Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Diaphragm Calculations

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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) writes:

<< Regarding  "G" values for OSB:
 Testing of shear walls has indicated that the G values are slightly higher 
 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 

<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 
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.

Dennis S. Wish PE