When simulating dynamic events in ANSYS, engineers often choose between Transient Structural Analysis and Rigid Body Dynamics (RBD). While both solve time-dependent problems, they are fundamentally different in assumptions, solver strategy, and application.

In this article, we’ll break down the differences, use cases, and when to combine both methods for a powerful simulation workflow.

What is Transient Structural Analysis?

Transient Structural Analysis solves how flexible bodies deform over time under dynamic loads. It calculates:

• Displacements
• Velocities
• Accelerations
• Stresses and strains

The solver accounts for mass (inertia), damping, stiffness, and can include nonlinearities such as plastic deformation, contact, and large deflections.

It’s ideal for problems where structural response matters, like:

• Drop tests
• Blast and crash simulations
• Time-dependent thermal loading
• Vibration analysis of flexible components

What is Rigid Body Dynamics?

Rigid Body Dynamics (RBD) assumes all parts are perfectly rigid—no deformation occurs. It focuses purely on motion:

• Translation and rotation of bodies
• Reaction forces at joints
• Contact impacts and friction

The solver is built for large rotational motion, complex joint systems, and multibody mechanisms, using advanced integrators like the Moreau-Jean scheme.

RBD is the go-to choice for:

• Robotic linkages and mechanisms
• Gear assemblies
• Suspension systems
• Motion studies with contacts and joints

Key Differences at a Glance

Feature	Transient Structural	Rigid Body Dynamics
Body Behavior	Flexible (deforms under load)	Rigid (no deformation)
Output Focus	Stress, strain, deformation	Motion, contact forces, joint loads
Nonlinear Capabilities	Large deflection, contact, plasticity	Geometric nonlinearity only
Solver Strategy (Implicit vs Explicit analysis)	Implicit/Explicit (Newmark, HHT)	Moreau-Jean, Lagrange multipliers
Best For	Structural integrity, detailed stress	Motion simulation, multibody systems

Combined Workflow: Best of Both Worlds

Many engineers run both RBD and transient structural in a hybrid workflow:

1. Start with Rigid Body Dynamics
   Simulate the full motion and extract joint/contact forces.
2. Transfer Results
   Export forces as time-dependent loads.
3. Run Transient Structural Analysis
   Apply loads to flexible components and analyze stresses and failures.

This workflow is faster and more accurate, avoiding unnecessary mesh deformation computations for parts that don’t need them.

When to Use Each

Use Transient Structural if:

• You need stress, strain, or deformation results.
• The structural response affects performance or safety.
• You’re simulating soft materials or large deflections.

Use Rigid Body Dynamics if:

• You’re analyzing mechanisms with rigid links and joints.
• Motion and forces are the primary concern.
• You want efficient simulation of large, rotating assemblies.

Final Thoughts

Choosing between Transient Structural and Rigid Body Dynamics depends on what you care about most: stress or motion.

For complex projects—like vehicle crash tests, robotic motion, or blast impacts—combining both methods gives you the best balance between performance and fidelity.

Let ANSYS do the heavy lifting: rigid where you can, flexible where you must.

Want to Go Deeper?

If you’re serious about mastering simulation workflows in ANSYS, check out our eBook:

All Models Are Wrong: Structural Analysis with Ansys Workbench (Third Edition) available for $12.50.

It’s a practical, example-driven guide that goes beyond theory — covering real-world modeling decisions, convergence issues, boundary condition tricks, and validation techniques.

Check out the free preview : All Models are Wrong