Deflection of square steel tubing

Are you trying to determine the deflection in the mid-span or the deflection/angles on the ends?

How is the tubing supported/held? A point load in the middle?

Yes a point load in the middle, it would be like putting a rolling floor jack right in the middle and jacking it up. how much would the center bow up before the weights came off the ground?

Thanks for your quick response

Paul

Okay - like you mentioned this type of detail is risky over an internet forum - so DO NOT TAKE THIS VERBATIM.

Because we do not know what exactly this is going to be used for, nor do we know of any live/impact loadings that will occur. I looked at it from simple static point of view, not designed to any civil code - and in the smallest wall thickness you should be safe with the amount of stress (about 1/2 yield strength based on low strength A36)

These numbers are a little conservative (actually deflection will be slightly less) because I came across a discrepancy in the moments of inertias stated by some suppliers and the AISC manual.

Having said all this:

You can expect a deflection mid-span of approximately:

0.46" for the 3/16" wall

0.37" for the 1/4" wall

0.33" for the 5/16" wall

Hello Guest

Can't agree with your figures. First reaction on seeing the OP was 3" x 3" won't stand 2000lb at 37 ft. Checking it it for 80 x 80mm, 5mm thick gives stress ~ 2900 MPa, 187 ton/in² i.e. much too high, and theoretical deflection 50 ft. (obviously it breaks long before that). Unless I've done something wrong.....

Cheers..........Codey

Yeah - I think you have done something wrong.

Where did you get 37 ft?

The OP said 74".

OK good point! I mistook " for '. Makes all the difference. Apologies!

Codey

Good morning Klotski. I'm assuming the situation is like a beam supported by a pin and roller connection, with a point load in the center. Also that the beam is made of regular A36 steel. I could not find the 0.313 wall tubing in the AISC, so I derived a section modulus from the strucalc. I get a moment of 74,000 in #. Without using the industry standard derating factor of 90%, I get a bending stress in the two smaller-wall tubes of over the specified minimum yield strength, and the other too close for comfort. Therefore, the conlusion is you need a member with about double the section modulus. One that would work with only slightly more cross-sectional area is S5x10, which has A=2.94in^2, S=4.92in^3, and a comforable bending stress of 15,000psi < 36,000 psi.

Whoops - Guest here again. Good catch Flyinghigh - I made a mistake in my stress calculation ......... 4(1.64)=6.656 ......... NOT = 16.656

Klotski - you are going to need a bigger member to not overstress it.

........ or a higher strength grade such as:

ASTM A500 Grade B - Fy = 46 ksi; Fu = 58 ksi

ASTM A500 Grade C - Fy = 50 ksi; Fu = 62 ksi

However, the stress on the thinnest of those walls is 44,500 psi - which is pushing the limit.

Note that the deflections will not change because they do not depend on strength - and I hope I didn't make a mistake in them.

Hello Klotski

A demo programme which is useful for estimating deflections of a range of materials, sections and load configurations can be downloaded from

http://www.mitcalc.com/

download the beam calculator section - I think you get 30days trial - and it very reasonably priced if you choose to buy it.

Regards

Thanks to all of you, for the help.

I have been out of town since the original post.

Regards

Paul