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Type of fastener to use to fasten Aluminum Purlins to Structural Steel

My name is Tom, I work for a medium sized Window and Door company in West Seneca, NY. We are currently involved in a project at a downtown high school that involves a large skylight. The skylight is made of Polycarbonate panels in an Aluminum frame. The panels sit on Aluminum purlins, that have to be fastened to the structural steel below. We had planned on using steel self-drilling/thread-forming 3/8" screws to go through the flange on the bottom of the purlin, securing it to the steel.
The steel will be painted and the aluminum is mill,no anodic finish. The architect wants us to use an "isolator" between the steel and aluminum, and to use SS screws. The use of SS screws means we can't use self-drilling screws because they would break. I'm looking for some supporting documentation to show that there would not be an issue if the fasteners are cadmium coated. Or if someone who knows what they're talking about to tell us we should use SS screws or bolts.
Thanks,
Tom

Tom Hailand
Installer - West Seneca, NY, USA
June 22, 2009

Hi, Tom. Your isolators are meaningless unless there are insulating bushings and washers to prevent the screw from touching the aluminum. If there are such bushings and washers, then, from a galvanic standpoint, it doesn't matter to the aluminum what the screw is made of.

If that be the case, painted fasteners like they use for deck screws might be best because stainless screws could theoretically galvanically corrode the tapped holes in the steel.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 23, 2009

I suggest contacting one of the more technically competent fastener suppliers such as Elco (part of Acument) or SFS Intec to discuss your application. Both of these companies offer stainless steel self-drilling screws specifically for this type of multi-metal application. The shank and head are stainless steel, while the drilling point is a hardened carbon steel. A coating is used for better galvanic compatibility with aluminum. Another option is to use a steel fastener that is electroplated (zinc is more common than cadmium these days) and maybe features an elastomeric insulator under the head.

Toby Padfield
Automotive module supplier - MI, USA
July 1, 2009