Special moment frame in etabs 1

[compe_ad]

Hi all! I am designing a simple moment frame using etabs. I am using autoselect section for both columns and beams. For the given load, all of the members are failing at stress/capacity check. Columns are displaying error message of - "Stress check message - l/r>60, check bracing at beam-column connections" and for beam - "Stress check message - Lb/ry>0.095ERy/Fy" Can someone please explain this and how can I fix this?

 

[jayrod12]

Are you sure your units are correct? If it's saying that the columns are exceeding allowable slenderness at the sizes indicated on your screenshot, those columns would be ridiculously tall. This assumes that there's some form of weak-axis bracing of those columns, maybe a series of wall girts or something?

If the units are all correct, are your unbraced lengths of the members entered properly?

 

[TheDW]

See section D1.2b of the provisions of AISC Seismic Design manual. You need to have beams framing into the side of your moment frame beams, to reduce their unbraced length for weak axis stability

 

[compe_ad]

Thank you! I am doing just 2d analysis. In 3D model, there will be beams on the other direction too. How do we incorporate this in my 2D model. Do I just restrained the top joint of columns on the direction perpendicular to the screen?

 

[TheDW]

I'm not too familiar with etabs. I would try modelling lateral support points where you will have beams, or try to manually adjust their unbraced lengths based on your beam spacing

 

[jayrod12]

I'm only vaguely familiar with etabs, but I believe there's a way to adjust the unbraced lengths of the columns and beams to account for other forms of restraint.

 

[Luceid]

Design - Steel Frame Design - View/Revise Overwrites when you have a frame element selected. You can specify the unbraced length ratio for LTB, minor and major axis buckling relative to the beam length. So if you had bracing beams at 8' o.c. for a 24' long beam, you could input 0.33 for your LTB unbraced length ratio.