Solved: Seismic Equivalent Lateral Force / Non-linear uplift released elastic ground

EduardoMeruvia · ‎08-24-2018

Hi all,

I'm having an issue running a Seismic Equivalent Lateral Forces analysis of a Steel Frame structure on top of a slab on ground foundation.

The issue I have is that my slab is currently defined as a uplift free panel (i.e. the slab is free to move upwards under uplifting forces) and thus a non-linear analysis is required as soon as this non-linear behaviour is set up in the model. Model Set up - Slab Foundation

After setting up my Equivalent Lateral Force Method analysis parameters and load combination I run the model and as expected the Equivalent Lateral Force cases will not converge under non-linear condition as this is an unstable load condition by itself. Normally do to get around this "issue", I set up the Equivalent Lateral Force load cases as Auxiliary primary cases but then include them fully in my load combination hoping they would come through/compile in that stage and ultimately converge as a nonlinear load combination.

However, while going through the process above I realised that although convergence was achieved on all load combination NONE of the automatically generated Equivalent Lateral Forces were created i.e. the Equivalent Force cases were "empty". I tried a few variations of the procedure mentioned above but none with positive results. I just seem unable to generate the Equivalent Lateral Forces whilst having a uplift free slab foundation.

I have look in the forum for possible previous answers but unfortunately couldn't find much that could aid my case.

Found the post below somewhat related, and also tried using 'Definition of a Starting Case for Non-linear Analysis" to see I could address my issue but again I seemed unable to.

https://forums.autodesk.com/t5/robot-structural-analysis-forum/no-convergence-with-uplift/m-p/343727...Starting Case Def.

Just to make sure that its nothing wrong with the model, I run an analysis with the slab foundation having NONE uplift released direction (i.e. linear analysis) and the Equivalent Lateral Forces work perfectly as expected.

Getting nowwhever at the moment, so if you know of a solution or method that can help me I would appreciate your help!

Thanks in advance!

Artur.Kosakowski · ‎08-24-2018

Hi @EduardoMeruvia

If you allowed for the uplift then you have some vertical loads that counteract the overturning moment due to the horizontal loads. In other words you shouldn't run the ELF case alone but at with all acting dead loads together.

See this article and let me know if it needs any improvement so that it answers the similar questions in the future.

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

Artur Kosakowski

EduardoMeruvia · ‎08-25-2018

Hi Artur.Kosakowski,

As always thank you for your prompt reply and information link sent on your message. The information on the link is very useful and a good point of reference.

However, regarding my specific case of running a ELF on a uplift free slab I still seem to have the same issue. When I put the primary load cases of ELF as Auxiliary so they are only analysed on my combination cases acting together with the dead load, it appears that the "automatically generated forces" per story level that ELF should generate ARE NOT created. Thus, when I interrogate the combination that have the ELF cases, these ELF cases appear to be "empty" within the combination case. Apologies, if I'm not describing my issue clear enough. I can't upload the original model due to confidentiality protocols but I have made a 'test' model and uploaded for your reference. Might be easier if you see the model.

Thanks again for your help!

Artur.Kosakowski · ‎09-04-2018

Hi @EduardoMeruvia

For the load cases which are needed for actual load generation (as ELF) just allow them to diverge (and do not check their results as if they were set as auxiliary) rather than set them as auxiliary.

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

Artur Kosakowski

EduardoMeruvia · ‎09-04-2018

Artur.Kosakowski Thanks for the response!

I ended up going with Modal - Seismic which is just as easy but was still wondering about a solution to the question above. Just tried it and seems to work that way!

mgaafar · ‎03-19-2019

Hi Artur,

I did what you said but what happened that the analysis stopped after it did not converge and did not analyze the other load combinations with seismic. and when I made the seismic cases (Equivalent lateral) as an Auxilary case it does not generate the automatic load generation.