IBC/UBC seismic warning

"**WARNING: IF THIS UBC/IBC ANALYSIS HAS TENSION/COMPRESSION OR REPEAT LOAD OR RE-ANALYSIS OR SELECT OPTIMIZE, THEN EACH UBC/IBC CASE SHOULD BE FOLLOWED BY PERFORM ANALYSIS & CHANGE."

 I did not understand anything from that warning but it contd to analize the structure. I wanna ask you that is my seismic analysis correct?

P.s: I entered 0.8s period values on purpose. And i know my modal analysis is wrong.

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  • In STAAD, when you analyse a model containing UBC, IBC, IS1893, and various other seismic loads, the program has to first generate lateral loads per the rules of that code.

    If subsequently, there are other load cases which refer to the above cases through the means of a REPEAT LOAD command, as in cases 21 and 22 below

    LOAD 1
    IBC LOAD X 1.0
    LOAD 2
    IBC LOAD Z 1.0
    LOAD 3 GRAVITY
    SELF Y -1.0
    LOAD 4 LIVE
    MEMBER LOAD
    ..
    ..

    LOAD 21 DEAD + SEISMIC IN X
    REPEAT LOAD
    1 1.0 3 1.0

    LOAD 22 DEAD + SEISMIC IN Z
    REPEAT LOAD
    2 1.0 3 1.0

    then, load cases 21 and 22 must have access to the lateral load values generated in cases 1 and 2 respectively.

    This means that STAAD must be told to retain those generated load values. If it is not told to do so, it will simply "forget" those load values once it has finished processing cases 1 and 2. Consequently, cases 21 and 22 will not reflect any effects of the seismic loads.

    Another instance where STAAD needs to remember those generated lateral loads due to seismic cases is when a re-analysis has to be done following a member selection in steel design, as in

    LOAD 1
    IBC LOAD X 1.0
    LOAD 2
    IBC LOAD Z 1.0
    LOAD 3 GRAVITY
    SELF Y -1.0
    LOAD 4 LIVE
    MEMBER LOAD
    ..
    ..
    PERFORM ANALYSIS
    PARAMETER
    CODE AISC
    ..
    SELECT ALL
    PERFORM ANALYSIS

    The analysis instruction which follows the SELECT ALL command is called re-analysis. During re-analysis, all previously defined load cases are re-analysed using the member properties which resulted from the member selection.

    The warning message you saw earlier is merely intended to remind you of these facts.

    The means by which you can tell STAAD that it has to "remember" the generated load values is to specify a PERFORM ANALYSIS and CHANGE commands following those individual seismic cases, as shown below.

    LOAD 1
    IBC LOAD X 1.0

    PERFORM ANALYSIS
    CHANGE

    LOAD 2
    IBC LOAD Z 1.0

    PERFORM ANALYSIS
    CHANGE

    LOAD 3 GRAVITY
    SELF Y -1.0
    LOAD 4 LIVE
    MEMBER LOAD
    ..
    LOAD 5
    ..
    LOAD 21 DEAD + SEISMIC IN X
    REPEAT LOAD
    1 1.0 3 1.0

    LOAD 22 DEAD + SEISMIC IN Z
    REPEAT LOAD
    2 1.0 3 1.0

    PERFORM ANALYSIS
    CHANGE

     

    Sudip Narayan Choudhury

  • Hello Dear Sudip,

    I made the model in physical modeler and during the analysis in analytical modeler work flow i had the same error. I am trying to modify the order as you mentioned to the above comments (perform analysis - change after ubc loads case) but when i am in to the editor i cannot make any change.

    Thank you very much in advance for your valuable support.

    Kind Regards,

  • If the model is created in the SPPM, that geometry part in the STAAD Editor will be locked, unless you break the link between the analytical and physical models by going to Utilities -> Drop Physical Model.

    However, the same order of the commands can be set up in the SPPM too. You would need to create a separate analysis set for each seismic load case by going to Loading -> Analysis Commands menu. The PERFORM ANALYSIS and CHANGE commands will be added to the input file automatically after each seismic load case then.



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