# Special Moment Frame - Column Design



## itsmemario (Apr 9, 2014)

When designing a SMF column, I checked section 21.6.1 of ACI 318-11, which states..."_Requirements of this section apply to special moment frame members that form part of the seismic-for-resisting system and that resist a factored axial compressive force Pu under any load combination exceeding Agf'c/10_"

What if my Pu does not exceed Agf'c/10, can I ignore the transverse spacing and Ash requirements of 21.6.4.3 and 21.6.4.4?

Can I follow the spacing requirements set by section 21.5., which would make the max spacing 6" oc, where currently we can be anywhere from 3"-5".

I would still design for the strong column weak beam requirement of 21.6, just wondering if I can forego the transverse requirements of 21.6

What if at the bottom floor my Pu does exceed Agf'c/10, however my top floor do not, can I use 21.6 at the bottom floor and use 21.5 at the tops for the transverse reinforcement.

If I can't follow the spacing requirements of 21.5, then what other section? The code doesn't specify this, any one else run in to this?

Here in the west coast the majority of the column costs come from the transvers reinforcement, so anything we could do to reduce that cost would be a savings.

Your thoughts on the matter would be greatly appreciated

Thank you.

Paul


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## TehMightyEngineer (Apr 9, 2014)

Good question, I've wondered this myself.


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## TehMightyEngineer (Apr 9, 2014)

As best I can tell you are correct, if Pu does not exceed Agf'c/10 then you design as if it were a flexural only member in a special moment frame. This seems super counter-intuitive to me but I can't find anything that says we can't. That said, would you really want to do this for a building? Seems like quite a risk to take if the building actually had a failure.


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## itsmemario (Apr 14, 2014)

NO I would definitely not do this. I would still design per section 21.6, but with a more relaxed confinement criteria.


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