Comparison of damage identification algorithms on experimental modal data from a bridge

Over the past 25 years detecting damage in a structure from changes in dynamic parameters has received a considerable amount of attention from the aerospace, civil, and mechanical engineering communities. The general idea is that changes in the structure`s physical properties (i.e., stiffness, mass, and/or damping) will, in turn, alter the dynamic characteristics (i.e., resonant frequencies, modal damping, and mode shapes) of the structure. Properties such as the flexibility matrix, stiffness matrix, and mode shape curvature, which are obtained from modal parameters, have shown promise for localizing structural damage. In this paper, several different techniques for damage assessment are demonstrated and compared using experimental modal data from an undamaged and damaged bridge

Lessons learned from applications of vibration-based damage identification methods to a large bridge structure

Over the past 30 years detecting damage in a structure from changes in dynamic parameters has received considerable attention from the aerospace, civil, and mechanical engineering communities. The general idea is that changes in the structure`s physical properties (i.e., stiffness, mass, and/or damping) will, in turn, alter the dynamic characteristics (i.e., resonant frequencies, modal damping, and mode shapes) of the structure. Properties such as the flexibility matrix, stiffness matrix, and mode shape curvature, which are obtained from modal parameters, have shown promise for locating structural damage. However, the application of these techniques to large civil engineering structures is limited because of the inability to find structures that the owners will allow to be damaged. Also, the cost associated with testing these structures can be prohibitive. In this paper, the authors` experiences with performing modal tests on a large highway bridge, in its undamaged and damaged state, for the purpose of damage identification will be summarized. Particular emphasis will be made on the lessons learned from this experience and the lessons learned from recent tests on another bridge structure

Field verification of a nondestructive damage location algorithm

Over the past 25 years, the use of modal parameters for detecting damage has received considerable attention from the civil engineering community. The basic idea is that changes in the structure`s properties, primarily stiffness, will alter the dynamic properties of the structure such as frequencies and mode shapes, and properties derived from these quantities such as modal-based flexibility. In this paper, a method for nondestructive damage location in bridges, as determined by changes in the modal properties, is described. The damage detection algorithm is applied to pre- and post-damage modal properties measured on a bridge. Results of the analysis indicate that the method accurately locates the damage. Subjects relating to practical implementation of this damage identification algorithm that need further study are discussed

Computation of structural flexibility for bridge health monitoring using ambient modal data

Issues surrounding the use of ambient vibration modes for the location of structural damage via dynamically measured flexibility are examined. Several methods for obtaining the required mass- normalized dynamic mode shapes from ambient modal data are implemented and compared. The method are applied to data from a series of ambient modal tests on an actual highway bridge. Results indicate that for the damage case examined, the flexibility from the ambient mode shapes gave a better indication of damage than the flexibility from the forced-vibration mode shapes. This improved performance is attributed to the higher excitation load levels that occur during the ambient test

A probabilistic risk-based decision framework for structural health monitoring

Obtaining the ability to make informed decisions regarding the operation and maintenance of structures, provides a major incentive for the implementation of structural health monitoring (SHM) systems. Probabilistic risk assessment (PRA) is an established methodology that allows engineers to make risk-informed decisions regarding the design and operation of safety-critical and high-value assets in industries such as nuclear and aerospace. The current paper aims to formulate a risk-based decision framework for structural health monitoring that combines elements of PRA with the existing SHM paradigm. As an apt tool for reasoning and decision-making under uncertainty, probabilistic graphical models serve as the foundation of the framework. The framework involves modelling failure modes of structures as Bayesian network representations of fault trees and then assigning costs or utilities to the failure events. The fault trees allow for information to pass from probabilistic classifiers to influence diagram representations of decision processes whilst also providing nodes within the graphical model that may be queried to obtain marginal probability distributions over local damage states within a structure. Optimal courses of action for structures are selected by determining the strategies that maximise expected utility. The risk-based framework is demonstrated on a realistic truss-like structure and supported by experimental data. Finally, a discussion of the risk-based approach is made and further challenges pertaining to decision-making processes in the context of SHM are identified

DIAMOND: A graphical interface toolbox for comparative modal analysis and damage identification

This paper introduces a new toolbox of graphical-interface software algorithms for the numerical simulation of vibration tests, analysis of modal data, finite element model correlation, and the comparison of both linear and nonlinear damage identification techniques. This toolbox is unique because it contains several different vibration-based damage identification algorithms, categorized as those which use only measured response and sensor location information (non-model-based techniques) and those which use finite element model correlation (model-based techniques). Another unique feature of this toolbox is the wide range of algorithms for experimental modal analysis. The toolbox also contains a unique capability that utilizes the measured coherence functions and Monte Carlo analysis to perform statistical uncertainty analysis on the modal parameters and damage identification results. This paper contains a detailed description of the various modal analysis, damage identification, and model correlation capabilities of toolbox, and also shows a sample application which uses the toolbox to analyze the statistical uncertainties on the results of a series of modal tests performed on a highway bridge

