> For the complete documentation index, see [llms.txt](https://docs.enercalc.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.enercalc.com/sel-faq/using-the-software/calculation-modules/analysis/2-d-frame-analysis/assigning-proper-fixities.md). # Assigning Proper Fixities How to set joint restraints and member end releases for trusses, moment frames, and mixed-fixity 2D frame models. ## How should I set the fixities in a model that represents a truss with pinned joints? When working with **truss models** with pinned joints, consider setting the following Joint Boundary Restraints: | **Typical** | Unrestrained | Unrestrained | Restrained | Although rotation at the joints is restrained, moment transfer is prevented by member end releases. | | ------------- | ------------ | ------------ | ---------- | --------------------------------------------------------------------------------------------------- | | **Support 1** | Restrained | Restrained | Restrained | | | **Support 2** | Unrestrained | Restrained | Restrained | | Even though the support joints are modeled as Restrained against rotation, all members framing into those joints will have their Local z Rotational degree of freedom released at both ends, so no moment will be delivered to the supports from the members. There must be at least two "support joints" to create global stability of the entire truss. Typically, we see **two** support joints with Y translational restraints and **one** support joint with X translational restraint, but many conditions are possible. Next, consider the following settings the Member End Releases: | **Typical** | Unreleased | Unreleased | Released | Members are hinged at the joints. | | ----------- | ---------- | ---------- | -------- | --------------------------------- | ## How should I set the fixities in my moment frame model with fixed bases? When working with portal frames with moment connected joints and **fixed bases**, consider setting the following Joint Boundary Restraints: | **Typical** | Unrestrained | Unrestrained | Unrestrained | Allows joints to freely move in X and Y and rotate about Z. | | ------------ | ------------ | ------------ | ------------ | ----------------------------------------------------------- | | **Supports** | Restrained | Restrained | Restrained | Fixed base. | Consider the following settings the Member End Releases: | **Typical** | Unreleased | Unreleased | Unreleased | Members are continuous through the joints. | | ----------- | ---------- | ---------- | ---------- | ------------------------------------------ | ## How should I set the fixities in my moment frame model with pinned bases? When working with portal frames with moment connected joints and **pinned bases**, consider setting the following Joint Boundary Restraints: | **Typical** | Unrestrained | Unrestrained | Unrestrained | Allows joints to freely move in X and Y and rotate about Z. | | ------------ | ------------ | ------------ | ------------ | ----------------------------------------------------------- | | **Supports** | Restrained | Restrained | Unrestrained | Pinned base. | Consider the following settings the Member End Releases: | **Typical** | Unreleased | Unreleased | Unreleased | Members are continuous through the joints. | | ----------- | ---------- | ---------- | ---------- | ------------------------------------------ | ## How should I set the fixities in a model that includes a mix of some pin-connected members and some continuous members? When working with models with a mix of fixity conditions, consider setting the following Joint Boundary Restraints: | **Typical** | Unrestrained | Unrestrained | Unrestrained | Allows joints to freely move in X and Y and rotate about Z. | | ----------- | ------------ | ------------ | ------------ | ----------------------------------------------------------- | Consider the following settings the Member End Releases: | **Typical** | Unreleased | Unreleased | Released | Members are hinged at the joints. | | -------------- | ---------- | ---------- | ---------- | ------------------------------------------ | | **Continuous** | Unreleased | Unreleased | Unreleased | Members are continuous through the joints. | The next step is to achieve stability. Review the model and check for any joints where the joint boundary restraints are Unrestrained – Unrestrained – Unrestrained. These joints **must** be rigidly connected to at least one member end with an Unreleased Local z Rotational fixity. If any such nodes are found, they must be stabilized by ensuring that **at least one member end** connecting to the joint has an Unreleased Local z Rotational fixity. {% hint style="info" %} **NOTE:** Be cautious during this process. If more than one member end at a joint is Unreleased for Local z Rotation, continuity forms at that location, and the joint no longer behaves as fully pinned. Only one member end must be rigidly connected to the joint to ensure stability while preserving the fully hinged behavior. {% endhint %} Finally, you must focus on the support nodes. If **pinned** supports are desired, consider setting the following Joint Boundary Restraints: | **Pinned Supports** | 0 | Restrained | Restrained | Restrained | | ------------------- | ---------- | ---------- | ------------ | ---------- | | 1+ | Restrained | Restrained | Unrestrained | | If **fixed** supports are desired, consider setting the following Joint Boundary Restraints: | **Fixed** **Supports** | Restrained | Restrained | Restrained | | ---------------------- | ---------- | ---------- | ---------- | Just make sure that the appropriate member ends are modeled with Unreleased Local z Rotational fixities in order for those members to be able to transfer moment to the fixed bases. ## How can I model partially fixed bases in a frame model? At present there is no explicit method of defining a base with partial moment fixity, but it can still be modeled with a little creativity. Consider modeling something like the concept shown below: ![](https://1076748670-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2FtBL2fEQRwrlKSM2irFL0%2Fuploads%2Fgit-blob-698251d75f6530d6e99547bdb34fc532fad8136a%2Fclip0003.jpg?alt=media) In this model, members A and E are fictitious members that are only there for the purpose of providing partial moment fixity at the bottoms of members B and D. With this concept, it would be possible to experiment with the section and the member length for members A and E to achieve the desired rotational stiffness at joints 1 and 4. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. 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