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

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

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

(OP)
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.

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

OK:

A few things here:

1. This building has a 15 foot eave height, 12 foot bays, with a 4:12 pitch and no concrete floor. Depending on the wind zone, it seems highly unlikely that a 6X8 with only 3'-4" of burial will work. I would expect more like 5 to 6 feet.

2. I think that knee braces would be needed here.

3. I also think tht because of the size of the structure, the size of the poles and depth of burial needed will drive the lateral design from relying only on the cantilever effedct of the poles. Plywood will be needed here to work effectively.

In esssence, the type of construction would work on a smaller structure, but just not this one in my opinion.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

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

I have seen a similar detail proposed by a lumber company in Alberta who sold the barn package exclusive of the foundation. That detail was a six foot deep, two foot diameter gravel "pile" with no concrete footing on the bottom. Presumably, it behaved as a friction pile or 2'-0" dia. footing or both.

I have never understood how that design could work, so I have avoided pole barns like the plague. However, a few years ago I did have the opportunity to examine a riding arena which collapsed due to a snow load of about 12 psf. The collapse resulted from an absence of bracing in the 70' long roof trusses. The piers were four feet deep gravel piers and had nothing to do with the collapse, in fact they seemed to be performing quite adequately. They were spaced at 8' centers, so each pile was carrying 8x35 = 280 square feet of roof plus 16' high wall. Assuming 5 psf for roof and wall dead load, each pile was supporting 3360# of snow and 2040# of dead load for a total load of 5400# when the roof collapsed. Don't ask me how it could do that, but it did.

If an engineer is sealing a foundation plan, he must be confident that the design satisfies recognized engineering principles. It is not enough to say that it has been done before and it works. You have not mentioned anything about the soil at your site, the depth of frost penetration or the design snow load. At a depth of 4'-0" the piers would seem to behave primarily as circular footings so you might want to check the bearing value and compare that to your expectation of the soil bearing capacity.

BA

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

Soil beareing was one of my concerns too. There is a formula to get higher allowable soil bearing the deeper the holes, but assuming that 1500 psf bearing on a 24" diameter augered hole, and it must be augered, not dug, to use the lateral soil bearing pressures of the IBC, then you are looking at only about 4.2 kips of capacity at each hole. Assuming 10 psf dead load for the roof structure (whch may be slightly high), 12 foot bays and a 48 foot span, this leaves a net allowable for the snow load at only 1.3 kips (4.2 - 2.9) spread over an area of about 288 square feet, or about 5 psf for snow. NO way man, just no way, unless there is not a snow load provision in your area, which could be the case.

This fact has been one snafu I have found in pole barn foundations for years - the bearing pressure is too high for the loads. I've seen it way too often.

I did not have a calculator, so the figures are just off the top of my head, but still in the ballpark...

With that analysis, I would be suspect of uplift being considered too, and an adequate mass provided in the foundation to resist that. Just a pole buried in a gravel pier will not give that. The pier would have to be concrete, and of sufficient mass to resist the uplift. There is no connection shown between the pole and the 8" concrete footing, so the dead load of the soil would not be developed. Depemnding on the uplift seen, this structure could be an airplane waiting to lift off.

Ane the connection at the top of the pole must be developed too for that same uplift.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

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

Pole barns traditionally have been used outside of metro areas for farms and in other areas typically not covered by building codes and inspectors.

Therefore the whole pole barn concept has been developed over time based on long term gains in knowledge and trial and error - and typically doesn't have as much engineering design (if any) behind it.

In the few cases we've been asked to provide "sealed" drawings for a pole barn - we've found it difficult to get them to work (if not impossible) and we've since avoided them altogether.

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

You may want to get a copy of Post-Frame Building Design Manual from NFBA.org or take a look at the resources at ASABE.org.

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

(OP)
Great! Thank you for the info. I did leave off design loads:

Snow Load: 25 psf ground
Wind: Oregon: 110 mpg per the newest code iteration
Frost: 24 inches

I just get nervous when I see a pole being supported by gravel and not concrete for lateral. To me, gravel for lateral resistance of a pole is in the same category as using sheetrock for a shear wall...once it deflects and the nails/screws create space, you lose a boat load of your reliability.

