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Re: rigid wood diaphragms

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Thanks John, these were my impressions as well. The damage incurred at 
Northridge Medows Apartments was, I believe, a worst case soft-story design 
that proved to be inadequate. If anything, this would add evidence to support 
the dismissal of design by rotation or open-front design. I do not have much 
information at hand on NMA, but remember something to the affect that the 
cripple walls between the top of foundation and the first floor framing were 
inadequatly sheathed. This would support much of the work that Ben Schmidt 
did between Loma Prieta and to the present.
Ben proved by analysis that prescriptive methods of construction were 
inadequate when a raised first floor was introduced into the design. He ran 
the numbers on a number of one story and two story residences with raised 
cripple walls that were damaged after the Loma Prieta Earthquake.
Ben was a member of the Structural Methods committee which I started around 
1990. He represented SEAOC at the SSC's Triple R (Residential Retrofit and 
Repair) committee chaired by Fred Turner. Much of the information that he 
presented supported the need for cripple wall strengthening and anchorage to 
foundations.

Ben was also on a similar "crusade" at or slightly before Northridge when he 
ran some deflection calculations on wood piers in a condo project that, I 
believe, his daughter was considering purchasing a unit at.  His calculations 
matched mine which, using the 1991 UBC Standards for shearwall deflection, 
showed that wall deflections increased past the allowable story drift when 
the aspect ratio approached the 3.5:1 ratio and the walls were highly loaded. 
 I tended to agree with Ben that the amount of damage was caused by 
overloading these walls without consideration for deflection analysis - 
something that just was not common at the time.

Knowing what we know today, it seems that using better judgement and a few 
more design tools which we neglected to use prior to Northridge, would yield 
better results and reduced damage. I believe that we should be addressing 
these problems before we augment the code with far greater restrictions which 
result in costlier construction and higher engineering fees. Until we have 
definitive proof that the extent of damage is directly the result of 
inadequate design measures, we should not be so quick to penalize the public.

Dennis Wish PE

In a message dated 6/2/99 7:20:24 PM Pacific Daylight Time, 
jrose36(--nospam--at)earthlink.net writes:

<< Well, of course there was the Northridge Meadows apt. in which both loss 
of life
 and major structural damage occurred. And there were lots of other wood frame
 buildings (condos) which sustained substantial structural damage (see Ben 
Schmid
 case history for an example, of which there were many others). However, your
 point is well taken - many of these structures had questionable design
 assumptions or construction features, in my opinion, even though these were
 permitted by UBC at the time of construction. I could not think of an example
 where rigid or flexible diaphragm performance could be a factor in the damage
 that was observed, however.
 John Rose/APA, Tacoma >>