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RE: IBC Sec. 1617.1

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Brian,

I thought that the 1997 NEHRP would provide clear guidance on it...was
just too lazy to dig it out and really look.  ;-)

Regards,

Scott
Ypsilanti, MI


On Fri, 12 Sep 2003, Brian Harris wrote:

> Scott and Jason-
>
> The 1997 NEHRP commentary makes it clear that this is a vertical effect:
>
> "In Eq. 5.2.7-1 and 5.2.7-2 , a factor of 0.2SDS was placed on the dead load to account for the effects of vertical acceleration."
>
> These two equations are labeled as 16-28 and 16-29 respectively in the 2000 IBC.
>
> -Brian Harris
>
> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu]
> Sent: Friday, September 12, 2003 11:23 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: IBC Sec. 1617.1
>
>
> Jason:
>
> To me, the definition of E just after Equation 16-28 should help provide
> some clarification.
>
> It states:
>
> "E = The combined effect of horizontal and VERTICAL earthquake-induced
> forces."  (emphasis is mine)
>
> Add to that, that QsubE is defined as:
>
> "QsubE = The effect of horizontal seismic forces."
>
> To me, that makes the 0.2SsubDS*D term a vertical component term.
>
> Further more, this is consistant with how the 1997 UBC deals with the
> similar situation (and the 1997 UBC is to some degree at least a minimal
> basis for what is in the 2000 IBC).  The similar equation in the UBC (eq
> 30-1) is E = rho*Eh + Ev.  And Ev is clearly defined as the vertical
> component of the earthquake ground motion ("Ev = the load effect resulting
> from the vertical component of the earthquake ground motion and is equal
> to an addition of 0.5*Ca*I*D to the dead load effect, D, for Strength
> Design, and may be taken as zero for Allowable Stress Design.")
>
> And last, since the 2000 IBC Seismic provisions are basically based off of
> the NEHRP Provisions, you could get a hold of the 1997 NEHRP provision and
> see what the commentary says on this section.
>
> HTH,
>
> Scott
> Ypsilanti, MI
>
>
> On Fri, 12 Sep 2003, Jason Kilgore wrote:
>
> > I normally work with buildings with a seismic design category of A, B, or C,
> > so I rarely use Equations 16-28 through 16-31.  (rho = 1, I don't use
> > section 1617.1.2, and 0.2*Sds = about 0.02).
> >
> > Now I've got a problem with an Sds of 68%, so the second term in those
> > equations is much higher.
> >
> > It's obvious that the first term acts horizontaly (Qe).  I've always worked
> > on the assumption that the second term acts vertically.  For example, take a
> > drag strut with vertical and horiz. loads.  Assume rho = 1.5.
> >
> > E = rho*Qe +/- 0.2*Sds*D
> > E = 1.5*Qe +/- 0.136*D
> >
> > You would then plug this value of E into, for example, Eq. 16-5:
> > F = 1.2*D + 1*E + 0.5*L + 0.2*S
> >
> > F = 1.2*D + 1.5*Qe + 0.136*D + 0.5*L + 0.2*S
> >
> > F = 1.336*D + 1.5*Qe + 0.5*L + 0.2*S
> >
> > In other words, the "D" value from equations 16-28 through 16-31 are added
> > to the "D" in whichever final load equation you use.
> >
> > Recently it was pointed out to me that the D in those equations is actualy a
> > horizontal force, and should be directly added to Qe to get a purely
> > horizontal "E" force.  In other words, part of the horizontal load in a
> > member is from the seismic base shear, and part is caused directly by the
> > dead weight (and supported dead weight) of the member itself.
> >
> > I think this person is wrong, but would like other expert opinions.
> > Thanks,
> >
> > ----
> > Jason Kilgore
> > Leigh & O'Kane, L.L.C.
> >
>
>
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