Dynamic Analysis in RISA-3D is a critical finite element analysis framework used by structural engineers to simulate how complex, multi-degree-of-freedom structures react to real-world, time-dependent lateral loads. Rather than treating seismic events, heavy machinery vibrations, or blasts as static forces, dynamic analysis evaluates a structure’s changing inertia, stiffness, and damping over time. Core Analysis Phases
Solving a dynamic model in RISA-3D typically involves a multi-stage workflow: 1. Dynamic Modal Eigensolution
Purpose: Identifies the structure’s natural frequencies, fundamental periods, and mode shapes (deformation patterns).
Mass Definition: RISA-3D automatically converts vertical loads from a chosen load combination into a lumped nodal mass matrix using gravity acceleration (g). Custom joint masses can also be added manually.
Code Compliance: Per building codes like ASCE 7, engineers must extract enough modes to achieve at least 90% cumulative mass participation in the lateral directions. 2. Response Spectrum Analysis (RSA)
Purpose: Determines peak design forces, drifts, and deflections under earthquake loads.
Mechanism: It matches the extracted mode shapes against a specific design response spectrum curve.
Force Scaling: Once the overall dynamic response is calculated, forces are scaled appropriately to meet code-minimum base shear requirements. 3. Time History Analysis
Purpose: Simulates the step-by-step physical response of a structure subjected to specific, time-varying acceleration records (e.g., historical earthquake data or equipment vibrations).
Mechanism: Solves the complete equation of motion over a defined timeline. Basics of Dynamic Analysis in RISA-3D
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