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IRC Braced Panel Discussion

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I believe that IRC construction will perform well in seismic events for a couple of reasons.  The IRC places restrictions on the spacing of walls and requires minimum lengths of fully-sheathed wall for each 25 ft length.  The percentages of required sheathing do not calculate out using traditional rigid-body free-body diagram engineering analysis, that is true, but the model traditionally used for design of wood shear walls only works when the overturning forces are resisted by mechanical anchorage (i.e., segmented wall design) and are located on a rigid foundation (i.e., 1st floor).  A design using the IRC (if followed strictly) will result in a distributed force-resisting system rather than a concentrated system.  When a building is "engineered,” the design is often pushed to the limit with large openings and a concentrated later force resisting system with fairly large spacing between wall lines.  In other words, the "engineered" building often has reduced redundancy.


In a current Building Seismic Safety Council effort to update the FEMA 232 document "A Home Builder's Guide to Seismic Resistant Construction,” the assumption is made that the base level seismic performance of the IRC is life-safety. This is the level of performance was used when the IRC was drafted and is still considered by most members of the technical update committee to be the expected level of performance.  (Yes, there are a few provisions in the IRC that provide performance levels above life-safety, but the minimum expected performance of the code is life-safety.) 


The intent of the BSSC update for FEMA 232, however, is to go a bit further by providing  guidance for builders and prospective homebuyers on how to improve the performance of the house to levels above life-safety (i.e., damage control).  As part of the update effort, a model house was developed so that quantitative analysis could be conducted to compare the performance that would result from different "above code" recommendations.  The building was analyzed using a non-linear analysis program and results from recent research (including the CUREE/Caltech Wood Frame Project).  This effort to conduct a rational analysis of the building showed the inadequacy of many of the "traditional" design tools for predicting deflections; however, it also showed that the IRC does provide a reasonable level of life-safety performance.  Also, a cost analysis was performed by a respected builder to provide an ability to judge whether the benefits of many of the above-code recommendations are cost effective.


 The updated FEMA 232 document will be completed this year.  It is currently being revised to respond to comments made by reviewers.  It will go out for one more cycle of review and revision before it will widely available (both in paper and electronic form).  The analysis used to compare the different construction configurations will be available from the Building Seismic Safety Council to anyone that wishes to look through it.


Dr. J. Daniel Dolan, P.E.

Professor of Structural Engineering

Department of Civil and Environmental


Washington State University

P.O. Box 642910

Pullman, WA 99164-2910


Courier Deliveries: 101 Sloan Hall, Spokane Street


Tel: 509-335-7849

Fax: 509-335-7632


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