] [Thread] [Date] [Author]

["Dickey, David" <David.Dickey(--nospam--at)mhgrp.com>]

Could someone who is familiar with the site-specific procedure for determining ground motion accelerations provide some details about the analysis?

The IBC says, "A site-specific study shall account for the regional seismicity and geology;  the expected recurrence rates and maximum magnitudes of events on known faults and source zones;  the location of the site with respect to these;  near source effects if any;  and the characteristics of subsurface site conditions."

This sounds very complex to me and I didn't think that it was done very frequently.  But some recent posts make it sound like it is performed more than I thought.  If the geotechs are charging around $2000-$3000 for the analysis, it must be less in depth than I envisioned it.

The USGS's acceleration contours that account for the regional seismicity and geology are developed from a complex process including a large database of earthquakes, many attenuation relationships, paleoseismic investigations, and other complicated methods.  Can the geotech's $3000 analysis of the seismicity be more "accurate" than the USGS's method?  

Is the procedure really just a more detailed analysis of the site coefficients (Fa & Fv)?  Does the software used by the geotech just 'tweak' the data from the USGS?  What software is used?  Are the ground accelerations determined by a site-specific analysis lower than the IBC's because less conservative models and assumptions are used?

Thanks,
David Dickey



-----Original Message-----
From: Clifford Schwinger [mailto:clifford234(--nospam--at)yahoo.com]
Sent: Thursday, January 29, 2004 9:17 PM
To: seaint(--nospam--at)seaint.org
Subject: Stupid earthquake question of the day


IBC 2000 allows for two methods for computing ground
motion acceleration - the General Procedure of Section
1615.1 or the Site-Specific procedure of Section
1615.2.  Section 1615.2.3 tells you how to calculate
the deterministic maximum considered ground motion. 
The "actual determined" maximum considered earthquake
ground motion is 150% of the actual measured ground
motion accelerations at any know active fault
location.  The end result is that the Code seems to
say that if you follow the Site-Specific Procedure of
section 1615.2 you might be able to take advantage up
to a 20% reduction in ground motion accelerations. 
There is a clause in 1615.2 that says the ground
motion accelerations computed per the Site-Specific
procedure can't be less than 80% of the ground motion
accelerations computed per the General Procedure of
Section 1615.1.  Did I interpret this correctly? Is
the advantage of the Site-Specific Procedure that of
allowing for a potential benefit of up to a 20%
reduction in seismic design forces?

Now here is my real question:  What if your building
is located in an area where there has never been ANY
damage to building structures from earthquakes.  Can
you use the Site-Specific Procedure and reduce your
seismic forces by an automatic 20%?! Obviously if your
building is located somewhere where there's never been
any damage recorded from earthquakes, the ground
motion accelerations of ANY earthquakes ever recorded
in the area will be nil and the  provision that the
ground accelerations used for design be no less than
80% of those computed using the General Procedure
method would govern by inspection!  This would allow
engineers in areas of low seismicity to whack an
automatic 20% off their seismic forces using Section
1615.2 versus if they went straight to Section 1615.1
to compute the seismic forces. 

Has anyone on the list ever used the Site-Specific
Procedure?  I'm not trying to be a wisenheimer with
this question.  I would seriously like to know if
anyone agrees with my logic on this issue!

TIA!

Cliff Schwinger
 


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