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What is the difference between Direction and Coupled material component behavior?

In a layered shell section, there are two types of layers: Directional and Coupled. The Directional type of layered shell will apply the uniaxial stress-strain curve defined in the material property in the S11, S22, and/or S12 directions but each direction will be independent of each other. For example, yielding in the S11 direction will not affect the S22 direction and Poisson's ratio will be 0. The Coupled type of layered shell will use the coupled nonlinear data in the material property and the S11, S22, and S12 directions of the layer are nonlinear and are dependent on each other, hence "coupled". The behavior is dependent on the material type: if concrete, the coupled material is a biaxial concrete material, and if steel, the coupled material is a Von Mises plasticity material.

More information on this can be found at:

• Analysis Reference Manual Chapter "The Shell Element", Section "Layered Section Property > (8) Layer Material Behavior"
• "Modified Darwin-Pecknold 2-D Reinforced Concrete Material Model" technical note for the coupled concrete material
• "Coupled Nonlinear Material Data Form" and "Von Mises Plasticity Model" Help topics

See Also

• Strain localization within nonlinear shells article