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Re: Effects of the New Code on Wood design - good or bad?????[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: Re: Effects of the New Code on Wood design - good or bad?????
- From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dgs.ca.gov>
- Date: Tue, 27 Jul 1999 11:49:52 -0700
- Cc: "'mswingle(--nospam--at)earthlink.net'" <mswingle(--nospam--at)earthlink.net>
Charles Greenlaw wrote: .... By the above method, the preponderance of your Fri posting advocated as code policy use of extreme-limit bracketing for both rigid and flexible, and used that rationale to explain away the interpretation that Lynn said he got from DSA/SSS. But a minor element in it indeed floated the bracketing gambit as merely an escape method when one (as usual) can't accurately calculate shear wall and diaphragm deflections. Harmonizing your points (absent your later-posted clarifications) remains difficult, and I stand by what I posted. Lynn Howard wrote: .... Prior to our starting on the project, I met with the DSA and told them I was considering designing the diaphragms as rigid, because that is what the Code clearly said. With full knowledge of what the code said, the DSA plan checker said that if I wanted to do a rigid analysis on the diaphragm, that was okay, but that I would have to also do a flexible diaphragm analysis and the building would have to fully comply with the results of that analysis. They basically were not going to require the rigid diaphragm analysis, and in fact WOULD NOT ACCEPT that type of analysis alone. Mark Swingle responds: Charles: Re: advocating code policy: Apparently you haven't read Dennis' original posting on this subject, and you haven't read my two posts of last Thursday. There were two different items I was trying to present: 1) the fact that the code has not changed regarding horizontal distribution of shear, and 2) a method whereby one could find the worst possible load to each shear wall without doing a complicated analysis of the diaphragm. I admit that mixing the two concepts may have lead to confusion, and I apologize for the limitations of my writing skills. There were some concepts that were left unsaid due to lack of time. I admit that I am not good at writing compactly. My intent was not to advocate that the code required using the worst case from the two methods. Rather I was proposing that if one wanted to do more than only a tributary load analysis, that by doing a rigidity analysis as well, one can see the two extremes for each wall line, and then, after considering the rigidity of the diaphragm relative to the walls, one can choose the design forces based on professional judgement. My point about NOT considering the diaphragm is that one does not need to know or consider the diaphragm WHEN FINDING THE TWO EXTREMES. The diaphragm rigidity need only be considered when deciding the final design forces. If one wants to be conservative, one can choose the worst load from the two methods, however, it is not required by code. In buildings where the mass distribution is fairly uniform, the spacing between wall lines is nearly equal, and the length of walls in each line is nearly equal, the difference between the two methods will be negligible. In buildings that are more "irregular" or "unsymmetrical" with respect to these properties, the differences in some wall lines between the two methods will be greater. If the wall lines are spaced far apart, then the diaphragm will be relatively more "flexible", and the designer can choose a force for each line that is closer to the tributary load case than for the infinitely rigid case. If the building has diaphragm spans that are relatively small (such as many residences), one may choose a force for each wall that is closer to the infinitely rigid case, since the diaphragm in this case will redistribute forces based on the shear wall stiffnesses to a greater degree. However, straying too far from the tributary load case for wood buildings with wood shear walls will raise red flags with plan checkers, "expert" witnesses etc, regardless of the merits of your approach. You have spoken eloquently in the past about this subject and the inherent legal and professional pitfalls. It is unfortunate that the "wood diaphragms are always flexible" mentality is so pervasive. Re: the DSA plan checker: Lynn stated that he "told them I was considering designing the diaphragms as rigid, because that is what the Code clearly said." That statement implies that he wanted to consider the diaphragm to be infinitely rigid, unless I am misinterpreting his intent. That means that he was planning to go to the opposite extreme of what most designers do. This approach may lead to some shorter walls with relatively large tributary forces to be underdesigned. I did not intend to use my method "to explain away the interpretation that Lynn said he got from DSA/SSS." The code does not "clearly require" that the diaphragms be considered as rigid. I was simply pointing out that using either extreme is not correct. Charles, I think we are actually very much in agreement about most of these points, but we have such different ways of expressing them, have different professional experiences, and are both so stubborn, that we are having a difficult time understanding each other. Let's keep the dialogue going. Mark Swingle, SE Oakland, CA These views are my own and are not necessarily those of my employer.
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