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
Hi, Nandeep,
Gone through the link, my questions remain.
Can you elaborate? with some examples preferably.
thanks
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,
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.
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
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,
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,
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,
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
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Model 3[1].mcb
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DataImage23.png
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Steel frame.mcb
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Book1.xlsx
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DataImage72.png
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Model 3.mcb
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DataImage62.png
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DataImage7.png
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DataImage27.png
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DataImage52.png
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DataImage24.png
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RS Sign 1.mcb
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RS.mcb
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DataImage45.png
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