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Re: Rigid plywood diaphragms

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Lynn see inserted comments:

In a message dated 11/18/98 8:44:20 PM EST, lhoward(--nospam--at)silcom.com writes:

<< "Since most residential buildings typically have deep, short span
 diaphragms, it is virtually certain that these diaphragms can not be
 considered flexible under [1994 Uniform Building] code definition and
 that
 force distribution to the shear walls must be made on the basis of wall
 rigidities.  Engineers should adjust their design practices to conform
 to
 these 1994 and 1997 UBC provisions in order to avoid lawsuits at the
 later
 date" >>>

If you have designed condominiums using wood frame construction under the 1994
or 1997 UBC,  the plantiffs in a lawsuit will most likely use this against
you, along with everything else.  The structural engineering professing can
say that it was not the standard of practice at the time to consider this as a
rigid diaphragm, but then again we put it in the code as something that we
were suppose to evaluate.  I think we have shot ourselves in the foot
regarding this.
  
<<< Hahahahahahahaha!!!!!  That is a good one!
 
 Who is going to sue me and on what grounds.  Certainly it WON'T be
 because the diaphragm did not perform properly in an earthquake.  If we
 are sure about ANYTHING it is that our seismic design of plywood
 diaphragms (vertical and horizontal) perform very well on residential
 construction.  This will be even more true now that the height to width
 ratios are more restrictive.  
 I have a REAL hard time with this one.  There are plenty of structural
 issues that are of real concern that talented people could be spending
 their time on.  Plywood diaphragms perform well for residential
 buildings as currently designed, period. >>>

I would agree for one story buildings, but as you start to increase the number
of stories to two, three and four stories (apartment buildings) there is more
of a problem, especially since we are still typically using slender walls even
with the new minimum aspect ratio of 2:1 relative to the height of the
building.  Granted in older buildings where drywall was used in the corridors
this a real problem which would probably not occur in a new design with the
reduced values for drywall.  As the building gets taller, life/safety becomes
more of an issue, but in the single story building I would think we will still
have significant amounts of cosmetic damage and costs to fix all the drywall
cracks.  (cosmetic damage is what will probably get us involved in a lawsuit
as the homeowners try to recoupe their losses)>>.
 
 <<<Anybody can sue for any reason.  And some people do just that.  There is
 no protection from those kinds of people.  What should happen is that
 there should be an exemption in the Code for certain kinds of buildings
 with a proven history of successful performance during strong ground
 motion.>>>

Agreed
 
<<< I have a question for the engineers that are going to design future
 buildings to this new provision.  Are you going to contact all the
 Owners of buildings you have previously designed and notify them that
 the plywood diaphragm building they own may not provide adequate
 life-safety protection in an earthquake??
 I would guess that no one will do this, because we all know that it is
 simply not true.>>>

Regarding the question, no you don't necessarily contact the owners.  After
earthquakes we learns things and make changes to the codes, but not
necessarily inact ordinances requiring the mandatory upgrade of buildings.
See the following examples:

1.  Drywall is not worth more than  30#/Ft (Zero in my book), do you contact
the owners of  all the single family residences that have been built in the
last 30 years and tell them they have a problem since you orginally used a
design capacity greater than 100#/ft.

2.  Tilt-up building roof diaphragm anchorages, past designs are inadequate,
but how many building departments are mandating that these connections be
retrofitted.

3.  Reduction in Allowable Bending Stress for 4x Douglas Fir Larch lumber.
That old panelized roof system can't span what it use too.  Do you tell the
owner that he has a problem, no since the roof live load is never there unless
you are trying to add new mechanical equipment and using the live load to
justify that the member can support the added weight. 
 
<< Sorry to vent like this, but some things just really get to me :)
 
 Lynn
  >>

I understand the frustration.

I know we don't see eye to eye on the topic of rigid plywood diaphragms.  I
feel the diaphragm does not act as a rigid diaphragm, but more as a simi-rigid
diaphram  which does cause some load redistribution to adjacent parallel shear
walls.  If you compare shear wall deflections you can determine what type of
redistribution might be required between adjacent shear walls.  The end result
is probably that the shear wall nailing is still "ok" but maybe you need to
use the next larger holdown size.

I definitely don't want to have to design the building twice,  but my read on
the code is we will have to if the building agency requires it.  I would
expect it from an outside plan check agency hired by the building department
since they have a potential liability if they don't bring it to the attention
of  the building official.

My 2 cents worth

Michael Cochran