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## Autodesk Robot Structural Analysis

Distinguished Contributor
Posts: 183
Registered: ‎04-10-2012

# Non-linear analysis, P-delta analysis

789 Views, 13 Replies
10-02-2012 08:52 AM

These settings are described in two places in the help:

http://docs.autodesk.com/RSA/2013/ENU/index.html?url=filesROBOT/GUID-539BF9B3-3DA5-4E07-8BC6-15284F8...

http://docs.autodesk.com/RSA/2013/ENU/index.html?url=filesROBOT/GUID-FB3C86D3-0E30-43A6-82D6-6C50F42...

I always use Non-linear analysis, because I tested that this is in fact what is usually called a second order or P-Δ analysis. I tested this on:

• a cantilever column with axial load and horizontal load applied at the top
• a column with pinned support at the bottom and a horizontal support at the top

In both cases the additional bending moments were as I expected.

Now, the second option is called P-delta analysis and it is supposed to account for third-order effects, so I am confused why it is called P-delta.

From the help:

"P-delta analysis - takes account of the third-order effects, such as the additional lateral rigidity and stresses resulting from deformation. This effect considers additional forces arising in a deformed structure such as a beam with fixed supports on both ends, loaded by a vertical load, longitudinal forces arise and the deflection decreases."

I modeled such a beam described above, see the attached file, but I do see any longitudinal forces.

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Product Support
Posts: 365
Registered: ‎06-23-2008

# Re: Non-linear analysis, P-delta analysis

10-02-2012 09:30 AM in reply to: PatrickEC

Hi Patrick,

as concerns misleading names of analysis it was already explained there:

Repeating here: resulting from backward compatibility with previous versions of the software

As concerns not observing the 3rd order geometrical non-linear effects in your model it is necessary to divide the beam in at least 2 parts - to have one node free to move. Attached modified model corresponding to the sceen capture below. It was also discussed in this post:

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/problem-in-analysis-with-zero-displacements/...

The precision of this type of analysis depends on the number of divisions of bars.

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Best regards,

Pawel Pulak
Support Specialist
Product Support
Autodesk, Inc.

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Active Contributor
Posts: 32
Registered: ‎04-13-2011

# Re: Non-linear analysis, P-delta analysis

03-18-2013 07:31 AM in reply to: pp2008

Hello

I want to continue the theme of nonlinear analysis in the robot. if possible otvette how you can use the length of the bar elements relultatam obtained from non-linear analysis to the robot in the selection of sections of steel structures. If you can step.

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Product Support
Posts: 365
Registered: ‎06-23-2008

# Re: Non-linear analysis, P-delta analysis

03-18-2013 10:02 AM in reply to: Morozofff

Helllo,

your question is not clear for me.

When I have applied some  "reverse engineering" using Google Translate in English-Russian direction I have received more clear message:

"если это возможно ответте, как можно использовать длины стержневых элементов relultatam получена из нелинейного анализа роботу в выборе участков стальных конструкций."

Do you mean something similar to the point 5.2.2(7)a) of EN 1993-1-1 (Eurocode 3)?:

If second order effects in individual members and relevant member imperfections (see 5.3.4) are totally accounted for in the global analysis of the structure, no individual stability check for the members according to 6.3 is necessary

Or do you mean using Robot to calculate buckling lengths of individual members to be used in steel design?

Regards,

Pawel Pulak
Support Specialist
Product Support
Autodesk, Inc.

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Active Contributor
Posts: 32
Registered: ‎04-13-2011

# Re: Non-linear analysis, P-delta analysis

03-18-2013 09:25 PM in reply to: pp2008

Or do you mean using Robot to calculate buckling lengths of individual members to be used in steel design?

Regards,

Yes it is
Hello, Yes poorly translated the question.

Yes, I was referring to the lengths of derived for the design of steel structures. can be either manual or have a robot

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Product Support
Posts: 365
Registered: ‎06-23-2008

# Re: Non-linear analysis, P-delta analysis

03-19-2013 01:44 AM in reply to: Morozofff

Hello,

in such case it has nothing to do with non-linear P-delta analysis.

