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Re: Wood interior wall studs

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I do a lot of wood, and here's my take:

The 5psf should be at a Cd of 1.6.  Why? Unless I have an intentionally pressurized room (which is not the case here), there is no possible way that common construction techniques can _maintain_ that kind of pressure differential over an extended period of time, and if you have a constant load (say, a commercial kitchen with doors that seal tight...right...) making the pressure differential then the HVAC engineer should already be providing makeup air in that area.  The building code can call it a "live" load all it wants, but the actual duration of load will never reach 10 years in the 50 year design life of the building.

Roof live load should be given a Cd of 1.25; snow is 1.15.

I don't recommend a loadbearing wall of 10' in height be made of 2x4s. Why? Because the walls will not be sheathed before the floor joists are set into place. The code limits the L/d ratio to 75 for construction, and a 10' 2x4 has an L/d of 80. The contractor cannot build the wall unless he plans on bracing every stud without violating the code.

And, for the record, I agree with Don that 2x4s 10 feet long look a lot like spaghetti. And as a bearing wall they give me the willies. My answer to such a request is normally to do it as designed, or provide another PE to seal off on the change with full calculations, and provide me with a full release of liability should anything happen.  I've never been taken up on the offer.


Andy Heigley wrote:



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





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