If beams are designed In ETABS ,then the slab as shell or membrane makes a considerable difference.The shell model will give lesser design forces for beams as the slab is dragged-in, to contribute in beam section.In case of membrane, the slab effect on beam is almost nullified. Membrane option is conservative for beam design ,but why not take advantage of slab which is monolithic with beam.? probably shell with reduced stiffness is an answer.

Further when we transfer the model to SAFE with beam and slab integrated & design the beams ,the beams attract very less moments compared to "UDL" on beam situation.Here the dilemma again starts ,because the beams forces and the reinforcements are drastically different in these two cases. Again --why not take beam and slab together and stop thinking about this comparison because this is the reality.?(Probably in safe we should take beams as T sections.).

If we look at ACI 318 ,the stiffness modifiers to consider cracking (0.25 for slab),play a role here.When 0.25 is applied in global model ,it considerably reduces the slab participation in lateral load transfers but still considers the slab effect.

Hence I would say ;

For beam design in global 3D model ,consider shell with modifier 0.25.

For slab design in safe ,transfer the same ETABS model with slab.Take a call on whether you want to design the beam by safe or continue with ETABS beam design.

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Membrane: used for conventional slab with beam.

Shell: is used for flat slab and wall.

Plate thin: is used for raft foundation where bending is predominant.

Plate thick: is used where shear is predominant.

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Shell Vs membrane is a very commonly debated topic among Etabs users.I am sharing response I got from E-tabs tech support Please bear with long description:

The rigid diaphragm assumes infinite in-plane stiffness of floors and therefore reduce the

stiffness matrix. The semi-rigid diaphragm uses the inplane stiffness of slab and does not condense the stiffness matrix. For most of the concrete structure where slab is sufficiently thick and we do not expect any deformation, results for semi-rigid diaphragm will be same as rigid diaphragm. Can use membrane definition.

However, if you are expecting the slab deformations then modeling a semi--rigid diaphragm will be the correct way to handle such cases. A semi-rigid diaphragm is same as assigning no diaphragm except it allows you to assign the accidental ECC due to Wind or Seismic load cases. At the same time , wind loads can be applied at the center of masses and you do not need to use Area Exposure method to define Wind load case for semi-rigid diaphragm cases.Need to use shell definition.

There are three levels of modeling for floors in ETABS.

1. The simplest level is that the floor is modeled as one big polygon and is used only to

define the extent of the floor. It is assigned meshing type - "For Defining Rigid Diaphragm and Mass only (No stiffness - No vertical load transfer)" . It is assigned a rigid diaphragm so it connects all elements falling within laterally but is unable to transfer any vertical load. This model can be used for a quick study of the lateral load resisting system.

2. The second level is a step higher in that the floor is meshed coarsely and given only

membrane or deck properties. One could assign a rigid diaphragm if needed, otherwise

connectivity is still provided through the coarse mesh. The vertical load is transferred to

edges of coarse mesh and is either supported directly by columns or by beams and walls on the edges. This is commonly used with composite floors and can be used for thinner concrete floors where beams are designed for full gravity loads and slab bending stiffness is not important to the lateral analysis.

3. The third level is to have a decent mesh of the floor either done externally or internally

that connects the major structural elements and also models the correct bending stiffness of the floor for frame action with columns. This obviously is the correct model and the reason to go to the previous two levels is to avoid getting a huge model that takes too much time solving or cannot even be solved on current hardware.