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A Laser-Based Noncontact Vibration Technique for Health Monitoring of Structural Cables: Background, Success, and New Developments

A Laser-Based Noncontact Vibration Technique for Health Monitoring of Structural Cables:... Hindawi Advances in Acoustics and Vibration Volume 2018, Article ID 8640674, 13 pages https://doi.org/10.1155/2018/8640674 Review Article A Laser-Based Noncontact Vibration Technique for Health Monitoring of Structural Cables: Background, Success, and New Developments Armin B. Mehrabi and Saman Farhangdoust Department of Civil and Environmental Engineering, Florida International University, Miami, FL 33174, USA Correspondence should be addressed to Armin B. Mehrabi; Amehrabi@fiu.edu Received 13 March 2018; Accepted 7 May 2018; Published 13 June 2018 Academic Editor: Cheng-Wei Fei Copyright © 2018 Armin B. Mehrabi and Saman Farhangdoust. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Structural cables are susceptible to the effects of high stress concentrations, corrosion, and wind-induced and other vibrations. Cables are normally the most critical elements in a cable-supported structure and their well-being is very important in the health of the structure. eTh laser-based vibration technique discussed in this paper is a means for health monitoring of cables and therefore the entire cable-supported structure. This technique uses a noncontact remote sensing laser vibrometer for collecting cable vibration data from distances of up to several hundreds of feet and determines its dynamic characteristics including vibration frequencies and damping ratios. A formulation specifically developed for structural cables capable of accounting for important cable parameters is then used to calculate the cable force. Estimated forces in the cables are compared to previously measured forces or designer’s prediction to detect patterns associated with damage to the cable itself and/or changes to the structure elsewhere. eTh estimated damping ratios are also compared against predefined criteria to infer about susceptibility against wind-induced vibrations and other vibrations. The technique provides rapid, effective, and accurate means for health monitoring of cable-supported structures. It determines the locations and elements with potential damage and the need for detailed and hands on inspection. To date, the technique has been used successfully for evaluation of twenty-five major bridges in the US and abroad. o Th ugh originally devised for condition assessment of stay cables, it has been developed further to include a variety of systems and conditions among them structural hanger ropes in suspension, truss, and arch supported bridges, ungrouted stay cables, cables with cross-ties, and external posttensioning tendons in segmental bridge construction. It has also found a valuable place in construction-phase activities for verification of forces in tension elements with minimal efforts. Future endeavors for automation and aerial delivery are being considered for this technique. 1. Introduction cable-supported structures. Incidents of major damage to cables in some cable-supported bridges [1–4] have created Despite all the advantages associated with structural cables enough concern for the owners to initiate a series of investi- and ropes, their use has not been without concerns and gation resulting in development of health monitoring systems questions. These include consequences of high stress con- and preventive measures. centrations in anchorage zones, corrosion, and susceptibility General inspection of cable structures provides use- ful information but can only reveal visible and/or highly to wind-induced vibrations and other vibrations. Because of these concerns, structural cable systems are being con- advanced damage that normally necessitates extensive repairs tinuously modiefi d and standard cable systems have yet to and remedies. This is because, normally, main tension ele- be established. Monitoring the health and performance of ments of structural cables, especially in the case of stay existing structural cable systems can help in determina- cables of cable-stayed bridges, are covered by layers of tion of advantages and shortfalls of various systems and corrosion protection elements and their condition is hidden identification of proper remedies for problems observed for from inspectors using traditional methods of inspection. 2 Advances in Acoustics and Vibration Therefore, availability of nondestructive testing methods for frequencies and damping ratios are calculated. Using this detection of damage early in the service life of the structural information and a formulation specifically developed for cables is essential for health monitoring of structures that rely structural cables, tension forces in the cables are calculated. heavily on these elements for their integrity and load carrying The resulting force distribution array in cables is used to capacity. Owing to the unique structural system in cable- make much more ren fi ed judgments about the condition, supported constructions, cables and tension members are aging, reliability, and maintenance of critical facilities than very suitable as surrogate sensors for reliable structural health previously possible. eTh innovation was so practical and monitoring adaptation for detecting damage and deteriora- effective that it was put to work right away as the NDT (Non- tion. Variation of forces and other dynamic characteristics of destructive Testing) tool of choice for assisting evaluation cables not only are indicative of their condition but also can of cable-supported bridges nationwide and abroad. Since its be indication of damage at any other location in the structure. development, this technique has been used successfully for Damage and changes at any location on the cable-supported condition evaluation of twenty-five landmark bridges in the structure do induce a change in the force distribution of all US, including thirteen cable-stayed bridges, vfi e suspension members, including the cables. It is also understood that bridges, and seven arch bridge. the intrinsic damping of structural slender elements such as Furthermore, one