# ACI 318-05 Ch. 21 Seismic Provisions



## Bigwolf (Oct 18, 2008)

I just realized that anytime I worked a concrete problem with lateral loads, they were always seismic based lateral loads and I applied the text of Ch. 21. This begs the question.....if the wind load governs the lateral design, do we just use the general provisions of ACI 318-05 (i.e. Ch 10, 11, 12, etc) and NOT the provisions of Ch. 21 since it specifically says Seismic Design? :blink:


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## Jennifer (Oct 18, 2008)

Section 21.2.2 says in regions of low seismic risk to use 1-18 and 22...not 21. So, if wind controls, I would assume that means low seismic risk.


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## Bigwolf (Oct 18, 2008)

Thanks for the reply--that makes sense.

I also just noticed :reading: IBC Section 1908.1.4 that ammends the following:

ACI 318-05 Section 21.2.1.2 to be Seismic Design Category A/B

ACI 318-05 Section 21.2.1.3 to be Seismic Design Category C

ACI 318-05 Section 21.2.1.4 to be Seismic Design Category D/E/F

So I guess if wind governs the lateral load, then we steer clear of Chapter 21 :thumbs:


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## kevo_55 (Oct 18, 2008)

BW,

Beware of wind controlling over seismic.

See section 1604.10 of the 2006 IBC.

I don't mean to scare you, but this is a good thing to keep into perspective.


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## Bigwolf (Oct 18, 2008)

Kevo--

Thanks for the heads up--I hadn't seen that note before. :beerchug:


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## kevo_55 (Oct 19, 2008)

No problem man!!

Bring on that test!!


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## Casey (Oct 19, 2008)

Ok, I have a question regarding the ACI seismic provisions. In particular the 2006 IBC Seismic Design Manual Combo - Vol III.

At the top of page 309 of Vol III they are determining the Mpr for the bottom of the column where the column meets the foundation so there are no beam Mpr values that can be distributed to the column at the point. Instead they use a P-M interaction diagram (not given) to find the Mpr at a given axial load of 1200kips. Where does this 1200kips come from? I have scoured the problem trying to find out why they chose this value, as it doesn't appear in their load tables (pages 303 and 304).

Does anyone have an idea of where this came from? Is it possible that it came from a different load combination not shown in this example? At the moment it feels like it was arbitrarily chosen.

Thanks.


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## kevo_55 (Oct 19, 2008)

Casey,

"PCA Column" is a computer program for a beam-column interation diagram.

They simply used an interation diagram.


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## Casey (Oct 19, 2008)

kevo_55 said:


> Casey,
> "PCA Column" is a computer program for a beam-column interation diagram.
> 
> They simply used an interation diagram.


Kevo,

I understood that part, I just don't understand why they used 1200kips axial force to get the moment 2860kip-ft to be used in their calculation of Ve. Why not 650kip axial force? Or 900kip axial force?

Why did they decide to use 1200kips to get the required moment? Even if you use all the factored live and dead loads from all floors, the load on the column is far from 1200kips.

Thanks for any insight you have regarding this.


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## kevo_55 (Oct 20, 2008)

Casey,

I don't have the book in front of me but I believe that the moment was the more important part of the interation.

I believe that they calculated the moment and then looked at the interaction diagram to get the maximum compressive load.

I hope this helps!


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## hairpin (Oct 27, 2008)

Bigwolf said:


> Thanks for the reply--that makes sense.
> I also just noticed :reading: IBC Section 1908.1.4 that ammends the following:
> 
> ACI 318-05 Section 21.2.1.2 to be Seismic Design Category A/B
> ...


Just wanted to mention, regardless if whether wind or seismic governs, if you are in high-seismic zone there are special detailing requirements in Ch. 21 that still need to be used. Where I work all buildings are SDC D, E, or F. So everything in the lateral system needs to be a special design regardless of the governing load.


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