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RE: IBC Sec. 1617.1

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If you read the NEHRP and accompanying Commentary (section 5.2.7 of 1997
provisions) the 0.2Sds is intended to account for vertical accelerations, as
you have interpreted.  There have been similar provisions in other codes as
well (BOCA, etc.).

-----Original Message-----
From: Jason Kilgore [mailto:jkilgore(--nospam--at)] 
Sent: Friday, September 12, 2003 2:06 PM
To: seaint(--nospam--at)
Subject: IBC Sec. 1617.1

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.

Jason Kilgore
Leigh & O'Kane, L.L.C.

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