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Re: '97 UBC Lateral Design - Envelope Solutions???? Calmed down reply[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: '97 UBC Lateral Design - Envelope Solutions???? Calmed down reply
- From: Seaintonln(--nospam--at)aol.com
- Date: Fri, 24 Sep 1999 17:39:07 EDT
In a message dated 9/24/1999 12:42:28 PM Pacific Daylight Time, Seaintonln writes: << Subj: Re: '97 UBC Lateral Design - Envelope Solutions???? Calmed down reply Date: 9/24/1999 12:42:28 PM Pacific Daylight Time From: <A HREF="mailto:Seaintonln">Seaintonln</A> To: <A HREF="mailto:Mark.Swingle(--nospam--at)dgs.ca.gov">Mark.Swingle(--nospam--at)dgs.ca.gov</A> CC: <A HREF="mailto:seaint(--nospam--at)seaint.org">seaint(--nospam--at)seaint.org</A> Mark, see my comments below: In a message dated 9/24/1999 12:01:55 PM Pacific Daylight Time, Mark.Swingle(--nospam--at)dgs.ca.gov writes: <<1. Within item 1 above is a whole range of possibilities, not just one answer. The range includes 100% rigid at one extreme and 100% flexible at the other extreme.>> (DW) I don't see any provision in the code that explains alternatives to what you describe as the "gray area". In 1630.1 second paragraph, the code explains that if the diaphragm is not flexible "the mass at each level shall be assumed to be displaced from the calculated center of mass in each direction a distance equal to 5% of the building dimension at that level perpendicular to the direction of the force under consideration.." This is the method to use for a 100% rigid diaphragm since no other comprimise is suggested. In the fourth paragraph of this section it states "Diaphragms shall be considered flexible for the purposes of distribution of story shear and torsional moment when the maximum lateral deformation of the diaphragm is more than two times the average story drift of the associated stories. This may be determined by comparing the computed midpoint in-plane deflection of the diaprhagm itself under lateral load with the story drift of adjoining vertical-resisting elements under equivalent tributary load." This, in my opinion states that if the diaphragm passes this test, forces shall be distributed by tributary load. Theoretically, we argue the possiblity of something in between, but in my opinion there is a definate difference between the rigid and flexible until someone defines both the condition and the means to resolve the comprimise. Yes, the stiffness of the wall is the ultimate concern, but the load distributed to these walls can vary a great deal on a non-rectangular building. This is what prompted my initial concern. If chosing one solution, we may be ignoring the possiblity of the other. If this "gray area" were adequately defined, then the recommended solution might be an envelope solution. We simply can not assume the adoption of one method when the discrepency in results is great - and I man hundreds of percentage differnce not a small degree. These are the results I am getting in my analysis. Hard facts: in one wall, the flexible analysis yielded 21,371 pounds of shear into this lline of resistance. The same grid line, under torson analysis 1,523 pounds of shear. Wall 13 yielded 4,536 pounds from tributary distribution while only 598 pounds from torsion. Wall 8 yeilded 10,656 # of shear from flexible, but 15,025 from rigid analysis Wall 24 1,758 from flexible 12,476 from rigid Wall 25 1,670 from flexible 12,305 from rigid The building was a "U" shaped structure - 9500 square feet - with one portion (the large garage) skewed from the orthagonal. The flexible results were confirmed. The torsion or rigid analysis was done using Enercalc software. The center of Mass determined within Autocad. Results when the shape was rectangular yielded differences between rigid and flexible that was less than 30% at the max. This is reasonable. So the answer, in my opinon, was to design the "U" shaped structure using an enveloped solution. Mark, these are practical questions since another engineer will, ultimately face the same problem and not know how to address it. << What I hope is that we can move the discussion beyond what the code says and toward developing the analytical tools required to comply with what the code says. (I also think that we should continue to lobby to have this portion of the code changed, with whatever means there are available to us. That will be a different post.)>> (DW) How can we discuss simplified analytical tools when we analyze the results of the present code provisions and recommend to the user just what the authors intention was in the publication of a methodology. I may agree with you that the performance of wood diaprhagms is somewhere in between, but it is the responsiblity of the policy makers to address these problems and create policy to resolve them and not simply to leave an interpretation that creates more revenue for the legal profession. If the tools are not adequate for a project - replacement. (DW) Where in the code is their a proceedure that explains what to do if the diap I know you have spent considerable time in using the sw deflection formula for wood walls. I am interested in your thoughts and/or experience with the use of these formulas. The most important piece in this puzzle is the stiffness of the WALLS, not the stiffness of the diaphragms. I still don't know how one can determine the forces to the walls assuming 100% rigid diaphragm when the shear wall deflection equations are so poorly defined, limited in scope, subject to interpretation, and non-linear. I would be interested in your methodolgy with regard to determining wood sw stiffnesses. (DW) I agree and am not arguing the issue of resisting wall stiffness but there is obviously significant discrepensies in the distribution of shear through the diaprhagm when one wall calculates as needing to resist 1,500 pounds while the same wall, using another appropach is calculated at 21,000 pounds. If we follow your strict interpretation of the the code method so that if the diaphragm is "not-flexible" we distribute shear by torsional analysis - how many of us would feel justified in ignoring the flexible resulsts when the discrepancy between the two is 18 times larger than what we chose to resist. Remember, it is not a clear cut case of one analysis yielding more restrictive answers - this depends on the wall stiffness and the location of the applied loads. In my example there is sufficient different on some walls to warrant compliance with the flexible analysis while other walls require compliance with rigid. Following the code, which has not addressed these possiblities may make us liable for damages, loss of live or the simple headache of being accused of overdesigning. Dennis >>
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