Response spectrum - Structural Analysis and Design - Forum - Structural Analysis and Design - Bentley Communities

You must be signed in to start a thread or post a reply
Not a member? Register      Problems signing in? Check here


Response spectrum

Dear Reader,

Thanks for your attention!

I have got stuck up generating spectrum   in Staad using EC8 2004

                SOIL TYPE C ALPHA 0.36 Q 2

when i change the Q factor above from 1 to 2 in EC8 2004 definition, i do not see any change in my spectrum values.

Indeed if i use EC8-1994 definition . i could see the change in the spectra values. Im bit worried , i havent done spectrum analysis in staad pro earlier.

Could someone help me on this? I need spectrum for the following conditions. Soil type:C, Q=2; Elastic Type I Spectra, Ground acceleration =0.36g, X=1,Z=0.3

It would be much helpful if someone gives a try and provides me the spectrum(As per EC-8, part -I), Period vs Acceleration values


Many Thanks in anticipation

Kind regards,


3 Replies (Most Recent Reply)

  • Hello Maddy,

    You have specified the elastic response spectrum. To the best of my knowledge, the behaviour factor Q, does not have an effect on the elastic horizontal/vertical response spectrum. The equations for the spectral values for the elastic spectrum does not contain the term q. The same only has an effect if you use the design response spectrum. Please refer to the clauses, and of the EC8:2004 code.

    Try using the design response spectrum and let me know if you stil cannot see any change. If not, please upload the STAAD file and i will look into the issue ind details.



    Sudip Narayan Choudhury

  • In reply to Sudip Narayan Choudhury:

    Dear Sudip,

    Thank you very much for your valuable guidance. Its pretty clear now. I have identified where i went wrong. Thanks again.

    If i am not bothering you.Just for my satisfaction, could i ask   you to review my spectrum definition in staad and drop your comments? It would be of much help. I am attaching staad input file for your perusal. The basic loads are dead load, liveload, and crane loads, along with case A, Case B (seismic loads). In combinations , all the gravity loads + EQ loads .

    pls clarify me that i can relay on the results that are generated from the combinations for design of my pile sections.

    Many thanks in anticipation.

    Kind regards,


    staad-Response spectrum.txt
  • In reply to Maddy:

    Hello Maddy,

    From the STAAD's point of view, the Eurocode response spectrum specifications appear to be alright to me. Alos, the load combinatins appears to capture all possible situations. However, I think you need to specify the mass definitions only in the first response spectrum load case. I find that you have specified the mass definition twice, once for each response spectrum load case. The mass distribution of a structure is supposed to be constant.



    Sudip Narayan Choudhury