steel design query

Question

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.

farhat sheen · Answer

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?

Sye · Answer

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.

steel design query

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