Spider element connections

Video: Create a spider mesh

A spider type of connection is one in which you connect a single node (the core node) to multiple nodes (leg nodes) with a rigid or constraint element. The types of elements you can use to create a spider connection depends upon your specified solver.

When you create a spider connection:

• The first point you select becomes the core node.

• The selected edge or face define the locations of the leg nodes. A group of smart selection methods is available on the Top Border bar to help you select the leg nodes.

After you create the spider connection, you can make specific degrees of freedom active or inactive by editing the element attributes.

1D Connection types that create spider type connections

In the 1D Connection dialog box, the following options in the Type list allow you to create spider type connections.

• You can use the Point to Edge or Point to Face options to connect a single node (the core node) to multiple nodes (leg nodes) with a rigid or constraint type element,

• You can also use the Point to Point or Node to Node options to define a spider element. These connections provide more control over leg node locations and are especially useful when using spider elements to distribute mass or loads.

• In the Simcenter Nastran or MSC Nastran environments, you can use the Edge to Edge option to use a combination of RB2 and RBE3 elements to connect a source edge to a target edge.With the RBE2 and RBE3 to Element Edge option, the software projects the nodes from the source edge onto the target element edge along a direction that is normal to the source edge. The software then:Creates an RBE2 element from each source node to each projected node on the target element edge.Determines the element edge on which each projected node lies. The software then creates an RBE3 element between each projected node (this node becomes the dependent node) and the nodes on that element edge (these nodes become the leg nodes). If a projected node is coincident with a node on the target edge, then the software creates an RBE3 element between the projected node and the nodes of all element edges that are connected to the coincident node.With the RBE2 and RBE3 to Element Face option, the software projects the nodes from the source edge onto the target element free face. The software then:Creates an RBE2 element from each source node to each projected node on the target element edge.Determines the element face on which each projected node lies. The software then creates an RBE3 element between each projected node (this node becomes the source or dependent node) and all the nodes on the nodes on the element’s face (these nodes become the leg nodes). If a projected node is coincident with a node on the target edge, then the software creates an RBE3 element between the projected node and the nodes of all element edges that are connected to the coincident node.

Arcs to Centers connection method for Point-to-Edge connections

When Type is set to Point to Edge in the 1D Connection dialog box, you can click Arcs to Centers to create a connection between the edge of a circular or non-circular loop and its center point. With this option, you select the edge of the loop and the software automatically selects its center point. This option is useful, for example, when you want to create spider type elements, such as Nastran RBE3 elements, between the center of a hole and the edge of a hole.

Circular edge selected using the Arcs to Centers method.	Resulting Simcenter Nastran RBE3 element connecting the selected edge to the center point of the hole.

If you use the Arcs to Centers option to select a non-circular closed loop, the software tries to automatically compute the location of its center. For example:

• For loops consisting of a single circle or for single arcs, the software uses the center of the arc or circle.

• For loops that consist of multiple edges, none of which is an arc, the software internally constructs an single edge and takes the average coordinates of a few equidistant points along that edge.

Note:

The software cannot automatically calculate the center of all non-circular loops.

Proximity Spider connection method

In the 1D Connection dialog box, when you create a Node to Node connection, you can use the Proximity Spider option in the Method of Connection list to create spider type constraint element connections between core nodes and leg nodes based on the relative proximity of the leg nodes to the core node.

• Use the Select Core and Leg Nodes option to create one or more connection elements in holes where the core nodes already exist. You select the core node and leg nodes and specify a Search Distance. The software creates spider connection element between every core node and the leg nodes that lie within the specified Search Distance.The same leg node can connect to multiple core nodes if it lies within the Search Distance of those core nodes. If the software does not find any leg nodes within the search distance, it does not create a spider element for that core node.

• Use the Select Leg Nodes only option to create a single connection element in a hole where the core node does not already exist. The software determines the location of the core node by calculating the average location of the leg nodes. For example, you can use this option to create a spider element where the leg nodes lie on the free edges of elements that define a hole:Selected leg nodesResulting spider element with the location of the core node determined automatically by the software

Typical applications for spider connections

Typical applications for spider elements include:

• Representing a pin in a hole. In the example below, the pin is modeled using two point-to-face connections and a node-to-node beam element. The core node of the spider element is defined at the center of the hole, and the leg nodes connect to the mesh on the inner face of the hole.

• Representing a bolt. In the example below, the head of the bolt is modeled with a point-to-edge connection and the shank is modeled with a beam element. After modeling the bolt, you can apply a pretension load using the bolt pre-load boundary condition. For information about bolt pre-load, see Bolt pre-load.

• To add and distribute a mass or a load. In the motorcycle example below, a node-to-node connection of RBE3 elements distributes the rider's mass (represented by the concentrated mass element) to the seat and handlebars. The RBE3 is used in this example because it includes the mass without adding stiffness.

Modeling bolts and pins using geometry-based 1D connections

1D Connection allows you to use the Snap Point Tool to select points when defining Point to Edge, Point to Face, and Point to Point connections. To quickly and easily define the core node location for a spider element at a bolt hole, for example, you can use Arc Center to create a point at the center of a circular hole. When you use this method, the software first creates the centerpoint, and then creates a node at that point. When you generate the spider element, this node becomes the core node.

Creating a centerpoint using the Arc Center method

However, you must exercise caution when combining geometry-based spider elements with geometry-based beams used to model a bolt or pin. If you use the Point to Point connection type with the Arc Center point method to define the beam element representing the shaft, the software creates another point and node coincident with the core node. This node is not connected to your spider element. To ensure that the beam element is connected to the spider element:

• Use the Existing Point point method to reselect the centerpoint.

• Use the Node to Node 1D connection type to create a beam element connecting the core nodes.

How do I

Create a node-to-node connection

Create an element edge to element face connection

Create a point-to-point connection

Create a point-to-edge connection

Create a point-to-face connection

Create an edge-to-edge connection

Create an edge-to-face connection

Create a face-to-face connection

Learn more

1D Connection

Edge-face 1D connections

Point-to-point and node-to-node connections

Working with RBE2 and RBE3 spider elements

1D mesh to face connections

Understanding Samcef RBE and MEAN elements (Simcenter Samcef)

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

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Spider element connections, Simcenter 3D 2021.1 Series

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