spectral analysis with dynamic response module

Hi

With respect to the response spectrum analysis, please would someone clarify how to combine responses for x,y and z directions:

While inputting the spectrum with dyrinp file, do I need to specify the spectrum in 1 direction while setting the spectrum in other two directions as 0.0! Then repeat this for other two directions? Thus while doing this I will be creating 3 dyrinp files for spectra in x,y and z directions (therefore 3 common solution files!)! Subsequently, would I be correct in my approach to use combine module to create 3 load cases to combine responses using SRSS method? This will be achieved using leading response file as primary file ‘P’ while second file as secondary ‘S’ and third file as ‘3’ –repeat this for all 3 directions while changing primary and secondary files as below:

 

Combined loading 1: x response (P), y response (S), z response (3)

Combined loading 2: y response (P), x response (S), z response (3)

Combined loading 3: z response (P), x response (S), y response (3)

 

Please advise.

  • Kasiphon,

    That is a good point about using CQC or SRSS for modal combinations. It is never wrong to use CQC with this analysis and I would recommend using it over SRSS because of the reasons you stated. We have the SRSS option in there for a variety of reasons but there isn't really a reason to use it for a new design.

    Regards,

    Geoff
  • Yee,

    As Geoff said we have 2 combinations as following

    1) Modal combination - CQC, SRSS , or other etc.

    2) Directional combination - SRSS by default of SPLOAD command

    Modal combination is always exists while directional combination will come when you have earthquake direction more than 1 direction.

    Do not confuse between 'modal combination' and 'directional combination'. It is different combination.

    For modal combination,

    CQC is more modern than SRSS. CQC is SRSS plus with correction factor. See picture below.

    SRSS term is always positive while correction term can be positive or negative. 

    Then, CQC can be lower or higher than SRSS depend on correction term which is correlation between nearby mode.

    This correlation term can disappear quickly depend on frequency ratio. Normally, frequency ration between 0.67-1.50 is recommended to use CQC.

    For out of the range above , you will see than correction term is close to zero. Then, you can use either CQC or SRSS. It does not change your result.

    By, for me, I always use CQC. When, it is well-separated frequency , CQC will be transformed to SRSS automatically. Why don't we have to care about SRSS?

    For directional combination

    If you use SPLOAD command, then it is 'SRSS' combination by default for directional earthquake combination.

    SRSS is recommended by most of earthquake recommended practice.

    If you would like to use another method of directioncal combination. You have to generate directional directional earhquake separatedly and combined it by using combine module.

    regards,

    Kasiphon

  • Yee,

    There are two different combinations going on when performing a spectral earthquake analysis, the modal combination and the directional combination. Since the seismic analysis is a dynamic analysis, SACS calculates the modal response for each mode shape specified on the DYNOPT line. The modal responses for each direction are then combined using the method defined on the SPLOAD line and then the directional responses are combined using SRSS (sum of the root squares). The CQC (Complete Quadratic Combination) method is used when the modal responses are correlated (i.e. influence) each other whereas the SRSS method can be used when there isn't much interaction between the mode shapes. This occurs when the frequencies of mode shapes are close together. There is a more detailed discussion of this process in Section 3.1 of the Dynamic Response manual.

    Regards,

    Geoff
  • Hi Kasiphon,
    Thank you for the detailed explanation on how SACS combined the response for 3-direction.

    What 'CQC' does for modal type combination in the "SPLOAD" line? Any relation between this 'CQC' with the SRSS combination for directional EQ by SACS default?

    I am confuse as there are option of 'SRSS', 'PRMS', 'PEAK' and 'CQC' for the SPLOAD comment line.

    Thanks!
    Regards
    Yee KP
  • Hi aaa123,

    I understand that your question is

    Do you need to split SPLOAD command to be 3 commands for X Y and Z.

    The answer is "NO NEED".

    You can make it the same line. Directional spectrum will be combined by using SRSS method by SACS default. You no need to specify SRSS for directional combination, if you put all Spectrum in the same SPLOAD command.

    Nonetheless, if you still need to split into 3 SPLOAD commands. It is not wrong. Btw, you no need to create 3 dyninp. You can repeat SPLOAD in the same dyninp.

    Finally, you will have only 1 CSF which you also can combine the solution inside that CSF for directional combination. In your combine input file, all will be "P".

    If you go this way, you also have to combine your seismic solution with static load by using 'PRSC' and 'PRST' at the end.

    Geoff, please correct me. If I understand something wrong.

    Regards,

    Kasiphon

    Answer Verified By: Geoff McDonald