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.

 

[msquared48]

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

 

[BAretired]

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

 

[msquared48]

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

 

[JAE]

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.

 

[SteveGregory]

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.

 

[BAretired]

I was just cruising the internet and came on the attached link which is very similar to the foundation suggested by the OP, but the roof span and column spacing is considerably less.

BA

 

[dhoward26]

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.

 

[dik]

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

Dik

 

[dhoward26]

hahaha, indeed. gotta save the enviorment!!!

 

[XR250]

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.

 

[RFreund]

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

http://www.HowToEngineer.com

 

[msquared48]

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

 

[ztengguy]

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

 

[BAretired]

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

 

[MiketheEngineer]

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.

 

[MiketheEngineer]

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

 

[dhoward26]

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.

 

[msquared48]

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

 

[dhoward26]

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.