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Re: IBC 2000 Seismic maps

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Scott,

The 1994 Standard Building Code would be based off of the 1991 NEHRP
provisions (FEMA 222) and not the 1997 NEHRP provisions (FEMA 302) or the
2000 NEHRP provisions (FEMA 302).  While the 1997 and 2000 NEHRP
provisions are based off of the same philosophy, the 1991 NEHRP provisions
are not the same (although in a very rough manner they are similar).

The primary differance that you see between the 1991(/1994) and the
1997/2000 NEHRP provisions are two fold (at least the major differences).
The first are the seismic map values.  The 1997/2000 NEHRP provisions are
using newer maps that are more "complete" for the entire US.  Those who
operate in California would not likely notice too much difference in the
"level" of seismicity based upon the maps when comparing to the older UBC
(and SEAOC Bluebook) stuff.  The rest of the US, however, will notice some
"marked" increase in seismicity based upon the new maps.  I am probably
not explaining it well since I am not 100% fluent on the differences (yet)
but I am sure that someone else can elaborate more if you want.

The second main difference is in what factors into the determination of
the level of seismic design.  Under the "old" UBC method, one would only
consider the seismicity of the area that the building would be located it
(i.e. the UBC zones).  Thus, if you lived in an area that suffered from
large seismic events, then the structure would be designed for larger
seismic forces.

With the advent of the 1991/1994 NEHRP provisions, the design of the
structure had to account for both the seismicity level AND the
"importance" of the structure.  Thus, the detailing requirements for the
structure were now a function of how important the building was (i.e. its
use) and the potential magnitude of the seismic event.  The result was
that a structure in a moderate seismic "zone" that was a hosiptal could
end up with the same detailing requirements as a plain vanilla commercial
building in a high seismic "zone".

Now, with the 1997/2000 NEHRP provisions, the additional factor of the
soil type is added to the mix.  Now, the level of seismic detailing is a
function of the seismicity (i.e. zone), the use/importance, and the type
of soil that the structure sits on.  The result is that a hospital that
sits on really soft soil in a low seismic "zone" could be required to be
detailed similar to a plain vanilla commerical building sitting on
relatively firm soil in a high seismic zone.

Now, I am sure that you are saying to yourself, "OK, that is nice, but I
did not want a long, boring disseration on the evolution of seismic
design".

So, to your quesitons...

Question #1:

Soil Profile Type is not the same as the Site Class.  They are rather
similar, but not identical.  While I don't have the 1994 SBC, it should be
very similar to the 1996 BOCA (which I do have).  The 1996 BOCA calls it
the Site Coefficient, but it should be the same basic thing as the Soil
Profile Type.  The point is that the codes based upon the 1991/1994 NEHRP
(1993/1996/1999 BOCA and 1994 [and a few others] SBC) have four Soil
Profiles/Site Coefficients while the codes based upon the 1997/2000 NEHRP
(2000/2003 IBC and NFPA 5000) have 6 Site Classes.  A Soil Profile B under
the 1994 SBC (basically the same as an S2 Site Cofficient in the 1996
BOCA) would likely be roughly equivalent to a Site Class C (or maybe even
D) in the 1997/2000 NEHRP based codes.

Question #2:

The Seismic Hazard Exposure Group is basically the same as the Seismic Use
Group (not reversed...at least based upon comparing the 1996 BOCA to the
2000 IBC).  There might be some small differences but they are basically
the same.

The Seismic Performance Category is very similar to the Seismic Design
Category.  They serve the same basic purpose, but are arrived at
differently.  Both rough serve the same purpose as the "zones" in the UBC
in the sense that they determine the level of seismic detailing required
for a structure.  An SDC or SPC of either A or B are roughly like a Zone
1.  An SDC or SPC of C are roughly like a Zone 2.  An SDC or SPC of D, E,
or F (an SPC of F does not exist) is roughly like a Zone 3/4.  The
difference between the SPC and the SDC is how there are determined.  The
SPC is a function of the Seismic Hazard Exposure Group and the seismic
values from the maps.  The SDC is a function of the Seismic Use Group, the
seismic values from the maps, AND the Site Class (see the dissertation
above).

Question #3:

Neither (at least I assume so).  The seismic maps values from the
1997/2000 NEHRP provisions (which should be what you get from the USGS web
site) are NOT the same as Aa or Av that would come from the 1991/1994
NEHRP provisions (thus the 1994 SBC).

Question #4:

Yes, the Ss is the mapped spectral acceleration for the short (0.2)
periods and the S1 the mapped spectral acceleration for the 1 second
periods.

