Wood Post Unbraced Length 2

[XR250]

To determine the capacity of this slender post (without getting into composite action), would you consider it as the twice the capacity of a single stud with strong axis buckling as the limiting factor. (i.e the weak axis is braced to equal the strong axis)

 

[Smoulder]

Strong + weak if not composite. And the same for governing direction if composite because centroids align.

 

[phamENG]

would you consider it as the twice the capacity of a single stud with strong axis buckling as the limiting factor.

Yes. If the load is concentric and you have no bending, composite doesn't really matter. They are each constrained in their weak axis by one in the strong axis.

My only concern would be the nails in withdrawal to provide the necessary brace force. But then, you've got quite a few.

 

[XR250]

Duh. Not sure why I had twice the strength. Thanks.
So conservatively, the capacity of a single stud where strong axis buckling controls.

 

[GaStruct]

I vote strong + weak as well. The stud on the left would need a shear flow calc for the nail pattern to be considered braced in the weak axis. Similar to how a double top plate cannot be considered a 3" deep member in bending. Looks like you have enough nails to make it work though.

 

[phamENG]

so for those who want a shear flow calc, what shear are you using if the load is concentric?

 

[Tomfh]

Itotal = Istrong + Iweak

Same as a T shape bucking about its minor axis.

 

[DTS419]

Single capacity would be very conservative. Both studs will take compression stress so completely ignoring one of them is wasting capacity. I would consider the cross section of both together, and do something like a 12d or 16d nail @ 12" O.C. It won't take much. If you need to put a number to that, I would size and space the nails for 1-5% of the axial load.

 

[phamENG]

XR - I'd say that would be conservative, yes. It's a common bracing method for trusses that don't have web members aligned over a long enough distance to put in longitudinal braces.

But as long as both studs are bearing at the bottom and the load is applied to both evenly at the top (within reason - this is wood construction we're talking about), I'd have no problem using the capacity of both studs. I usually call for screws or ring shank nails, though.

 

[GaStruct]

I would need to check the NDS to see if they have any guidance on the bracing loads. I'm sure they do. Off the top of my head I would say on the order of 1% of the axial load which is the AISC requirement for column point bracing.

 

[XR250]

But as long as both studs are bearing at the bottom and the load is applied to both evenly at the top (within reason - this is wood construction we're talking about), I'd have no problem using the capacity of both studs. I usually call for screws or ring shank nails, though.

full strong axis capacity of both studs?
I use this to brace truss webs as well. But only to consider the truss web weak axis to be braced as it was strong axis.

 

[phamENG]

I'm an idiot. Strong + Weak is correct. You just have to be careful with that approach because weak axis is no longer allowed above 6'3" for a 2x. But If you take 0.01P brace loads at third points (to negate weak axis buckling), you get about a 30% reduction in axial capacity. For a 6' 2x4, two of those 'braced' studs equals one 6' 2x4's strong axis capacity + one 6' 2x4's weak axis capacity. So much larger than that and you'd probably be wise to just use a single stud's capacity because the weak axis will fade to zero pretty quick beyond that.

 

[XR250]

Yup, I figured for anything reasonably slender, the weak won't be contributing much.
Where are you getting the 6'-3" number from?

 

[phamENG]

l[sub]e[/sub]/d can't be more than 50. 1.5" * 50 = 75"

 

[Smoulder]

"so for those who want a shear flow calc, what shear are you using if the load is concentric?"

@Phameng Buckling is really bending moment reducing capacity. Pick your poison on how to estimate the moment. Three ways I'd look at: 1) Second order analysis with initial deflection. 2) Something like how British/Euro codes back calculate concrete p-delta from interaction curve. 3) Work out the moment that reduces section capacity to bucking capacity. Assume BM shape to convert to shear (for all 3 methods).

 

[driftLimiter]

I don't understand the strong + weak vote.

If the weak axis member buckles then there's nothing left but the strong axis member? If both members were sharing 50% of the total load then after the weak buckles the rest goes to the strong axis member.

Plus the nails @ 6" o.c. is pretty analogous to sheathing+nails on one side of the 'weak axis' member right??

 

[phamENG]

drift - you can simplify it one of two ways.

1) You're just putting load into one of 2x4s, and the other is bracing it in the weak axis (this is how T-braces work for trusses). You ignore one of them altogether as it's just there to keep the other from buckling.

2) You assume they are adequately tied together (10d nails at 6" is a lot, so probably safe assumption), and the two 2x4s will have to buckle the same direction. One will buckle about its strong axis, and the other about its weak axis (roughly - there will be some rotation there, too...)

As the member gets longer, the two simplifications end up merging together as, eventually, the one will be too slender to do much about its weak axis and will only help by bracing the other in its weak axis.

 

[driftLimiter]

Okay that makes a bit more sense. Not sure if I believe it buts tbh its way too complicated for a couple of 2x4's imo lol. I think I would stick with single stud fully braced on weak axis. If need more strength Double it up.

 

[phamENG]

I think I would stick with single stud fully braced on weak axis.

Agreed. I spouted off this morning before I'd had enough coffee, and planted my foot firmly in my virtual mouth. To convince myself, though, I ran a couple quick calculations with a more 'complex' situation of just how they brace each other and what the resulting capacity is - they match up really well but the simplest method is certainly the best for wood.

 

[ChorasDen]

I haven't put any pen to paper on this, and have just been a bit along-for-the-ride with reading this thread, but how are you all addressing eccentricity with the combined strong plus weak approach when addressing buckling? What eccentricity is everyone applying to the individual members, or are you assuming a global eccentricity?