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Re: Need feedback from those using strict interpretation for Woodstructures[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Need feedback from those using strict interpretation for Woodstructures
- From: chuckuc <chuckuc(--nospam--at)dnai.com>
- Date: Sat, 31 Jul 1999 13:32:23 -0700
Dear Dennis As I understand it, the major impetus for the changes in the 1997 U. B.C. seismic provisions was the poor performance of some wood structures in the Northridge earthquake. Northridge was classified as a ?moderate? seismic event and yet the damage totals into the tens of billions of dollars. At the load levels of the Northridge earthquake we were not supposed to get serious damage in our major structural elements. Northridge also coincided with a growing understanding that monotonic testing of shearwalls was inadequate and that narrow shearwalls performed poorly. Ben Schmidt's calculations in the city of L.A.'s Northridge Report demonstrate the problems of not adequately considering relative deflection in shearwall calculations. Rigid diaphragm calculations are a pain in the ass, but it is a way to make sure that engineers account for the problems inherent in mixing shearwalls of substantially different stiffness within a structure. As Mark pointed out, for a reasonably laid out structure it doesn?t change the loads too much. I've done a few homes now using the ?97 code, and it appears to be adding about 10 to 20 hours of additional calculations. I expect as I get more familiar with the process that will drop substantially. I've done a little shearwall testing myself and have tried to read most of the literature. I've also done a few parametric spreadsheet studies to try to get a better sense of the ramifications of the new code. If you make the assumption that Simpson?s PHD tiedowns produce acceptable performance with eight-foot shearwall's, you can back into a stiffness for them. I find that assuming a total tie down deflection of.2 inches when you reach nail capacity produces reasonable looking results (assuming linear behavior up to nail capacity). For a 4 foot wide wall the deflection is.7 inches, for an 8 foot wall .5 inches, and for a 20 foot wall.4 inches. This accounts for deflection of the holdown, slip in the fasteners, plate crushing, and general construction slop. Narrow shearwalls are undesirable because of their lack of stiffness. By my calculations the relative stiffness of a 4 foot wall is approximately 30% that of an eight-foot wall. . So far the analytical process seems to work fairly well, the narrow walls are soft and the long walls tend to pick up more load. So far the redundancy factor has not been a problem. The little spreadsheet work I?ve done so far indicates to me that for a normal range of floor areas, the redundancy factor is reasonably well behaved. To avoid penalty, in a 600 square foot house Rw may not exceed .8 and for a 3,000 square foot house.3. Which is to say a 10-foot wall could carry as much as 80 percent of the load in a 600 square foot structure and 30 percent in a 3,000 square foot structure. With five foot walls that would be reduced to 40 percent and 15 percent respectively. There does appear to be a significant penalty for narrow shearwall's, but that is as it should be. I was also surprised to see that Josh Kardon?s paper on the subject of Standard of Care didn?t get more comments. As I read it, the legal definition is based on an average practioner?s best judgment (whatever that mean) but the practically definition is that it comes down to whatever a judge or jury decides to with the testimony of dueling experts. Ten years from now those experts will be looking at some of our design work, the test will be whether or not our structures are considered structurally adequate?and not necessarily how well we did the arithmetic. If you keep a uniform nailing pattern for shearwalls, distribute the shearwalls reasonably, avoid too many narrow walls, and use good tiedowns I think both the analysis and the structure will be well behaved and well regarded. Chuck Utzman, P.E.
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