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Report on Wood Diaphragm Issues[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Report on Wood Diaphragm Issues
- From: "Ron O. Hamburger" <ROH(--nospam--at)eqe.com>
- Date: Sat, 11 Dec 1999 12:24:37 -0800
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 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-
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