Natural frequencies
The frequencies at which the structure tends to vibrate.
Technical guide · Structural monitoring
Modal analysis and dynamic identification of structures
Identifying how a structure characteristically vibrates from measured vibrations.
Modal analysis returns the dynamic parameters of a structure in one or more measurement campaigns; it is not the same as continuous monitoring, which follows quantities over time.
What modal analysis is
Every structure has natural frequencies and mode shapes describing how it responds to vibration. Modal analysis identifies them from accelerations measured with accelerometers.
With operational modal analysis (OMA) identification takes place under service conditions, using ambient excitation; techniques include Stochastic Subspace Identification (OMA-SSI).
What it can identify
Mode shapes
The deformed configuration associated with each mode.
Damping
Estimated dissipation associated with the identified modes.
Dynamic response
Reading the behaviour of the structure under operational excitation.
Comparison with the FEM model
Where relevant, comparing measured parameters with the numerical model to calibrate it.
Baseline for later observations
Reference parameters to compare with future campaigns or monitoring.
From measurement to identification
- 01Objective and measurement layout
- 02Accelerometer placement
- 03Vibration acquisition
- 04Processing (OMA-SSI)
- 05Modal parameter identification
- 06Optional FEM comparison
Modal analysis and continuous monitoring
- Modal analysis identifies parameters in a campaign
- Dynamic monitoring follows the response over time
- The two can be combined when the question requires it
Identified modal parameters must be read in the context of the structure, the measurement layout and the operating conditions at the time of acquisition.
Documented cases
Bridge deck — OMA-SSI with Artemis
Identification of a deck's frequencies and mode shapes from operational vibrations.
Historic tower — FEM model with Midas
Mode shapes and natural frequencies of the numerical model for comparison with measured parameters and calibration.
Related methods
Further topics in the monitoring cluster and the comparison with point-in-time diagnostics.
Dynamic monitoring
Vibration response observed over time.
Static monitoring
Deformations, rotations and cracks over time.
SHM sensor networks
Designing and managing the system.
Non-destructive testing
Point-in-time surveys of materials and geometry.
Frequently asked questions
- What does modal analysis measure?
- It identifies a structure's natural frequencies, mode shapes and damping from vibrations measured with accelerometers.
- What is OMA-SSI?
- An operational modal analysis technique that identifies modal parameters from vibrations under service conditions, without dedicated artificial excitation.
- Why compare with a FEM model?
- Where relevant, comparing measured and computed parameters makes it possible to check and calibrate the structure's numerical model.
- Are modal analysis and dynamic monitoring the same thing?
- No. Modal analysis identifies dynamic parameters in one or more campaigns; dynamic monitoring observes the structure's response over time and may use modal analysis as a processing tool.