Lightly Loaded Concrete Moment Frame

[shaneelliss]

I think this is a simple question, just want to get an opinion because I haven't run across this before. I have a concrete moment frame that supports a small elevated platform and it is in earthquake country where Chapter 21 of ACI applies (special reinforced concrete moment frame). Chapter 21.5 is for flexural members of SMFs (typically beams) and 21.6 is for members subjected to bending and axial loads (typically columns). However, my columns have a light enough load that they don't meet the minimum axial load required by that section to be considered a column. Do I just follow the design requirements for flexural members? There are provisions in 21.6 for columns that try to ensure strong column-weak beam type of action. Do I ignore those requirements and others with regard to tie spacing etc. because it is lightly loaded (vertically)? I think that is what the code is telling me to do. I would therefore just design it with the minimum steel and ties as if it were a beam, but it seems weird to do so. Thoughts?

 

[KootK]

I see your dilemma. For a special moment frame, your columns need to resist axial forces consistent with the development of over strength plastic hinges in the beams. Is that axial load still very small? Usually it is significant. If there's any way out of employing a special moment frame, that's what I would do.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[shaneelliss]

I guess looking at the intermediate concrete moment frame, it is written more explicitly that any member that doesn't meet the minimum axial load, should be treated like a beam (in regards to minimum reinforcement, etc.). I will probably switch to using the intermediate moment frame design. Thanks.