Norris Stubbs

Damage identification in beam-type structures: frequency-based method vs mode-shape-based method

This paper presents a methodology to nondestructively locate and estimate the size of damage in structures for which a few natural frequencies or a few mode shapes are available. First, a frequency-based damage detection (FBDD) method is outlined. A damage-localization algorithm to locate damage from changes in natural frequencies and a damage-sizing algorithm to estimate crack-size from natural frequency perturbation are formulated. Next, a mode-shape-based damage detection (MBDD) method is outlined. A damage index algorithm to localize and estimate the severity of damage from monitoring changes in modal strain energy is formulated. The FBDD method and the MBDD method are evaluated for several damage scenarios by locating and sizing damage in numerically simulated prestressed concrete beams for which two natural frequencies and mode shapes are generated from finite element models. The result of the analyses indicates that the FBDD method and the MBDD method correctly localize the damage and accurately estimate the sizes of the cracks simulated in the test beam.

A global non-destructive damage assessment methodology for civil engineering structures

This paper describes a potential methodology to continuously assess the safety of civil engineering structures, e.g. bridges, frames and offshore platforms. The proposed damage assessment methodology not only detects and sizes damage, but also evaluates the impact of the damage on the performance of the structure. The proposed methodology deals with two broad activities: (i) periodic non-destructive damage localization and severity estimation; and (ii) the assessment of structural safety based on the results of the non-destructive damage detection. To accomplish these activities, first an established methodology which yields information on the changes in localized sectional stiffness properties (e.g. axial, bending and torsion) from changes in dynamic properties (i.e. mode shapes and frequencies) of the pre-damaged and post-damaged structures is reviewed. Next, an efficient technique to directly assess the reliability of a structural system using reliabilities of the components of the structure is developed. The efficacy of the combination of the non-destructive damage detection and the structural reliability evaluation is demonstrated using pre-damage and post-damage modal data obtained from numerical simulations of a rigid frame.

Field Verification of the Damage Index Method in a Concrete Box‐Girder Bridge via Visual Inspection

The structural condition of a concrete box-girder bridge is monitored twice by detecting and localizing potential damage in the bridge superstructure. Experimental field data were collected on the bridge in December 1997 and 9 months later in September 1998. Modal parameters for the structure are extracted from the measured frequency-response functions, and the resulting resonant frequencies and modeshapes are fed into a proven systems identification procedure. Modal parameters from the identified baseline structure and the modal parameters determined in the field are used as input to a field-tested nondestructive damage-evaluation method (the damage index method) to localize damage in the bridge superstructure. To provide some evidence of the veracity of the predictions of the possible damage locations, a visual inspection was performed on the bridge in May 1999, and surface cracks on the deck were recorded. A comparison of the predicted damage locations on the superstructure with the surface cracks documented via visual inspection is provided. The results indicate that a strong correlation exists between the predicted damage locations and the observed surface crack pattern.

Documentation of Changes in Modal Properties of a Concrete Box-Girder Bridge Due to Environmental and Internal Conditions

This paper presents the results of 2 modal tests performed on a concrete box-girder bridge. The 2 modal tests were performed sequentially with a 9-month time interval between tests. This sequence of tests form the initial phase of a modal-based level IV nondestructive damage-evaluation process implemented to establish the rate of structural deterioration and remaining service life of the tested structure. A level IV process detects, locates, and sizes damage and determines the impact of the damage on the performance of the structure. A novel feature of the field testing is the execution of an incremental and minimally intrusive (to traffic flow) modal test procedure. The instrumentation, experimental test plan, and resulting modal analysis are discussed. The changes in the 2 modal data sets are also discussed.

Nondestructive damage detection algorithms for 2D plates

The objective of this paper is to develop appropriate nondestructive damage detection algorithms for 2D plates. Two methods, one using the coal compliance and the other using local modal strain energy, are presented and compared in this paper. The compliance method is derived simply from the governing differential equations of motion presented in the classical plate theory. The second method is developed from expressions for the elastic strain energy of a plate. A damage index, used to indicate local damage, is defined and expressed in terms of modal displacements that are obtained numerically from mode shapes of the undamaged and the damaged structures. The possible damage locations in the structure are determined by the application of damage indicators according to previously developed decision rules. The damage indices, which are obtained for each mode, are transformed to probability space and superposed as weighted general means (WGM). In the WGM, the fraction of modal energy for the element is used as the weight of each mode. Each of the two methods is demonstrated by using a numerical example of a simply supported plate with simulated damage. Finally, the relative performances of the two methods are compared.

