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Re: Plywood rigid diaphragms
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- Subject: Re: Plywood rigid diaphragms
- From: ErnieNSE(--nospam--at)aol.com
- Date: Sat, 29 Aug 1998 10:42:38 EDT
Michael Cochran says: "Los Angeles has adopted a new voluntary ordinance called Division 93: Voluntary Earthquake Hazard Reduction In Existing Wood Frame Residential Buildings with Soft, Weak or Open Front Walls. In this ordinance, you are required to calculate shear wall deflections to show that the deflection is less than .005H. The shear wall deflection equation is from the UBC Volume 3." I think this is good but they could have gone further by requiring that, for walls in the same direction, if the relative rigidity is such that one wall line is 50%(or any number thay want) or more rigid than any other wall line, the distribution of lateral loads to the shear wall line should be by relative rigidity assuming a rigid diaphragm. But they should require that the distribution at each wall line should not be less than the value obtained using the traditional tributary width-flexible diaphragm analysis. You said "Using this equation, the biggest part of the calculated shear wall deflection is due to holdown slip (minimum 1/16" oversized hole in post for holdown bolts,.............................." How about requiring that all holdowns be tightened such that we finger tighten the nuts on the bolts to the posts or studs first, then, finger tighten the bolt on the anchor and then turn it three(or less/more) revolutions with a wrench before finally tightening the post bolts. This should reduce the holddown's contribution to the shear wall deflection. You said "This becomes a nightmare when you try to distribute the load by rigidity though, since you do not know the rigidity of the plywood shear walls, ........." Yes we do not have an exact formula for plywood shear wall deflection considering the contribution of the holdown (and don't forget the slip in the sill bolt on the sill plate hole). But if we consider the "relative" rigidity using the inexact but approximate deflection formula to distribute the lateral forces, since they will all be subjected to approximately the same unpredictable and unquantifiable conditions(assuming we are analysing all plywood shear walls), then this should be good enough, at least better than not considering relative rigidity at all. That is, until we have more test and results converted to formulas which the engineering community will agree upon and accept. You said "I think the concerns are that if you design strictly by tributary area, you may end up underdesigning certain walls. In a simple box shaped building with perimeter shear walls only, the short wall on one side is designed for 50 percent of the tributary area, the long wall on the other side is designed for 50% of the tributary area. The short wall deflects (never sees the full 50%) and the long wall now sees 70-100% of the load when it was actually just designed for 50% of the load. Now the question is how to come up with a reasonable solution, which can be implemented within the design fee structure that as engineers we typically get for designing wood structures. Any suggestions?......." See my first suggestion about checking the relative rigidity of shear walls in the same direction. Let's do some trial calculations of relative rigidity of different combinations of full length plywood shear walls at the back and a series of short walls at the "soft" front, using the only deflection formula we have so far. Then pick a combination that we feel warrants a distribution other than our standard "50-50"distribution, convert it to a percentage of relative rigidity difference, round it off to a "nice" number(which I notice a lot on our code) and use it as a criteria when relative rigidity calculations should be required. Again, use the greater of the values obtained by using both the relative rigidity distribution and the traditional distribution. These are my suggestions. And I'm intersted in finding out what SEAOSC or the presenters in the November seminar on this topic are going to say. Ernie Natividad
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