Response Spectrum Analysis

Question:

Can you send me an example of Response Spectrum Analysis illustrating the difference of ticking or unticking the "Add Signs"  check box? (see below)  thanks

---

Answer:

Hello,

Kindly refer the image below on how the results would vary.  The details of how this works could also be referred in the online help manual (Link).

Hi, Nandeep

Thanks for your quick reply.

But what we have is 2018 V1.2, your models were created under higher version.  so I cannot open them.

Can you somehow save the models for lower versions?

thanks

Hi, Nandeep.

I create my own RSA models with and without "Add signs".  ... seems to be working....

Questions I am thinking:

1) when you do a modal combination, say SRSS, you will lose the signs automatically by the rule of SRSS.  What exactly do you mean by "add signs"? How do you know what sign to add?

2) What do you mean exactly by "along the major mode direction?" and what is the def of a major mode?

3) what do you mean by "along the absolute maximum value"

4) what do you mean by the checkbox "select mode shapes" and what do the mode shape factors mean?

Need more elaboration, please.

thanks

Hello Jack,

Glad you were able to generate models at your end!  Now, about the questions this time around, details on each of these is provided in our online help manual (Link).

Hi, Nandeep,

Gone through the link, my questions remain.

Can you elaborate? with some examples preferably.

thanks

Hello,

I can't find better words to describe the feature!  However, I'll try...

Thanks for your reply, Nandeep. Now I understand the issue better. But if I think deeper, I still have some issues:

1) Say I have a model with only 2 mode shapes, 1st being major at 51% mass contribution .  The sign of the 1st mode shape will be taken as the sign of the SRSS/CQC results if I tick "Along the Major Mode Direction".  I suspect, there will be some nodes that will have signs same as the 2nd mode shape, given the 49% contribution.  How does Midas handle this situation?  It sounds the graphic results can not ALWAYS be correct.

2) Presumably, I will get different results if I tick the other radio button.  Which one will be the CORRECT one?  How do I know which button to tick? keeping in mind there is ALWAYS some torsion in any model. 

Sounds like it is a work of art, not of science.

I have to say, the language used by Midas is vague either in the help or in the various dialog boxes.  I don't understand it most of the times.

You will see what I mean if you compare the language used by MS in EXCEL, for example.

Thanks


Hello,

Glad the response helped a bit.  To further clarify, kindly refer my comments below:
After performing RS analysis and combining modal results, signs are definitely lost.  Now, engineers are left with 2 choices.  Whether to add signs or not.

Thanks again, Nandeep.

In terms of the language, I can give you an example here:

Refer to the screenshot, I'd change "modal combination type" to "modal combination method".

I'd also change "select mode shapes" to "select modes for modal combination" and remove the "mode shape factor": it 's not clear what is a "mode shape factor" and I doubt anybody would bother changing them.

Cheers and have a good day.

Hello,

Thanks for the suggestions!  Personally, I am very used to what the options mean, so I tend to overlook such a part. A bad habit developed due to daily exposure!
I'll definitely update our development team about this.  Kindly let us know if further assistance would be required... :)

Hi again, Nandeep,

Refer to the attached model.

I understand:

1) By modal combination, the signs are lost, either for displacement or force.  But the absolute values of all analysis results should remain.

This appears to be true with displacement, and not with, say, reaction. See screenshot:

Pier reaction without tick at CBMAX 2475 load combo.

Pier reaction with check box ticked at CBMAX 2475 load combo

can you please explain?

2) I'd imagine Midas would do a modal combination on displacements to get the total displacement.  And similarly a modal combination on force to get the total force.  In another word, if I were to apply a set for forced displacement statically, I will not get the same force.  please confirm.

thanks

Hello,

The model file was not attached.  Kindly try sending the same again.

Regards,
Nandeep
Technical Manager, MIDAS

Hi, Nandeep,

I figured out the answers to my questions from yesterday. No worries.

Now one new question: (See model attached.)

When I check the reaction in spreadsheet format, the reported summary does not match my hand summation (8203  reported vs 20919 by hand).  Can you please shed some light here? thanks

see screenshot below:

Hi, Nandeep,

Refer to the same model.

For some reason, some beam element shear forces changed as I switch from "no sign"  to "add sign".  This cannot be true.  But why?

