Modeling with Confidence: Understanding Nodes in RISA-3D

Nodes may be small, but they have an outsized impact on your RISA-3D model. They control how loads are transferred through your model. Missing or misplaced nodes can change analysis results if you don’t understand how they influence element behavior. Here are some of the most common ways nodes can affect your design behind the scenes. 

1. Plate Loading & Framing

How it works: 

Plates only transfer load and stiffness through their corner nodes. 

Key considerations: 

• Apply loading as plate surface loads or as nodal loads at the corners
• Framing elements, such as beams or adjacent plates, must connect at corner nodes to transfer load correctly
• Check out the articles below to learn more about plates interacting with other elements

Tip: 

Use the Auto Submesh Plates tool to mesh the plate into smaller sections and make sure there is a corner node at crossing members. This tool can be used after the plate is already drawn or automatically while drawing in the plate.  

2. Crossing Member Interaction

How it works: 

Members can only transfer load at node locations. Members will behave independently if a node is not located at an intersection. 

Key considerations: 

•  Add a node at the intersection if crossing members should be transferring load based on your design  

Tip: 

Use the Model Merge tool to automatically add nodes at member intersections.

3. Rigid Diaphragm Behavior

How it works: 

RISA-3D connects all the nodes at the diaphragm plane together using a hidden set of rigid links. 

Key considerations: 

•  Determine what diaphragm behavior aligns with your design intent

• If the beams are tied into the diaphragm, add nodes along the member to provide additional points of rigid connection

• If specific nodes in the plane are not supposed to be rigidly connected, use the Detach From Diaphragm checkbox in the Node Coordinates Spreadsheet to exclude that node from the hidden rigid links

Tip: 

Use the Add Nodes tool to add nodes along a member without needing to draw multiple members. 

4. Unbraced Length Calculations

How it works: 

RISA-3D assumes members are fully unbraced unless specified otherwise. The Segment command is a quick way to limit the unbraced length by assigning brace points at node locations.  

Key considerations: 

• Review the Help File for additional information on unbraced lengths and the Segment command.
• Use Segment for the unbraced length where applicable such as columns or girders with beams framing in
• Extra nodes without framing members can unintentionally reduce the unbraced length and unconservatively increase capacity

Tip: 

Review members using the Segment command for stray nodes that might be inaccurately bracing your member. 

5. P-Delta Analysis

How it works: 

RISA-3D uses the Standard Nodal Shear Method to approximate the effects of P-Delta on the member analysis. This method relies on the end-joint displacements which can be referred to as P-Big Delta analysis. P-Little Delta forces are based on the rotation of the member along its length.  

Key considerations: 

• Check in the Help File to get more detail on the analysis and assumptions
• Node placement influences how the internal rotation is captured by providing another consideration point for the program

Tip: 

To better account for P-Little Delta, strategically add nodes where displacement is greatest along the column.  

6. Thermal Load Application

How it works: 

Node temperatures assigned in the Node Coordinates Spreadsheet serve as the ambient temperature of the structure. 

Key considerations: 

• Ambient temperatures may vary over the structure causing stress in the members, plates, and wall elements
• Add thermal loads to the members, plates, or walls to induce additional stress in the elements. Stress will be based on the temperature difference between the applied load and ambient node temperature
  

Tip: 

Use thermal loads to simulate prestress loading in your members by defining intentional temperature differentials. Learn more about this application in the Help File below.