of the primary parameters which affect cables is normally very low [5], making them vulnerable to the susceptibility of the cables to aerodynamic vibration prob- large oscillations in presence of wind-induced vibrations and lems is the cable damping. Structural cables have minimal other source of excitations. intrinsic damping that is not normally adequate to suppress A rapid, laser-based vibration technique for force and wind-induced vibration in the cables. eTh y can potentially be damping measurement, along with numerical algorithms driven to large amplitude oscillations by a variety of wind- developed through federally and privately funded research, related mechanisms. Of the various wind-related mecha- has provided a practical, cost-effective tool to address imme- nisms, the structural cables are generally susceptible to three diate concerns and determine the need for action in relation main phenomena, namely, rain-wind-induced vibrations, to structural cables and cable-supported bridges [6, 7]. As galloping of various types, and vortex excitation [1, 5]. Of described above, force estimation in time provides an excel- these, the rain-wind phenomenon is a widespread problem lent means for detection of damage in structural cables and for stay cables resulting in large amplitude vibration of the cable-supported bridges. Figure 1 shows the laser vibrometer cables under moderate wind and light rain and has been targeting cables and a sample comparison of estimated forces. reported on several bridges around the world. Formation To also address vulnerability of structural slender elements of water rivulets under such conditions is related to this against wind-induced vibrations, damping ratios can be phenomenon. estimatedusing thelaser-based techniqueandcompared In recent years, there has been new developments on to predefined thresholds for verification of susceptibility application of various types of noncontact sensors to vibra- to excessive vibration. eTh damping measurement is also tion measurement. One such promising method is holo- employed to verify the damping provided by external devices graphic interferometry with ability to target several moving designed to suppress large oscillations. In all, the laser-based objectsfromasingleviewpoint[8,9].Vision-basedmon- vibration technique provides an excellent tool for damage itoring devices have also been used with some success for detection, determination of susceptibility to damage, and vibration measurement applications [10, 11]. Regardless of verification of design of mitigation methods. the type of technology, it is essential ensure assure adequate To date, this technique has been used successfully for accuracy and to avoid object identification errors during eld fi twenty-five cable-supported bridges and has continuously measurement. A comprehensive QC/QA for field measure- been adapted and evolved for new applications. With the mentprocessesaswellasfor postprocessingis critical for advent of new wave of technological advances in delivery of viable data. Noncontact sensor devices are only the tool for testing and inspection methods in the form of automated collecting the vibration data from slender tension elements. aerial and ground vehicles, an entirely new endeavor has The major task for force and damping estimation still remains opened for novel application of this already tested technique. with postprocessing and employment of simple yet accurate formulation. 2. Background 3. Field Implementation and Procedure As part of a research project sponsored by the Federal Highway Administration to develop a quantitative condition- Field application of vibration-based methods normally poses assessment technique for cable-stayed bridges, an innovative problems related to attachment of sensors (accelerometers) noncontacting, laser-based vibration technique was devel- to measurement points and interruption of bridge function oped to measure vibration, forces, and damping in structural and traffic flow. For periodic cable force measurements, the slender elements [6]. With this technique, cable vibration manual method of sensor attachment involves additional from ambient sources can be recorded from a distance by cost and difficulty. The use of laser-based sensors therefore targeting the cable with the laser beam. The laser can then oer ff s many advantages. The laser technique includes the turn to the next cable and continue measurement for all cables use of a noncontact laser vibrometer aimed at cable from a in direct view. By analyzing the vibration record, the cable distance. It eliminates the need for accessing cables for sensor vibration characteristics including fundamental vibration installation and normally avoids major interruption to traffic. Advances in Acoustics and Vibration 3 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Span 1 Center Span Stay Cable Number Designer’s (Minimum Force) Designer’s (Maximum Force) 2009 North Measured Forces 2009 South Measured Forces Figure 1: Laser vibrometer targeting cables (left), sample force comparison (right). eTh laser-based measurement technique provides a noncon- location is not available on the bridge, the laser vibrometer tact remote sensing ability, and for periodic measurements canbestationed offthebridgewithintheeeff ctive range. andforcableswithamoredicffi ultaccessibility,thetechnique The effective range of laser vibrometer, depending on the has proven to be an extremely useful tool in reducing the type of the equipment, can be from 100 to 300 ft (30 to time and eo ff rts required for measurements. In most cases, 100 m). Usually, several cables are targeted from one laser ambient excitations sources such as traffic and wind induce station. Although the high sensitivity of the laser vibrometer enough vibration and there is no need for manual excitation. allows flexibility in laser positioning and targeting, the ideal However, if these sources are not present or not adequate, is to have the laser beam as perpendicular to the cable chord cables are excited manually. Depending on accessibility and as possible. Also, targeting the 1/3 of the cable length from cable stiffness and size, manual excitation can be performed the deck provides better clarity and at the same time better with slight impacting or the use of ropes, where a rope is participation of rst fi and second mode of vibration in the passed around the cable and pulled repeatedly with a rhythm recorded response. in concert with cable’s natural vibration frequencies [2]. The vibration time history of each cable is recorded in this manner which contains data showing the amplitude of velocity with respect to time for a predefined time interval. 