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Young engineers often ask about whether to select membrane or thin shell or thick shell for modelling area elements such as RCC slab. I am going to talk about the difference in the results when choosing thin-shell or membrane. This is what makers of ETABS says about the topic. I will quote the following to clear out the structural difference between the two: "Load which is applied to membrane objects transfers directly to supporting structural objects, whereas meshed shell objects have bending stiffness and therefore resist a portion of the load through flexural deformation. As a result, less load will be available to transfer to beams located under a shell, while 100% of the load will transfer through a membrane". Lets see how much can the results differ. Figure below shows the partial framing plan of a level in the building I will discuss how the modelling of S11a as thin-shell or membrane will effect the design shear and moment on the beam B54a, see figure above. The figure below shows the design forces obtained against the gravity loads for the membrane case: The figure below shows the design forces obtained against the gravity loads for the thin-shell case: Based on the results presented above, B54a's design shear increased by 155%, and its design moment by 147% as compared to S11a's behavior as membrane. Now the question, that a Structural engineer needs to ask is: Which result is more appropriate or which result captures the actual behavior more realistically? And that is where you engineering judgement comes into play.