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RE: Code changes to amplification of ground acceleration

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


I will attempt to baffle with  B.S. and here is how I have rationalized it
in my little world. The 97 code was being adopted when I took a night class
on this stuff and it has been a while but...

There is no big difference between static-dynamic "response spectrum" in the
newer codes.  The old codes required you to scale dynamic results by the
static force base shear anyway.  I thought the reason for the 94 UBC
2.75-->2.5 difference had to do with the 1/T^.66 static method C value
relative to the 1/T value used for the dynamic response spectrum.  The newer
codes use 1/T for static and dynamic methods and are also more accurate so
there was no need for the 2.75 instead of 2.5.

1994 UBC Figure 16-3 was the response spectrum for modal analysis.  2.5 is
the "fault amplification factor" for normallized fraction of the maximum
pseudo-acceleration to effective peak ground acceleration for 5% damped
elastic single degree of systems w/ varying fundamental periods of vibration
oscillated with the N-S 1940 El-Centro time-history.  The code spectrum is
derived from an ensemble of earthquake records 2.5 is the amplification
factor that ends up being typical.  You could take this 2.5 and multiply it
by your effective peak ground acceleration and it will give you your
"pseudo-acceleration" that you multiply by your building mass to get the
design force resisted by an elastic building as it deflects.  The response
modification factor "R" then converts the force that the building can
dissipate and remain standing.  The 2.5 amplification factor is the same in
the 94 UBC and 97 UBC and is built into the Ss and S1 values in the IBC.

Pseudo acceleration is the pseudo velocity divided by the buildings
fundamental period.  The 94 UBC static analysis formula limited C to 2.75
with C=1.25*S/T^.66 and the dynamic "response spectrum" limited the maximum
value to 2.5 with a 1.25*S/T relationship.  The 2.75 accounted for the
1/T^.66, the effects from other modes and the uncertainty of the soil
characteristics.

The soil type and distance from faults are handled in the Cv and Ca of the
97 UBC.  There is a limit of 2.5 on the spectral amplification with
[V=minimum(2.5*Ca,2.5*Cv/T)/R]. The IBC accounts for site conditions with
Sms=Fs*Ss & Sm1=Fv*S1 and handles the 2.5 stuff inside the Ss and S1.

Another lunch break wasted for the profession.


Scott M Haan P.E.
Plan Review Engineer
Building Safety Division
Development Services Department
Municipality of Anchorage
http://www.muni.org/building
phone:907-343-8183
fax:907-249-7399
mailto:haansm(--nospam--at)ci.anchorage.ak.us



-----Original Message-----
From: Sherman, William [mailto:ShermanWC(--nospam--at)cdm.com]
Sent: Tuesday, February 05, 2002 5:32 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)cdm.com

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