Prestress-Force Estimation in PSC Girder Using Modal Parameters and System Identification:

In this paper, a vibration-based method to estimate prestress-forces in a prestressed concrete (PSC) girder by using vibration characteristics and system identification (SID) approaches is presented. Firstly, a prestress-force monitoring method is formulated to estimate the change in prestress forces by measuring the change in modal parameters of a PSC beam. Secondly, a multi-phase SID scheme is designed on the basis of eigenvalue sensitivity concept to identify a baseline model that represents the target structure. Thirdly, the proposed prestress-force monitoring method and the multi-phase SID scheme are evaluated from controlled experiments on a lab-scaled PSC girder. On the PSC girder, a few natural frequencies and mode shapes are experimentally measured for various prestress forces. System parameters of a baseline finite element (FE) model are identified by the proposed multi-phase SID scheme for various prestress forces. The corresponding modal parameters are estimated for the model-update procedure. As a result, prestress-losses are predicted by using the measured natural frequencies and the identified zero-prestress state model.

Relative performance of clustering-based neural network and statistical pattern recognition models for nondestructive damage detection

The objective of this paper is to compare and contrast the capabilities of neural networks and statistical pattern recognition to localize damage in three-dimensional structures. A theory of damage localization, which yields information on the location of the damage directly from changes in mode shapes, is formulated. Next, the application of statistical pattern recognition and neural networks for nondestructive damage detection (NDD) is established. Expressions for classification using linear discriminant functions and a two-stage supervised clustering-based neural network are generated. Damage localization is applied to a finite-element model (FEM) of a structure which contains simulated damage at various locations. A set of criteria for comparing and contrasting statistical pattern recognition and neural network models is then established. Finally, the evaluation of the two models is carried out using the established criteria.

Bridge diagnostics via vibration monitoring

In this paper nondestructive damage detection (NDD) in large/complex structures is demonstrated via simulating vibration monitoring of such structures. The theory of NDD for truss type structures is formulated. To examine the feasibility of the theory, a finite element model of a 3-D truss structure, which consists of sixteen bays and includes 246 elements, is developed to simulate damage. Four damage cases are simulated numerically. The cases range from the structure being damaged in one location to the structure being damaged in three locations. For the given modal parameters, this study reveals very good results for small aounts of damage as well as large damage.

Loss of prestress prediction based on nondestructive damage location algorithms

As damage accumulates in structure, the stiffness of the structure changes. Changes in stiffness are reflected in changes in the frequencies and mode shapes vibration of the structure. A theory of structural damage evaluation in which changes in the dynamic characteristics of a structure are used to predict the location and severity of damage has been developed by Stubbs et al. The objective of this study is to investigate the feasibility of using such technique for detecting loss of prestress in a prestressed concrete bridge. In order to achieve this goal the following tasks were performed: (1) a review of the state of the art modeling of prestressed concrete using finite elements, (2) a finite element model of a 3-D prestress beam using a commercial finite element code (ABAQUS/PATRAN) was developed, (3) dynamic modal analyses of the undamaged and damaged (less prestress) models are performed, (4) evaluation of an existing damage location algorithm for predicting location and severity of damage. The inability of the algorithm to predict loss of prestress is observed and explained. An improved version of the algorithm is qualitatively presented.

Automated decision support systems for hurricane mitigation planning

The risks posed by hurricanes are significant and rising. Recent advances in computer technology have stimulated interest in the potential applica tion of automated decision support systems to hurricane mitigation planning. This article discusses why computer technology should be used in hurricane mitigation planning. Reviews of the several different, though often interrelated, lines of advance in automated decision support systems and hurricane risk assessment modeling are presented. Recent research on organizational concerns that should be ac counted for in the design of such systems is also reviewed. Guide lines are then provided showing how future research might be directed to effectively design and implement decision support systems.