Insulation between Metal Deck and Purlins

[TonyES]

For some thermal break issues the contractor wants to install a 1" thick layer of insulation on top of metal purlins and than screw down the 1.5" 20g metal deck on top of the insulation. This is for a large tilt up concrete building with steel girders every 20' and metal purlins spanning the 20'. My gut tells me this will screw up my diaphragm and require another system to be the diaphragm such as rods (similar to a PEMB). Thoughts?

 

[msquared48]

I would say no way... It will negate any diaphragm action using the metal deck, plus screw up the lateral support for the top flange of the purlins by the metal deck in my opinion.

Mike McCann, PE, SE

 

[hokie66]

This is typical practice in Australia, but not when diaphragm action is needed. The insulation, in the form of batts, not rigid stuff, is placed first, usually over a safety mesh, and the steel roofing (not metal deck, but metal roofing) is screwed through into the purlins. You could not do this if diaphragm action is intended.

 

[strucguy]

See if you can address diaphragm action by using either tie rods or flat plates as horizontal bracing.

 

[jgailla]

I did steel working for about a year and every, or almost every, building I constructed had roll insulation over the purlins and below the roof deck.
There were usually tie rods connecting the purlins.

 

[ztengguy]

No. No and No.

 

[BUGGAR]

If you substitute tension braces for the diaphragm loads, who designs this system? Are the building frames sized for the compression forces that will be induced in them?

As you noted, "The Contractor wants......" How badly doe$ the contractor want?$?

 

[TonyES]

The contractor says he'll save over $200,000 by doing it this way but I don't see it. Either way I agree with the comments and appreciate the responses.

 

[msquared48]

Tell the contractor that he needs to put the savings into extra insurance.

Mike McCann, PE, SE