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RE: Old Seismic Values

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Bill,
Good input on the Av stuff. The Aa and Av concept was first presented in the NEHRP before it got into the BOCA code. The background for the Aa & Av is contained in Appendix A "Background for Maps 1 through 4" of the 1994 NEHRP Commentary (FEMA 223A).

The 1994 NEHRP Provisions (FEMA 222A) contains a good section worth reading "Appendix to Chapter 1, Development of Design Value Maps".

Regards,
Harold Sprague


From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: RE: Old Seismic Values
Date: Fri, 13 Aug 2004 09:31:23 -0400

Jason W. Kilgore wrote: "But I'm still stuck on the 'peak velocities of 1.5
inches per
second' part. Where does this come in?"


I'll take a stab at it:

spectral acceleration = w*(spectral velocity) = (w^2)*(spectral
displacement)

where: w = 2*pi*f = 2*pi/T 		(in radians)
f= natural frequency of structure
T = natural period of structure

A tripartite response spectrum has a "constant velocity" range in the
mid-range of natural frequencies, at the peak velocity response. So the peak velocity and natural period should be able to be used with the above formula
to determine effective acceleration:

w*V = 2*pi*V/T

where V = spectral velocity

If you divide this quantity by 386 in/s^2, you get the result in terms of
"g's":

2*pi*1.5/(386*T) = 0.024g/T

The value 0.024g should roughly correspond to BOCA's "effective peak
velocity-related acceleration", Av.


William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com


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