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Report on Wood Diaphragm Issues

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On December 4, 1999, during the SEAOC Seismology Committee meeting, a session
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

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

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

Ron Hamburger
SEAOC President