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Code changes to amplification of ground acceleration

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I have been trying to track down the basis for changes to the
code-prescribed amplification to ground acceleration between the 1994 UBC
and the 1997 UBC (which carried over into the 2000 IBC). I've looked thru
the NEHRP Commentary (1994 and 1997) but could not find an explanation for
one of the changes made:

1. In the 1994 UBC, Section 1628.2.1 placed a maximum value of 2.75 for the
value of C as used in V=ZICW/Rw. This would indicate a peak response of 2.75
times the ground acceleration. (However, it is curious to me that Figure
16-3 showed a maximum amplification to ground acceleration of 2.5?)

2. In the 1997 UBC, Section 1630.2.1 places the maximum base shear value at
V=2.5CaIW/R. (For soil type B, Ca is similar to Z from the 1994 UBC.) This
would indicate a peak response of 2.50 times the ground acceleration.

What is the basis for the reduction in the peak response amplification
relative to ground acceleration, when using the static force procedure?

William C. Sherman, PE
Camp Dresser & McKee, Inc.
Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

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