Pole barn column

[JStructsteel]

Looking at a pole barn, and trying to decide a Ky factor for my column. I know its not fixed, and i know its not free. Somewhere in between? Ky 1.6 keeps the column in check. Whats your opinions?

 

[msquared48]

With sufficient embedment per the AITC formula, why would you not achieve base fixity?

With knee braces and proper embedment, you will also get double curvature in the column.

With no embedment, a “K” of 1.0 is correct, but you will need shear walls.

Mike McCann, PE, SE (WA, HI)

 

[RWW0002]

If this is a polebarn without diaphragm/shearwall action, it better be fixed! I do not design many polebarns, but I would be looking for a k of 2.1 unless some of the load is tracking through diaphragms to the endwall columns or shearwalls.

I recommend the NFBA Post-Frame Building Design Manual for good overall discussion of pole barn design and some good guidance on the post and foundation design.

 

[JStructsteel]

Sorry, didnt explain, I have the base fixed, but was contemplating the top. Is it free, or is it fixed somewhat, since there is a truss at each column. the columns are 3 ply 2x6's, and the truss is let into the center with the outside plies going up to the top of the truss top cord. There are also knee braces each column. I figure that gives some fixity. Thats why I am trying to justify not 2.1 for fix-free, and not 1.2 for fix-fix, but somewhere in between.

 

[msquared48]

Free unless there are knee braces.

Mike McCann, PE, SE (WA, HI)

 

[JStructsteel]

I do have knee braces. Thanks.

 

[BridgeSmith]

If the knee braces and the column to truss connection are sufficient to prevent substantial rotation at the joint under loads imposed assuming a fixed joint, then using 1.2 for the K factor (fixed bottom, translation free/rotation fixed at the top) would seem reasonable to me. So, as long as the knee braces and their connections at both ends, and the connection of the column to the truss, are designed, detailed, and constructed with adequate strength, the lower K factor would seem appropriate, to me at least. Granted, it's been a long time since timber design class, and I haven't done much since, so I could be way off base.

Rod Smith, P.E.

 

[KootK]

Are the knee braces and colmns this buildings lateral system or are there shear walls etc?

 

[JStructsteel]

Its a pole barn, so columns and kneed braces. Its a 40' span. Truss at each column.

 

[XR250]

Is it enclosed?
The detailing of knee braces in this situation has always been dicey at best for me.
There is inevitably a bolt end distance issue where the beams splice over the columns.
Also, if you are knee bracing from the column to a roof truss, that would require accounting for in the truss design and the bottom chord bracing to keep it stable.
I generally try to flagpole these structures or use the metal wall sheathing if it is enclosed.

 

[KootK]

Its a pole barn, so columns and kneed braces.

I apologize if you feel that I've wasted your time with my question. I expected that the knee braced frames were part of the lateral system but I wanted to know that with some degree of certainty before investing my time trying to help you given that:

1) I've been involved in new build pole barns that utilized shear walls in the transverse direction.

2) I've be involved in forensic/retrofit pole barn investigations that utilized transverse shear walls out of desperation.

3) I don't know enough about your background or your practice region to know your level of experience with pole barns and whether or not [pole barn = lateral knee brace frame] in your mind.

Like XR250, I'm inclined to flagpole. And the excerpt below from the NFBA manual leads me to believe that is becoming the dominant practice.

...and trying to decide a Ky factor for my column

To possibly waste more of your time, I'd like yet another clarification. Doesn't [Ky] mean that you're investigating post buckling perpendicular to the plane of the knee braced lateral frames? If so, I suspect that most of the responses thus far have been geared towards buckling in the plane of the knee braces. Or is the longer side of your posts actually running parallel to the wall?

 

[JStructsteel]

KootK, I suppose I really dont know the system name I am using. I think of a pole barn as a barn with a 4x4, etc buried into the ground providing the lateral resisting system

My Y direction is across the building, parallel to the trusses. Maybe I am backwards, but thats the direction i am talking about. My posts have the longer direction in my Y direction, across the building.