3.1. Procedure. Prior to field measurements, physical prop- Using Fast Fourier Transform (FFT) analysis, the frequency erties of the cables including length, cross-sectional area, spectrum for each cable vibration is calculated and the mass per unit length, and mechanical properties are sought fundamental vibration frequencies are identified. Sampling andgatheredfromdesignoras-builtdrawingsandother rate and duration of vibration measurement are selected to sources. This information is utilized for sensor selection and provide the required frequency resolution and preventing the utilization planning, and for the selection of cable excitation aliasing eeff cts. For example, a 100 second recording duration methods and means of safe access to the cables. Normally, will offer 0.01 Hz resolution in the calculated frequency minimum traffic interruption is desired by the owners. spectrum. Also, to avoid the aliasing eects ff the data needs eTh refore, employing lift equipment to install contact sensors to be sampled at a rate at least twice the expected frequency (e.g., accelerometers) needs to be avoided, and the use of (Nyquist Theorem). Figure 2 shows a sample time history laser vibrometer is definitely preferred. Depending on the record and results in frequency domain. eTh force versus roadway shoulder width and other access limitations, the eld fi frequency relationship described later in this paper is then measurement can be performed with only one shoulder or used to calculate the force corresponding to each frequency one shoulder and a lane closure. In some instances, for cable- for cables. eTh accuracy of the force estimation using this stayed bridges with one plane of stay cables, the roadway median has been used for measurement operation without formulation is dependent on the accuracy of geometric any lane closures. For cables with customary mechanical and and physical/mechanical properties of cables provided for measurement process, as well as on the consideration and geometric properties, e.g., those with first-mode frequencies of up to 2 Hz, ambient sources normally provide adequate knowledge of the actual boundary conditions for cables. Although higher accuracies are achievable, the sampling rate excitation and there is no need for external sources. For stiffer cables that either have large cross-section or shorter lengths, and recording duration are normally selected to limit the manual excitation may be required. For measurements, the maximum error in frequency measurement to one percent, equivalent to about two percent error in the force mea- laservibrometerisstationedon thebridgedeckandcablesare targeted. For construction-phase measurement, when proper surement. For damping measurement, an excitation source Force (kips) 4 Advances in Acoustics and Vibration 1.464844 −1 −2 2.919922 −3 −4 0 10 20 30 40 50 0123456789 10 Time (Sec) Frequency (Hz) Figure 2: A vibration time history (left) and result in frequency domain (right). is required to generate a “considerable” movement in the (vi) All datasets for cables tested are stored and identified cables. It is realized that the damping ratio calculated for a uniquely with cable numbering. Other relevant infor- cable may be dependent on the initial excitation amplitude; mation such as date/time, wind, and temperature is however, to this end there is no guideline or standard as to also recorded. what this amplitude should be. The common way of exciting (vii) The time domain vibration data are transformed the structural cables for damping measurement is manual to a record of amplitude versus frequency by Fast excitation such as impacting with rubber mallet or using rope Fourier Transform processing. Dominant (funda- to apply resonating motion in the cable. Mechanical shakers mental) modes of frequency are identified and have also been used as source of excitation, but because of recorded for use in computation of tension force for concerns for damaging the cables, their application has not each individual cable. been favored by the owners. In any case, the damping ratios (viii) Individual cable forces are computed based on the test are calculated from the free vibration time history recorded data analysis protocol and mathematical formulation aer ft releasing cable using available methods. developed specicfi ally for cables of various character- Following steps are normally ensued for a typical force istics. measurement program: (ix) Time history records are also used for estimation of (i) To properly characterize physical properties of the cable damping ratios and compared to preden fi ed thresholds for vericfi ation of susceptibility to wind- cable, data related to its mass, length, geometry, stiffness, and component materials are sought and induced vibration. gathered from the client. The structural significance of cable array force distributions is studied through comparison of measurements with design- (ii) The attachment condition of the cable to other com- estimated cable forces, construction records, and/or previous ponents of the structure is obtained and studied, as measurement results. Comparative assessment seeks cursors are results of previous measurement programs for for cable force redistribution from the prior measurement or comparison. from baseline data. Signicfi ant changes of forces, e.g., lower- (iii) The data from steps one and two above are reviewed than-anticipated forces in a cable element, can be indicative of to assess their specific relevance to force measurement potential deterioration or damage. Additionally, the pattern procedures and accuracy, and the test data analysis of force changes in cable array can also indicate damage or protocol is defined for each individual cable variant changes in other elements of the bridge structure. Estimation on a structure. of damping ratios and comparison to predefined thresholds also determine the need for additional damping or adequacy (iv) The structure’s configuration is reviewed, permitting of existing damper devices. sensor selection and utilization planning and selec- tion of cable excitation techniques and safe access methods. 