You would use the 5% damping values (at least that is what the 2000 IBC
uses).

Question #5:

I'll take a stab at it, but someone may need to correct me.

The PGA will be peak acceleration that a particle of soil sitting at
surface of the earth would see during an earthquake.

The spectral values (Ss or S1) are the peak acceleration that a particle
on a building would experience.

You can get some more info from the USGS FAQ page:

http://geohazards.cr.usgs.gov/eq/html/faq.html

To wrap it up, I would strongly suggest that you obtain the 1994 SBC.
First, the NEHRP provisions are the basis for the seismic portions of the
new codes, but there are not just reprinted in the codes.  The NEHRP
provisions are not completely written in mandatory language (i.e. "code
speak") so they do get modified for mandatory language as well as
"tweaked" (i.e. slight changes in some provisions) by the individual
codes.  The NEHRP documents are a really good source for background
information of seismic design (for example, they have a commentary that
will explain a lot of the "rationale" behind the provisions and seismic
design), but are not really code documents.  The NEHRP provisions can help
if you need to think "outside the box" of the codes.

You should not need to get your hands on a ASCE 7-95...the 1994 SBC should
have all the seismic stuff printed in it.  The exception would be is you
need other information (i.e. wind loading) from the ASCE 7-95.

HTH,

Scott
Ypsilanti, MI

On Sat, 16 Mar 2002, Scott A. Dunham, PE wrote:

> >From one of the dumb old guys that learned about being an Engineer back in
> the days before the powers that be thought that there were such things as
> earthquakes on the east coast - I'm attempting to make some sense out of a
> building design that states the following:
> Effective peak velocity-related acceleration Av = 0.06
> Effective peak acceleration Aa = 0.06
> Seismic Hazard Exposure Group I
> Seismic Performance Category B
> Soil Profile Type B
>
> The referenced codes are 1994 Standard Building Code & ASCE 7-95
>
> Along with my lack of experience with earthquake design, I've just been in
> business on my own for about a year, so I don't have the referenced
> documents.  I do have a copy of FEMA 368 & 369, but the terminology is
> slightly different.  I also went to the USGS Hazard Map site & got a
> printout , but quite frankly I unsure of what the numbers are telling me.
>
> Will some of the more knowledgeable of you please lend a hand and comment on
> the following:
>
> 1) Is the Soil Profile Type the same as Site Class in FEMA 368 (i.e., good
> rock)?
> 2) Is the Seismic Hazard Exposure Group similar (but reversed) to the
> Seismic Use Group in FEMA 368 (i.e. this structure is a III), & the Seismic
> Performance Category the same as the Seismic Design Category in FEMA 368
> (i.e. this structure is a B).
> 3) What is the %g from the USGS Map site (Av or Aa)?
> 4) I assume that the FEMA Ss is the USGS 0.2 sec value, & S1 is the 1.0 sec
> value; but which value do you use, the 10%, the 5%, or the 2%?  And how do
> you decide?
> 5) What are the PGA values on the USGS site?
>
> Before you flame me for getting in over my head, let me explain that I'm not
> designing the building.  I design stairs & design/check connections for
> fabricators.  I have this project to design the stairs & out of the blue,
> the contractor asked if the stairs were designed for earthquake.  My gut
> tells me that there is no problem, but guts don't do you much good in court,
> so I'm trying to learn a new (& exciting) aspect of this business.
>
> Thanks in advance for any assistance you may be able to lend.
>
> Scott A. Dunham, PE
> Dunham Engineering Services
> Dothan, AL
> 334-678-6948
>
> ----- Original Message -----
> From: "Roger Turk" <73527.1356(--nospam--at)compuserve.com>
> To: <seaint(--nospam--at)seaint.org>
> Sent: Saturday, March 16, 2002 4:17 PM
> Subject: RE: IBC 2000 Seismic maps
>
>
> > Jason Kilgore wrote:
> >
> > . > For more accurate Ss and S1 values, click on "Hazard by Zip Code" or
> > . > "Hazard by Lat/Lon". Then enter the USPS zip code or
> Latitude/Longitude
> > . > and the page will return values that are more accurate than reading of
> a
> > . > large-scale map.
> >
> > And how accurate is the value you get when one building has one value, and
> > another building on an adjacent lot has another value because they are in
> > different ZIP code areas?
> >
> > The answers that you get with such precision are wholly inaccurate.
> >
> > A. Roger Turk, P.E.(Structural)
> > Tucson, Arizona
> >
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