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# Re: Plywood rigid diaphragms

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Plywood rigid diaphragms
• From: chuckuc <chuckuc(--nospam--at)dnai.com>
• Date: Fri, 11 Sep 1998 17:31:15 -0700

```Mlcse(--nospam--at)aol.com wrote:
>
> In a message dated 98-08-31 03:00:06 EDT, you write:
>
> << Subj:         Re: Plywood rigid diaphragms
>  Date:  98-08-31 03:00:06 EDT
>  From:  jgkorff(--nospam--at)ix.netcom.com (james korff)
>  To:    seaint(--nospam--at)seaint.org
>
>  Where does the .005H limitation come from ?
>
>  jgkorff
>   >>
>
> The .005H is from the Los Angeles City Division 93 Ordinance.  This was the
> maximum limit in the 1994 UBC for structures with a period less than 0.7
> seconds, which could be even less if  0.04/Rw governed.
>
> I am don't know how this now correlates to the 1997 UBC, so hopefully someone
> can explain the comparison, if there is still one.  If you use the 1997 UBC
> the maximum deflection limit would be based upon section 1630.9.2.  The
> maximum inelastic drift is (delta m) = 0.7R(delta s).  Per section 1630.10.2
> the (delta m) drift can not exceed 0.025H for structures with a period less
> than 0.7 seconds which most bearing wall wood structures will be (R= 5.5).
>
> The (delta m) drift is then = (0.7)(5.5)(0.025)(H) = 0.0963H.  The questions I
> have are do you somehow still account for the 1.4 conversion (97UBC Rw = 5.5
> vs 94 UBC Rw = 8).  Is (delta m) suppose to be similar to the (94 UBC
> deflection )(3Rw/8).
>
> Michael Cochran
>
Michael-
I think your math is wrong.  You should divide by .7R not multiply.  For
a shearwall that reaches ultimate load at 2.5" deflection (.025H) the
allowable capacity will now be the test load at at a deflection of .65"
--as compared with the old citeria of .5" (.005H).  As far as I know
there's nothing magical about .005H or .025H, its just a way to set a
reasonable deflection limit which will allow us to achieve deflection
compatabily between the shearwalls at each floor level.
If all the walls are the same height and construction, Ed Diekmann's
suggestion to apportion load based on wall length x capacity makes for a
pretty reasonable approach. (In effect, the wall's stiffness is assummed
proportional to the number of nails. Probably as good as we can do for
now, given the paucity of test data on tie down deflection.)
Chuck Utzman,P.E.

```