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RE: Report on Wood Diaphragm Issues

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thank you  Mr. hamberger , this Issue was becoming  a  dilama for The plan
check and cause of contious discusion with the design engineers. 

I would also  like to share the following with you  ;

As a result of my recent plan check request under section 1630.6 of the 1998
CA Building Code on a tract of 2 story homes of up to 3000 square feet the
engineer sent me the following result of analysis ; 

line          Shear lb. /ft Flexible analysis   	Shear lb./ft Rigid

A	518					578

B                      656

C                      654

D                     245

E                     185

1                      379

As it can be seen at shear wall lines A , D , E and 1 of this two story
tract house the rigid diaphragm analysis resulted in higher loads while at
lines B and C ,  the flexible method resulted in higher loads.
 The solution was to use flexible analysis results as usual however when
rigid analysis show load deficiencies and higher shear loads to go ahead and
use the higher loads as calculated by the rigid diaphragm analysis for the
same wall.

Although I am glad that the problem is finally resolved I would like to hear
your comments on the above analogy and results that were indeed performed in
a two story tract home and showed that the flexible analysis could be
deficient at certain shear lines.

once again thank you for your continuous guidance to our profession.

Sid Danandeh

Senior Plan Check Engineer

> From: 	Ron O. Hamburger[SMTP:ROH(--nospam--at)]
> Sent: 	Saturday, December 11, 1999 12:24 PM
> To: 	seaint(--nospam--at)
> Subject: 	Report on Wood Diaphragm Issues
> On Decembe r 4, 1999, during the SEAOC Seismology Committee meeting, a ses
> sion
> was held to discuss the continuing concerns of much of the membership with
> regard to literal interpretations of the Uniform Building Code that
> require
> rigid diaphragm type analysis of light frame wood construction.  As
> President of
> the Association, I attended the meeting, as did Alan Robinson, chair of
> the
> SEAOC Code Committee.  Martin Johnson, chair of the committee has prepared
> a
> full summary of the discussion, and will post this separately.  The
> following
> are my observations:
> The committee was unanimous in the following opinions-
> 1- One and two-family residentail buildings of light frame construction
> have
> traditionally performed very well in earthquakes, and have not created
> conditions threatening life safety, despite the fact that most have either
> been
> designed using flexible diaphragm assumptions or the somewhat more
> permissive
> conventional construction provisions.
> 2- When the Seismology Committee developed the defintiions of rigid
> diaphragms
> for the 1988 UBC, based on relative deflection of the diaphragm and
> supporting
> vertical elements, it had not intended that those provisions to apply to
> small
> residential buildings employing light frame construction.
> 3- Use of flexible diaphragm assumptions for light frame residential
> construction can actually induce a designer to provide a more favorable
> and
> safer configurations of the lateral force resisting system than would
> occur
> using rigid diaphragm assumptions, by encouraging the placement of shear
> walls
> around the perimeter of the floor areas, minimizing the need to have
> diaphragms
> work hard and reducing potential torsional systems.
> 4- There are some cases when flexible diaphragm assumptions can result in
> potentially unsafe conditions.  In particular, the committee was concerned
> with
> cases where cantilever column elements were used as vertical elements of
> lateral
> force resisting systems, and multi-story structures where shear walls
> landed on
> beams or other flexible elements.  In such cases, relatively little of the
> load
> that would be assumed to accrue to these elements would actually be
> resisted by
> the elements, forcing more load to go to elements which may be undersized
> for
> these loads.
> 5-  Flexible diaphragm assumptions are probably not appropriate for larger
> residential type construction such as multi-story apartment buildings and
> hotel/motel type structures, particularly when concrete topping slabs are
> present on the floors.  The poor performance of many such structures in
> the
> Sherman Oaks area during the Northrigde earthquake, that included a number
> of
> partial collapses, confirms the importance of using more rigorous design
> practices for such buildings.
> The Committee resolved, with the concurrence of the Code Committee chair
> to act
> as follows:
> 1- Submit a code change to specifically except 1 and 2 - family
> residential
> buildings using light frame construction from requirements for rigid
> diaphragm
> analysis.
> 2- Prepare an article (joint with the Code committee) for the ICBO
> Building
> Standards publication to explain the Committees' interpretation as to when
> rigid
> diaphragm analysis is appropriate, and when not necessary.
> 3-  Request that the example of a residential building design  in the
> pending
> Volume 2 of the Design Manual be revised to indicate that flexible
> diaphragm
> assumptions are the traditional and apporpriate design approach for such
> construction.
> On Thursday, December 9, a teleconference was held with the Seismology
> Chair and
> Chairs of the Design Manual  committees, as well as myself and the author
> of the
> example solutions for wood frame construction.  In this conference, it was
> confirmed that the Design Manual would be revised to reflect the above
> decisions.
> Regards,
> Ron Hamburger
> SEAOC President
> 3-