Evaluation of a pole barn for snow loading - opinions sought

[RHTPE]

I have been tasked with evaluating an existing barn for snow loading. The owner is reconstructing an arena that was added to the original barn which failed under last winter's snow loading. The pertinent facts are:
- Original barn building permit pulled in 1973.
- Original arena was constructed before the original barn was complete.
- New arena roof is 4' higher than the existing barn, as was the previous arena.
- Eave-to-ridge distance is 43.5' along the 4:12 slope.
- This is a pole-barn style construction with principle roof members at 5' centers.
- Purlins are 2x4s at 24" centers running over top of the roof members.
- Roofing is sheet metal.

Using the snow load recommendations of ASCE 7-10 and the 2005 NDS design methodology, I find that the principle members are a bit overstressed under the maximum unbalanced snow load.

Using the building code snow loads of 1971 (drifting & unbalanced not specifically addressed) and the 1968 NDS design methodology, I find that most members look good.

Worthy of note: The 1971 building code has a higher magnitude of uniform snow load than does ASCE 7-10. However, the 1971 building code does not specifically address unbalanced & drift loading and, I suspect, leaves the evaluation of drifting and unbalanced load to the judgement of the designer.

Obviously this roof has sustained almost 30 seasons of snow loads with no ill effects. There is no evidence that last season's record snowfall caused any distress in the roof framing or connections.

My question for those who follow this forum: Would you use the snow loading of ASCE 7-10 (lower overall magnitude but higher drift loading) with the 1968 NDS design values and design methodology for wood (most likely what the original building was designed around)?


Ralph
Structures Consulting
Northeast USA

 

[msquared48]

If the members are the original grades from 1968, I have no problem using those allowable values with the current snow drift loading stipulations.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

That building is almost 40 years old! Most pole barn guys warrant them for 20 years and 50 years is the usually the most you can expect from them.

Drifting snow was not a consideration back then and that may have added to the demise of the building.

IMHO - building is probably OK for now. And your calcs are probably OK also. Sound reasonable. I have been designing these since about 1973!!

But you cannot offer any guarantees. The poles may well be rotted under the soil, the purlins infested with rot or termites or whatever, the fasteners may be shot.

I would tell the owner that "You have gotten good life from this building, it may last another 10 or 20 years, but be prepared to replace it at some time - sooner or later."

 

[RHTPE]

Mike, The snowfall we received last season certainly was the demise of the arena portion. Interesting that the barn portion was unaffected. They have just completed framing of its replacement. My report is required by the suilding inspector for them to receive their CO.

Ralph
Structures Consulting
Northeast USA

 

[woodman88]

RHTPE - What building code are being looked at for the Original Barn UBC, BOCA, etc. and which IBC for the current design?
Why are you looking at the existing barn? Is the new arena changing the loads to it or is the owner adding loads more than 5% additional loads to it? If no changes and you are using the IBC then I would not touch it, rather use IBC Chapter 34 to grandfather it. If there are changes than I would repair it for the current code requirements which include the current NDS. Unless you can get approval of the building department to use the older version of the NDS.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[JAE]

Can you clarify which failed (the barn or the higher arena)?

 

[RHTPE]

Mike - thanks for the insight RE: the life expectancy of these kinds of structures. I will indeed comment on that issue in my report.

Garth - The CT Code in place at the time the permit for the original barn was issued was BOCA + CT Supplement. Today it is 2003 IBC + CT Supplement. There is no change to the original barn structure. The original arena was constructed as an addition to the barn, but was constructed before the barn had been completed. The local inspector has required an evaluation of existing barn as a condition of the building permit. I have discussed the matter of which Code(s) are acceptable with the inspector.

JAE - See the previous post - it was the arena area that collapsed under snow load.


Ralph
Structures Consulting
Northeast USA

 

[woodman88]

Ralph - Quote "...The local inspector has required an evaluation of existing barn as a condition of the building permit..." If no changes are being done to the Barn, than I would grandfather it per the 2003 IBC (if the CT Supplement does not remove or change Chapter 34) and evaluate it per the original building code and NDS.
For the inspector's evaluation, are they looking for you to do a site evaluation? If so, I would get a clear statement of what they want you to look for to avoid conflicts later about whether you met the your legal responsibility in the site evaluation. For a site evaluation requirement, if they have not made a clear statement of what they want, I would send a letter to the building department outlining two or three proposed means of doing it. Asking them which of them they would approve. This way you would have a clear statement of what the Building Official required from you. I have done this in the past and usually they will pick one and make some changes to it.
Do include Mike's comment about the life expectancy.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.