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RE: Wood interior wall studs

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You post has prompted quite a bit of good responses.  I’m going to shoot from memory right now because I don’t have my NDS in this office.  Regarding which Cd factor to use, I think you need to decide the actual time duration of the force and base your decision on the table given in the NDS.  There is such a wide range of time durations and factors in the table.  We know wood has a much higher strength for short load durations.


You made a statement below that prompted me to write this email.  You stated “Applying 60% more to the allowable axial stress makes a huge difference.”  I’m not sure if you are applying the NDS correctly.  Once you have a slender element the Cd factor has very little influence on increasing the allowable axial load.  I’m referring to ASD here.  The Cd factor is only applied in the Cl & Cp factors.  It is not multiplied by the resultant Fc value.  It specifically states this in the NDS, but since I don’t have my copy here I can’t give you the reference.  I have a spreadsheet I made for combined axial and bending.  For a 9’-10” 2x4 stud I get an allowable axial stress of 397 psi for Cd=1.0 and 407 psi for Cd=1.6, using SP No. 2 wood.  That is only a 2.5% increase in axial load.  I just want to caution you since it does not appear anyone else questioned this statement.




From: Andy Heigley [mailto:aheigley(--nospam--at)]
Sent: Friday, July 13, 2007 2:39 PM
To: seaint(--nospam--at)
Subject: RE: Wood interior wall studs




Thanks for your responses…


Here are my responses to some of your questions back:


  • I would design for LL and LLr if the wall were supporting both the floor and roof loads.


  • I am designing to ASD.


  • I guess I’m a little leary of using the Cd of 1.6 for this reason.  The duration factor is applied to both bending and axial capacities.  Applying 60% more to the allowable axial stress makes a huge difference.  And if you have a 4 story building, for example, you are going to be approaching the capacity of the stud just due to DL and LL…  you then add a “little bit” of short term horizontal loading to the stud and increase the capacities by 60% seems non-conservative.


  • Scott:  I haven’t found the rated wall design reduction factors you’ve mentioned.  Can you tell me the code section that is in?


  • 10’-0” high 2x4’s… exactly why I initially called for 2x6 stud walls, but the GC is flipping out about it… I get the old, “I’ve been doing this for 30 years, and never had to do this before”…



Andrew Heigley, PE