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11 comments
Member

Join Date: Nov 2008
Location: Moorhead MN USA
Posts: 5

Deflection of square steel tubing

11/27/2008 11:57 PM

I am trying to determine how much deflection a piece of 3" steel square tubing 74" long will have. I need it for 3 different wall thickness .188, .250, .313. the best way I can explain it, is to picture a dumbbell from a weight set, you know how the outside edges bow down when it is picked up. my dumbbell would use the 3" tubing for the center piece and would have a 2000 lb weight on each side. I know from reading other posts that people don't care to offer structural design information this is for some preliminary design work only, if it goes any further then this, I would have a engineer approve it.

Regards Paul

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Anonymous Poster
#1

Re: Deflection of square steel tubing

11/28/2008 12:05 AM

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?

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Member

Join Date: Nov 2008
Location: Moorhead MN USA
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#2
In reply to #1

Re: Deflection of square steel tubing

11/28/2008 12:14 AM

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

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Anonymous Poster
#3
In reply to #2

Re: Deflection of square steel tubing

11/28/2008 12:38 AM

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

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Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
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#4
In reply to #3

Re: Deflection of square steel tubing

11/28/2008 8:58 AM

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

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Anonymous Poster
#5
In reply to #4

Re: Deflection of square steel tubing

11/28/2008 9:04 AM

Yeah - I think you have done something wrong.

Where did you get 37 ft?

The OP said 74".

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Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
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#6
In reply to #5

Re: Deflection of square steel tubing

11/28/2008 9:35 AM

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

Codey

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Join Date: May 2008
Location: Arizona
Posts: 346
Good Answers: 4
#7

Re: Deflection of square steel tubing

11/29/2008 9:15 AM

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.

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Anonymous Poster
#8
In reply to #7

Re: Deflection of square steel tubing

11/29/2008 7:15 PM

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.

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Anonymous Poster
#9
In reply to #8

Re: Deflection of square steel tubing

11/29/2008 9:18 PM

........ 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.

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Commentator

Join Date: Oct 2008
Location: Gaborone, Botswana
Posts: 74
Good Answers: 18
#10

Re: Deflection of square steel tubing

11/29/2008 10:30 PM

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

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Member

Join Date: Nov 2008
Location: Moorhead MN USA
Posts: 5
#11

Re: Deflection of square steel tubing

12/07/2008 9:42 PM

Thanks to all of you, for the help.

I have been out of town since the original post.

Regards

Paul

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Anonymous Poster (5); Codemaster (2); flyinghigh (1); johnbots (1); Klotski (2)

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