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Re: Plywood rigid diaphragms
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- Subject: Re: Plywood rigid diaphragms
- From: Mlcse(--nospam--at)aol.com
- Date: Mon, 31 Aug 1998 03:07:52 EDT
In a message dated 98-08-29 10:44:11 EDT, you write: <<< 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.>>> I agree that the design load to wall line should not be less than what you get by tributary area, and that you can not reduce the load because of rigid diaphragm redistribution. I don't think that the use of the term rigid is completely appropriate. Lets say you have 5 parallel wall lines separated by 30 feet between each wall line, as you go from left to right, with initial calculated deflections of 0.25", 0.3", 0.35", 0.2" and 0.4" respectively. I don't imagine the deflection of the far right wall line with 0.4" defleciton is going to transfer any (very little) load to the left wall which initially had a deflection of 0.25". The wall with 0.2" will see more load, and its deflection will increase as the far right wall with 0.4"calculated deflection initially moves. The stiffer wall lines will pick up load as the adjacent wall lines deflect, but not like a rigid diaphragm which has a center of rigidity. <<< 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. >>> This is a good idea, but if you still have some wood shrinkage at the sill plate or the floor joist, now the bolts are no longer tight in the post holes and you may have the 1/16" oversized hole effect again. In evaluating existing buildings, you may find that the actual post hole is greater than 1/16" and that the combined shrinkage of the rim joists and wall sill plate and double top plate may be 1/2" or more. This shrinkage can play havic with 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. >>> Remember that some walls along a given wall line may have significant gravity loads acting on a wall, while at other locations there may be little loading since the joist are running parallel to the wall. The gravity loads greatly influence the wall overturning resistance and holdown size. It is hard to determine the relative rigidity when the holdown movement could account for 50% or more of the shear wall deflection for some walls, but significantly less for other walls. The closest I think you can come to using your suggested approximation method would be to set all holdown deflections to the same amount, lets say uplift is always 1/4" which then the holdown assembly component of deflection would be (1/4)(h/d). This would take into affect wall length effects. <<< 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 >> I like the idea of making parallel shear wall lines have similar deflections, lets say within 10% of the adjacent wall line. You then balance the load between wall lines until they all have the same deflection within 10%. When redistributing loads to adjacent wall lines, you only check that wall line for the added load. The wall line design which you take load away from remains unchanged knowing that when this wall line deflects, the adjacent wall line now has the capacity to resist the additional forces dragged over by the diaphragm. Any more thoughts? Michael Cochran
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