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- To: seaint(--nospam--at)seaint.org
- Subject: Re: IRC Braced Panel Discussion
- From: "Dennis S. Wish, PE" <dennis.wish(--nospam--at)verizon.net>
- Date: Fri, 14 Jan 2005 18:58:21 -0800
Dan,First let me welcome you to this discussion - I know your work very well and the contribution you have made when you were at (I assume you were at) Virginia Tech at the time. I won't argue the life-safety aspect of the IRC as I agree with you. Historically light-frame construction has performed exceptionally well to protect the lives of the occupants. The failures in structure that we have seen and have address where with cripple-wall collapse and with older homes not adequately connected to their foundations. So, from this perspective I agree with you.
Performance is another issue entirely and I have tried to steer this discussion into the idea that adding hardware in an area of high risk is a small investment that I believe strongly will improve the performance and reduce the damage to a home in a high risk region. Unlike commercial buildings, the spacing of shearwalls is not as large as I think you indicated. This is one reason why I am not in favor of rigid diaphragm analysis on residential homes. Shearwalls are used within the structure to control diaphragm movement and performance whereas in commercial buildings the space is expected to be as open as possible to allow for creative redistribution of interior walls to maximize leasible space.
The next point hits on your comment "if followed strictly". In nearly 20 years of practice with both seismic retrofit of URM and Wood buildings as well as the design of light-framed custom homes (and low income housing) I have found very few framers who have the skills we associate with builders (or should I say craftsmen) of the past. Skill is not productive or cost effective and tract home developers hire low income "grunts" to learn as they go how to assemble a home. The developer backs off 15/32" plywood or OSB in favor of 3/8" to save a few bucks, and then ignores over nailing which the APA published is not allowed for more than 20% of the panel installation without losing values. Framers used kicker plates to locate the centerline of the anchor bolts and few today invest in this relatively cheap tool in favor of simply placing the mud-sill over the anchor and pounding from the top to set the imprint of the anchor and then they over drill the holes. I've even seen (and documented) framers who did not want to spend the time locating anchors so they cut the plate from the side - slicing out a rectangular wedge and slipped the plate into place. Drywall and Plywood sheathing is paid for by the panel and not by the hour so the more plywood that can be nailed into place in a day, the more the laborer earns and the cost does not include care to back off the pressure of the gun. Nor does it care if the nails are common or sinkers until and if caught.
We know what happened with Simpson (and others) HD products that required bolting - the bolts were oversize holes and this caused splitting of the posts. Buildings did not want to use 4x or 6x material to secure PHD so they disregarded the warnings from Simpson and used double 2x's which are not recommended for the SDS-1/4" self-tapping screws used in the PHD's. To make matters worse, in our area those who build prescriptively using the UBC Section 2320 are sold on the PAHD strap holddowns that bend and enter the slab edge, but a survey of the installation rarely lines the strap up with the foundation and spalling occurs. Furthermore, we live in an area of extremely high mineralization of the soil and exposed straps corrode through in less than five years making the alternate braced panel ineffective.
Blocking between trusses or roof framing is required by code to be in-plane and above the top plate of the wall, however, historically it is placed outside the plane of the wall to act as a nailer for the eave sheathing or stucco connection. Simpson and others have developed modifications to the A35 that secures the connection but the boundary nailing is missing and in most cases h1 or H2 hurricane clips are substituted for the A35 replacement and does not have the capacity to resist rotation without the blocking.
The IRC does not design beams or headers - so who does? The framers generally use the 1-inch rule; 1-inch of depth for each inch of width. Shearwalls (or braced panels) are not positively connected to the roof as there is no requirement to align the truss with the wall and to make a positive connection.
Dan, some of this comes from my experience with the BSSC TS-7 committee that I believe you were participating on. When I offered to establish a private listservice between members Phil Lines of AF&PA who was chairing the committee reprimanded me for this. He warned other members not to discuss any of the committee work on my list and that if I had any questions or wanted any information as a non associate member (not representing a professional association or material association) I was to direct my questions to him and wait until answer came months later at scheduled meetings. What kind of way is this to work through issues on a committee? Also, why must this work be held in private from the professional community unless it was to enforce what the committee determines as deemed necessary rather than what the professional community in practice see as practical.
My arguments go the same route with SEA and are well documented. I've been active on many committees and chaired my own for many years with SEAOSC. I also co-founded the SEAINT List and website with Shafat Qazi who took over as Chair of the Computer Applications committee when I left Los Angeles (I took it over from Mike Brooks of Enercalc in 1988 or 1989. So this is not unique to BSSC but is an industry problem.
When CUREE set up their shake table tests it was obvious that the IRC WAS being adhered to and this is where the problem lay - it was not typical of what was done in the field and this is why we needed additional and alternate directions to go.
I believe you worked on the Perforated Wall design for AF&PA and NAHB that was to be used for HUD designed homes when you were at or collaborated with Virginia Tech. The first time that the perforated wall design was suggested to the engineering community at the Wood Fair around 1997 or so, it was me with much criticism and still is within a high risk region. This is not to say that Perforated Wall design does not work when the risk is low in seismic country as it seems effective in static load applications, but there are the questions about how to treat walls that terminate at the plate in door openings.
I can go on but I would only support the accusations that I am verbose :>). However, this is not a perfect world (was this a Walgreen commercial) and we have not ventured into the political arena necessary to force framers who are responsible for constructing the structural system to have better skills or be certified. If this were to happen then I might compromise better - but the fact is that the cost of repair and replacement of damaged structures irregardless of life safety measures can be improved and we are not doing this. Having the IRC in place is only support for a historic method of construction that is known to protect lives. We live in an age where cost of repair can force an owner to stay or walk away from his investment as many homeowners did in a moderate Earthquake as Northridge was.
Dan, I am always willing to work toward improving upon construction so that we may reduce the complexity and ambiguity of the building code for light-framing, but we can't fight the system if the lobby against us is so powerful. Someone has to bend and we are not. I remember, as my last comment, when I debated the report issued by NAHB on Prescriptive Design performance during Northridge. It was dribble and when I brought up that the examples taken were homes that had not suffered damage, the argument was statistics - the statistics taken of homes at random near the epicenter of the quake on culdesac streets showed that homes performed well that were constructed by conventional prescriptive construction. This really bent the rules as statistics can be produced to bias any problem. In this case it focused on statistical analysis of all buildings at random and ignored the majority of those on the same street hard hit because of orientation to the earthquake and the conformance to the building code. So where did the $30-billion in damages come from? I was told by the same NAHB-RC manager that this was an inflated figure by contractors taking advantage of the public. I represented both sides and I can attest that there was an equal amount of advantage taken by both sides that balanced this figure out and made it realistic.
Dan, I am willing to become more involved in the work being done as I have in the past, but not as a silent member who is only given the task of checking for spelling and grammatical errors in code proposals.
Thanks again for you comments and your past work that I do highly respect. Sincerely, Dennis S. Wish, PE. J. Daniel Dolan wrote:
Dennis: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., 1^st 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 Engineering 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 Visit our website: http://www.ce.wsu.edu
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