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- To: <seaint(--nospam--at)seaint.org>
- Subject: Barn Structure
- From: "Gerard Madden, SE" <gmadden(--nospam--at)maddengine.com>
- Date: Tue, 7 Oct 2003 14:58:02 -0700
I am working on a Barn Structure that was built in the late 50’s.
The roof structure is simple gable shape with corrugated metal roofing over 2x4 sub-purlins that sit on 2x8 scissor trussed rafters every 10 feet o.c. with collar-tied rafters in between (5 feet away). There are 6x posts every 10 feet in the long direction of the building. End bays are 10 feet wide (wall to post), center bay is 25 feet wide (post to post).
The owner wants to convert the barn into a storage area. The dirt floor will have a new concrete slab. The owner wants insulation and humidity control and therefore needs to insulate the roof and walls.
I have checked the roof structure and it doesn’t work (by calculation) for existing gravity loads when considering roof live load. The other main problem I can see is wind load in perpendicular to the long direction of the building. There doesn’t appear to be a real diaphragm, only the corrugated metal roof fastened to the 2x4’s. They sit on top of the trussed rafters and there is no blocking at the eave.
The problem is the owner wants it to look as is – worn and old from the inside looking up. Another problem is the exterior look must be the same as it is now (exterior walls have vertical boards and the metal roof is painted white)
The architect is considering a foam roofing/insulation over the existing metal roof, with a new metal roof over the foam. My concerns are pretty obvious: weight of the new roofing material and creating a diaphragm. Screwing through foam and existing metal to create a diaphragm seems kinda iffy (engineering term).
Can others share how they have approached these problems? I was thinking some sort of diagonal braces in plane with the diaphragm, either blocking or some kind of rod system. For the gravity, I’m hand strung with the requirement to maintain the look (exposed old looking lumber). If I strengthen rafters with new wood, it won’t look so old anymore… It’s the live load requirement that kills it, because the dead load is very small (existing) and probably no more than 4 to 5 psf with the new roof.
Also, if anyone can point me to some design criteria for agricultural buildings (i.e. historic barns, although this isn’t considered historic) that I can use a basis for my concerns. I’d like to have something to back me up if I have to break the bad news to the owner.
Comment appreciated in advance.
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