Engineering Post Frame Buildings

[medeek]

I'm getting ready to do an analysis of a post frame building and I am still hunting down every resource I can find to educate myself since this is the first post frame building I have ever done. I've managed to acquire a copy of the 1999 Post-Frame Building Design Manual published by the NFBA, which appears to be the defacto standard for pole building engineering. However, online I have found other misc. papers describing a "simplified" method for designing post frame buildings.

I am wondering what others typically use as their reference and what are your thoughts on the simplified methods (Don Bender and Drew P. Mill).

I've also just noticed that the second edition of the NFBA manual has now come available.

http://www.nfba.org/index.html

Has anyone had a chance to purchase it and compare it with the 1999 edition (First Edition)?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[msquared48]

I've always understood post frame to be different than a pole structure. In the pole structure, the fixing is at the ground level, and in the post frame it is at the juncture of the beam and column, leading to totally different foundations.

I suppose you could combine the two also. What do you have?

Mike McCann, PE, SE (WA)

 

[jayrod12]

Sliderule's website has another pole-building reference on it if I remember correctly. I think it's by Donald Patterson.

I also use:
[ul]
[li]ANSI/ASAE EP 559 - Mechanically Laminated Columns[/li]

[li]ANSI/ASAE EP486.1 - Shallow Post Foundation Design[/li]

[li]Research papaer FPL-RP-528 by the United States Department of Agriculture[/li]
[/ul]

Might I also recommend if you are really stuck, Try contacting Dr. Kris Dyck at the University of Manitoba (you can get his info from their staff directory page). He is the Pole shed and hay bale house specialist.

 

[medeek]

6x6' posts, 4' deep. Post frame structure not a pole barn.

Is there any specialized software out there already, my ignorance may have me reinventing the wheel here.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

nope.

At least not that I've found. To be completely honest you are in for a battle with the contractor again.

These will not calc out the way he's expecting. You are going to need to use your 'What's reasonable?' hat a lot when designing these.

You can basically throw deflection requirements out the window and be prepared to take every member right to it's limit.

How tall? Dirt Floor? 4 foot embedment is unlikely to work unless the posts are spaced tightly.

 

[medeek]

I haven't started running any numbers yet but its not very large so I'm thinking it should calc out, we'll see...

It's only a 20x30 with posts at 10' on center, so four posts per side. A 5-1/8 x 10-1/2 Glulam beam spanning the posts on each side with mfg. trusses at 24" o/c.

14' eave height, 4/12 pitch, 24" overhangs all the way around. Originally an open design but the owner has framed in walls between the posts, so its now an enclosed structure.

Post depth is 4' with 18" Dia. holes, back filled with concrete to grade. The original plans called out #4 rebar drilled through the posts for uplift.

This is a case of the structure being built without a permit and now the building dept. has required it be engineered. The owner had assumed that he did not need a permit since the square footage was under 800 sqft.

The addition of framing and sheathing to the walls will add to the lateral strength of the structure however I think I will first neglect that contribution and see if it works with just the capacity of the poles and roof diaphragm.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Forgot to add dirt floor.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

My bet is it won't calc out at those spacings, but that's just my initial thought. It really will depend on height.

Make sure to use low importance when determining your loading.

Good luck with this, you're in for another tough one.

 

[medeek]

I've been doing some reading of different resources and I am now at a cross roads with my analysis.

I am trying to determine whether I should analyze the structure with or without diaphragm action. I am looking for pros and cons of each method and what others typically do for simple post frame structures.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[msquared48]

Sounds more like a pole barn to me with the 6X6 post cantilevering out of the 18" diameter X 4' deep concrete footings.

A true post-frame structure would have let-in diagonal braces framing from the columns to the beams above, columns not extending into the foundation, and hence pinned at the footing.

Mike McCann, PE, SE (WA)

 

[medeek]

Another question I have with the post embedment design is how to handle a post that is fully encased in concrete vs. a more traditional collar design. Would such a post be considered constrained vs. unconstrained, it seems to me that the mass of concrete encasing the post below grade will have some effect on the resultant soil forces and the bending moments and shear of the post itself.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[msquared48]

Depending on the wind load seen, the 14 foot eave height may kill the 6X6 columns, let alone the required embedment with only an 18" diameter footing. You will just have to run the numbers and see where you are. Hopefully you know the grade of the columns. Also check to see if they are rough cut members or not.

Mike McCann, PE, SE (WA)

 

[Splitrings]

ASABE EP486 will answer all your post questions.

A post would be considered constrained at the ground surface if a concrete slab or other means was present. A post with a full collar acts just like a post without a collar, except it is much wider, therefore reducing the lateral stress on the soil. Again EP486 will answer all these questions.

A true pole structure will likely need knee and wye braces at the top of the posts, otherwise the posts act as a cantilevers. Fixed at about d/3 below the ground and pinned at the top. The Ke factor of 2 for this condition increases post sizes drastically. This may work with smaller structures. My experience is mostly 40-50' wide structures with peak heights of 20' or so. These structures typically require 8x8 or 8x10 SYP posts. These structures are designed for 60 psf gsl.

 

[BUGGAR]

"Originally an open design but the owner has framed in walls between the posts, so its now an enclosed structure."

From a pole frame to a shearwall structure. I used to make a pretty good living off of contractors who did that.

 

[Jerehmy]

Use diaphragm action and shear walls + the cantilevered columns to resist lateral loads. They show an example in the NFBA. They have some program called DAFI you can download for free. I don't like it.

In the NFBA they have tables for diaphragm and shear wall assemblies that they tested just using the metal decking as the sheathing for the walls and roof.

In my experience designing pole barns is a pain. The owner always thinks it should be cheaper.

 

[Splitrings]

A professor at Cornell University did some research on the diaphragm action from light gauge sheet metal siding and roofing. I am not sure if he published the research.

 

[medeek]

This job is probably a bit more headache than I'm going to get paid for but once again it is an educational tool so I can accept that.

I've been doing a lot of reading the last couple of nights (last night up until 4:00AM, not sure how productive that was). DAFI is used to analyze the structure if your assuming diaphragm action, which I will probably want to do in this case since I can get some of the transverse load off of the post frames and into the gable end shearwalls.

Unlike conventional framing there are not holdowns but I'm assuming that the embedded posts at the corners of the structure can serve the same purpose.

My other burning question right now it how to deal with the tributary area for lateral loads due to wind. With conventional stud walls and roofs half the wall height and the roof height is tributary to the top of the walls and the shear load is applied at the top of the walls. For a standard post frame structure using girts the load distribution would seem to be different, wouldn't all of the wall area be considered for lateral loading to the posts in addition of course to the lateral loads coming from the roof.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

Yes your embedded posts can act as the tie-downs, but beware that it may not work as well as intended. The last time I needed belled piles to enact enough soil weight.

The NFBA manual goes through that as well.

 

[jayrod12]

And for the last question,

If you are designing the posts to cantilever then your thoughts are correct, when designing the posts you need to account for the entire wall, and the roof contribution.

If you're trying to dump some of the lateral shear load into the end walls the hardest part is determining how much makes it there as opposed to how much the columns end up taking.

 

[msquared48]

Very true in that the lateral deflection of the roof diaphragm and the lateral deflection of the columns in one line transverse to the diaphragm
must equal at each column location.

Simpler to design the columns for their tributary area in the short direction and the long sidewall shearwall for half the endbay area force.

Mike McCann, PE, SE (WA)