4. Theoretical Background (v) Laser vibrometer is stationed in appropriate location Duetothecomplexnatureofthe problemoffreevibration on the deck (or off the bridge) and the cables are tar- of structural cables with vibration characteristics that are sig- geted under ambient (wind or traffic), or if necessary nificantly different from strings, accurate yet simple analytical manual excitation, and time history of cable vibration relationships that take into account all pertinent parameters is recorded. are needed. In many cases (such as stay cables), the use of Amplitude Amplitude Advances in Acoustics and Vibration 5 the string equation leads to excessive oversimplification and L =L(1 +(mgLcos𝜃/H) /8), unacceptable increased errors in force predictions. I= equivalent moment of inertia, g= acceleration of gravity, 4.1. Force Measurement. eTh laser-based technique devel- 𝜃= inclination angle of cable chord, oped for health monitoring of structural cables is a vibration- L= chord length, based method. It has been completed and enhanced by development of mathematical formulation and simplified m= mass per unit length of cable, relationships that take into account the effect of parameters H= tension force, not considered in the taut string equation [6]. This technique E = equivalent modulus of elasticity of cable cross- has been validated by extensive laboratory experimentation section, andfieldvericfi ation,andhasbeenusedsuccessfullyfor evaluation of dozens of bridges. A= equivalent cable cross-sectional area. Formulation—For the tension members that deviate from a taut string by their different dynamic characteristics, 4.2. Damping Measurement. Normally, for measuring damp- the taut string theory needs to be augmented to be able ing, cables are excited manually and let free to allow recording to account for the differing characteristics of cables from of the time history. Figure 4 shows manual excitation of cables those of theoretical taut string. Mehrabi and Tabatabai [6] using a rope for stay cables and impacting with hammer introduced a n fi ite dieff rence formulation for vibration anal- for hanger ropes. With the vibration time history, various ysis of structural cables. This formulation incorporated the methods can be used to calculate the damping including effects of bending stiffness of cable and its sag-extensibility decay method and half-power bandwidth method [13]. If characteristics into a unified solution and provided a tool the measurement is performed in windy condition, wind for accurate determination of vibration mode shapes and speed and direction is also measured for calculating the frequencies. Various cable end conditions, variable cross aerodynamic damping that is used in calculation of eeff ctive sections, and intermediate springs and/or dampers were damping for cables [5]. taken into account. Using a nondimensional form of this formulation, a parametric study was conducted on the effects 5. Health Monitoring and Damage Detection of sag-extensibility and bending stiffness. Figure 3 shows Using Laser-Based Vibration Technique variation of the rfi st-mode inplane vibration frequency of a cable with respect to sag-extensibility and bending stiffness eTh laser-based noncontact vibration technique described parameters. This formulation was veriefi d with available abovehas theability todetectdamageinindividualcables theoretical solutions and compared with n fi ite element anal- and elsewhere in the structure that would aeff ct the cable yses. A simple relationship among nondimensional cable response. It can also be employed to determine the vulner- parameters was also introduced for the range of parameters ability of the cables against wind-induced excessive vibration applicable to stay cables in cable-stayed bridges. This simple that has the potential for damaging the cable and the structure relationship provides an accurate tool for measurement of by overstress, fatigue, and collision. tensionforcesinstaycablesusing thevibration method. eTh simple equation introduced by Mehrabi and 5.1. VulnerabilitytoWind-InducedVibration. One of the Tabatabai [6] is expressed as primary parameters which affect the susceptibility of cables to aerodynamic vibration problems is the cable damping. =𝛼𝛽 −0.24 (1) Cables normally have minimal intrinsic damping that is 𝜔 𝜁 ns not normally adequate to suppress wind-induced vibration. where Cables can potentially be driven to large amplitude oscilla- tions by a variety of wind-related mechanisms. Of the various 2 2 2 𝛽 =1+2/𝜁+(4+ n 𝜋 /2)/𝜁 [12], wind-related mechanisms, the structural cables and stays are generally susceptible to three main phenomena, namely, rain- 𝜔 = n𝜔 = n𝜋/𝐿 𝐻/𝑚, ns 1s wind-induced vibrations, galloping (dry and wake gallop- 2 2 𝜆 = L((WLcos𝜃/𝐻) /(HL/𝐸𝐴)), ing), and vortex excitation. Of these three, the rain-wind 1/2 phenomenon is a widespread problem resulting in large 𝜁= L(𝐻/𝐸𝐼) , amplitude vibration of the cables under moderate wind and W= cable weight per unit length, lightrainandhasbeenreportedonseveralbridgesaroundthe world. Formation of water rivulets during light rainfall with 𝜔 = First mode frequency as a taut string, 1s 2 moderate winds is associated with this phenomenon [1]. Dry 𝜇=𝜆 for n=1(in− plane), and wake galloping normally occur at higher wind speeds and 𝜇=0 for n>1(in− plane), require certain angles of attack with respect to the cable axis and position of adjacent cables with respect to each other. 