Its a flagpole column, buried 4' in the ground. But, at the top of each column, there is a truss bearing on top and a knee brace. In the opposite direction (what I call Kx), the column I would say is effectively braced against buckling by the wind girts from column to column.

In the Y direction, if just a flag pole column alone, I would use a K value of 2.1, since its fixed at the base, free at the top.

My dilemma is since the truss, and knee brace provide some fixity, its not a fixed-free column anymore, but is it a fixed-fixed, fixed-pin, etc. I see it somewhere in between. So a K value from 1.0 up to 2.1.

Thoughts?

 

[BridgeSmith]

The knee braces, if they are adequately stiff and strong, make the connection at the top fixed for rotation but free to translate, so k = 1.2. If the restraint against rotation is ignored, then the knee braces, their connections, and their affect on the truss members, do not have to be analyzed (as indicated in KootK's excerpt). I would guess the axial loads for a pole barn are generally small enough relative to lateral bending on poles/posts, that using k = 2.1 makes no difference to the required post size.

Rod Smith, P.E.

 

[KootK]

KootK, I suppose I really dont know the system name I am using.

Frankly, I don't care what we call it. That's why I wanted clarification on whether or not the braces were part of your lateral system. That way, my understanding of function here is terminology independent.

Thoughts?

1) Once you decide to have the knee braces, I think that it's sensible to design that path all the way through for the required strength and stiffness. Otherwise, failure might occur at a connection that would impair your ability utilize a backup, flag pole load path.

2) I agree with Hotrod's K-1.2 for the most part. As shown below, there will be many sources of flexibility that likely will not be rigorously accounted for in your design.

3) Critically, note that the real column length for buckling is the distance from the foundation to the brace point rather than from the foundation to the underside of the truss. If you ran your numbers using the full length of the post as the column length for buckling, I can definitely get behind K=1.6 if that makes things pleasant for your post design.

 

[BAretired]

With columns extending only 4' below grade, it seems to me that the point of fixity would be some distance below grade. That would be variable depending on the properties of the native soil. Significant passive pressure is questionable at that depth. I think the usual procedure is to fill around the bottom of the columns with gravel. Some folks might use concrete fill but the inflection point could be a couple of feet below grade.

BA

 

[JStructsteel]

Thanks,
Its strange how the cheapest of buildings for a consumer provide the most questions/dilemmas for us engineers.

 

[XR250]

Its strange how the cheapest of buildings for a consumer provide the most questions/dilemmas for us engineers.

Exactly the reason I stay away from these jobs as there is usually not sufficient fee to make it worthwhile.
To do it correctly without flagpoling takes a shit-ton of analysis and detailing that will often never get performed properly in the field.
Historically, however, they seem to hold up fine regardless of how great or un-great they are constructed.

 

[BridgeSmith]

As I went back and reread this thread I realized that each pair of posts is supporting the lateral loads for a 40' span. I presume the frame/truss spacing is at least 8'. Assuming 4' of embedment of the posts to be adequate to achieve fixity (or even lateral capacity) under that loading seems suspect to me, unless the posts are to be embedded in a fair sized concrete shaft.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[jayrod12]

In my experience, the posts tend to be at 4ft on-centre, and it's a double truss at each post. 4ft of embedment still seems suspect if the clear height of the barn is of standard dimensions +/- 14ft or more.

 

[JStructsteel]

XR250,
I think its the fact that there is more structural resistance than a straight up flag pole column. You have the vierendeel truss action of the roof purlins, the metal roof panel diaphram capacity that is ignored, the fact that with kickers the posts and truss act as a sudo-frame, corner braces and roof braces, and the ignored action of diaphagm/end wall resistance.

I know its a research paper to quantify all that, and thus why they stand up so robustly even if put together by a horrible contractor.

I think in my county 36ft span is the limit for a non engineered structure. Perhaps thats why so many 36ft buildings are built!