/ Perma-Columns #11
Do you have any diagonal bracing at the corners of your barn? The bracing will give you more strength then the metal, which is limited to the thickness of the metal and the ability of the screws to hold it secure. Most screws are just snug so the rubber will keep out water, they do not secure the metal to the wood beneath it enough to actually be structural.
I do have diagonal bracing in the corners and I'm not saying the bracing is not needed, just that the higher significant shear resistance is provided by the metal. Rubber washers don't prevent the metal from being structural. The shear load is perpendicular to the screw and the screw shank is in contact with the metal for it's full circumference - that's the resistance to movement in the shear plane.
There are plenty of pole barns out there that rely on the metal siding for shear resistance even though building codes may not fully recognize that fact. The OP should contact an engineer specializing in post frame construction if he is in doubt. I'd also talk to Perma-Coulmn to see what they say. A lot depends on building dimensions (wall heights and lengths especially), exposure to prevailing wind, wind direction, door opening size and location. There are a lot of variables in play.
I've researched this a fair bit and there is a number of folks that are professionals in post frame engineering who will substantiate the diaphragm influence of steel panels as light as 29ga. They state that the columns do about 10% of the work in resisting lateral loads. My building was on my property when I moved here 11 years ago (it's about 22 years old now) and I was concerned about the construction method. I've since added some strapping from the bottom plate to the posts to prevent uplift just to be safe. We have recently redone all the metal to match the color of the house and that's when I experienced first hand the stiffening effect of the metal after we stripped the building completely and redid the metal coverings.
s219: Was your engineer a specialist in post frame construction? If not, it's likely he's not familiar with the nuances of this construction method, nor is his software specialized to handle these conditions. A lot of engineers that are not intimately familiar with a specific construction method will over specify just to protect their judgment - CYA. Their licence is VERY important to their livelihood and they if they aren't intimately familiar with a construction method, they won't spend the time to fully understand it, it's easier and quicker to over specify. They need to get done quickly and move to the next paying customer, they don't want you to be upset with their fees, so they transfer the costs to the construction method. I've seen this way to many times in many different engineering disciplines. I won't argue that OSB or plywood provide better shear resistance, but if they are ultimately not needed why would we absorb the additional costs?
I've seen plans where an engineer wanted 5/8" plywood nailed on 3" centers on a barn end wall (crazy over kill). Further evaluation by a post frame specialist determined that NO sheathing was required, the girts and metal siding provided the necessary shear resistance. The National Frame Builders Association has done extensive testing on the type of roof and wall panels which we are discussing, and they show good diaphragm and shear wall capacity. They have analyzed this extensively and provided a free calculator: DAFI Calculator | content
I copied this from an engineering forum:
"there is SIGNIFICANT diaphragm strength and stiffness out of simple 29ga corrugated steel roofing when screwed through the flats into 2x4 SPF purlins setting on top of wood trusses. I've also done simulated wall diaphragm tests as well where we've screwed through the steel into the wide face of 2x4 SPF girts. I'm talking "Pole barns" here as many call them, but those types of structures are very highly engineered buildings.........when done right "