Warning when using imperfection case

[Matthew]

Hi! I am designing a steel gridshell and want to carry out a global buckling analysis using Dlubal RFEM6. First I am carrying out a stability analysis of load combination 1 using a linear analysis. Then I use the obtained first global buckling mode shape from load combination 1 to create an imperfection case. I then use this imperfection case to carry out a non-linear stability analysis of load combination 2.

Unfortunately, I am getting a warning when Dlubal is working out the imperfection case. The warning reads as "The required accuracy of the iteration calculation was not reached | The results may be unusable | Increase the 'Maximum Number of Iterations.' Otherwise, the structure cannot support the applied loads or the iteration does not converge."

Attached below please find a screenshot of the calculation screen showing the instance that the warning occurs, and also a screenshot of the settings I am using for the imperfection case.

You help is greatly appreciated. Thanks in advance.

[sonja.vonbloh]

Hi Matthew,

thanks for your message!

To analyse the problem more precisely, the model file would be very helpful:

Click on File → Save as and choose the following settings to reduce the file size:

Then upload the file here (e.g. *.rf6, *.rs9) – this way, the community can also contribute to the solution.

Prefer not to share the file publicly? No problem – send it to me via direct message: click on my profile picture or user name → Message.

Best regards
Sonja von Bloh

[Matthew]

Hi Sonja,

I sent you the Dlubal file via private message. Thanks for your help.

Best regards,
Matthew

[sonja.vonbloh]

Hi Matthew,

Scaling the imperfection shape in the global Z-direction would result in a very large spatial displacement since the normalized displacement uZ is very small (see Figure 1).
I therefore recommend scaling the imperfection in the spatial directions S or –S instead (see Figure 2).

Figure 1

Figure 2

Best regards
Sonja von Bloh

[Matthew]

Hi Sonja,

This fixed it. Thanks for your help!

Matthew