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
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,
<< Well, of course there was the Northridge Meadows apt. in which both loss
and major structural damage occurred. And there were lots of other wood frame
buildings (condos) which sustained substantial structural damage (see Ben
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 >>