steel design query

hello everyone 

                  i am design the steel building i staad pro . now if the beam is continuous say 10 m and i am having the node at the ends. the say i'm getting the beam section as pass with iswb 450 and if i am inserting the node in between the beam say one at mid point the the staad gives beam as pass with value less than the above written value. say ismb 400 what is the reason. i have check all the parameters and they are ok . but design  is not sufficient as per requirement.

123333.std
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  • Himmatlal,

    Your question is not clear.

    Can you attach both the models? You have attached only one.

  • STAAD uses the length of the member when calculating the allowable bending compression stress. If you use a node in the middle, you are reducing the effective length of the member. Hence, you are getting higher allowable stress which will result with a smaller pass ratio.

    You have to give Ky and Kz values for the members to help STAAD identify what is the actual effective length to be taken for calculating allowable stresses. If there is a crossbeam at the middle node which provides restraint for the compression flange, then you can introduce the node and use the default Ky and Kz values. If not, then you have to enter Ky and Kz as 2 for the members with new node in center.

    Calculation of Ky and Kz is Effective length factor x Actual Effective length of the beam of which the member is a part of / Length of the member...

    For example, if you take a 10 m long member and cut into 4 and 6 m parts, and if you can take effective length as 0.85 x L, then

    Ky and Kz for 4m part = 0.85 x 10 / 4

    Ky and Kz for 6m part = 0.85 x 10 / 6

    Hope that if you add these factors, your pass ratios come same in both cases...

    Arun

  • Dear Mr. Arun,

       I believe either one should be sufficient. If you use both, the last one specified will be used in design...

    Arun

  • I am REALLY AND FULLY appreciated you. Cuz now, I could understand that Kx, Ky, Lx, Ly, UNT, UNB command.

    Thank you so much!

  • Dear Suresh,

    As far as I know, the Lx, Ly, & Lz value will be used to calculate the Kl/r ratio. In the current case, we do not check slenderness for flexural members. So from my point of view, there shouldn't be any assignment of Lx, Lz, Ly for the beams.

    If anyone assigns Lx, Ly, Lz for beams then they should assign 'Main1' as well. In the end, the result will be similar if you don't assign L values and if you assign L values with Main 1 for beams. 

    Can you solve if I'm incorrect. ? 

  • Seems like this thread is quite old. Answering the last question. Although flexural members may not be subjected to significant axial forces but design codes usually require flexural members to satisfy the axial check as well as interaction of axial + bending checks. Any steel member designed in STAAD.Pro is therefore checked for both axial and axial+flexure interaction. You would need to specify Ly Lz etc. unless the default lengths for the analytical members represent the correct unsupported lengths that should be considered for the flexural buckling calculations. MAIN 1 should only be used if you want to bypass the slenderness check for some reason. The software would still check the axial capacity as required by the code and hence you should always specify Ly Lz irrespective of whether you want the slenderness check to be by-passed or not. 



  • Dear Suresh, 

    I couldn't find any slenderness check (kl/r <200 for compression) for fluxural members in any code or textbook. 

    If I use Main 1 for Sub beams, Main Beams and web tappered Rafters to bypass slenderness check, is it okay ? 

    Can you tell for which member and when we should use Main 1 parameter? 

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