Pole Barn beam sizing

[Jmeng1026]

I was asked to design a pole barn and I have always passed on these type of projects because my calcs don't seem to match what the contractor says they want to do and what has worked for them in the past.

The contractor would like to install 4x6 columns 8' o.c. He wants to use a 2x12 beam on each side of the post to support attic trusses.

The building is 30' wide x 48' long. The trusses span the 30'. Trusses spaced 24" o.c.

Ground snow load in my area is 60 psf. Metal roof with 8:12 pitch so the snow load can be reduced to around 33 psf.

When I run the calcs it says a triple 2x12 is needed to span 8' but this contractor and a few other contractors (and online searches) say double 2x12's are fine.

I realize what contractors say and have done in the past does not mean it is correct but I have seen a lot of pole barns built the way the contractor wants to do it and they look great (not structural issues).

Am I missing something? Is there something different about designing a beam for a pole barn.

 

[XR250]

This is why I pass on these among other reasons. It probably used to work before the NDS dropped the design values and the GC's have not caught up yet.

 

[RWW0002]

I doubt you are missing something.. Pole barns can be a bit difficult to get local practices/performance to mesh with current code. Wait until you run a check on the posts.. Depending on height, they can be very difficult to get to work (strength or deflection) without really sharpening the pencil with regards to the contribution of siding to lateral strength and stiffness.

I pass on these whenever possible also, but have been drug into them from time to time for an existing client.

Good luck.

 

[ANE91]

I have seen a lot of pole barns built the way the contractor wants to do it and they look great (not structural issues)

Your calculations require a larger beam, not because the pole barn will otherwise immediately fall down the moment anything smaller is erected, but because that size has been shown to fail so rarely (e.g., < 0.015% annualized) that it can be considered sufficiently reliable by code. Plenty of code-incompliant structures get built and “never have structural issues” merely because the wood knots were distributed more favorably, or because they never experience design-level events. We simply don’t exercise the control we think we do over these structures. Without a load test, I’d never accept something that far below code.

 

[Jmeng1026]

Thank you for the responses.

The columns would be 10' tall. I will run the calcs but they will probably end up being 6x6's or 8x8's.

 

[RWW0002]

My money is on 8x8's coupled with another "are you crazy!" look or comment from owner/contractor.

There are some companies that make glue-lam posts for pole barns with pressure-treated plys below grade and a bit higher-graded lumber that might be worth checking out depending on your area and post size.

 

[LittleInch]

Your calculations require a larger beam, not because the pole barn will otherwise immediately fall down the moment anything smaller is erected, but because that size has been shown to fail so rarely (e.g., < 0.015% annualized) that it can be considered sufficiently reliable by code. Plenty of code-incompliant structures get built and “never have structural issues” merely because the wood knots were distributed more favorably, or because they never experience design-level events. We simply don’t exercise the control we think we do over these structures. Without a load test, I’d never accept something that far below code.

This is exactly the issue.

The other way to think about it is that all the design calcs etc have inbuilt or explicit factors of safety against a design event (wind, snow, rain, load) which may never happen. Hence why non calcualted designs end up essentially using up the factor fo safety plus they never get to the max design load. Hence 2 2 x 12's "work", but you need 3 to achieve the required FoS against the design load.

What happens when you use 2 x 12s? What FoS do you end with or how much over the max loading are you? I.e. do you really need 2 and a half 2 x 12s?

And why pole barns with all their uncertainties and variances and local practices are "still standing", though no one ever tells you about the ones which fell down in a gale / snow storm or cracked and needed reinforcement.

All depends who is looking at the calculations I suppose - the local authority or the insurance company or both?

 

[Deadblow]

OP, did you use machine stress rated (MSR) grade lumber for your beam checks? I've seen some pole building designs that are specified to use MSR for the beams.

 

[gte447f]

OP, for your own edification, I suggest you recheck your analysis and design for the beams using only marginal dead load and a drastically reduced live and/or snow load relative to the code required loads. The results of this alternative, non-code-conforming, analysis and design will most likely explain why the contractors think that the 2-2x12's are fine. Many structures never experience code level loads. But, you have to design for the code required loads.

 

[tolchijb]

My state has very specific requirements for "Residential Accessory Post and Beam Structures". This only appears in the residential code and is severely limited in size and height. It is a prescriptive method that will allow you to be code compliant. Beyond that, I tell people if they have a building department, pole barns are not for them.

I agree that pole barns do not work on paper and would never stamp one because the liability is not worth the hassle. I have personally observed pole barns built with no engineering and, frankly, limited know how, that have stood for over 40 years. There is no convincing an "I do it all the time" contractor that he needs pressure treated parallams or 8x8 posts. That is a battle not worth fighting.

 

[RontheRedneck]

My main worry in this case would be fastening the 2X12s to the posts. It would take a hell of a lot of fasteners to support that much weight.

Some areas around here allow lower live loads for trusses that humans aren't typically in. That might possibly make a difference in the building you're looking at.

Of all the building failures I've known about where we built new trusses to replace some that failed - All of them were post frames. (And a few stick framed houses)

 

[Greenalleycat]

I've done a few pole bars, they're not too bad
Never one with attic trusses though
Agreed with everyone else: I'm sure the contractor saying "it's never been an issue before" has no bearing on actual design calculations, code-intended FOS, and doesn't relate to a building that experiences a design snow + storage load that we are required to design for
His thoughts are not relevant if they disagree with your engineering design - you are the expert

That said, you should double check to make sure that your design has sharpened the pencil where appropriate
Have you used wet or dry timber properties?
Is this strength or deflection governed - if deflection, is your deflection criteria reasonable for this type of structure?
Have you assumed single or multi span support as this will significantly improve the deflection?

Something to be careful of when you get to 3/ beam build ups is how the load and support conditions are applied in reality
A double beam I imagine will be split either side of the post? But with a triple beam you'll need a 2/ on one side and a 1/ on the other - but the critical connection loads will come out through 1 piece of timber on the double side
Also if the loading is primarily coming through trusses, how are you sure that the loads are getting evenly split between the two/three members?

 

[gte447f]

Depending on who/where/what will be using this structure, pole barn plus attic/loft trusses screams "stacked full of square bales" (of hay that is) to me. If so, then 2-2x12 (spaced) definitely ain't gonna cut it. Better get a good handle on the intended use and the appropriate design loads.

 

[BAretired]

If the pole barn is a farm building and if your location has a farm code, use that because its structural requirements are usually less stringent than the building code.