# 3.4.3: Line-Joint

The **"Line-Joint"** component allows the definition of linear hinges within or at the boundaries of shell patches. **Figure 3.4.3.1** illustrates an example with two shell patches, **"A"** and **"B,"** each composed of two shell elements. Patch **"A"** is fully fixed on one side and connected to **"B"** via a linear joint, represented by a purple cylinder. **Shell "B"** can rotate around the joint’s **X-axis**, indicated by the red arrow. A line joint introduces additional degrees of freedom (DOFs) at the connected vertices—one for each hinge DOF.

 

<figure><img src="/files/p22X3lHmj6R2wOF1ivVB" alt=""><figcaption><p>Fig. 3.4.3.1: Line-joint symbolized by a purple cylinder between two shell patches "A" and "B"</p></figcaption></figure>

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## Inputs for Defining Non-Rigid Shell Connections:

The "Line-Joint"-component provides these inputs to specify the non-rigid connections between shells:

* **"J-Curve"**: A line-like curve lying on the nodes of the shell that are part of the joint. The nodes do not have to be the endpoints of the curve. The curve's direction defines the joint’s **X-axis**.
* **"Elem|Id"**: Specifies the shell patch or its identifier where the joint should be placed. If left empty (default), the joint can be attached to all shells in the model.
* **"Y-Ori"**: Defines the local **Y-axis** of the joint, pointing towards the shell patch to which the joint is attached. When **"Joints"** is enabled in the **"ModelView"** component, the **Y-axis** appears as a green arrow on the hinge line. The size of the axes can be adjusted using the **"Local Axes"** slider in **"ModelView"**.
  * By default, **"Y-Ori"** is a zero-vector, which is sufficient in most cases, such as for naked edges or surfaces without additional attachments. Explicit **Y-orientation** is only needed in more complex scenarios where multiple placement options exist.
  * The shell where the line-joint is placed is indicated by a **purple cylinder**. If multiple line joints with **zero rotational stiffness in the X-direction** are present along a line, at least one surface must remain without a joint. Otherwise, a rigid rotation of the DOFs at the joint line may occur.
  * If the **Y-direction** of the joint changes along its length, specify its direction at the **start point**, and it will be automatically updated across elements.
* **"Z-Ori"**: If the joint’s **Y-axis** is parallel to the joint-line direction, the right-handed local coordinate system is defined by the **X-direction** of the joint line and the **Z-orientation**. The joint is attached to the elements that the **Y-axis** points toward. When displayed in **ModelView**, the **Z-axis** appears as a blue arrow. Explicit joint orientation is generally only necessary for geometrically complex connections.
* **"DAlpha"**: Specifies the maximum angle **(in degrees)** between a mesh face and **Y-Ori** for the joint to be applied.
* **"Ct"**: A vector defining the **translational spring stiffness** of the line joint. Values only apply if the corresponding DOF is set to **hinged** in the **"Dofs"** input or via the **"Joint Definition"** submenu radio buttons.
* **"Cr"**: A vector for **rotational spring stiffness**, functioning similarly to **"Ct"**. In complex setups with multiple line joints, assigning a small stiffness for **rotation around the local X-axis** helps prevent rigid body modes at the connection nodes without significantly affecting the structural response.
* **"Dofs"**: A list of DOF indices to be released, in addition to those selected via checkboxes in the **"Joint Definition"** submenu. The DOF indices are as follows:
  * **0: Tx** (Translation in X)
  * **1: Ty** (Translation in Y)
  * **2: Tz** (Translation in Z)
  * **3: Rx** (Rotation about X)
  * **4: Ry** (Rotation about Y)
  * **5: Rz** (Rotation about Z)
  * Use a **"ValueList"** component for convenient selection or enable **"Expand ValueLists"** in the component’s context menu.

The **radio buttons** under **"Joint Definition"** indicate which DOFs are released when enabled. To visualize the joint’s **local coordinate system**, enable **"Joints"** in the **"Display Scales"** submenu of **"ModelView"**. The **"Local Axes"** slider in the same submenu allows for scaling the displayed arrows.

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