Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Old Seismic Values

[Subject Prev][Subject Next][Thread Prev][Thread Next]

I think the key word that is missing from your description is "spectral".
Peak GROUND acceleration or velocity is different than the peak SPECTRAL
acceleration or velocity.  It sounds like the information in the report you
were given was for peak ground values.  Aa, Av, Ss, & S1 are spectral
values.  The theory is that an ideal oscillator (think of a bowling ball on
a thin steel rod stuck in the ground) with a particular fundamental
frequency will experience a certain peak acceleration and velocity in a
given seismic event.  These accelerations and velocities, which are the
spectral values,  will be different for oscillators of different
frequencies, and will be different from the values at the ground surface.
Aa & Ss are the values for an oscillator with a 0.2 sec. period (period =
1/frequency).  Av & S1 are for an oscillator with a 1 sec. period.

In the older UBC, Z is the approximate peak ground acceleration.  0.2
spectral acceleration is approximately Z*C with C=2.75.  The 1.0 spectral
acceleration is approximately Z*C with C calculated for T=1.0 => C=1.25.

Of course, all these values vary depending on the confidence level.  The
current codes use a 2% probability of exceedence (98% confidence) in 50 yrs.
I'm not sure what the older UBC values were based on.  Up to date values for
any location in the US are available free at

The FEMA 273 or 368 commentaries, or the ASCE7 98 or 02 commentaries have
very good information on this subject.

I hope you send a summary of your (general) findings to the list, once you
have finished your investigation.  This is a very relevant topic that most
structurals, at least in the west, will be dealing with for years to come.

Dmitri Wright, PE
Progressive Consultants Inc.
Portland, OR

From: "Jason W. Kilgore" <jkilgore(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: Old Seismic Values

I have a project in a low/moderate seismic zone (moderate to high wind), =
the client specifically wants seismic calculations and details.  I was =
given this information as part of a larger report:

"The site is in seismic zone one (1).  However, the area is free of =
active fault structures and the region is relatively free of seismic
activity.  According to recent investigations, the area will experience
seismic forces with a maximum effective peak acceleration of 5% of =
and effective peak velocities of 1=BD inches per second during any =
period, at the 90% confidence level."

The 50-year/90% confidence is the design level in the UBC codes and the =
NEHRP / ASCE 7-95.  I'm pretty sure I know what "maximum effective peak
acceleration of 5% of gravity" and "effective peak velocities of 1.5 =
per second" mean.  However, is there a relationship between these =
and UBC design values?  What about Aa and Av (older NEHRP)?  Ss and S1
(current NEHRP)? =20

Any guidance as to where to look?

Specifically, I would like to give the client a comparison between the
values they gave me and values from FEMA 450/2003 NEHRP (Ss =3D 14%, =

Thanks in advance.

Jason Kilgore
Leigh & O'Kane, LLC
Kansas City, Missouri

******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at:
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*   Questions to seaint-ad(--nospam--at) Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: 
******* ****** ****** ****** ******* ****** ****** ********