Membrane Stiffness of Bare Metal Decks

Membrane stiffness of bare metal deck is based on the deck shear stiffness available in deck property data. Refer to the following sample calculations for Verco deck on how to choose deck shear thickness. Similar calculations can also be performed for other type of decking. 

From Verco Floor Deck Catalog: Composite Slabs General Information Section 12. 
F is the deflection in microinches for a span of 1 foot under a shear of 1 pound per foot.
To convert to equivalent shear thickness for ETABS use the following:
The shear deflection d is given by:   d= force*span/(shear modulus*width*t)
or the required thickness, t = (force/width) *span /(shear modulus * d)

d = F/1e6 inches  
span = 12 inches
force/width = 1/12 pound/inch
shear modulus = 11150000 psi
therefore t = (1/12) * 12 /(11150000*F/1e6)  = 1/(11.15*F) inches So if F=5 for an unfilled deck then t = 1/56 inches.