𝜇=0 for all n(out− of− plane), The amplitude of vibration for ordinary structural cables (e.g., n= mode number, stay cables and hanger ropes) subject to vortex excitation is 𝛼=1+0.039𝜇, normally small and structurally insignica fi nt. 6 Advances in Acoustics and Vibration showsanoticeable drop duetoapotentiallossofcross- section, slippage, or similar anomalies, the cables adjacent to it should takethemajorityofthedroppedloadandtherefore wouldshowanincreaseintheirforces.This(local) pattern / offorcechanges wouldbeindicativeofapermanentforce variation and a potential damage in the cable showing force 2 drop. Accordingly, inspection of pattern for force variation can lead to detection of potential damage to an individual cable. Figure 6 shows a sample force comparison with a pattern indicating potential damage in a cable. This, at a minimum, will warrant a special inspection of suspect cable with the use of various available NDT methods [2]. 410 0.1  0.01 0.001 5.3. Global Damage Detection. Structures when exposed to Figure 3: Parametric study for first-mode inplane vibration fre- varying environmental conditions can undergo changes in quency of cables with respect to sag and bending stiffness. stiffness, material properties, and boundary conditions over time. Damage can initiate from various sources. With a sys- tematic and rational method, in general, measured changes in structural parameters reflected by individual sensor outputs In addition to the external sources such as wind, live load, canberelatedto damagesourcesand locations. eTh se andgroundmotion, theinteraction betweenthe structure parameters may include member forces, geometry profiles and supporting cables can generate excitation in one caused by vibration of the other. Cables may experience noticeable (deflections), support reaction forces, structural strains, and oscillation by “motion-induced” and/or “parametric excita- support settlements. This forms a general framework for most tion” [14, 15]. This phenomenon is characterized by motion global damage detection techniques. of the cables induced by motion of the structure, e.g., deck Several investigations have been performed on damage and towers of a bridge, during operational response of the detection of structures including vibration-based modal structure (i.e., wind, tracffi , pedestrian, excitation, etc.). analysis [16], dissipated energy density method [17], and To investigate the vulnerability of cables to wind-induced parameter estimation methods [18, 19]. These methods usu- and other types of excitations, various measures and criteria ally require a relatively large computational eor ff t and a have been introduced. Comparison of dynamic characteris- knowledge of exact loading configuration. An analytical tics of cables measured with the laser-based method with procedure, Precursor Transformation Method (PTM), was the respective criteria determines the adequacy of existing proposed by Mehrabi et al. [20] identifying the location(s) measures or the need for additional means for preventing and relative significance of possible damage sources based excessive and damaging oscillations. For example, in the case on measured changes in structural response parameters over of stay cables of cable-stayed bridges, Scruton Number, Sc time. This method oers ff advantages in sensitivity and cost (shown in (2)) is a nondimensional parameter calculated efficiency when compared to other available methods. Devel- based on the measured damping ratio, air density, and cable oped originally for cable-supported structures, this method diameter, and mass and is used as a measure of susceptibility takes advantage of the fact that cables and tension members of cables to rain-wind-induced vibrations. areverysuitablefor reliablestructural health monitoring adaptation as surrogate sensors for detecting damage and 𝑆𝑐= deterioration. (2) Variation of forces and other dynamic characteristics of cables not only are indicative of their own condition but In thisequation,ScistheScrutonNumber,𝜉 is the damping also can be indication of damage at any other location in ratio,𝜌 is the air density, and D is the cable outer diameter. thestructure.Damageandchangesatanylocation onthe Lower Scruton Numbers correspond to higher susceptibility cable-supported structure do induce a change in the force to wind-induced oscillations. Normally, a minimum Scruton distribution of all members, including the cables. Therefore, Number of 10 is recommended for stay cables to avoid large- in this method, changes in the state of the structure are amplitude rain-wind-induced vibration [5]. If this number experimentally assessed through measurement of structural cannot be achieved for a stay cable, each external measure response parameters such as displacement, strain, or internal or a combination external measures such as cable surface cable forces for the case of cable-supported structures at modification, viscous and other type of dampers, and cross- discrete points on the structure at a reference time and ties will need to be implemented to suppress the vibration. later at any desired time. For application of this method, Figure 5 shows Scruton Numbers calculated for a cable array the external loading state at different measurement times and comparison with the criteria discussed above. should be constant. This loading could be the dead load of the structure alone or augmented with additional live 5.2. Damage to Individual Cables. In general, it is expected load. To uncouple the effects of different sources of damage that if dead load force (or a constant loading state) in a cable and to determine their locations and relative significance 𝜌𝐷 𝑚𝜉 Advances in Acoustics and Vibration 7 Figure 4: Manual excitation for damping measurement; using rope (left) and making impact with rubber mallet (right). 