The automatic buckling length in steel design is discussed in this forum topic:

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Robot-2013-buckling-length-calculation/m-p/3...

There were also some discussions related to using buckling analysis to calculate buckling lengths of individual members. The general conclusion is that buckling analysis is suitable for global stability (buckling) of whole structure. Using it for individual members is difficult and inefficient. See these forum topics:

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Buckling-Analysis/m-p/3299643/highlight/true...

http://forums.autodesk.com/t5/Autodesk-Robot-Structural/Bucking-analysis-of-column/m-p/3709322/highl...

---------------------------------------------
If this post answers your question please click "Accept as Solution". It will help everyone to find answer more quickly!

Regards,

Pawel Pulak
Support Specialist
Product Support
Autodesk, Inc.

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Active Contributor
Posts: 32
Registered: ‎04-13-2011

# Re: Non-linear analysis, P-delta analysis

03-19-2013 07:17 AM in reply to: PatrickEC

Thanks, Pawel, we probably each other misunderstood

The calculation is carried out on the deformed scheme (2nd order theory) on a given combination (RSN). In this calculation, an initial imperfection laid schemes (OBLI), resulting, for example, the analysis of buckling (task Buckling Eigenvalues, EIGE). This technique, to my knowledge, is given in the Eurocodes for steel structures.
In fact, in this calculation indirectly addressing the real flexibility and a gauge length of the elements according to their place in the design scheme.
To determine the safety factor in this case there is a possibility of the iterative selection of the critical load (ULTI). In SOFiSTiK it works. In Robot???

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Product Support
Posts: 365
Registered: ‎06-23-2008

# Re: Non-linear analysis, P-delta analysis

03-19-2013 10:21 AM in reply to: Morozofff

Hello,

I have searched some documentation of SOFiSTiK and it seems (see close to page 53 and then chapter 5.7 of this document) that it is the approach I have mentioned before: similar to the point 5.2.2(7)a) of EN 1993-1-1 (Eurocode 3):

If second order effects in individual members and relevant member imperfections (see 5.3.4) are totally accounted for in the global analysis of the structure, no individual stability check for the members according to 6.3 is necessary

In case of Robot such approach can be used when 2nd order analysis ("Non-linear analysis" check-box active for appropriate design combinations) is used and local and global imperfections are considered in the structure. Global imperfections can be considered using for instance Buckling Deformation (last tab of Analysis Type window) or changing horizontal coordinates of nodes.

Regards,

Pawel Pulak
Support Specialist
Product Support
Autodesk, Inc.

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Contributor
Posts: 22
Registered: ‎09-24-2008

# Re: Non-linear analysis, P-delta analysis

03-20-2013 01:06 AM in reply to: pp2008

Pawel wrote:

"In case of Robot such approach can be used when 2nd order analysis ("Non-linear analysis" check-box active for appropriate design combinations) is used and local and global imperfections are considered in the structure. Global imperfections can be considered using for instance Buckling Deformation (last tab of Analysis Type window) or changing horizontal coordinates of nodes."

Question is:

Can we use in Robot 2013, Global Imperfections from Buckliong Deformation (last tab of Analysis Type window) together with local imperfections on bars.

It didn't work correctly in Robot 2012 - on January 2012 KW  from Robobat wrote me that problem was signed as RM-26704.

JC - Legar

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Product Support
Posts: 365
Registered: ‎06-23-2008

# Re: Non-linear analysis, P-delta analysis

03-20-2013 01:57 AM in reply to: legar

@JC

Unfortunately RM-26704 is not fixed yet

But it is related to SIMULTANEOUS use of buckling deformation and local imperfections.

Point 5.3.2(11) of EN 1993-1-1 states that buckling deformation is used for global AND local imperfection, so there is no need to additionally define local imperfections:

Regards,

Pawel Pulak
Support Specialist
Product Support
Autodesk, Inc.

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