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RE: Code changes to amplification of ground acceleration

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The intent of the code authors was to have similar performance with either
the 1994 or the 1997 UBC.  You will note that the Rw and the R values are
also different.  When you plug in the values for any particular system, you
will find very similar base shears.  The 1997 UBC was an incremental way to
introduce some of the changes promulgated in the 1997 NEHRP.

The big changes (using the spectral ordinates to get an MCE and a DBE and
the demise of "seismic zones") are in the 1997 NEHRP and in the 2000 IBC.

We are moving toward a performance based design, and we will be taking some
steps in that direction in the 2003 NEHRP cycle.  This was a major topic of
discussion at the NEHRP annual meeting held in Charleston last month.

Harold O. Sprague

> -----Original Message-----
> From:	Sherman, William [SMTP:ShermanWC(--nospam--at)]
> Sent:	Tuesday, February 05, 2002 8:32 AM
> To:	SeaInt Listserver (E-mail)
> Subject:	Code changes to amplification of ground acceleration
> 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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