# Surfaces \[FEA]

Surfaces are 2D shell finite elements used to model slabs, decks, walls, webs, and other plate/shell structural components. They support quadrilateral (4-node) and triangular (3-node) element formulations with both membrane and bending behavior.

## Geometry

**Node 1:** The first corner node of the shell element. Nodes should be defined in a counter-clockwise order when viewed from the positive normal direction.

**Node 2:** The second corner node of the shell element.

**Node 3:** The third corner node of the shell element.

**Node 4:** The fourth corner node of the shell element. For triangular elements, leave this empty — only Nodes 1 through 3 are required.

**Thickness:** The shell thickness of the element. This defines the out-of-plane dimension used to compute the element's bending and membrane stiffness.

**Material:** The material assigned to this surface element. Provides the modulus of elasticity, Poisson's ratio, and unit weight for stiffness and self-weight calculations.

**Area (readonly):** The computed surface area of the element based on its corner node positions.

**Weight (readonly):** The computed self-weight of the element based on its area, thickness, and material unit weight.

## Global Offset

Global offsets shift the element corner points from the node locations in the global coordinate system. These are used to model eccentric connections where the shell mid-surface does not pass through the node (e.g., a deck slab offset from the girder top flange nodes).

**Node 1 Offset X:** Global X direction offset at Node 1.

**Node 1 Offset Y:** Global Y direction offset at Node 1.

**Node 1 Offset Z:** Global Z direction offset at Node 1.

**Node 2 Offset X:** Global X direction offset at Node 2.

**Node 2 Offset Y:** Global Y direction offset at Node 2.

**Node 2 Offset Z:** Global Z direction offset at Node 2.

**Node 3 Offset X:** Global X direction offset at Node 3.

**Node 3 Offset Y:** Global Y direction offset at Node 3.

**Node 3 Offset Z:** Global Z direction offset at Node 3.

**Node 4 Offset X:** Global X direction offset at Node 4.

**Node 4 Offset Y:** Global Y direction offset at Node 4.

**Node 4 Offset Z:** Global Z direction offset at Node 4.

## Local Offset

Local offsets shift the element corner points from the node locations in the element's local coordinate system. The local axes are defined by the shell element's orientation.

**Node 1 Offset X:** Local x direction offset at Node 1.

**Node 1 Offset Y:** Local y direction offset at Node 1.

**Node 1 Offset Z:** Local z direction offset at Node 1.

**Node 2 Offset X:** Local x direction offset at Node 2.

**Node 2 Offset Y:** Local y direction offset at Node 2.

**Node 2 Offset Z:** Local z direction offset at Node 2.

**Node 3 Offset X:** Local x direction offset at Node 3.

**Node 3 Offset Y:** Local y direction offset at Node 3.

**Node 3 Offset Z:** Local z direction offset at Node 3.

**Node 4 Offset X:** Local x direction offset at Node 4.

**Node 4 Offset Y:** Local y direction offset at Node 4.

**Node 4 Offset Z:** Local z direction offset at Node 4.

## End Releases

End releases control the connection behavior at each corner node of the shell element in the global coordinate system. A value of 0 means the DOF is rigidly connected (default). A value of -1 means the DOF is released (free/hinged). A positive value represents a semi-rigid spring stiffness. Right-click any end release cell for actions: *Set to Free* releases the DOF; *Set to Fixed* fixes the DOF; *Edit Stiffness...* opens a stiffness editor for defining nonlinear spring curves.

**Node 1 Tx (global):** Translation release in the global X direction at Node 1.

**Node 1 Ty (global):** Translation release in the global Y direction at Node 1.

**Node 1 Tz (global):** Translation release in the global Z direction at Node 1.

**Node 1 Rx (global):** Rotation release about the global X axis at Node 1.

**Node 1 Ry (global):** Rotation release about the global Y axis at Node 1.

**Node 1 Rz (global):** Rotation release about the global Z axis at Node 1.

**Node 2 Tx (global):** Translation release in the global X direction at Node 2.

**Node 2 Ty (global):** Translation release in the global Y direction at Node 2.

**Node 2 Tz (global):** Translation release in the global Z direction at Node 2.

**Node 2 Rx (global):** Rotation release about the global X axis at Node 2.

**Node 2 Ry (global):** Rotation release about the global Y axis at Node 2.

**Node 2 Rz (global):** Rotation release about the global Z axis at Node 2.

**Node 3 Tx (global):** Translation release in the global X direction at Node 3.

**Node 3 Ty (global):** Translation release in the global Y direction at Node 3.

**Node 3 Tz (global):** Translation release in the global Z direction at Node 3.

**Node 3 Rx (global):** Rotation release about the global X axis at Node 3.

**Node 3 Ry (global):** Rotation release about the global Y axis at Node 3.

**Node 3 Rz (global):** Rotation release about the global Z axis at Node 3.

**Node 4 Tx (global):** Translation release in the global X direction at Node 4.

**Node 4 Ty (global):** Translation release in the global Y direction at Node 4.

**Node 4 Tz (global):** Translation release in the global Z direction at Node 4.

**Node 4 Rx (global):** Rotation release about the global X axis at Node 4.

**Node 4 Ry (global):** Rotation release about the global Y axis at Node 4.

**Node 4 Rz (global):** Rotation release about the global Z axis at Node 4.

## Group

**Group:** The FE group to which this element belongs. Groups are used to organize elements for load application, result extraction, staged construction, and filtering purposes.

## Time Dependent

**Casting Day:** The day on which the element's concrete was cast, used for time-dependent staged construction analysis. The element's age at any construction stage is computed as (Stage Construction Day − Casting Day). This age is used to determine concrete strength, modulus of elasticity, creep, and shrinkage behavior per the CEB-FIP 1990 code.

## Overrides

Property overrides allow you to manually specify material properties for this element, overriding the values from the assigned material.

**Section Thickness:** Overrides the shell thickness for this element.

**Material Modulus of Elasticity:** Overrides the Young's modulus (E) of the assigned material for this element.

**Material Poisson's Ratio:** Overrides the Poisson's ratio (ν) of the assigned material for this element.

**Membrane Fxx Modifier (local):** Scale factor applied to the membrane stiffness in the local xx direction. Use values less than 1.0 to reduce membrane stiffness (e.g., for cracked section analysis). Default is 1.0.

**Membrane Fyy Modifier (local):** Scale factor applied to the membrane stiffness in the local yy direction.

**Membrane Fxy Modifier (local):** Scale factor applied to the membrane shear stiffness in the local xy direction.

**Bending Mxx Modifier (local):** Scale factor applied to the bending stiffness in the local xx direction. Use values less than 1.0 to reduce bending stiffness (e.g., for cracked section analysis). Default is 1.0.

**Bending Myy Modifier (local):** Scale factor applied to the bending stiffness in the local yy direction.

**Bending Mxy Modifier (local):** Scale factor applied to the bending twisting stiffness in the local xy direction.


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