Pole Barns - Pier Review - Haven't Seen This Method Before

[dhoward26]

I recently got asked by a drafter to do a pier review of a pole barn he drew and had engineered back in 2011/2012. Reason is, he is wondering what design is the best because he keeps getting different designs from different engineers and wants another opinion.

I have never seen the method that I am attaching before. Quick summary:

1) There isn't any wood sheathing on the entire thing. Just metal roofing and metal wall sheathing with no call out for attachment requirements or gauge of steel.
2) The walls utilize 2x6 horizontal "girts" spaced vertically at 24" o.c. The roof utilizes 2x6 purlins at 24" o.c.
3) 6x8 Wood Posts at 12" on center. Haven't ran a calc on these yet.
4) Foundation: 2ft diameter x 3'4" deep compacted 3/4" gravel, on top of an 8" x 24" concrete pad.

I haven't ever seen a foundation designed like this before? Any ideas on the validity of it? My first thought is no way.

 

[Archie264]

It seems to me that one of the keys to this type of construction (i.e. post-frame, pole barn, etc.) is that the bases are embedded in the ground providing a large degree of base fixity. Yet I've always thought that embedding posts in the ground, or even into concrete, was to be avoided. And in fact the AITC Timber Construction Manual has many suggested details showing columns bases pinned to concrete and virtually every detail has a note saying not to place concrete below finished concrete floor level.

Can anyone help me reconcile this? I know that light, unoccupied agricultural buildings can be held to a different standard from other buildings, and obviously thousands of pole barns have been constructed and function successfully. But, as I understand it, even commercial, residential, and industrial buildings are built this way. So...what's the story here, for lack of a better way of asking that. It seems then that both approaches are legitimate, but can you learned folks help me resolve this seeming conflict?

Thanks.

 

[msquared48]

It has been done for years and successfully, but with PT material. I also use a gravel base at the end of the pole so water will wick awy from the end (low water table assumed here).

30 years ago I used 4X4 PT posts to erect my fence with concrete around the base in the same matter described and save the damage from the squirrels and cats, they are in the same condition as when I put them into the ground. No worries.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Archie264]

Well, it's hard to argue with a successful track record...

 

[BAretired]

It depends to some extent on the drainage to be expected at a particular site. Embedding posts in solid concrete is not good because there is nowhere for the water to go after it enters the wood below the top of concrete.

Embedding posts in crushed rock or gravel might work well in a well drained soil, but in an impervious clay, there is nowhere for the water to go after it enters the gravel. This could result in wood rot and/or frost heaving.

BA

 

[focuseng]

Postframe design values are historically based on empirical diaphram values of the metal cladding stiffness. The center frames lend stiffness but most of the load is taken by the end wall frames which have bracing and cladding (again usually metal but wood can be used). You need to get a slew of publications by ASAE or now the ASABE and review them. A lot of published material there. A lot of real world full scale testing has been done to verify these designs and results. Additionally the pressure treatment and drainage around the embedded posts is dealt with by the design and all. Some buildings don't even embed the posts anymore. Postframe buildings are quite stout and given a good manufacturer and erector they are superior in cost and performance to other building material at a certain scale. There are a lot of real engineers doing work on these buildings but most of them work for a manufacturer. I recommend if you are designing a post frame building to look one up and converse with the engineering department.

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MAP

 

[dhoward26]

focuseng: Thank you for the info and recommendations. I purchased the "Post-Frame Building Design Manual" from NFBA.org and have been reviewing it in further detail. There is a lot of good information out there, just have to dig it up and most of the time it is found through word of mouth. Thank you.

 

[focuseng]

Yes. I have the manual too and you really should dig into those ASAE/ASABE papers which are referenced in the Manual. The following documents are good assets
ASAE paper # 004006: post foundation designs (2000)
ASAE paper EP484.2: Diaphragm Design of Metal-Clad, Wood-Frame Rectangular Buildings (Feb 2003)
ASAE paper EP486.1: Shallow Post Foundation Design (Oct 2000)
ASAE EP558: Load Tests for Metal-Clad, Wood-Frame Diaphragms (DEC 1998)
ANSI/ASAE EP559: Design Requirements and Bending Properties for Mechanically Laminated Columns (FEB 2003)
ASAE Paper 01-4003: Investigating Building Failures (1993/2001)
ASAE Paper 024007: UW & LBS Full-Scale Metal-Clad Wood-Frame Diaphragm Study. Report 1: Project Introduction and Building Design Details (2002)
ASAE Paper 024008: UW & LBS Full-Scale Metal-Clad Wood-Frame Diaphragm Study. Report 2: Frame Loading and Data Acquisition Systems (2002)

Another source would be to contact David Bohnhoff at the UW Madison. He is one of the primary authors of most of these documents.

http://bse.wisc.edu/Directories-Faculty_Staff_Directory.htm.

Most columns are built up 2x8,2x10 with the bottom lams being PT for perm ground contact.


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MAP

 

[dhoward26]

Great! Thank you for the information I really appreciate it!