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RE: Low Seismic Design Catagory

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I think that your interpretation is correct. Chapter 21 of ACI 318-02 is
very difficult to discern what applies or does not apply. In Chapter 1,
Table R1.1.8.3 indicates that your Seismic Category B qualifies as "Low
Seismic". Section indicates that Chapter 21 does not apply but
Section implies that some provisions of Chapter 21 may apply. 

The code side is not very clear, but Table implies that the
special diaphragm provisions in Chapter 21 only apply in high seismic
zones. Thus, your interpretation is reasonable based on the Commentary
to the code. 

-----Original Message-----
From: Jake Watson [mailto:jwatson(--nospam--at)] 
Sent: Tuesday, March 29, 2005 3:46 PM
To: seaint(--nospam--at)
Subject: Low Seismic Design Catagory

We are working on a parking garage in Seismic Catagory B.  I am a Salt
Lake native and find it hard to turn off my seismic intuition.

If I use an Ordinary Reinforced Shearwall lateral force resisting
system, do I have any need to go to Chapter 21 in ACI 318?  If I read
the definitions section correctly, there are no special detailing
provisions for my walls.  

Secondly, if I use the Ordinary system, do I need to meet the diaphragm
provisions of Chapter 21?  This is a little more grey to me.  I may
chose to meet them anyway, but it is always good to have options.

Any thoughts?

Jake Watson, P.E.
Salt Lake City, UT 


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