Can you use a metal roof as a diapragm?

[arcoiris74]

I am new to structural engineering and have been assigned to design a barn structure. Is there literature or design examples on using metal roofing to calcualte roof diaphragm forces, etc.? The metal roofing will be screwed to 2x6 purlins @ 24" o.c. (lying on weak axis) against tied rafters. The designer does not want to use wood sheathing, or any manufactured wood for the structure for that matter. Any suggestions or direction would greatly be appreciated.

 

[strangeoddity]

When you say metal roofing...what kind of product are you referring too? Standing seam or 9/16" corrugated 20ga roof deck? Typically a standing seam roof isn’t used as a diaphragm (if it can be avoided). However, if you are designing a barn with a good diaphragm aspect ratio, I believe almost any roof deck will provide adequate diaphragm properties. As far as calculating the allowable permitted forces…with a tight roof purlin spacing, your fasteners between the 2x’s and the metal deck will control your design. Determine the shear flow forces in the diaphragm and the allowable shear values of your fasteners to determine fastener spacing.

As far as published values between steel deck and wood rafters, I don’t recall any…sorry. I suppose I have been fortunate with my barn designs.

 

[DaveAtkins]

What you are talking about are steel roof panels, and their diaphragm capacities can be found in the Post-Frame Building Design Manual, available from the National Frame Builders Association. This Manual also covers steel wall panels used as shear walls, and the design of the purlins as diaphragm chord members.

DaveAtkins

 

[dpa]

Contact the National Frame Builders Association

http://www.postframe.org

(800) 557-6957. Get the Post Frame Building Design Manual. There are several pages of test data in there for metal diaphragms that were actually constructed and tested in a lab. The book was pricey as I recall but worth it. The last chapter is an actual set of calculations sort of fill in the blank for a pole building. Should be very helpful for barns also. Personally I always prefer to have plywood sheathing first. In earthquake zones screw heads can pop off because the steel in a screw is much more brittle than the steel in a nail. Severe winds could conceivably cause the same problem. I have seen lots of metal roofs fastened with screws but they were all laid over plywood or OSB.

Good luck.

DPA