Orthotropic membrane - eesd-epfl/OpenSees GitHub Wiki
To model floors with elastic membrane elements, as is common practice in equivalent frame models, one can use in OpenSees standard shell elements coupled with a suitable sectional model. An orthotropic elastic section model, implemented for this purpose, replicating exactly the Tremuri formulation, can be defined as follows. Since sectional models cannot be read from external libraries, the section type is integrated directly into the OpenSees executable (download binary -64 bit, compiled for Windows- or build your own). The syntax is:
section OrthotropicMembraneSection $secTag $E1 $E2 $ni $G $h <$rho $angle>
element ShellMITC4 $eTag $nI $nJ $nK $nL $secTag
Where:
| secTag | material tag |
| E1, E2 | stiffness in material direction 1 and 2 |
| ni | Poisson’s ratio |
| G | Shear modulus in 1-2 plane (independent) |
| h | height of the section |
| rho | density per unit area (optional, default=0.0) |
| angle | angle of material axis 1 with respect to the local axis x (°degrees) |
The section model defines only the membrane stiffness components, providing zero bending stiffness. Bending degrees of freedom can be fixed imposing node constraints. If also the bending stiffness is of interest, the user can adopt any other section model, including any plate fibre section, possibly nonlinear.
The orientation of material axes--1 and 2--with respect to the local axes of the element to which it is applied can be passed to the model through the parameter "angle". The local orientation depends on the definition of the shell element. Local axis x is the “average” of the directions defined by the vectors IJ and LK; axis 2 is in the perpendicular direction in the element plane.
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