30.0 25.0 20.0 Pattern indicating potential damage to Cable NE55 15.0 10.0 5.0 0.0 Cable No. Cable No. Estimated Forces (Period 2) Figure 5: Sample Scruton Number calculated for cables using data Baseline Estimated Forces (Period 1) recorded with the laser technique. Figure 6: A sample of force comparison with a pattern indicating potential damage in cable. based on the experimental data, an analytically determined transformation matrix is utilized. From an analytical standpoint, the sources of damage can remaining service life in cable-supported bridges. Examples be characterized as precursor events (or damage precursors) are force measurement of hanger ropes in the Hoan Arch that precipitate changes in the state of the structure. Precur- Bridge (Figure 8) and the Bosporus Suspension Bridge sors are externally imposed and are therefore independent of (Figure 9). thestructure andthe subsequentchangesinthe stateofthe structure. Examples of damage sources that can be modeled as precursor events include loss of material or stiffness, 6. Success in Implementation joint slippage, support settlements, and loosening of bolts. For the case of cable-supported structures, the precursor eTh laser-based noncontact vibration technique for structural transformation matrix contains patterns of force changes in health monitoring was initially developed for condition cablearrayeachgeneratedanalyticallyforaspecicp fi recursor assessment of cable-stayed bridges. Its application was limited [20]. Figure 7 shows force variation calculated for cables of a to stay cables of certain congfi uration. However, the method cable-stayed bridge (Figure 7(b)) and calculated precursors as has been evolving in time to include various stay cable damage identified for three of its cables (Figure 7(c)). conditions, and its application was extended into evaluation Laser-based vibration measurement technique has been of cables and ropes in suspension and arch bridges. It can, in usedalsoforinvestigationofcauseofdamageorestimationof general, be utilized for any structure with slender structural Scruton No. 1n 2n 3n 4n 5n 6n 7n 8n 9n 10n 11n 12n 13n 14n 15n 16n 17n 18n 19n 20n 21n 22n 23n 24n 25n 26n 27n 28n 29n 30n 31n 32n 33n 34n 35n 36n 37n 38n 39n 40n 41n 42n Force (kips) NE41 NE42 NE43 NE44 NE45 NE46 NE47 NE48 NE49 NE50 NE51 NE52 NE53 NE54 NE55 NE56 NE57 NE58 NE59 NE60 NE61 NE62 NE63 NE64 NE65 NE66 NE67 NE68 NE69 NE70 8 Advances in Acoustics and Vibration Table 1: List of bridges for which the laser-based vibration technique has been employed. Cable-Stayed Bridges Location Scope Date(s) Second Vivekananda Bridge Kolkata, India Stay Cable Force Measurement 2016 Leonard Zakim Bridge Boston, MA Stay Cable Force Measurement 2015 Sunshine Skyway Bridge St. Petersburg, FL Stay Cable Force and Damping Measurement 1999, 2009,2015 Dames Point Bridge Jacksonville, FL Testing and Evaluation 2008,’10,’12,’16 Luling Bridge Luling, LA Stay Cable Force and Damping Measurement 2002-2006 Queen Elizabeth II Dartford, UK Stay Cable Force Measurement 2008 Maumee River Crossing Toledo, OH Stay Cable Force Measurement 2006 Varina-Enon Bridge Henrico, VA Stay Cable Force Measurement 1999 & 2007 C&D Canal Bridge Middletown, DE Stay Cable Force Measurement 2005 Fitchburg Bridge Fitchburg, MA Stay Cable Force Measurement/ Construction Phase 2003 Talmadge Memorial Talmadge, GA Stay Cable Force Measurement 2000 Cochrane Bridge Mobile, AL Stay Cable Force and Damping Measurement 1998 Weirton-Steubenville Weirton, WV Stay Cable Force Measurement 1997 Suspension Bridges Bosporus Bridge Istanbul, Turkey Hanger Force Measurement, Failure and Fatigue Analysis 2004 Tazlina Pipeline Bridge Glennallen, AK Hanger Ropes Force Measurement 1999 & 2004 Tanana Bridge, AK Delta Junction, AK Hanger Force Measurement 2001 Carquinez Bridge Vallejo, CA Hanger Force Measurement/ Construction Phase 2003 Paseo Bridge Kansas City, MO Hanger Force Measurement 2002 Arch Bridges Hart Bridge Jacksonville, FL Hanger Force Measurement/ Construction Phase 2016 Sherman-Minton Louisville, KY NDE Testing 2011 Troup Howell Rochester, NY Hanger Force Measurement/ Construction Phase 2006 & 2007 Telegraph Road Taylor, MI Hanger Force Measurement/ Construction Phase 2007 Cass Street Bridge La Crosse, WI Hanger Force Measurement/ Construction Phase 2004 & 2005 Belle-Vernon Bridge Belle-Vernon, PA Hanger Force Measurement/ Following Accident 2003 Hoan Bridge Milwaukee, WI Hanger Force Measurement /Failure Analysis 2001 elements. A list of twenty-five major bridges for which this structures where access to the cables is extremely limited. method hasbeenutilizedisshownin Table1. Table 1 identifies the bridges for which the technique has been used during construction to verify the forces and apply adjustments if necessary. 6.1. New Developments. Following describes some of the later applications for this technique. 6.4. Extradosed Bridges. In 2016, the laser-based vibration technique was used to measure forces in stay cables of the 6.2. Pipeline Bridges. In 1999, 2001, and 2005, engineers were Second Vivekananda Bridge in Kolkata, India, as a part of able to stand on the spectacular Tanana and Tazlina River routine maintenance and inspection program. This bridge, banks in Alaska and measure remotely the forces of suspender shown in Figure 11, is the first extradosed bridge for which and other cables of two suspension bridges carrying Trans this technique has been utilized. One of the unique features Alaskan Pipeline System with great efficiency, accuracy, and of stay cables in an extradosed bridge is their relatively short speed. This was a part of scheduled evaluation project to length and higher bending stiffness. determine the safety and soundness of this major pipeline to continue carrying crude oil in Alaska [21]. Figure 10 shows laser being used for force measurement of hangers in one of 6.5. Ungrouted Stay Cables. Most of the newer generation these bridges. of stay cables in the US and elsewhere do not use grout filling as corrosion protection method. Having unbonded 6.3. Construction-Phase Force Verification. The exceptional and normally detached cover pipe was thought to introduce value of the laser-based vibration technique for construction- complications in vibration measurement of the stay system, phase cable force vericfi ation was quickly recognized. eTh therefore resulting in difficulties for estimating the force techniqueoeff redarapidand economic yetaccuratemethod from uncoupled vibration characteristics. The laser-based as alternative to logistically cumbersome methods such as technique adapted for taking into account the noncomposite li-ft offs. Additionally, the remote noncontact nature of the action of the cable cross-section was used successfully for technique made it the only reasonable choice for unn fi ished the rfi st time in 2015 for force measurement of ungrouted Advances in Acoustics and Vibration 9 1A 1B 17 24B 24A North Side South Side (a) 300 500 CABLE 15 CABLE 21 CABLE 3 −100 200 −200 −300 −400 NORTH SIDE NORTH SIDE −100 −500 SOUTH SIDE SOUTH SIDE (b) (c) Figure 7: Damage detection using precursor transformation method. (a) schematic of a cable-stayed bridge superstructure, (b) force variation in cables, and (c) damage detected in three cables. stay cables of the Leonard Zakim Bridge in Boston, MA inspection, and health monitoring program. The technique (Figure 12). wasalsousedin2015forforcemeasurement oftied stay cables of the Leonard Zakim Bridge in Boston, MA (Fig- ure 12). 