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Please see the following links for a good explanation of the difference: 1. http://www.sepakistan.com/topic/165-slab-elements-in-etabs/ 2. http://www.slideshare.net/neikrof/membrane-plate-shell 3. https://wiki.csiamerica.com/display/etabs/Membrane+vs.+shell+slab Regards.

W/Assalam. According to BCP Table 5.13, R value for an Undefined Structural System should be selected as per procedure given in BCP Section 5.29.9.2. However, IMO the problem may be tackled in the following manner, as well. In case structural details about the existing RC structure are not available, it may be considered as an Ordinary Moment Resisting Frame (OMRF) at the most, with a tabulated R value of 3.5. On the other hand, third storey, proposed to be constructed in steel (cold-formed or hot rolled) may be assumed & designed as Steel OMRF (R = 4.5), Steel EBF (R = 5.6) or Steel SMRF (R = 8.5). However, in absence of reliable structural information about the existing structure, it would not be advisable to design the third storey for a higher R value, than the RC structure supporting it (the third storey). In such a case you may assign an R value of 3.5 to whole structure, to be on the safer side. However, in case you do have exact or reliable structural data for the existing structure, and you are satisfied that it can be classified as an IMRF or SMRF, you may use a higher R value for the whole structure, including the additional storey. I hope that before proposing the third storey, you would have already checked that the footings are capable of taking the load of the additional storey safely. HTH Regards.