How to Assign Fixed Support in ETABS

Assigning a fixed support in ETABS is a straightforward process that ensures your structural model accurately reflects real-world conditions where a joint is fully restrained against all movement. This crucial step is essential for accurate structural analysis and design.

To apply a fixed support, you will navigate through the assignment menu after selecting the relevant joints.

Step-by-Step Guide to Assigning Fixed Support

Follow these steps to successfully assign fixed supports to the joints in your ETABS model:

Select the Joints: First, you need to identify and select the specific joint(s) in your model where you want to apply the fixed support. These are typically the base joints of columns or walls that connect to the foundation.
- You can select individual joints by clicking them.
- For multiple joints, use a window selection or select them by properties.
- Ensure no other elements (like beams or columns) are accidentally selected.
Access the Assignment Menu: Once the desired joints are selected, go to the main menu bar at the top of the ETABS interface.
- Click on "Assign".
Navigate to Joint Restraints: From the "Assign" dropdown menu, hover over "Joint".
- Then, click on "Restraints (Supports)". This will open the Joint Restraints dialog box.
Choose Fixed Support: In the Joint Restraints dialog box, you will see several predefined support types.
- Click on the "Fixed Support" icon. This icon typically depicts a solid triangle with all translational and rotational degrees of freedom locked (represented by small 'x' marks or locked symbols next to Tx, Ty, Tz, Rx, Ry, Rz). Selecting this option will automatically check all six restraint boxes (three translations and three rotations).
Apply the Support:
- After selecting the fixed support, click "Apply" to see the changes immediately without closing the dialog, or click "OK" to apply the support and close the dialog box.

Once you click "OK," the fixed support will be applied to the selected joint(s), indicated by the specific symbol appearing at the base of the supported elements.

Understanding Fixed Support

A fixed support (also known as a rigid support or encastered support) is a type of structural support that completely restricts all forms of movement at the joint. This means:

No Translation: The joint cannot move in the X, Y, or Z directions (Tx, Ty, Tz are restrained).
No Rotation: The joint cannot rotate about the X, Y, or Z axes (Rx, Ry, Rz are restrained).

In practical terms, fixed supports are idealizations of real-world connections that are designed to resist both forces and moments. Examples include:

Column bases rigidly connected to a large, stiff foundation.
Beams rigidly connected to very stiff columns or walls.
Cantilevered structures at their point of connection.

Verifying Support Assignment

After applying the supports, it's good practice to verify them:

Visual Inspection: Look at the base of the elements where you assigned supports. ETABS displays distinct symbols for different support types. A fixed support will have a unique symbol, typically a solid triangle with additional lines indicating moment resistance.
Display Options: You can also go to Display > Show Object Load Assigns > Joint > Restraints to clearly see the applied restraints.
Check Joint Properties: Select a supported joint, right-click, and open the "Joint Object Information" window. Go to the "Assignments" tab to confirm the restraints.

Common Support Types in ETABS

Understanding the different support types is crucial for accurate modeling. Here's a brief comparison:

Support Type	Translational Restraints	Rotational Restraints	Description
Fixed	All (Tx, Ty, Tz)	All (Rx, Ry, Rz)	Prevents all movement and rotation. Transmits forces and moments. Ideal for rigid connections to stiff foundations.
Pinned	All (Tx, Ty, Tz)	None	Prevents all movement but allows rotation. Transmits forces but no moments. Often used for simple connections or where a hinge is desired.
Roller	Two (e.g., Ty, Tz)	None	Prevents movement perpendicular to the rolling surface and possibly vertically, but allows movement along the surface and rotation. Transmits forces only in restrained directions. Used to account for thermal expansion or movement along a specific axis.
User	Customizable	Customizable	Allows the user to select specific translational and rotational degrees of freedom to restrain or free. Useful for complex boundary conditions.

Best Practices

Model Simplification: While real-world connections are rarely perfectly fixed, using a fixed support is often a valid and conservative simplification for rigid connections to a large, stiff foundation.
Check Releases: Ensure that if you've applied fixed supports, you haven't accidentally applied any joint releases at the same location, which could contradict your support assignment.
Review Assumptions: Always review your support assumptions based on actual structural behavior and connection details.

By following these steps, you can confidently assign fixed supports in your ETABS models, laying the groundwork for a robust and accurate structural analysis.