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## Robot Structural Analysis

Active Contributor
Posts: 29
Registered: ‎01-26-2010

# Critical moment-zg value

351 Views, 12 Replies
07-11-2012 12:52 AM

As you can see in the attached image zg is considered positive in columns_left analysis and negative (same value) for columns_right....thus, different Mcr values have been calculated...(verified by hand as shown here http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Critical-moment/m-p/3335273/highlight/true

Is it possible to have negative values for zg since it's a distance?

Shouldn't both left and right columns have the same zg value since both are loaded on the upper flange which is also the restained one...

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Posts: 4,297
Registered: ‎12-17-2010

# Re: Critical moment-zg value

07-11-2012 02:23 AM in reply to: arailos

Could you send the rtd file please?

Artur Kosakowski
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Posts: 29
Registered: ‎01-26-2010

# Re: Critical moment-zg value

07-11-2012 04:05 AM in reply to: Artur.Kosakowski

Please find zip file attached...The rtd has been saved without results...

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Registered: ‎12-17-2010

# Re: Critical moment-zg value

07-11-2012 07:07 AM in reply to: arailos

Case 9 component 42 - approx. the same internal forces for both columns (207 and 208).  The values of Mcr are 2342 kNm for bar 207 and 28834 kNm for bar 2068 respectively. This difference is caused by the fact that you set that for bar 207 load is applied at the top flange level whereas for bar 208 at the bottom one. Setting the top flange for both bars you will have the same value of Mcr. The plus and minus sign for zg is due to the fact that they are measured from the middle of the section in the opposite directions.

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Artur Kosakowski
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Posts: 29
Registered: ‎01-26-2010

# Re: Critical moment-zg value

07-11-2012 11:36 AM in reply to: Artur.Kosakowski

Ok I figured out which is the upper and lower flanges according to Robot so I have no problem with Mcr now?

But something that confused me when trying something is that when I give a gamma angle of 180 degrees to the columns which are symetrical (HEA) I have different critical load components for steel analysis as you can see from the images attached....The rtd file is the same as previous...

Why is that?

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Posts: 29
Registered: ‎01-26-2010

# Re: Critical moment-zg value

07-11-2012 12:03 PM in reply to: arailos

Mea culpa... the rtd file of the previously attached images is not the same. It's the same structure with simplified wind loads according to DIN code...I upload the file just in case you have some time to have a look....

But the same happens with the one I uploaded in my previous post, if you rotate the sections, compute the calculations,verify once, change some code parameters needed (upper lower flange loading and detention) and verify again....

Thanks a lot

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Posts: 4,297
Registered: ‎12-17-2010

# Re: Critical moment-zg value

07-11-2012 11:40 PM in reply to: arailos

But something that confused me when trying something is that when I give a gamma angle of 180 degrees to the columns which are symmetrical (HEA) I have different critical load components for steel analysis as you can see from the images attached....The rtd file is the same as previous...

Why is that?

This seems to be reasonable as the loads are in the opposite directions which causes different intenal forces on the windward and lee sides of the building. You can see that the results are symmetrical.

Artur Kosakowski
Active Contributor
Posts: 29
Registered: ‎01-26-2010

# Re: Critical moment-zg value

07-12-2012 12:11 AM in reply to: Artur.Kosakowski

I think that I was not quite elaborative so you didn't understand exactly what I mean...

Please have a look at the following attachments...

The different critical load case component happens for the same columns when I change the gamma angle...due to symmetricity I think that the critical component should be the same irrespective of the column orientation.....

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Registered: ‎12-17-2010

# Re: Critical moment-zg value

07-12-2012 12:48 AM in reply to: arailos

One question - after rotating the bar have you redefined the LTB  parameters assigned to the top and bottom flanges? They should be 'reversed' too.

Artur Kosakowski
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Posts: 29
Registered: ‎01-26-2010

# Re: Critical moment-zg value

07-12-2012 12:57 AM in reply to: Artur.Kosakowski

They are not reversed because the critical component as you can see from the images has different direction...

In both case components the load is applied to the upper flange...

That's why I have marked the LB parameters on the images attached.....

The upper flange is defined according to the local axis of the bar isn't it?Positive z gives upper flange....That's why I get the same Mcr in both cases as well....