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# Re: UBC 97 Drift

• To: seaint(--nospam--at)seaint.org
• Subject: Re: UBC 97 Drift
• From: Eddie Gonzalez <Eagonzal(--nospam--at)ENG.CI.LA.CA.US>
• Date: Fri, 26 Mar 1999 08:30:45 -0800

```Mr. Serhal,

Based on my continually developing understanding of UBC97, drift calculations
are primarily based on load combinations found in 1612.2.1, particularly 12-5 &
12-6, as noted in Section 1909.2.3.  Exception 2 of 1612.2.1 is not recommended
to be applied per Seismology Committee per Dr. Ghosh's paper in Building
Standards.  Section 1612.3 do not apply.  1612.4 is for increasing forces on
special elements.  One may use Section 1928 as alternate but 1) alternate
strength reduction factors must be used 2) Correct error of 1.5 load factor for
E in LC 5 & 6.

Section 1630.9.1 specifically calls for a mathematical model in accordance with
Section 1630.1.2 which specifically requires that "1. Stiffness properties of
reinforced concrete and masonry elements shall consider the effects of cracked
sections."

One must also account for tranlational and torsional deflection, as well as
panel zone deformation (steel bldgs) when calculating Ds

I believe your I=0.35Ig  you obtained from recommended values when calculating
magnification factors of slender columns, Section 1910.11.1.  If this is what
you are checking than it seems in accordance with the code.  For Shear walls,
you must comply with Section 1921.6.6.5 where Dt may equal Dm and you must
calculate the curvature requirements.   I believe there is a requirement of
limiting the stiffness of the boundary elements but I can't find it right now.

The draft Blue Book seems to recommend a revised formula of Dt as  Dt = RDs,
such that Di = Dt - Dy

You are right.  The requirements are very stringent.  Compare the UBC 97 with
UBC 94 (incorrectly assuming I=Ie=Ig):

D97   =    0.7RDs       =     0.7R(1.4D)     =    0.7       =   1.87
D94       0.375RwD          0.375(1.4R)D        0.375

Some expect that this large deflection results may pose a challenge when
design.

My 0.02 cents,

ed gonzalez

>>> Sleiman Serhal <mony(--nospam--at)destination.com.lb> 03/26/99 12:51AM >>>
When computing Ds for a shear wall according to the UBC 97, do u use in
your mathematical model an I of 0.35Ig if the wall cracks under elastic
forces - due to E only

It seems to me that this would be too penalizing especially that we
multiply Ds by the R factor to get Dm which is the value actually used
to determine if the allowable drift has been exceeded.

Some engineers tell me they use an I = 1Ig !

And is the 1.2D + f1L + E including P-D  the only combination to use for
drift calculations ?

Moni Serhal

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