Has anyone been inspecting these hybrid metal buildings? They are not specifically addressed in the IRC building codes. The town I live in requires construction to conform to the IRC code.
Last week I was asked to investigate drywall cracks on a 40x60 Shouse in a rural area with no code enforcement. The building is on a concrete slab with 2x6 studs walls on 16" centers with engineered trusses. Based on the drywall cracks it was apparent the building was moving due to wind as low as 30 mph. Most metal barns and shops are post-frame buildings with 2x4 roof purlins and 2x4 wall girts. They are classified as agricultural buildings and are exempt from building codes in my area. This building has sheet metal siding attached horizontally on the exterior wall studs, i.e. no plywood wall sheathing. Plywood wall sheathing resists lateral forces due to wind and seismic forces, however the metal siding screwed to every other stud was inadequate. After doing extensive searching and reading there were no published guidelines for sheet metal shear walls. I found some university studies where sheet metal siding attached to cold-form steel metal studs were tested in the lab in an effort to establish minimum metal thickness and screw spacing. Turns out it took really close screw spacing to prevent the sheet metal from buckling. Unfortunately, this data was not transferable to wood studs. So the solution was to have the siding removed and 7/16" OSB plywood attached.
Hmm, interesting… there are few in my area as well.
Where did that 30mph number come from?
I’m also curious about how the OSB solution was determined… which I assume means the entire outer wall “skin” of the building comes off, OSB goes on, then skin goes back on? …what about moisture from the metal condensation? …What kind of insulation? …and is there a vapor barrier?
The interior was in the process of being renovated and new drywall was being installed. Two days later several drywall cracks developed mainly at the ceiling and wall joints. They were repaired and in a week some cracks reappeared. This is highly unusual on a concrete slab with no visible signs of movement. We were expecting high winds with gusts up to 50 mph. I suspected the wall framing was moving so I had a plumb bob attached to the ceiling and marked the point on the floor. The winds gusts the next day only peaked out around 25 mph. The homeowner took a video showing the plumb bob swaying 1/8” back and forth. Double the wind speed to 50 mph will increase the wind pressure four times, theoretically pushing the walls 1/2”. Cyclic movement like this racking the walls could easily cause drywall joints to crack. Metal pole barns typically don’t have drywall so this movement is not a problem. The poles embedded into the ground provide lateral restraint. Prefabricated steel framed buildings rely on wind X-bracing to provide lateral support, and offices or living spaces built inside are constructed independently from the exterior framework. This is what started my quest to do some research on metal shear walls. To design shear walls you have to consider wind and seismic forces. The design wind pressure on this house is 20psf. At this location the seismic forces were small so the wind controlled the design.