CQC-SRSS and element verification

[394-98236683]

Hello,

I have doubts how RSA uses the seismic combinations and forces to check the member.  I've been checking the simple bar enclosed with EC8 and create seismic combinations X and Y (cases 6 and 10)

I checked the element with the different cases I've got and see the following:

• If I check the element with the case 6 or 10, RSA uses the forces from the  SRSS combination to verificate the seccion.  I've defined the the main mode in the analysis type "combination Sign" tab and define CQC combination but it ignores it.  Why RSA ignores the CQC combination if I define a main mode and the CQC? I see RSA doesnt work with CQC forces unless we use the define direction tab when you create a seismic case.

So, afterwards I defined the signed CQC combination of X and Y (7, 8, 11 and 12) and then I saw RSA uses the CQC forces to check the element.  I understand that RSA will work with this CQC forces if I combine them with other cases. Do we always need to define CQC combination in that way when we create the seismic case and work with the new cases created? Can we do a non linear calculation with the CQC forces created?

Thanks in advance.

[Artur.Kosakowski]

The intended workflow is that you combine seismic case (e.g. 10) with other loads and then you decide on the type of quadratic combination (CQC or SRSS). It is rather hard to imagine that there is only seismic load (without e.g. self-weight) therefore in the steel design module the default type of combination is assumed (there is no option to select SRSS or CQC e.g. for code checking for case 10). 

Can we do a non linear calculation with the CQC forces created?

I'm not sure if I understand you correctly. Theoretically you could convert pseudostatic forces from CQC of e.g. case 10 but not as signed (as signing is done at the stage of display of results) and include such created case in nonlinear analysis but I'm not sure if it would make sense. Perhaps it would be better to convert pseudostatic forces from selected mode instead. 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[394-98236683]

Thanks for your answer.

 I understand that normally the seismic case is combined with others and then you must select the type of combination (SRSS, CQC or others) for the seismic. , so it's not a problem when you combine it with other forces.  I only wanted to understad how RSA works with the different inputs in the analysis type tabs. As I see:

• The defaul type of combination assumed it's only to define which one RSA will use by default when you create combinations with others manually. But you can modify it later in  manual combinations
• In case I don't combine the seismic case it works with the SRSS forces in the member verification
• if I define the main mode of each seismic case it defines the sign of the CQC or SRSS force

Regarding the CQC calculation I want to confirm how RSA works when displays the reactions at the supports  in case I have a structure with more than 1.   In some codes we have a limitacion of total base shear force and sometimes it's necessary to apply a factor to increase the seismic forces. I'd like to confirm the following in the table of Reactions of the CQC mode case for a one direction seismic case :

• The Sum of val. displays the sum of the different reactions at the supports calculated each reaction accoding the CQC
• The sum of reac. displays the CQC calculation of the sum of the reactions at the supports for every modes.

It's important because as fas as I undersand the method of calculation from books and how RSA works, the total base shear  would be the sum of reac.  and not the other one and sometimes those values are different.  For some codes RSA includes base shear calculation but if you define a new spectral case for a code this value must be checked.

Thanks in advance

[Artur.Kosakowski · Accepted solution]

The answer is YES for all the points.

In addition for 

• The Sum of val. displays the sum of the different reactions at the supports calculated each reaction according the CQC

mind that this is the sum from support nodes currently displayed in the table of reactions (filter applied). 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[394-98236683]

Thanks for your time.