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RE: Ca factor in the 1997 UBC

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This was first presented in the 1997 NEHRP.  I posed the same question to
the chairman of the BSSC Technical Subcommittee that developed this.  And
you surmised the reason.  The softer soils are limited on how much ground
motion they can transmit for short period spectral accelerations.
Regards,
Harold Sprague


> -----Original Message-----
> From:	Harper, Mark [SMTP:MHarper(--nospam--at)hdrinc.com]
> Sent:	Monday, February 26, 2001 6:57 PM
> To:	'seaint(--nospam--at)seaint.org'
> Subject:	Ca factor in the 1997 UBC
> 
> In the 1997 UBC the Ca factor for soil type Se is .36Na which is less then
> for soil types Sb, Sc,& Sd.  What is the reason for this?
> 
> I know that short period structures are acceleration controlled and that
> long period structures are velocity controlled but what does that have to
> do
> with the soil coefficients?
> 
> It would appear that soft soil types are less prone to acceleration,
> therefore there is a reduced value for Ca.  Is this the reason for the
> reduced value?
> 
> Mark Harper  SE
> HDR INC.
> mharper(--nospam--at)hdrinc.com
> Phone: (626) 564-2820
> Fax: (626) 564-2873
>