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Re: Effects of the New Code on Wood structures - good or bad?????[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Effects of the New Code on Wood structures - good or bad?????
- From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
- Date: Fri, 23 Jul 1999 15:55:21 -0700
I think everybody has gone astray on this supposed code requirement -CG At 10:36 AM 7/23/99 -0700, Mark Swingle, SE, wrote in response to Lynn Howard: >Lynn, that is not what the code clearly says. What I am saying is that the >code requires an accurate analysis of the ACTUAL stiffness of the diaphragm, >but since we don't know how to do that, we can be conservative and cover all >possible forces. Doing ONLY the INFINITELY STIFF diaphragm analysis is not >sufficient (and by the way is not sufficient for many buildings other than >wood). [C.G.:] I presume we are referring to Sec 1630.6 of 97 UBC or 1630A.6 of CBC for DSA (same thing) on "Horizontal Distribution of Shear"(?) If so, I don't see where the code requires an accurate analysis of anything. The first para presumes that some kind of an estimate of proportionate rigidities of shear walls will be made, and that the rigidity of the diaphragm above them will be "considered". Then one must distribute story shear in accordance with how one has estimated and considered those rigidities. The second para again merely requires that the effect of a prescriptive calculation be "considered". The third (and final) para mandates in its first sentence when a diaphragm MUST be "considered" (ie, "deemed", a different meaning of "considered") flexible, but the scenario one must use is ambiguous to the point of being useless. The second sentence gives a presumed clarifying method one "may" use, but that too is ambiguous beyond usefulness. Any number of interpretations are possible. This method came from Ed Zacher and SEAONC Seismology in mid 1986 pursuant to giving relief from doing TORSIONAL effects, and that was all. It removed a burden and did not add one. It DID NOT RELATE to the provisions in the first para of today's 1630.6, on which walls get how much shear, and it STILL doesn't. I posted a lot on this about two months ago, when a recent Seismology Committee member told us an untrue version of it. >My point was that since there is no rational way to model the diaphragm >accurately, then by doing both a flexible analysis and a rigid analysis, you >always cover the worst cases for all walls, and so as I said, NO >CONSIDERATION OF DIAPHRAGM STIFFNESS IS REQUIRED. Let me explain what I >mean. So "you always cover the worst cases for all walls" (?) and you do this "since there is no rational way to model the diaphragm accurately" (?) As Professor Hom at Cal State Sacramento always said, "You can do it that way too". But since the code ONLY requires you to "consider" rigidities, it leaves as a matter of discretion how wide one wishes to bracket the uncertainties, if at all. So you do it your way and I will do it my way, and we're both in compliance, because we both "considered" the relative rigidity matter. >In a "flexible diaphragm" analysis, all diaphragms are considered simple >spans from wall to wall. The forces to the walls come from a simple span >reaction, which does not depend on the stiffness of the diaphragm (reactions >are independent of EI in simple spans). ..."considered" simple spans...(?) Obviously I am mocking the way the word "considered" is used. But words in their plain meanings count hugely in law. Engineers are very careless in use of words in their legally enforceable writings. Please consult Webster's. "Considered" does not mean a standardized or habitual judgment imposed by others, it means "arrived at after careful thought" and from "having directed the mind to something in order to understand it or make a decision about it". There is a fresh contemplation or inquiry implied when something is considered. Otherwise one should say "customarily deemed" if one is not going to think it out anew. Further, this simple span habit for flexible diaphragms is not mandated anywhere in UBC that I have ever seen, however it is on deck in the 2000 IBC, as I posted on June 1.(q.v.) (Watch out when that one takes effect. It's really stupid.) >In a "rigid diaphragm" analysis, the diaphragm is considered infinitely >stiff, and the forces to the wall depend ONLY upon the stiffness of each >WALL, and their relative spacing. The resulting forces to the walls are >independent of the diaphragm in this case also. Right, except they're "customarily deemed" infinitely stiff... >Taking the highest load for each wall from the two cases will cover all >possible forces from any possible diaphragm system. Some walls will have a >higher force under the flexible diaphragm analysis, and some walls will have >a higher force under the rigid diaphragm analysis, and that is most likely >why the plan checker does not want the flexible analysis thrown away. Yes, that covers it all. But there is no basis in code for arbitrarily requiring use of "worst case" results from these two artificial and extreme versions of diaphragm rigidity, just because "accurate" deformation calculations elude us. You can choose to go that broadly for your own design, but not to inflict it on another engineer, as that would go beyond the code's own terms. Rather than your conclusion that "NO CONSIDERATION OF DIAPHRAGM STIFFNESS IS REQUIRED", ONLY such a consideration is required, and if it HAD to bracket the worst case limits the code simply would say so instead of what it does say. >My previous posts pointed out that there still remain serious problems with >determining the stiffness of plywood shear walls, so we are still left >without a rational way to comply with what I am proposing. Well, we haven't a real accurate way to determine stiffness, but that doesn't thwart making reasonable rigidity estimates pursuant to complying with the first para of UBC 1630.6, which never was contingent on any particular degree of accuracy, back in the slide rule days when it originated. It simply codifies a good principle. Further, Blue Book Commentary and common ductility sense says that inadvertently overloaded plywood shear walls soften and deflect more so as to limit what they endure, while walls underutilized stay stiffer and thus pick up the load, and it all works out. As Alfred E. Neuman (untroubled by screwball litigation experts) always said, "What, me worry?" I've got my story figured out, what about the rest of you? Charles O. Greenlaw SE Sacramento CA
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