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Primary code or not, the D 100-96 is wrong in its current form. And not just by a little bit.

Be very careful using the RF of 2.5. The use is predicated on "Where information is not available to determine a recurrence level accurately, ....". With the USGS maps, that information is available throughout the US. Apparently you have a response spectra with a 10,000 year mean recurrence interval, and you want to scale to a 475 year mean recurrence interval.

It was frustration with the tank industry that led the lead tank developers to use the NEHRP / ASCE / IBC process to circumvent the tank industry system and update the D 100. The authors are from Chicago Bridge and Iron, Tank Industry Consultants, Black & Veatch, and Preload Tank. The NEHRP specifically states that the intent is to augment the D 100, not replace it. NEHRP created a "bridge" to use the D 100-96 and update the seismicity to the maps and probabilities originally developed for the 1997 NEHRP. And now D 100 is finally getting the upgrade it was forced into.

When prudent engineering indicates a problem with the prevailing code, it is incumbent on engineers to use prudent engineering. The D 100-96 is predicated on the old seismic zone map that was in the old UBC and roughly predicated on a 10% exceedance in 50 years (mean recurrence interval of 475 years). The old zone maps used 1960's technology for seismic ground motions. In some areas, studies indicated that you can be off by a factor of 12 on base shear calculations using the map contained in the D 100-96.

Using the bridging equations contained in the NEHRP / ASCE / IBC will put the seismic ground motion probability to 2% exceedance in 50 years (mean recurrence interval of about 2,500 years). Actually the design ground motions will be 2/3 of the 2% in 50 year event. Read the NEHRP and the commentary. It explains it much better than I can.

Harold Sprague

From: "Al Grathwol" <AGrathwol(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: RE: AWWA D100
Date: Tue, 21 Dec 2004 14:34:19 -0800

I'm talking about RF per D100 Section 13.4.  My response spectra has a
475 year return period versus 10,000 for RF = 2.5.
D100-96 is still the primary code used in the industry until the new one
is published.

> -----Original Message-----
> From: Harold Sprague [mailto:spraguehope(--nospam--at)]
> Sent: Tuesday, December 21, 2004 2:22 PM
> To: seaint(--nospam--at)
> Subject: RE: AWWA D100
> Albert,
> In response to your particular question, the answer is no.
> You really have me confused.  If you are looking at the D
> 100-96, the "force
> reduction factor" is Rw (not Rf) which is really a response
> factor and is
> independent of the ground shaking / seismicity.
> It sounds like you are trying to fine tune a tank design.  If
> so the "out of
> the can" D 100-96 is not the proper tool.  Besides, it is the
> Z that defines
> seismicity (recurrence periods) and the ground shaking.  All
> that aside, the
> rest of the seismic code world left much of the D 100-96
> behind several
> years ago without MAJOR tweaking.
> To get what I think you want, use the ASCE 7-02, Section
>  If you
> want to modify the seismic probabilities, do it within the
> SDS and SD1
> calculations.  Don't modify the R.  You will probably want a
> site specific
> response spectra developed for the probability that you desire.
> I would also suggest that you read the FEMA 450-1 and FEMA
> 450-2, Chapter 14
> for the commentary that led to the development of the tank provisions
> contained in ASCE 7-02.  You will find similar changes in the
> ACI 350.3.
> BTW, the D 100 is in the process of a major upgrade.  I
> believe that it is
> being balloted now.  Seismic is one of the MAJOR overhaul areas.
> Regards,
> Harold Sprague
> >From: "Al Grathwol" <AGrathwol(--nospam--at)>
> >Reply-To: <seaint(--nospam--at)>
> >To: "SEAOC List (E-mail)" <seaint(--nospam--at)>
> >Subject: AWWA D100
> >Date: Tue, 21 Dec 2004 12:00:35 -0800
> >
> >According to AWWA D100, the force reduction factor Rf = 2.5
> for a 10,00
> >year recurrence period.  Is there a methodology for adjusting Rf for
> >smaller recurrence periods?
> >
> >
> >Albert H. Grathwol
> >Phone (949) 476-3562
> >Fax (603) 971-0200

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