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Re: Diaphragm Calculations
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: seaint(--nospam--at)seaint.org
- Subject: Re: Diaphragm Calculations
- From: Seaintonln(--nospam--at)aol.com
- Date: Thu, 8 Jul 1999 14:56:52 EDT
Doug, It was not meant to be a personal attack. The members of the SEAINT list have been debating these issues for almost a year and I have written to Bill Nelson no less than three times asking for specific verification as to the dimensions and units used in the two (and possibly only) examples provided to educate engineers on the new code provisions. I have not received a response and not one of you have addressed the concerns of those on the List even though Bill Nelson conveniently posts messages indicating that he has some contact with the list. You do not seem to realize that there are a great many engineers out there who need to learn this methodology, some are converting it to productivity tools such as spreadsheets and others are trying to backtrack through your examples to figure out what you have done. You make assumptions that all engineers should not only understand the methodology but should understand how the units in each formula are converted. Please remember that if an engineer inputs this information into Mathcad, the program will automatically convert terms to common units. In the first part of the diaphragm deflection formula, the results come out as feet - not inches. If what you say is true, then the 5/8 constant is a conversion factor which should be explained. If not, Mathcad users will notice that their answers are 12 times greater than your examples. I don't believe that I am making a personal attack when I expect the same level of error checking that I learned in school many years ago - always list your units and show how they balance. Remember, it is your name that appears on the design example sold to member of SEAOSC for this miniseminar. I expect that if your name appears as the author of the information you will be responsible for the results. If the example were this simply done, without the assumptions that riddle it (and I am speaking of the Seminar notes from February 1998) then this would not be an issue. You can't take for granted that everyone is at the same level of experience or understanding of this method as your are - you need to bring the level down to those of us who are learning this for the first time. Personally, I worry more about my liability than I do about hurting someones feelings for pointing out an error. Please indicate to those of us on the list how the units are balanced. I request that you simply address the diaphragm deflection formula so that we know how the first term (bending) balances to inches and where the conversion occur in the third term for nail slippage and bending. I apologize for my frustration, but I have clients that expect the work to be done and are not particularly concerned with how I accomplish this as long as it is within their time frame. Dennis Wish PE PS: See my comments embedded into yours below. In a message dated 7/8/99 7:38:03 AM Pacific Daylight Time, SThompsonB(--nospam--at)aol.com writes: << Regarding "G" values for OSB: Testing of shear walls has indicated that the G values are slightly higher in OSB than in plywood but not enough to warrant the use of different values. Use Plywood values. Regarding the E value of 1,700,000 psi: I hope that you are not specifying "stud" grade lumber for chords. ******************************************************** <Dennis Responds> This depends upon what you consider the chord to be. I only specify walls to be stud grade or construction grade materials. I don't believe that I have a specific stress specified for double top plates and assume that they are the same materials used in the construction of the bearing walls. In this case, they very well could be stud grade. If you are recommending that the chord is the rim joist or joist parallel to the wall, this is a different story - but then I don't think it will have an area of 5.25 sq. inches as indicated in your design example. ********************************************** Regarding the diaphragm span "L" being in feet: The seminar notes are CORRECT in using FEET for the diaphragm span. Refer to the equation definitions on page 3-41 of Vol 3 of 1997 UBC. There is however an ERROR in the UBC in units for metric conversion. The diaphragm width "b" should be M not mm and the diaphragm length "L" should be in M not mm. ******************************************* <Dennis Responds> Let's look at your example from the 1998 Feb. 21 Wood Seminar - I don't at the present have a copy of Volume 3 of the UBC to compare to: The first term is 5 v L^3 / 8 E A b v is in plf l works out to be in ft^3 E is in psi A is in in^2 and b is in ft. When all units cancle, the resulting unit is in FEET while the rest of the formula yields results in inches. Unless the 5/8 term is a conversion factor (not indicated in your design example) the results of the first term needs to be multiplied by 12 to convert it to inches which dramatically increases the deflection in the beam term. Please indicate where I am incorrect. ***************************************************** Regarding the 0.188 number in the third part of the equation: The number is a constant and not related to units. ************************************************ <Dennis Responds> Then please tell us how the units balance so that we can explain this to Mathcad that automatically converts and issues an error when units do not match? When looking at the basic terms 0.188 L en, L = feet en = in en is taken from Table 23-2-K "en" Values (inches) for use in calculating Diaphragm Deflection due to nail slip (structural I). Once again, the units do not balance since this term yields either in^2 or ft^2 and all other terms are in inches. Please clarify for us who are confused. ******************************************************* Dennis: you need to lighten up on your personal attacks. Doug Thompson S.E. >> Doug, I think you need to realize that our livlihood depends upon getting this correct. At this point in time, I would suggest that you not be so sensitive to the comments and simply address the answers for those of us who rely upon your examples. Respectfully, Dennis S. Wish PE
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