I will definitely check out the articles later tonight though.

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

mpg... must be a hybrid structure...

Dik

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

(OP)
hahaha, indeed. gotta save the enviorment!!!

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

Metal roofing and wall sheathing provide fairly effective diaphragms and shearwalls - even without detailing.
I believe it is on the order of 100 plf (I believe i saw this in the aforementioned Post-Frame Building Design Manual)
This is how many pole barns are built around here and they seem to hold up fine.

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

As mentioned above the NFBA is the place to look.
They have a manual which publishes allowable shear values for metal sheathing and roofing. They also have seminars every so often presenting the design concepts for the style of construction that you have described.
Basically pole embedment and shear walls are both utilized to resist lateral forces. Also the diaphragm is not the usual 'deep beam' analogy; rather they use 'multiple beams' where the purlins act as chords.

I think it is a situation, like JAE mentioned, where construction trial and error came first and now we are trying to justify why it works through engineering principles.

EIT
www.HowToEngineer.com

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

There is just no way that two 6X8's at a 12 foot bay spacing with a 15 foot eave, 4:12 pitch, in a 110 mph wind, even if it is Exposure B, will work. That is about an average of 15 to 20 kip feet of moment to each 6X8... just no way unless the columns are steel members - HSS 6X8.

I would really like to see the numbers on this one...

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

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

Perhaps the drafter is just looking for free engineering from you?

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

Mike,
I think the design is based on the assumption that the roof diaphragm transfers the wind load to the end walls, so that the 6x8 posts are not behaving like 15' high cantilevers. That seems like a reasonable assumption to me.

BA

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

Go to NFBA or See any publications from Frank Woeste - U of Virginia. These are done all the time and perform remarkably well. 3'-4'' is not deep enough but 4'-5' usually is.

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

BTW - I have been designing them for over 30 years - never lost one except to a tornado!!

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

(OP)
I bought the manual and perused through some examples I found. I'm not providing this drafting any design information, only doing a pier review and noting deficiencies that I see.

The one major thing that I see with the drawings is the foundation. The NFBA manual recommends providing a concrete collar around the post to deal with uplift. I would think a person would rely on frictional resistance of the post to prevent uplift.

Mike: Do you typically provide a collar? Or is uplift not usually an issue for most cases?

Thanks.

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

dhloward26:

Yes, uplift is usually a problem, particularly in large riding arenas - open structures. It can also be the case for closed structures such as these. I would use concrete for the pole footing, not gravel as I mentioned before. The uplift will drive the diameter and depth of the footings as well as the bearing and any imposed lateral wind forces.

BA:

I agree for this structure, particularly considering the "X' bracing in the roof structure, and I have done the same many times, but with a plywood roof diaphragm and plywood shearwalls, not 28 or 29 gage sheeting. I this case, the pole footing columns act as holddowns. Dhoward26 did not mention the gage of the sidewall sheeting, but I have seen too many of these with metal sheeting of the gages I mentioned. I guess it would help to know.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

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

(OP)
msquared48:

I have always encased the columns in concrete with the open pole buildings I have done in the past...it's how I was taught. If I wasn't encasing them then I had parts of walls/corners sheathed with plywood and some continuous footings.

The NFBA manual uses footings that aren't encased in concrete the full depth, but they do use a collar tie to handle the uplift. I like the idea and am going to run through some old designs and see what difference it makes.

The design that I'm reviewing had no mention of the guage of steel or the screw pattern for the steel sheathing.

As a side note: This region that I have expanded into for engineering practice is an area that has been greatly under served. I have lost track of the amount of insufficient designs that have been done in this area by the same engineer...it's actually quite unnerving. I'm doing my best to help educate the drafters and the building department. Even the building department has been letting some pretty significant things slide as well that are in the code, but are being told are "okay" due to the engineers interpretation...I'm assuming because they just don't know any better.

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

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.

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

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

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

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

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

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

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

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.

______________
MAP

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

(OP)
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.

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

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_Dire....

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

______________
MAP

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

(OP)
Great! Thank you for the information I really appreciate it!

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