Specifically, I was checking the Fz of "Fixed bearing" at CBmax 2475 and CBmin 2475.

thanks

Hello,

First of all, about the reactions.

Thanks again Dandeep.

This is what I expected but good to have your confirmation.  This is what it should be for summation.  I don't see why it is the same for envelope.

On the shear force question, for example, I understand when I switch from "No sign" to "Add sign" and check the CBmax2475/CBmin2475, the signs will change and hence the max/min values will switch.  But the absolute values should not change.  Can you comment on this?

Numerically, for member ID48. When I have "add sign" Fz=-5399/822 for CBmin2475/CBmax2475... but when I have "no sign", Fz=4544/7009.

This cannot be true, right?

If true, why and what value should I use  for design?

Further thoughts:

1) To make it simple on the summation, I think we can say this is a matter of taking CQC before summation (which is wrong) or after (which is correct).

2) In my model, I defined a load combo "1.0X+0.3Y @2475" after the analysis and there is no need to rerun the analysis. Presumably, Midas simply combines the relevant results as the results of the combo is there right away.  True?  If yes, it is then taking CQC before summation (in this case, combination) and this would be wrong.  Or is Midas taking CQC after summation (in this case, combination), this should take some computing time and it doesn't seem to.

Looking forward to your comments...

Thanks

Hi, Nandeep,

Another simpler model attached for your perusal.

If you take a look at the reaction, you will see it makes a big difference selecting "no sign" vs "add sign".

Which is the correct one?

Thanks

Hi, Nandeep

Refer to the "Steel frame" model.

It seems to me for load combo "1+0.3", Midas simply adds up the relevant reactions to report the combo reaction.  This is wrong as it is taking  CQC first and then adding up.  Should take CQC last as we discussed earlier. 

Similarly with the other combo"0.3+1".

Comments?

Hello,

Your understanding is correct.  Actually this is pertaining to how the combinations are made.  Right now, the combinations are made with Type as Add.  For combinations, we have 4 options.  Add, Envelop, ABS and SRSS.  We do not have CQC here.  But for the results obtained after CQC of response spectrum, SRSS type combination should be used, if these have to be combined.  This would give results closer to what is expected.

Hi, Nandeep.

Thanks for your reply/explanation.  With the  combination type switched to SRSS, I got some results that are seemingly good.

Additional thoughts:

1) By CQC/SRSS, we know the signs are lost in the 1st place.  There is no bringing back.  Giving users the choice of "add sign" seems to be a trick that gives plausible results which are never correct.  Better remove this feature in future releases. Agree?

2) Even with SRSS as the combination type, say for the 1.0x+0.3Y combination, the results are still wrong in theory (may be close enough practically).

Reasons #1, the code requirement of "100% in one direction+30% in the other direction" should NOT be interpreted as SRSS of the results from RSA in x and RSA in y directions, it should be: i)ADD within each of the modes with 2 RS's applied simultaneously and then ii) SRSS/CQC the results from i) between all modes.

Reason #2, this would require some computing time after the combo is defined.  It require to run another RSA load case where 2 RS's are applied simultaneously.  Midas does not seem to have this feature?  Good to have in the future.

The recommendation of using SRSS between RSA results in x and RSA in y directions appears to be a Band-Aid Solution. Agree?

Thanks very much.

Hello,

Glad the earlier suggestions helped!  About the current comments, kindly refer the response below.
Regards,
Nandeep
Technical Manager, MIDAS

Hi, Nandeep,

Attached please find another model.

In the RSA's, I have 475x100% without signs and 675X100% with signs added.

Take a look at the deformed shapes and let me know what you think. (one is obviously wrong that yet Midas seems to suggest using.)

Thanks

Hellok,

As we discussed before, Midas can just provide methods that engineers want to use to suite their practical needs.  I totally agree with your observations that with sign and without sign would give completely un-comparable results.  As midas technician, I can only say that both the methods are correct and its up to the engineer which one they want to go for.
However, as an engineer, personally I feel that if I can be sure of the quality of execution, I'd prefer the method to add signs as that tends to give more realistic results, whereas the conventional method without signs tends to give very conservative results.  Again, I reiterate though, that none of the method is wrong.  Its just like how Working Stress Method is not wrong for design.  Its just way too conservative as compared to Limit State Method.  Thats it.

Regards,
Nandeep
Technical Manager, MIDAS