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RE: UBC Eq. 30-1

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Since I am not really a UBC user (although I did look at the section), so
this may be a naive question...would this not depend on which load
combination you are considering and what you are checking?  I would think
that vertical accelerations from an earthquake could act either upward or
downward (while I have yet to really experience an earthquake except when
I was a little baby living in CA, I find it a little hard to believe that
the vertical earthquake accelerations/forces would "want" me to only go
up), especially since an earthquake does not only act in a "left"
direction but also a "right" direction (i.e. back-and-forth).

Thus, for a load combination like 1.2D+1.0E+(f1L+f2S) the Ev would be a
downward vertical component that would add to the gravity (DL and LL and
SL).  Now for a load comindation of 0.9D+/-(1.0E or 1.3W) and checking of
overturning, it would make sense for the Ev component to be upward, thus
reducing the DL effect.

Now, to be honest, my question is somewhat loaded, because the 2000 IBC
has a similar provision but is a little more specific.  There are two
versions of the horizontal plus vertical seismic effect:

E=rho*Qe+0.2*Sds*D for use with
	1.2*D+1.0*E+f1*L+f2*S (LRFD)
	D+(W or 0.7E)+L+(Lr or S or R) (ASD)
	D+L+S+E/1.4 (Alternate ASD)

and

E=rho*Qe-0.2*Sds*D for use with
	0.9*D+(1.0*E or 1.6*W) (LRFD)
	0.6*D+0.7*E (ASD)
	0.9*D+E/1.4 (Alternate ASD)

Thus, since the UBC and IBC are supposed to more or less point in the same
direction for seismic design (but not necessarily travel down the exact
same road), it would seem that my question may not be too naive.

HTH,

Scott
Ypsilanti, MI


On Wed, 11 Sep 2002, Gerard Madden, PE wrote:

> Curt,
>
> The dead load helps you to resist overturning/uplift. This factor takes
> away from that contribution. It is in addition, but Ev is upward and DL
> is downward.
>
> -gerard
>
> -----Original Message-----
> From: LaCount, Curt [mailto:Curt.LaCount(--nospam--at)jacobs.com]
> Sent: Wednesday, September 11, 2002 3:33 PM
> To: Seaint (E-mail)
> Subject: FW: UBC Eq. 30-1
>
> Gerard,
>
> I think that Ev is only used as an addition to the dead load and not a
> reduction.  The reduction of dead load is another factor in a different
> section and is limited to cantilevers and prestressed members.  For
> those of you using ASD, Ev can be ignored.
>
>
> Curt La Count
> Jacobs Engineering
> Portland, OR
>
> -----Original Message-----
> From: Gerard Madden, PE [mailto:gmadden(--nospam--at)attbi.com]
> Sent: Wednesday, September 11, 2002 1:41 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: UBC Eq. 30-1
>
>
> Ev is only used in non-working stress design. It is a reduction factor
> for dead lead and acts Upward. SO combine this term with your DL and
> you'll get something smaller than 0.9D in your load combo.
>
> -gerard
> Santa Clara, CA
>
> -----Original Message-----
> From: RainCat1(--nospam--at)aol.com [mailto:RainCat1(--nospam--at)aol.com]
> Sent: Wednesday, September 11, 2002 1:18 PM
> To: seaint(--nospam--at)seaint.org
> Subject: UBC Eq. 30-1
>
> In the UBC Eq. 30-1 E=rhoEh + Ev, what is the direction for Ev?
> Are we suppose to combine numerically the expressions rhoEh & Ev?
> Thanks!
>
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