Vibration based structural damage detection for structural health monitoring of civil infrastructure system

1. introduction 2.Dynamic testing of structures 3.Overview of vibration-based damage detection 4. Application to a fiber reinforced polymer rehabilitated bridge 5.Extension to prediction of service life 6.Other applications 7.Challenges in Vibration-based SHM

Schematic showing application of SHM Mitigate deterioration and efficiently manage maintenance efforts on structure: 1.Extend the service life of structures 2.monitor performance changes of structure SHM provides the methodology that evaluates the condition of a structure. 1. introduction

1.Occurred 2.Location of damage 3.Damage severity 4.The impact of damage on the structure or the remaining useful life of the structure Schematic of SHM structure Levels for SHM 1. introduction

2.Dynamic testing of structures Sources of excitation: input-output methods  Advantage: Suppress the effects of extraneous noise in the measured structural response  Primary disadvantage: The test is conducted outside conditions of normal operation and thus necessitates disruption in use and does not allow for continuous monitoring of a structure. Impact hammerDrop weight impactor Shaker

2.Dynamic testing of structures Sources of excitation: input-output methods

2.Dynamic testing of structures Sources of excitation: output only methods  Advantage: Cost effective Continuous monitoring  Sources for ambient vibration methods include: Vehicular traffic Wind Pedestrian traffic Ocean waves Low seismic activity

2.Dynamic testing of structures Sources of excitation: output only methods  Assumptions for ambient excitation test method: The excitation forces are assumed to be a stationary random process, having a flat frequency spectrum. The recorder response of the structure alone is sufficient for extraction of modal parameters.

2.Dynamic testing of structures Sources of excitation: output only methods

Vibration Excitation Equipment Quick release device to excite free vibration by pulling the structure and releasing Image courtesy of LANL & Anco Engineers

Vibration Excitation Equipment Pulse load generated by running a car (with pre- determined mass) over a bumper: pulse duration depends on the speed of the car Instrumented impact hammer Bumper Image courtesy of LANL Instrumented impact hammer

Vibration Excitation Equipment Eccentric mass shaker (electrically powered) Electromagnetic shaker Eccentric mass shaker

Vibration Excitation Equipment Servohydraulic linear inertia shaker Linear inertia UCLA Image courtesy of J. Wallace, UCLA & Servotest

2.Dynamic testing of structures Transducers Accelerometers velocity Displacement transducers Acceleration Velocity Displacement Main transducers mass The transducer provides the type of data required for structural assessment.

2.Dynamic testing of structures Experimental modal analysis Time domain Frequency domain Input-output Output only algorithms techniques

3.Overview of vibration-based damage detection A summary of features used to identify, locate and quantify damage in a structure.

3.Overview of vibration-based damage detection

4 Application to a fiber reinforced polymer rehabilitated bridge

Transverse crack Longitudinal crack visual 4 Application to a fiber reinforced polymer rehabilitated bridge Vibration-base damage detection procedure Application of CFRP SHMS Output only modal accelerometers

Sample frequency: 200samples/sec

The acceleration response of the structure is : If N samples are measured the filtered acceleration time history containing, the ith mode is mode is described by:

The output energy matrix is a real semi-positive symmetric matrix, which can be decomposed into:

Time domain decomposition Singular value decomposition

5.Extension to prediction of service life A time-dependent measure of reliability is developed for service life estimation.

LAX Theme Building Monitoring by UCLA EMA (Experimental Modal Analysis) done before & to be done after seismic retrofit of the structure The purpose of EMA is to measure the dynamic properties of a real structure for comparison with and validation of computer models of the structure Mode Frequencies Mode Damping Mode Shapes Transfer Functions Permanent real-time monitoring to be installed for earthquake and SHM research 40

Theme Building Experimental Modal Analysis The LAX Theme Building is a uniquely difficult structure to model: Complex geometry Complex connections Older materials EMA adds confidence to the modeling of earthquake and wind response EMA estimates in-situ damping EMA helps in the design of the proposed TMD system 41

Measurements UCLA’s small shaker, with 10,000 lb maximum force, installed on east side of observation deck. Force set to (100 x f 2 ) lbs. 51 channels of accelerometers installed at 18 locations Very high resolution digital recording to measure ambient through earthquake levels (micro-g to 2g) 42

Sensor Locations 43 Shaker Location

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46 Sensor Recorder

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Data Recording Thursday Oct. 18: Installation Friday Oct. 19: E-W (X) shaking Friday–Sunday: Ambient Vibration, Santa Ana winds on Saturday Oct. 20 evening to 20 mph Monday Oct. 22: N-S and E-W shaking Monday–Friday: Ambient vibration, continuous 48

49 Sample Data: Location 14, observation deck, vertical, 1-hour, ambient & shaking Peak~0.01g

Sample Data, Acceleration (g) 50 Ambient Shaker Sweep Shaker at 2.6Hz

Sample Data, Displacement (inch) 51 Ambient Shaker Sweep Shaker at 2.6Hz

Sample Ambient Vibration Spectra, Top of Core, X and Y Directions 52 First Modes Dominate Core Motion

Results FrequencyShapeDamping, Ambient Damping, Shaker 2.5E-W1%5% 2.7N-S2%5% 4.7Torsion + Legs 5.7Legs 7.0E-W 9.4N-S 53

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Challenges in Vibration-based SHM Many technical challenges are identified in vibration-based structural health monitoring techniques, including Better use of the nonlinear response characteristics of the damaged system Development of methods to optimally define the number and location of the sensors Identification of the features sensitive to small damage levels, The ability to discriminate changes in features cause by damage from those caused by changing environmental and/or test conditions The development of statistical methods to discriminate features from undamaged and damaged structures, Performance of comparative studies of different damage-detection methods applied to common datasets (or benchmark problems). and many others

Equation of Motion (EOM) for MDOF System EOM for MDOF system is a set of ODEs that can be expressed in the following matrix form where, M, C, K are the mass, damping and stiffness matrices of the MDOF system (e.g., a multi-story building structure) respectively. L is the identity vector with all its components equal to one.