6.6. Tied Stay Cables. Cross-ties, designed as a measure for vibration suppression of structural cables, alter dynamic characteristics of the cables, making it difficult to iden- 6.7. Structural Health Monitoring Using Periodic Force Mea- tify their individual vibration frequencies for force calcu- surement. eTh laser-based vibration technique is being used lation using the available algorithms. Dames Point Bridge by some bridge owners for periodic force measurement as in Jacksonville, FL, is a cable-stayed bridge whose stay part of bridge health monitoring and maintenance programs. cables aretiedtoeachotherusingcross-ties(Figure 13). The Sunshine Skyway Bridge in Florida shown in Figure 14 After a series of verification experiments and research in is one of these bridges. For this bridge, in addition to force 2008 to 2010, a new procedure for efi ld application was measurement, the laser technique is also used for damping developed and numerical formulation was complemented measurement of the cables with and without contribution of for taking into account the connectivity of cables. eTh laser the external damping devices. This ensures proper function- techniquehassincebeenusedfor twosuccessfulperiodic ing of the dampers and adequacy of the overall damping for cable force measurements as part of its routine maintenance, suppression of wind-induced vibration. Force Changes (kN) Precursor 10 Advances in Acoustics and Vibration Figure 8: Hoan Bridge in Milwaukee, WI. Figure 9: Bosporus Bridge in Istanbul Turkey. 6.8. External Posttensioning Tendons. Currently, the laser- methods for structural health monitoring. Undoubtedly, the based method is being considered for a new application to laser vibration technique discussed in this paper has a great external posttensioning tendons of a segmental concrete box potentialasatriedandveriefi dmethodtobeimplemented girder bridge. Most external posttensioning tendons utilize with an automated delivery, let it be aerial or ground vehicle. a system very similar to grouted stay cables; therefore, their force estimation can be performed using the same eld fi and 7. Summary and Conclusion analysis procedure described earlier. Tendons however are normally stressed to much higher stress levels than stay cables A laser-based vibration technique for health monitoring and hence possess dynamic characteristics that are different of cable-supported structures was discussed in this paper. fromthoseofstaycables.Tendonsareexpectedtohavehigher The technique includes field implementation of noncontact frequencies owing to their shorter lengths and higher forces remote sensing laser vibrometer to record the vibration in comparison with stay cables. and calculate the dynamic properties and a formulation developed specicfi ally for structural cables for calculation 6.9. Future Applications. Technological advances in the appli- of their tension forces. eTh comparison between estimated cation of automated unmanned vehicles and robotics have cable forces using this technique and previously measured created new opportunities for application of nondestructive or expected forces can be used to establish a pattern of Advances in Acoustics and Vibration 11 Figure 10: eTh use of laser-based vibration technique for pipeline bridges. Figure 11: Extradosed cable-stayed bridge for which the laser-based technique was used. Figure 12: Laser technique was used for evaluation of ungrouted stay cables of the Leonard Zakim Bridge. 12 Advances in Acoustics and Vibration Figure 13: eTh forces of tied stay cables of the Dames Point Bridge are periodically evaluated using the laser technique. Figure 14: eTh Sunshine Skyway Bridge in Florida. changes indicative of location, type, and intensity of the segmental bridge construction and force estimation of exter- nal posttensioning tendons. Future endeavors for automation potential damage to the cable and the structure elsewhere. and aerial delivery are being considered for this technique. eTh damping measured for cables with this technique is used for comparison against predefined thresholds and to determine their vulnerability to various types of wind- Data Availability induced and other oscillations. Though developed originally Data underlying the findings of the research work described for condition assessment of cable-stayed bridges, the laser- here is available through the references cited in this paper. based vibration method was adapted for use in other types Availability of data generated during bridge evaluation of bridges. The technique has been used successfully for projects referenced in this paper is with discretion of the evaluation of twenty-five major bridges in the US and funding authorities and bridge owners. abroad that include cable-stayed, extradosed, suspension, pipeline, and arch supported bridges. In recent years, this vibration method has been complemented with operational Disclosure and formulation features to apply to differing conditions of cable systems including ungrouted stay cables and cables Opinions expressed in this paper are those of the authors and do not necessarily represent those of the sponsors. connected to each other with cross-ties. It has also found a valuable place in construction-phase activities for verification of forces in tension elements with minimal efforts. eTh Conflicts of Interest technique has proven to provide a rapid, cost-effective, and accurate method for evaluation and health monitoring of eTh authors declare that there are no conflicts of interest cable-supported bridges. Its application is also extending into regarding the publication of this paper. Advances in Acoustics and Vibration 13 Acknowledgments [15] E. Caetano, Dynamics of Cable-stayed Bridges: Experimental Assessment of Cable-Structure Interaction [Ph.D. thesis],Univer- The original development of the technique presented in sity of Porto, Porto, Portugal, 2000. this paper was performed in the Construction Technology [16] E. Aktan, D. Brown, C. Farrar, A. Helmicki, V. Hunt, and J. Yao, Laboratories (CTL Group), Skokie, IL, and was supported by “Objective Global Condition Assessment,” in Proceedings of the Federal Highway Administration under Contract no. DTFH- 115thInternationalModalAnalysisConference,vol.1,pp.364– 373, Orlando, FL, USA, February 1997. 