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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: Lynn <lhoward(--nospam--at)silcom.com>
- Date: Sat, 31 Jul 1999 17:00:34 -0700
Chuck- Who classified the Northridge earthquake as "moderate". True, this earthquake was not the "big one", but we do not design structures to go through the big one without significant structural damage anyway. We are supposed to be designing for the earthquake that has a 10% chance of reoccurring every 50 years. From the recorded ground accelerations, it was my understanding that this was indeed close to a design base earthquake. Certainly this was not a "moderate" earthquake. If you have some reference as to where you could point me to people or organizations who have classified this as a moderate earthquake, I would be interested in hearing about it. I do not agree with the philosophy that the Code should require that structures should be able to go through a design based earthquake and suffer little damage. The Engineer and Owner have the prerogative of designing to a higher standard than the Code requires, and we often do that here in our office. However, in my opinion, the minimum requirements of the Code should only provide life-safety protection for the occupants. Almost any one or two story wood framed house with plywood horizontal diaphragms and vertical plywood shear walls will provide life-safety protection of the occupants regardless of all these issues we have been discussing. FEMA 310 is a standard that provides guidelines for the evaluation of life-safety performance of existing structures. Section 3.2 and Table 3-1 of FEMA 310 states that any wood light framed building, or wood framed commercial or industrial building designed and constructed in accordance with the 1976 UBC will provide life-safety protection for the occupants. If this is true, and I believe it is, then all of this hoop-la will do nothing but reduce the damage in a building, and not necessarily increase life-safety. I do not know that there is any kind of a mandate that the minimum requirements of the Code provide for the control of damage. Our office has run a few residential building through the mill on the rigid diaphragm, redundancy and Rho factor issue, to try and see how this will all come out. The calculations say the diaphragms are rigid, when they are OBVIOUSLY not, and the Rho factor has been greater than 1.0, all the way up to 1.5 for all of these building also. Design base shears for plywood building are approaching TWICE the base shear we used on structures based on the 1994 UBC. You have to add the Rho and the near source factors into your base equations around here, making the design shears very large. If this is really the intent of the Code developers, then I think I would be morally obligated to tell the all of our past clients, including the clients of building that are currently under construction, that their buildings pose a life-safety threat to them an all who enter these structures. I guess I should see this as an opportunity to do a seismic retrofit of all of our past wood framed structures. Lynn chuckuc wrote: > > 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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- From: Ed Fasula
- AASHTO Lane Loads
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