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RE: '97 code seismic map arrived from ICBO

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>From a engineer also struggling to figure this thing out I offer the
following comments.  The first thing to note is that the forces you have
calculated are ultimate strength, and not allowable stress design
forces.  If you are designing in timber, the seismic forces can be
scaled down by a factor of 1.4 per the load combinations in section
1612.3.1 or 1612.3.2, unless you are using LRFD for timber design.  The
load combinations for concrete can be per section 1928 provided the phi
factors from this section are also used.  These factors allow you to
have a uniform set of load combinations for LRFD design in concrete,
steel, and timber.  Something did get simpler!  In general the loads are
a little higher if you are not near the fault and a lot higher if you
are near the fault.  Things can really blow up if your rho factor goes
to 1.5, so spread your loads to multiple walls if you can.

Thomas Castle, S.E.
> ----------
> From: 	Harris3803[SMTP:Harris3803(--nospam--at)]
> Sent: 	Monday, March 16, 1998 8:03 PM
> To: 	seaoc(--nospam--at)
> Subject: 	'97 code seismic map arrived from ICBO
>      I just received the new seismic maps for the '97 code from ICBO.
> I
> noticed most of the cities in my area ( Ventura County ) are in
> seismic source
> type B areas. The next step was to see what difference this is going
> to make .
> Most of the buildings i work on are plywood shearwalls or concrete
> tilt up.  
>      For an assumed soil type D ( sec 1630.2.3.2 )  i get E = .22 with
> rho at
> 1.0 to E=.33 at rho=1.5  ( sec 1630.1.1 ) . This is 1.6 to 2.4 times
> the old
> V=.138W base shear . I must be making a mistake somewhere. Can someone
> clarify
> the base shear for a two story plywood shear wall house under the
> described
> conditions. Quoting code sections would help.
>      The next question is when to start using this. If the '97 code is
> adopted
> three years after the '94 code that would be Jan 1999. If you were
> working on
> a tract that had multiple phases i would think we should be advising
> clients
> now so they don't have to pay for a redesign in future phases or if
> the
> submittal is delayed.
>      Thanks in advance.
>      Tom Harris,SE
>      Thousand Oaks, CA