I am in the permitting process for a small 20’ by 24’ cabin in Northern Arizona. The building has a 12’x20 loft over the back half. The building is being carried on 4 - 12”x12” piers along each side and 4 down the middle, each resting on 2’ square footing. The question has to do with the bottom outboard rim-joists carrying the back half of the bottom floor, loft area and roof.

The span between the piers is 6’8”. The rim-joists in that area will be carrying 40 lbs live load each per floor and roof and 15lbs dead load each per floor and roof for a total load of 165 lbs per square foot. The rim-joists will be carrying half the load from the outside of the outer piers to the center = 5’ x 165 lbs = or 825 lbs per running foot evenly distributed over the span of the rim joist or 6' 8".

The building is scheduled to have either 7/16th or 1/2 inch OSB sheathing installed either across or parallel to the 2x4 @ 16” oc framing. The sheathing will start at the bottom of the outer rim-joist which I calculated to be doubled 2x8 Doug-fir.

If the sheathing is installed horizontally, the break between the panels on the first course would occur over the centerline of the footings, and the sheathing and framing system in my opinion would serve as a box beam or truss. I am willing to install blocking as a top cord at the top of the break between the sheets at 4’ or 8’ depending on the installation, and am willing to nail the sheathing on at some nailing schedule designed to accommodate the load. The top is in compression and the bottom is in tension.

This building is small and will only be used seasonally but the Coconino County Permitting Authority will allow no compensation for the sheathing and originally specified 5 1/8 x 10 glu-lam but now is down to a 3 1/8 x 9” Glue-lam or 4 - 2x12s bolted together to span the 6’8”.

Can someone provide me with the mathematics to demonstrate the load carrying capacity of the wall system including the sheathing?

Thanks,

Bill Cowan
Rimrock, Arizona