False Local Modes in Seismic Analysis

What are false local modes

False local modes are modes that appear in the modal extraction but do not represent meaningful dynamic response. They may arise from trivial masses (e.g. a small mass on a stiff spring producing a high-frequency local mode), from modelling artefacts (e.g. a poorly constrained node), or from mechanisms in the model. False local modes can inflate the mode count without contributing to mass participation, and can distort modal combination results if they have non-trivial participation.

Identification

False local modes are identified by examining the mode shapes and the effective modal mass. A mode with a very low effective modal mass in all directions is likely a local mode. A mode shape that shows deformation concentrated at a single node or a small region, while the rest of the structure is stationary, is a local mode. A mode at an unexpectedly high frequency, or a mode that appears only in one part of the model, should be investigated.

Causes

• Trivial masses on stiff springs (e.g. a small point mass on a very stiff element) producing high-frequency local modes
• Unconstrained or poorly constrained nodes that produce mechanism modes
• Incompatible element connections that create local flexibility
• Excessively refined mesh in a local region creating local modes that are not physically meaningful
• Mass assigned to nodes that should not carry mass (e.g. mass on a reference node for a rigid link)

Handling

False local modes should be removed from the modal combination if they do not contribute to the seismic response. This can be done by excluding modes with effective modal mass below a threshold, by removing the trivial mass that causes the mode, or by using a frequency cutoff that excludes modes above the rigid boundary. If the mode is a modelling artefact, the model should be corrected rather than the mode excluded.

Prevention

• Do not assign mass to reference nodes or nodes that are connected by rigid links to mass-carrying nodes
• Ensure all nodes are adequately constrained — no free nodes or mechanisms
• Use a frequency shift in the eigensolver to avoid extracting modes below a minimum frequency
• Review the mode shapes and effective masses before accepting the modal results