Nastran Grid Point Force Balance

Engineering Task

Verifying equilibrium in a Nastran model and extracting interface loads for use in local detailed models.

Software Implementation

• Request GPFORCE output in Case Control: GPFORCE=ALL or GPFORCE=SET
• The F06 file lists the force balance at each node: element forces, applied loads, SPC forces and inertia forces
• The sum at each node must be zero (equilibrium) — non-zero indicates a modelling error
• Use the free-body load extraction tool (e.g. in Patran or FEMAP) to sum forces across a cut section

Key Uses

• Verify that reaction forces (SPCFORCE) balance applied loads — the most fundamental check
• Extract interface loads at a cut section for use as boundary conditions on a local model
• Identify load path — which elements carry the most force at a node
• Detect hidden modelling errors — disconnected nodes or wrong load application

Related Knowledge

• FEA Best Practices: Model Verification
• FE Model Verification Checklist