Andrew,
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.
Rich
From: Andy Heigley [mailto:aheigley@jgaeng.com]
Sent: Friday, July 13, 2007 2:39 PM
To: seaint@seaint.org
Subject: RE: Wood interior wall studs
Everyone:
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