The displacements in seismic analysis are changing for the different q factor

[Anonymous]

As in the topic. I am finishing my master thesis and something is going wrong with calculations... When I changed the behavior factor for modal response spectrum seismic analysis  the displacements in stories dialogue as well as in displacement table are not the same... I was trying many times to find the reason but I really do not know what is going on... I opened the files in the repair mode but it didn't help... Please help, I stuck with this issue and can't go further..

In wetransfer link below I am sending the four models with 30 stories RC building with different types of structural system. The displacements for the 1st and 2nd are slightly the same, for the 3rd and 4th the situation is very similar but it doesn't make sense at all...

Additionally I am sending the excel screen with displacements values

https://we.tl/t-OwE3DsSLPV

Regards,

Ryszard

[Simau]

Hi @Anonymous

If you want to make comparison, try to use the same model and change only the behavior factor

Model 1 and 2 have different meshing, and idem for model 3 and 4, that's why the displacements are slightly different.

Model 1

Model 2

Model 3

Model 4

[Artur.Kosakowski]

HI @Simau

Thank you for your time and effort. Much appreciated.

[Anonymous]

The number of nodes is connected with additional walls as you can see in the model. The first model is moment frame system, the second one was improved/upgraded by adding shear walls in the core of the structure. 

In the third type the columns on the shorter edges of the building projection were replaced by shear walls and finally in the fourth type also but in the second direction. 

You can try for example change the behavior factor in the 2nd model and you will find that the displacements will change. 

Here are the pictures with planar view for the models:

I hope that now the situation is more clear for you but still thanks for the response!

[Artur.Kosakowski]

Hi @Anonymous

What are the values of q you compared the results for?

[Anonymous]

1,0 and 3,9

[Artur.Kosakowski]

Hi @Anonymous

Then check: https://forums.autodesk.com/t5/robot-structural-analysis-forum/ec8-reduction-factors-amp-displacements/m-p/5671369

Then check: https://forums.autodesk.com/t5/robot-structural-analysis-forum/displacement-calculation-under-seismic-analysis/m-p/3311215

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.

[Anonymous]

Sorry Artur, but I don't get it...

For example, for the second model, doesn't matter if we take behavior factor q=2,0 or q=3,9 the values of displacements should be the same? In my case they weren't the same...

Look for the case from different point of view. I have got 4 models, with different structural systems. I am analyzing the behavior of the structures respectively for 10, 20, 30 and 40 floors. For 10 and 20 stories I obtained more or less expected values. But then for 30 floors it doesn't make any sens.. 

Below the graphs for displacements in Y-dir for buildings with 10 and 30 stories.

What do you think about it?

[Anonymous]

Anyone?

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

I'm sorry. I posted the link to the other topic that I wanted 

This is the right one:

https://forums.autodesk.com/t5/robot-structural-analysis-forum/displacement-calculation-under-seismic-analysis/m-p/3311215

The displacements can be different depending where you are 'along'the spectrum.

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

[Anonymous]

Ok, now it make sense a bit. When I compare the models I should use the values of displacements from the stories table or displacement table from Robot? But still what do you think about the results I obtained for the 30-stories buildings? Because it is surprising that when I replace the columns for walls in 3rd model the displacements strongly decreased and when I did the same for the 4th model ( second direction) the values almost didn't change...

Thanks for help

Ryszard

[Artur.Kosakowski]

Hi @Anonymous

You can use both but depending on what kind of results you actually need to check. I'm not able to investigate the reason for the difference in results between different models.

[Anonymous]

Ok thanks a lot.

I have last question, in case of situation when behavior factor affects the values of displacements in seismic analysis, the values provided by stories table are the final one? Is there any case that the displacements obtained in Robot should be multiplied by the behavior factor q?

Regards,

Ryszard

[Anonymous]

@Artur.Kosakowski 

I got a little confused...

I run the analysis based on design spectrum for the 2 different values of the behavior factor and another one based on elastic spectrum for two kinds of structures(2nd and 3rd as shown in the message above):

In 2nd model we have q=3,9, in the 3rd one q=2,246. For elastic spectrum the displacements are almost the same. Could anyone tell me why in case of greater value of the behavior factor the displacements are bigger?  

Does it make any sense?

Ryszard

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

If we agreed that displacements can be different depending on where you are on the spectrum (which is the function of the eigenfrequencies of a structure) then there is no point in comparing the results for two different models (or it is not an easy task to do). Also for different models the base shear can be different too. As I wrote previously I'm not able to run such detailed comparison analysis. 

If one or more of these posts answered your question, please click Accept as Solution on the posts that helped you so others in the community can find them easily.

[Anonymous]

hi @Artur.Kosakowski

As we agreed in some cases displacements can be different. And that particular case according to EC8 is as follows( @Pawel.Pulak answer to the one of forum post) :

"Displacements in EC8 are in case of design spectrum already multiplied by behaviour coefficient q.

In case of T>TB it results (independently from q value) in displacements equal to displacements from elastic spectrum.

If some vibration modes with significant mass participation are observed for T<TB it may result in slight differences of displacements depending on q value - as it the screen captures attached to your post.

This effect results from comparison of analogous branches of elastic (formula (3.2)) and design (formula (3.13)) spectrum - the difference between them is more than just division by q, as in other branches of spectra."

In my seismic case, the value of TB is equal to 0,1 s. Hence natural periods and mass participation of the building for the first 10 modes are presented below:

I would like to understand this phenomena but again for me it doesn't make any sense...

Regards,

Ryszard