Weak column/strong beam 1

[rrmiv]

As a continuation of an archived thread (thread726-55027), I have a one-story retail building braced with intermediate steel moment frames in a high wind region that also is in seismic design category D. Wind drift controlled the member sizes. My question, in regard to Taro's comment at the end of the thread, is it acceptable to have the plastic hinge develop in the column instead of the beam? Does AISC or the IBC prohibit this weak column/strong beam type of system? If not, is the panel zone merely designed to develop the capacity of the column, not the beam?

 

[bjb]

As the originator of that thread, I feel your pain. I can't recall all of the research that we did for that project, but we ultimately ended up using dog bone moment connections. I seem to remember that it was very difficult to make my siuation work using the other types of moment connections. I also seem to remember designing the panel zone for the moment capacity of the beam. Be careful with the welding in the panel zone. There can be so many welds that you can have problems with weld cracking. Try emailing the AISC solutions center, they hopefully can give better guidance. If you do, please post the response in this thread. That was the only steel framed design category D building I have worked on, I would be interested in hearing what the latest is from AISC.

That was our first project using the IBC or performing any seismic design, because prior to 2003 NY didn't require seismic. We dove straight into the deep end, and it was painful. I still have a philosophical problem with having to design a seismic design category D structure in upstate NY.

 

[rrmiv]

I did receive a response from AISC. Here it is:


"The idea behind the moment frames found in the AISC Seismic Provisions (i.e. SMF, IMF and OMF) is to establish a design where we achieve strong columns/weak beams. Otherwise, a localized failure in a column can result in a global collapse scenario for floors above the localized failure. Hence all SMF and IMF prequalified moment connections are based on strong column/weak beam testing.

Please note that OMF does not require the use of prequalified moment connections. That is, any moment connection, even those for static loads (i.e. those found in the AISC Manual) may be used.

Hence one can conceivably design a single-story OMF with weak columns/strong beams and avoid the multi-story collapse scenario.

In such a case, the plastic hinges would form at the tops of the columns rather than in the beams. One would need to consider the column anchorage rigidity when attempting to determine when a plastic hinge will form at the top of the column, as the number of hinges in the frame will defined when the frame mechanism will develop and hence when collapse will occur. That is, one cannot treat the column anchorage as a hinge (pinned connection) or as a fixed (moment connection) unless it has been designed that way, which is critically important, as it affects the load which will develop a plastic hinge in the column top (i.e. column anchorage affects the moment diagram of the column.)

However, OMF have requirements for continuity plates within the column, as again, the Seismic Provisions address moment frames from a strong column/weak beam perspective. Having said that, you would need a building official to approve the design you mentioned as an OMF."


My problem is I cannot use OMF because of the seismic design category. Yet my member sizes are controlled by wind drift. In AISC 341, the Column-Beam moment ratio does not apply to IMFs, only SMFs (sections 9.6 & 10.6). This seems to be the step that would ensure strong column/weak beam - is this correct?. If this step is not required for IMFs, does that mean the strong column/weak beam requirement does not apply. In using the design procedure in AISC 358 for RBS connections, I don't find a step that checks the column capacity. I don't want to have to use W10x100 columns for a one-story building. Any thoughts or comments on AISC's reply.