Special Tube Moment Frame Requirements

[msquared48]

Can anyone explain to me the difficulty in using tube members for SMF's as opposed to W sections? I understand that it is in the detailing requirements, but specifically what code detailing requirement(s) is/are causing so much disconsternation among structural engineers and code officials with tube SMF's?

Is there a detail solution for it, or is it insurmountable?

Would solid non-shrink grouting the whole tube frame solve the problem, as I feel that is a joint detailing issue, possibly relating to local buckling?

Mike McCann
MMC Engineering

 

[JAE]

msquared48 - I don't know the answer to your problem specifically, but I wonder if it is simply that there are no physical tests that have been performed to verify these kinds of connections....i.e. no pre-qualified connections.

Don't know...just guessing.

 

[dougantholz]

I think JAE has it right, and it goes to one of my pet peeves of the code: it is written for the average 8 story building. Engineers who work on smaller structures get very little funding support.
To your specific problem - you can't use a tube because there are no pre-approved joints. I assume that you are forced into SMF, not a R=3 where prescriptive detailing requirements go away.

 

[LowLax]

I'm assuming you are talking about tube columns with moment connections to wide flange beams. In that case it's all governed by Chapter K in the new AISC spec. The connections are very tough to make if the column has a relatively thin wall. Punching shear, local yielding and sidewall yielding of the column are going to limit the amount of force you can deliver to the column. If the moment plate you are using is narrower than the column width, you will have punching shear and local yielding problems. Plus if you are designing it as a SMF in a highly seismic zone the forces you are generating may be extremely high.

 

[msquared48]

What I have is one portal frame designed of all the same tube members - 4X members to fit in a 5.5" wood framed wall. Due to the site class the Building official is saying that must be a SMF, where I am using the code exception for single story structures that allows for OMF frames. His problem is too that the one story area of the project is connected to a five story area that is all apartments. I maintain that they are not proximate, nor is the frame in a direct seismic force line to receive any forces from the five story area.

His interpretation of the code makes no reasonable sense to me and I believe that the exception applies here, and for more than one reason.

Mike McCann
MMC Engineering

 

[ssweetland]

As far as HSS as SMF, I do not believe there is sufficient testing, therefore there are no prequalified connections for them, refer to AISC Seismic Provisions for Structural Steel Buildings, section 9.2b. Chapter K of the standard provisions would not be sufficient to meet the seismic detailing requirements.