NDS 3.7.1.4 On studs Question

[sponton]

NDS 3.7.1.4 states that compression members should be limited to a slenderness ratio of less than 50. Are studwalls included in this rule? Why is this generally ignored, I mean, software does warn you when you do use a member that exceeds this ratio but it's not clear on the commentary whether this is applicable to studs; following this particular rule would be impractical since 2x4 walls would be limited to a height of about 6'-6".


Anybody has a thought on this?

 

[TehMightyEngineer]

The slenderness ratio of less than 50 is the same as AISC recommendation of KL/r < 200 (NDS uses 50 because they use KL/d).

This usually isn't an issue for stud walls as they have sheathing on one or both sides that laterally braces the weak axis. Thus, for a 2x4 stud you need a length of greater than 14.5 feet before you exceed the 50 limiting value. You are entirely correct that without weak-axis bracing stud walls are limited to 6.25'.



Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[sponton]

Well, I'm not worried about the weak axis buckling, as you said, the sheathing provides some restraint, but in the strong direction you don't really have any restraint so you're forced to use the full height of the stud [I don't think adding mid-blocking genuinely provides some real buckling stability in the strong direction, specially in short walls, I think Breyer talks about this in his book].

IBC 2012 states that the max. height for 2x4's [laterally unbraced] is 10 ft [Table 2308.09.1] or whatever is justified by analysis.

 

[TehMightyEngineer]

In the strong axis direction your "d" is going to be the depth of the member. So, for a 2x4 it's 3.5 inches and gets much better when you have a 2x6 or 2x8 stud wall.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[sponton]

No it wasn't that, never mind, i am using the new enercalc and they switched the wording of where you input the bracing so I was inputting it the other way around, bracing the strong direction rather than the weak direction. That happens when you don't read the instructions, nice to know about NDS 3.7 though. Thanks to you too.