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# Re: drift calculation, 1997 UBC using ASD

• To: seaint(--nospam--at)seaint.org
• Subject: Re: drift calculation, 1997 UBC using ASD
• From: "Robert J Bossi, PE" <rjbossi(--nospam--at)sonic.net>
• Date: Tue, 09 Nov 1999 17:43:45 -0800

```Jeff:
You are not alone.  The drift limitations are stated as 2 or 2.5% of story
height (1630.10.2).   DeltaM is .7 R DeltaS  When doing an ASD Design you still
must still use the USD load combinations in 1612.2 to calc DeltaS.  That is
because Delta M is the maximum Inelastic Displacement.  This should not be a
big thing.  Delta M will be about 5 times what you expected under the 1994 UBC
but note that the allowable deflection is also 5 times higher.

Now for the real problem in San Francisco.  See 1633.2.11.  If you allow DeltaM
to be 2.5%, you are looking at a 9 inch set back from the property line for a
30 foot tall building.  That would be giving up 18 inches on a 25 foot lot.  If
there is a building on the adjoining lot(s) at the property line, the root mean
square separation would be almost 13 inches. You need to stiffen up your
structure to reduce DeltaM.

John Rose at APA told me that 2:1 shear plywood shear walls with nailed or lag
screwed HDs should yield about 1% delta at table allowable loads.  1%, then
might be a good compromise delta limit.

Bob

Jeff Smith wrote:

> Can anyone help me out in understanding the drift requirements for OMRF
> steel frames or braced frames using ASD (3 story wood frame residence)?
>
> It seems to me that you have to use Strength Design load combinations to
> determine delta s. Do I *have* to use strength design to determine drift
> allowables and requirements and if so it seems there is no sense using ASD?
> (Maybe this is what the code writers wanted). Am I the only one confused?
>
> Jeff
>

```