Effects of measurement statistics on the detection of damage in the Alamosa Canyon Bridge

This paper presents a comparison of the statistics on the measured model parameters of a bridge structure to the expected changes in those parameters caused by damage. It is then determined if the changes resulting from damage are statistically significant. This paper considers the most commonly used modal parameters for indication of damage: modal frequency, mode shape, and mode shape curvature. The approach is divided into two steps. First, the relative uncertainties (arising from random error sources) of the measured modal frequencies, mode shapes, and mode shape curvatures are determined by Monte Carlo analysis of the measured data. Based on these uncertainties, 95% statistical confidence bounds are computed for these parameters. The second step is the determination of the measured change in these parameters resulting from structural damage. Changes which are outside the 95% bounds are considered to be statistically significant. It is proposed that this statistical significance can be used to selectively filter which modes are used for damage identification. The primary conclusion of the paper is that the selection of the appropriate parameters to use in the damage identification algorithm must take into account not only the sensitivity of the damage indicator to the structural deterioration, but also the uncertainty inherent in the measurement of the parameters used to compute the indicator

Variability of modal parameters measured on the Alamosa Canyon Bridge

A significant amount of work has been reported in technical literature regarding the use of changes in modal parameters to identify the location and extent of damage in structures. Curiously absent, and critically important to the practical implementation of this work, is an accurate characterization of the natural variability of these modal parameters caused by effects other than damage. To examine this issue, a two-lane, seven-span, composite slab-on-girder bridge near the town of Truth or Consequences in southern New Mexico was tested several times over a period of nine months. Environmental effects common to this location that could potentially produce changes in the measured modal properties include changes in temperature, high winds, and changes to the supporting soil medium. In addition to environmental effects, variabilities in modal testing procedures and data reduction can also cause changes in the identified dynamic properties of the structure. In this paper the natural variability of the frequencies and mode shapes of the Alamosa Canyon bridge that result from changes in time of day when the test was performed, amount of traffic, and environmental conditions will be discussed. Because this bridge has not been in active use throughout the testing period, it is assumed that any change in the observed modal properties are the result of the factors listed above rather than deterioration of the structure itself

Transmissibility of non-linear output frequency response functions with application in detection and location of damage in MDOF structural systems

Transmissibility is a well-known linear system concept that has been widely applied in the diagnosis of damage in various engineering structural systems. However, in engineering practice, structural systems can behave non-linearly due to certain kinds of damage such as, e.g., breathing cracks. In the present study, the concept of transmissibility is extended to the non-linear case by introducing the Transmissibility of Non-linear Output Frequency Response Functions (NOFRFs). The NOFRFs are a concept recently proposed by the authors for the analysis of non-linear systems in the frequency domain. A NOFRF transmissibility-based technique is then developed for the detection and location of both linear and non-linear damage in MDOF structural systems. Numerical simulation results verify the effectiveness of the new technique. Experimental studies on a three-storey building structure demonstrate the potential to apply the developed technique to the detection and location of damage in practical MDOF engineering structures

B→ϕπB\to \phi \pi and B0→ϕϕB^0 \to \phi\phi in the Standard Model and new bounds on R parity violation

We study the pure penguin decays $B \to \phi\pi$ and $B^0 \to \phi\phi$. Using QCD factorization, we find ${\cal B}(B^\pm \to\phi\pi^{\pm} )=2.0^{+0.3}_{-0.1}\times 10^{-8}$. For the pure penguin annihilation process $B^0 \to \phi\phi$, analyzed here for the first time, ${\cal B}(B^0 \to\phi\phi)=2.1^{+1.6}_{-0.3}\times 10^{-9}$. The smallness of these decays in the Standard Model makes them sensitive probes for new physics. From the upper limit of $B\to \phi\pi$,we find constraints on R parity violating couplings, $| \lambda{''}_{i23}\lambda{''}_{i21}|<6\times10^{-5}$, $| \lambda'_{i23}\lambda'_{i21}|<4\times10^{-4}$ and $| \lambda'_{i32}\lambda'_{i12}|<4\times10^{-4}$ for $i=1,2,3$. Our new bounds on $|\lambda{''}_{i23}\lambda{''}_{i21}|$ are one order of magnitude stronger than before. Within the available upper bounds for $| \lambda{''}_{i23}\lambda{''}_{i21}|$, $|\lambda'_{i23}\lambda'_{i21}|$ and $|\lambda'_{i32}\lambda'_{i12}|$, we find that ${\cal B}(B\to\phi\phi)$ could be enhanced to $10^{-8}\sim 10^{-7}$. Experimental searches for these decays are strongly urged.Comment: 5 pages, 3 figures embede