Re: Code changes to amplification of ground acceleration

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: Code changes to amplification of ground acceleration
• From: "Mark D. Anderson PE" <mark(--nospam--at)alaskaengineer.com>
• Date: Tue, 5 Feb 2002 17:18:06 -0900
```Perhaps it is clearer to rephrase the original question as > "What is the
basis for the amplification of the ground (or zero period) acceleration to
obtain the maximum or peak response of the system, when using the static
force procedure?"

One resource that gives a succinct answer is the EERI Monograph "Earthquake
Spectra and Design", by Newmark and Hall.  On page 35 the various
amplification factors are provided as a function of % critical damping, and
also as a function of what may be called "confidence limits".  For
acceleration, at 5% damping, at one sigma, the amplification factor is given
as 2.71, and at the median value (50 %) the amplification factor is given as
2.12.

The vacillation in the amplification factor between 2.5 and 2.75 is usually
about tweaking the confidence limit used as the basis for the criteria,
since we are ordinarily presuming 5% damping.

Mark D. Anderson PE

----- Original Message -----
From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, February 05, 2002 11:57 AM
Subject: RE: Code changes to amplification of ground acceleration

> "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
> 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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