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Low, Moderate/Intermediate, or High Seismic Risk

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Hi All,

One of our engineers is trying to determine the relationship between a
structure's Seismic Design Category and Seismic Use Group as defined by
IBC and the seismic designations used in ACI 318 Chapter 21.  ACI 318
Chapter 21 makes designations on seismic design requirements based on
"Low", "Intermediate", and "High" seismic risk.  

"Design of Concrete Structures 13th Edition" by Nilson suggests the
following relationship between SDC and seismic risk level:

high seismic risk - SDC D, E, and F
moderate / intermediate seismic risk - SDC C
low / no seismic risk - SDC A and B

Another publication "Seismic and Wind Forces" written in accordance with
IBC 2003 by Alan Williams seems to suggest the following relationship
between SDC and seismic risk:

high seismic risk - SDC D, E, and F for SUG I, II, and III provided that
SDS > 0.5g and SD1 > 0.2g
moderate / intermediate seismic risk - SDC B and C for SUG I, II, and
III provided that 0.167g < SDS < 0.5g and 0.067g < SD1 < 0.2g
moderate / intermediate seismic risk - SDC D for SUG III provided that
0.33 g < SDS < 0.5g and 0.133g < SD1 < 0.2g
low seismic risk - SDC A for SUG I, II, and III provided that SDS <
0.167 g and SD1 < 0.067g

These are slightly different in that a SUG III building in SDC D falls
into the moderate category according to Williams. Is anyone aware of an
"official" ACI definition of low, moderate and high that would apply
here?

Chuck Hanson, PE, LEED AP
Principal
chuckh(--nospam--at)dpsdesign.org
Dekker/Perich/Sabatini
Albuquerque, NM 



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