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Active Contributor tim
Active Contributor
tim
Posts: 37
Registered: ‎10-10-2012

beam deflection error

607 Views, 5 Replies
10-19-2012 01:09 AM

Hi,

today i have been having problems getting the correct deflection on a simple Universal column (203UC46.3).

The stresses are spot on at 8.3 MPa in the middle of the beam.  I have applied a UDL of 840.3 N/m.  The beam is 5.95 m long.

 

I have midplane meshed this.  The deflection however is wrong at 1 481 327 mm!  It is about 1 million times too much.  Hand calcs give 1.5 mm.

 

I have made sure my model units = my display units as I had a problem earlier this week with shrink fits.

 

Has anyone got some insight on this?

 

 

 

 

 

 

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Distinguished Contributor
ilyas
Posts: 141
Registered: ‎06-13-2012

Hi Tim,   I opened up your fem file, set the plate thickn...

10-19-2012 01:57 AM in reply to: tim

Hi Tim,

 

I opened up your fem file, set the plate thickness to be the midplane mesh thickness and material as Steel(ASTM-A36). I ran the analysis and got the maximum deflection as 1.5mm (which is what you expected). In short, I did not find any problems with your settings(unless you specified too thin a thickness or too weak a material). What version of the software are you using?

 

Perhaps you might want to update to service pack 2

http://forums.autodesk.com/t5/Autodesk-Simulation-Mechanical/Autodesk-Simulation-Mechanical-2013-Ser...

 

Regards

Ilyas

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Active Contributor tim
Active Contributor
tim
Posts: 37
Registered: ‎10-10-2012

Re: beam deflection error

10-19-2012 03:04 AM in reply to: tim

Thanks,

I spotted the error thanks to your tip.  I used the default alloy steel as a quick check.  It has a youngs modulus of 0.205 N/mm^2.  It should be a million times bigger, hence my million times error!  This should be changed by you guys especially because its a default!

 

I'm using sp2 by the way.

 

Deflections are now correct.  Thanks!  By the way, If I select "use mid plane mesh thickness" does this pick up the different thickness in the web and the flange?

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Distinguished Contributor
ilyas
Posts: 141
Registered: ‎06-13-2012

Re: beam deflection error

10-19-2012 05:25 AM in reply to: tim

Yes, the Use mid-plane mesh thickness picks up the different thicknesses of plates provided that they are different surfaces. One surface can only have one thickness.

 

Regards

Ilyas

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Valued Mentor
AstroJohnPE
Posts: 510
Registered: ‎08-30-2012

Re: beam deflection error

10-19-2012 06:37 AM in reply to: ilyas

Either my memory or the Help documentation is wrong. But I thought the midplane mesh actually used the calculated thickness for each element, not just the same thickness for all elements on the same surface. So if you had a rolled beam where the flange thickness changed, you would see that "taper" by the varying thickness in each element. ("Results Contours > Other Results > Element Properties > Thickness of plate" in the Results enviornment.) Of course, the change in thickness would be a step change from element to element, not a smooth change.

 

This capability, if it exists, may be different for linear stress, MES, and thermal.

 

John Holtz, PE
Mechanical Engineer
Pittsburgh, PA

16 years experience with Simulation Mechanical
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Active Contributor tim
Active Contributor
tim
Posts: 37
Registered: ‎10-10-2012

Re: beam deflection error

10-19-2012 08:06 AM in reply to: tim

Thanks both of you for your info.  I've been on a good learing curve with ASM these last few weeks.  I've just installed sp2 on my home machine.  I'm now trying to mesh a much larger group of assembled beams.  I'm getting quite consistent crashes where the Autodesk error report comes up, sends a report and then it continues to mesh the model. 

 

Has anyone seen this error?

 

Regards

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