I assume you're trying to model corrugated metal deck. The following works OK:
1. Determine G' for your deck/fastener combination and compute the thickness that results in the correct in-plane shear stiffness. This thickness is usually a tiny fraction of the nominal thickness. Maybe 10-50x smaller.
2. Define a shell element with the G and t from #1. At this point, your in-plane shear stiffness is correct, but the weight and orthotropic bending stiffnesses are way low.
3. Get the psf from the deck catalog. Compute the psf of your very thin shell. Divide these to get the property modifier on mass and weight.
4. I usually leave the membrane property modifier 1.0, but you can play around with that also, if you'd like. Beware that if you use an area PM and a mass/weight PM, you might be double-dipping. Some experimentation will be necessary to know if SAP2000 applies the area PM and the computes the mass/weight, or if it uses the original thickness to compute the mass/weight.
5. Get the moment of inertia from the deck catalog. Determine the MOI for the shell from #2. Divide these to get the PM for bending in the direction parallel to the deck ribs. This will be m11 or m22 depending on your local axis orientation. This PM will be enormous, like 10^5 or larger.
6. Compute the MOI for bending in the direction perpendicular to the deck ribs. The section is a rectangle with the nominal thickness. Determine the property modifier and add that to the shell definition.
7. Make sure your local axis is in the correct direction for the shells you add to the model.
8. Experiment with your shell in a simple span condition to make sure it's giving you the EA and EI values that you want.
