# Concrete Seismic Questions, ACI 318 Chapter 21



## ARLORD (Sep 21, 2008)

Am I correct on the following regarding Chapter 21 of ACI 318:

1) Unlike the Steel Seismic Provisions, ACI Chapter 21 does not refer to the building code overstrength load combinations, therefore ACI does not dictate when to use the overstrength factor for concrete design. If the overstrength factor is to be used in concrete design, then ASCE 7 will direct you to do so.

2)ACI Section 21.12 discusses concrete IMF. It addresses beams and columns, but not beam-to-column connection design like Section 21.5 for SMF. Am I corect to assume that there are no special seismic requirements for IMF Joint design.


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## Mcgill (Sep 22, 2008)

Ans (2)

Yes, - I just checked my PCA book- it says :there is no similar requirements for IMF/OMF.


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## kevo_55 (Sep 24, 2008)

ARLORD,

My take on #1: You are 100% correct, you do not use the Em load combo for concrete beams/columns. The reason that there is still an overstrength factor for concrete frames is if there were "other" portions of the structure which would not enter into the concrete moment frame system but still require the Em load combo. Example, maybe a rood lintel is used in a SCMF for some odd reason over a doorway. The Lintel would be concidered a drag strut and would use the overstrength factor for the main building system.

I hope this helps.


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## ARLORD (Sep 24, 2008)

Yes kevo_55,

Thanks for the input. I also found a similar/related discussion in the commentary of ASE7-05, Section C12.4.3.


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