I thought metal deck was considered a flexible diaphram.  Concrete on metal deck would be rigid.  I'm an EIT, so I could be completely wrong.

LowLax,

You are correct. You have to have concrete (as in concrete slab or concrete on top of metal deck) to achieve rigid diaphragm.

Regards,

Lutfi
www.cdeco.com

Turizo,

Although concrete would be my personal preference, diaphragm action can also be achieved with certain types of steel deck using vermiculite aggregate and rigid insulation. Also note that deck lap conditions and weld patterns greatly effect the diaphragm action.

You should check the deflection of the deck against the deflection of the shearwall to determine if it's rigid or flexible.  Flexible deck deflects more than its supporting shearwall.  Rigid deflects less.  I think it's the same if the deck is supported by a moment frame.  Almost always steel deck with no concrete is flexible for typical construction spans and loads.

Given proper detailing and engineering you can use the steel deck for a diaphragm even if the roof is curved.  What kid of curve is it? Does it curve in plan view?  I had a case once where I had a small building with a curved side.  I was able to cantilever the steel deck past a shear wall to pick up the curved end.