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.

  • SACS uses SRSS to combine modal responses to spectral loading. Refer to Section 3.1.3 of the Dynamic Response manual for more information.

  • 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 

  • 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
  • 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