Floor Slab in ETABS

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When floor plate elements are modelled in ETABS, they can be either modelled as:
(i) Plate element, which contains out-of-plane stiffness only;
(ii) Membrane element, which contains in-plane stiffness only;
(iii) Shell element, which contains both in-plane and out-of-plane stiffness;

It shall be noted that if the shell element is adopted, in normal situation, all the loading would only be transmitted to the shell's node points instead of its supporting beam/wall elements.

In order to allow gravity load transfer, the following modelling procedure shall be adopted (for normal floor slab):

(1) Use Membrane Element:

  • Model the floor slab as Membrane element;
  • Assign rigid diaphram for lateral load transfer;
  • In "Floor Meshing Options", choose "Default";
  • For the beams supporting the slab, choose "Use Line For Floor Meshing" (Especially important for using large slab element in model)

OR

(2) Use Shell Element or Plate Element:

  • Model the floor slab as shell element or plate element;
  • Assign rigid diaphram for lateral load transfer;
  • In "Floor Meshing Options", choose "Auto Mesh Object into Structural Elements" with small element size (say 1m);

For Transfer Plate, the following procedure should be adopted:

(1) Define the transfer plate as a shell element using the actual thickness for "membrane" and "bending" properties.

(2) Draw meshing lines using "NULL" lines. Make sure all the column that is being transferred are connected.

(3) Go to Assign --> Shell/Area -->Area Object Mesh Options. Under Floor Meshing Options, choose Auto Mesh into structural elements and check mesh at beams and mesh at wall and ramp edges. Leave the other two unchecked.



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