.035 round by 36" and also 48" long ?
I assume that 0.035 is half of 0.065 , but I'm using 2024-t3.
I didn't want to ask, but the book is outta my reach. I'm a tech, not an ENG & might not grasp it
from the book.

Thanks
Good luck

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