Rigid structrual shape

[pkpavv]

Hi,

I would like to ask one fundamental question. It would be great if somebody lit my light again.

Which is the strongest section for same cross sectional area (say for some load bearing frame weldment) among 'Square tube and hallow tube'???

What is the parameter to judge this???

I would be greatful, if somebody help me.

Regards,
mkp

 

[Lion06]

not sure I understand your question. Strongest for flexure? Strongest for axial compression?
What is a load-bearing frame weldment?
What is the difference between a square tube and a hollow tube? Is the square tube solid?

 

[pkpavv]

Hi StructuralEIT,

It seems I posted question thinking only in my prospect.I am sorry for that.

1.I am looking at a weldment which should be capable of taking good flexural loads (Basically not going to have much axial loads). I

2. I am trying to find best structural member for my application either Circular tube or Square tube. Which one is better for same cross-sectional area??

What is the parameter to judge this?? I mean how can I know this theoretically??

Hope now my question is clear. Anyway thanks for your response.

Regards,
mkp

 

[civilperson]

Greatest I, (moment of inertia), for the axis in flexure is what you want. Since the depth cubed is in the formula, try for the greatest depth with a given cross section area. The W shapes have more strength than rectangular HSS for the same depth and same area.

 

[PMR06]

Set area equal to 1, solve for square dimension b and circle diameter d. Then calculate I and S for both and compare.

Square: A=1.000, b=1.000, I=0.0833, S=0.167
Circle: A=1.000, d=1.128, I=0.0795, S=0.141

The square appears both stiffer.

 

[PMR06]

... forgot and stronger at the end of my last sentence.

 

[pkpavv]

Thank you so much guys, I really appreciate your responses.

Regards,
mkp

 

[frv]

One caveat about wide flanges..

They are subject to lateral-torsional buckling, so the statement that wide flanges are stronger is true only if it is properly braced.

For the sake of completeness it is worth mentioning that HSS members are also subject to LTB, but the span to depth ratios necessary for this to control are completely unrealistic.

 

[Dinosaur]

When pursueing a question like this there are unfortunately several things to be considered. As already stated, member strength loaded in flexure increases substantially with increasing depth in the plane of the load. So, one may conclude that for equal area, the deepest section always wins. But this is not so because there will come a point where the thickness of the material will cause the shape to buckle reducing its strength. This is closely related to the comment about adequate bracing. With some member proportions the entire shape may buckle. In other cases, the thin wall of the section may buckle. Either of these will reduce the strength.

Having said all that, a hollow square will have a greater moment of inertia and section modulas in bending than a hollow circular section of the same depth. You may want to consider the rounded corners of the square tube as special situations and decide if you want to make an allowance for the residual stresses there. But, for a given wall thickness and cross sectional area, the circular tube might be stronger because it will be deeper. A quick hand calc showed me the circle had a higher I but a smaller S.

Finally, I do not think doubly symetric shapes are subject to LTB. LTB causes a shape to roll over to its weak axis. Doubly symetric shapes do not have a weak axis.

 

[frv]

Dinosaur-

That's right. It's the same principle as a wide flange bent about its weak axis- it will yield long before you can reach the point of LTB.

 

[rb1957]

going back to the post on 9th Apr ...

the square tube is going to be superior in bending as it has more material at the extreme fiber (compared to a round tube) and therefore can absord more moment (or produce smaller stresses/deflections for the same moment).

this is easy to show with basic section properties.

but doesn't "weldment" make the problem more complex ? again, a square tube is going to be easier to weld to.

if you know your load directions a rectangular tube would be better yet (from the same simple section properties, more material further from the neutral axis).

 

[hokie66]

"Weldment" is indeed a very important word in the OP's question. In order to develop the capacity of a closed section, the welds will have to be performed with the use of backing bars.