Ke Value for Pole Barn

[gshurd84]

Hello,

I wanted some opinions on what the Ke value of a pole barn post would be. The post is imbedded in concrete and the free end is connected to steel truss spanning to another wood post.

For me it is something between a true flagpole situation (ke = 2.1) and is the most conservative approach and a Ke value of 1.2 (rotation fixed translation free).

 

[driftLimiter]

I suppose you are suggesting that there is a partial rotational restraint from the steel truss to the top of the post. It will depend on the detail of the joint, and the relative rigidity of the connected members. Although I agree that it is somewhere between these two values for most practical connections, unless I specifically detail the connection to restrain rotation I would consider it a pin for this purpose. In general I try not to get to skinny with the posts, they are the main load carrying element and without those its kaput, for a polebarn this is even more true. That being said I understand that the Ke factor = 2.1 seems like a penalty, but its just physics, cantilever columns are prone to stability issues what are we gonna do.

 

[gshurd84]

Yes I agree with your post and the conservative approach and appreciate your input. For an open pole barn I allow up to 14 feet of unbraced length mainly because the contractor I talk to says after that the barn moves too much during construction. Otherwise the wind loading and gravity loading is well within tolerance for most Areas using a 6x6 ground rated post embedded in concrete.

With the 2.1 we can only go to about 10.5’ and using 1.65 (right in the middle of 1.2 and 2.1)

Actually I think the 2018 NDS code allows something greater than 50 for construction loading but could not find anything since they changed their book and you can’t download and search the whole thing. I do know the actual equation only needs unbraced length member size and the Ke value so not sure why loading would even matter.

Not disagreeing with you just a discussion point and using Ke values (or other constants) with custom amounts based on experience.

 

[driftLimiter]

@gshurd84 I am in shock that AWC changed their policy and now is charging for the PDFS, I am also quite saddened by this. I believe codes should be accessible. Anyway here is the temp unbraced length provision :

The slenderness ratio for solid columns, le/d, shall not exceed 50, except that during construction le/d shall not exceed 75.

I guess the best advice would be to take advantage section 3.7.1.2 where it says ..."in accordance with engineering mechanics." and take a stab at calculating the effective length according to the actual members and detail. Time to sharpen the pencil?

When I find myself in similar situations I am tempted to just assume the preferable value (i.e. Ke = 1.6 seems to match what the customer wants, I will just use that since I have a feeling that the Ke would be right around there.

But I think the better approach is to resist that urge and endeavor to make your calculations and your approach more refined. If indeed your feeling is correct you can document it, if you find that you were wrong then you at least you will know where to draw the line.

Also worth noting that IBC, or locally adopted provisions might have specific regulations or exceptions for odd structures like pole barns.