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Re: Effective Peak Acceleration to Velocity Related Acceleration

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Many good responses were provided on this topic.  It is the intent of
this response to provide some additional opinion.

Eqbal M. Kassam wrote:

> I am looking for any formulas or rule of thumb to convert "Peak Ground
> "
> accelerations to "Effective Peak Velocity Related Accelerations ". I
> have ASCE 7-93 which provides definition of these accelerations, i.e
> Aa
> and Av, the difference being the periods.

The effective peak acceleration (Aa) and effective peak velocity-related
acceleration (Av) were introduced in the National Earthquake Hazard
Reduction Program (NEHRP) Recommended Provisions.  These provisions are
still in a status of flux as evidenced in the changes from 1988 edition
to 1991, to 1994 editions.  The forthcoming 1997 edition will have even
more changes.  The Aa maps were determined from seismic risk as in most
seismology reports.  Av maps are much more complicated.  They are
derived from Aa map taking into account large distant earthquakes and
the duration of shaking.  In general, they are obtained from the
relationship of

                0.1 g of peak acceleration to 3 in/sec of peak velocity,

                0.2 g of peak acceleration to 6 in/sec of peak velocity,
                0.4 g of peak acceleration to 12 in/sec of peak

Then, the peak velocity values are modified regionally depending upon
regional geology and seismology.  Finally these peak velocity maps are
converted back into Av maps.  Thus, it is difficult to apply a simple
rule of thumb for conversion.

> Actually I have a "Seismic Response Assessment Report" from a
> consultant
> done long time ago for my project, located in Middle East. This
> consultant is no longer providing any services to the project.
> The report states  "For 1000 Average Return Period (criteria for this
> project), the computed Mean Peak Ground Acceleration is 0.183g". The
> report also provides Mean Ground Accelerations for other return
> periods
> - 500 years is 0.127g, 5000 years is 0.354g and 10,000 years is
> 0.452g.
> It does not provide any information on Velocity Related Acceleration.

Since the effective peak velocity-related acceleration is a new concept,
it is not a surprise to
find that it is not in most seismology reports.  If there is no large
distant earthquakes about 100 km away, Av can be approximated by Aa.
This is definitely not the case for Mexico City.

> How does one determine the Equivalent UBC Zone with the above
> information ? Is it UBC Zone 2B or Zone 3 ?  (UBC Return Period is 475
> years versus 1000 years on my project). The AISC Seismic provisions
> and
> ASCE categories is based on "Effective Peak Velocity Related
> Accelerations". UBC Zoning is also based on this.

The data and references presented in the seismological report may give
you some clue to perform the conversion from 1000 years to 475 years..

> Is it safe for me to assume that this site (not located in US) is
> equivalent to UBC Zone 2B ? According to "Project Guidelines" prepared
> prior to my arrival on the project, the site was classified as UBC
> Zone
> 3. From my drawing reviews, detailing of concrete structures,
> ductility
> requirements, for Zone 3 has not occurred. It is a Lump Sum Turn Key
> Contract and the Contracor is trying to convince the owner that
> detailing for UBC Zone 2B is adequate and that there was an error in
> the
> "Project Guidelines". The majority of the structures that get effected
> are concrete and some steel connections are doubtful.

If your Project Guidelines follow the definition of UBC-88, Z value for
Zone 2B is 0.2 which is larger than the 1,000 years return period
acceleration of 0.183 g.  It appears that the Turnkey Contractor does
have a good basis.  However, one still needs to examine the contract to
determine whether contractually it could be done.

Hope the above helps.


Chang Chen, P.E., Ph.D.
Apollo Consulting, Inc.