Modeling a steel moment frame
Modeling a steel moment frame
(OP)
When modeling an ordinary steel moment frame (I'm using RAM Frame), should I pin the beam ends in torsion?
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Modeling a steel moment frame
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Modeling a steel moment frameModeling a steel moment frame(OP)
When modeling an ordinary steel moment frame (I'm using RAM Frame), should I pin the beam ends in torsion?
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RE: Modeling a steel moment frame
RE: Modeling a steel moment frame
If it is a three dimensional frame with significant torsion, which would be more common, I would have to think about it. I might model it both ways here and look at the difference.
Mike McCann
MMC Engineering
RE: Modeling a steel moment frame
RE: Modeling a steel moment frame
RE: Modeling a steel moment frame
I'm not sure how we could achieve moment fixity at the ends of the member without having some level of torsional resistance as well. Maybe one of the partially fixed moment connections which are supposed to be pinned for gravity loads, but fixed for lateral loads? I've never really use those types of connections, but that would be the only way I could see releasing torsion for a moment connection.
RE: Modeling a steel moment frame
I'd have tangent question.
Do most of folks out there use "beam offsets" when modeling moment frames? Say you have a moment frame with two columns and one beam and the beam is framing between column flanges, do you offset the beam and in order to account for dead load moment in the columns due to the connection eccentricity?
It amazes me how many people ignore this effect in frames that dont see balanced loading.
RE: Modeling a steel moment frame
RE: Modeling a steel moment frame
I have had a few projects where the connection eccentricity and the shear loads from massive girders created a pretty significant effect on the columns.
RE: Modeling a steel moment frame
RE: Modeling a steel moment frame
I always ignore that "rigid offset" for moment frames. It doesn't significantly end up increasing the moment in the columns.
IMO, the biggest argument for using those offsets relates to reducing the drift of the frame. But, in seismic you should not normally use it because (for steel frames) you are probably also ignoring panel zone shear deformation. According to NEHRP commentary, if you ignore the rigid region, then you can also ignore panel zone deformation.
For shear connections, it's a more interesting question. I usually feel perfectly comfortable ignoring the effect for interior frames. For exterior columns, it's not such an easy call... though I still usually end up ignoring it. I just might take a closer look at my column size, my connection eccentricity, my total shear and my column footing support.
If I do want to consider it then I just put in a rigid end offset like you suggested. Odd that rigid end offsets were originally intended for moment frames, but current structural practice makes using them for shear connections more common.
RE: Modeling a steel moment frame
IMO, the biggest argument for using those offsets relates to reducing the drift of the frame. But, in seismic you should not normally use it because (for steel frames) you are probably also ignoring panel zone shear deformation. According to NEHRP commentary, if you ignore the rigid region, then you can also ignore panel zone deformation. "
Do you know the logic behind this?
RE: Modeling a steel moment frame
That Panel Zone Deformation section of the code has always bugged me. If the commentary is going to say "this effect can be ignored" then how / why does a provision like this ever makes it's way into the code?!
RE: Modeling a steel moment frame
RE: Modeling a steel moment frame