61-96-C-00029. eTh technique was further developed in the course of several bridge evaluation projects. [17] P.W. Mast,J.G.Michopoulos,R.Badaliance, H.H.Chaskelis, and J. S. Sirkis, “Dissipated energy as the means for health monitoring of smart structures,” in Proceedings of the 1994 North References American Conference on Smart Structures and Materials,pp. 199–207, Orlando, FL. [1] N. Telang, C. Minervino, and P. Norton, “Retrofit of Aerody- [18] M. R. Banan, M. R. Banan, and K. D. Hjelmstad, “Parameter namic Cable Instability on a Cable-Stayed Bridge: Case Study,” estimation of structures from static response. I. computational Transportation Research Record, vol. 1740, pp. 61–67, 2000. aspects,” Journal of Structural Engineering (United States),vol. [2] A. B. Mehrabi, “In-service evaluation of cable-stayed bridges, 120, no. 11, pp. 3243–3258, 1994. overview of available methods and findings,” Journal of Bridge [19] M. Sanayei and M. J. Saletnik, “Parameter estimation of struc- Engineering,vol.11,no.6,pp.716–724, 2006. tures from static strain measurements. I: formulation,” Journal [3] A.B.Mehrabi,“Amonumentalbridgewithaproblemcausedby of Structural Engineering,vol.122,no. 5,pp.555–562,1996. oversights in design,” Bridge Structures,vol.2,no.2, pp.79–95, [20] A. B. Mehrabi, H. Tabatabai, and H. R. Lotfi, “Damage detection in structures using Precursor Transformation Method,” Journal [4] A.B.Mehrabi,“Performanceofcable-stayedbridges:Evalua- of Intelligent Material Systems and Structures,vol.9,no.10,pp. tion methods, observations, and a rehabilitation case,” Journal 808–817, 1998. of Performance of Constructed Facilities,vol.30, no.1,2016. [21] A. B. Mehrabi and A. T. Ciolko, “A non-destructive method [5] “Wind-Induced Vibration of Stay Cables,” Publication FHWA- for structural evaluation of pipeline suspension bridges,” in HRT-05-083, US Department of Transportation, Federal High- Proceedings of the Pipelines, ASCE, Chicago, IL, USA, August way Administration, Research, Development, and Technology, Turner-Fairbank Highway Research Center, McLean, Va, USA, [6] A. B. Mehrabi and H. Tabatabai, “Unified finite difference formulation for free vibration of cables,” JournalofStructural Engineering,vol.124,no. 11,pp.1313–1322,1998. [7] W.-H. P. Yen, A. B. Mehrabi, and H. Tabatabai, “Evaluation of stay cable tension using a non-destructive vibration technique,” in Proceedings of the 15th Structures Congress, pp. 503–507, ASCE, New York, NY, USA, April 1997. [8] R.Kulkarni andP.Rastogi,“Simultaneousestimationofmul- tiple phases in digital holographic interferometry using state space analysis,” Optics and Lasers in Engineering,vol.104,pp. 109–116, 2018. [9] A. M. Beigzadeh, M. R. R. Vaziri, and F. Ziaie, “Modelling of a holographic interferometry based calorimeter for radia- tion dosimetry,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,vol.864,pp. 40–49,2017. [10] D.Feng,T.Scarangello,M.Q.Feng, andQ.Ye,“Cabletension force estimate using novel noncontact vision-based sensor,” Measurement,vol.99, pp.44–52,2017. [11] Y. Xu, J. Brownjohn, and D. Kong, “A non-contact vision-based system for multipoint displacement monitoring in a cable- stayed footbridge,” Structural Control and Health Monitoring, vol. 25, no. 5, p. e2155, 2018. [12] J. L. Robert, D. Bruhat, J. P. Gervais, and J. Chatelain, “Mesure de la Tension des Cables par Methode Vibratoire,” Bulletin de liaison des laboratoires des ponts et Chaussee,vol.173,pp. 109– 114, 1991. [13] R. W. Clough and J. Penzien, Dynamic of Structures,McGraw- Hill, 2nd edition, 1993. [14] J. L. Lilien and A. P. Da Costa, “Vibration amplitudes caused by parametric excitation of cable stayed structures,” Journal of Sound and Vibration,vol.174,no.1, pp.69–90,1994. 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A Laser-Based Noncontact Vibration Technique for Health Monitoring of Structural Cables: Background, Success, and New Developments

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Copyright © 2018 Armin B. Mehrabi and Saman Farhangdoust. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Advances in Acoustics and Vibration Volume 2018, Article ID 8640674, 13 pages https://doi.org/10.1155/2018/8640674 Review Article A Laser-Based Noncontact Vibration Technique for Health Monitoring of Structural Cables: Background, Success, and New Developments Armin B. Mehrabi and Saman Farhangdoust Department of Civil and Environmental Engineering, Florida International University, Miami, FL 33174, USA Correspondence should be addressed to Armin B. Mehrabi; Amehrabi@fiu.edu Received 13 March 2018; Accepted 7 May 2018; Published 13 June 2018 Academic Editor: Cheng-Wei Fei Copyright © 2018 Armin B. Mehrabi and Saman Farhangdoust. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Structural cables are susceptible to the effects of high stress concentrations, corrosion, and wind-induced and other vibrations. Cables are normally the most critical elements in a cable-supported structure and their well-being is very important in the health of the structure. eTh laser-based vibration technique discussed in this paper is a means for health monitoring of cables and therefore the entire cable-supported structure. This technique uses a noncontact remote sensing laser vibrometer for collecting cable vibration data from distances of up to several hundreds of feet and determines its dynamic characteristics including vibration frequencies and damping ratios. A formulation specifically developed for structural cables capable of accounting for important cable parameters is then used to calculate the cable force. Estimated forces in the cables are compared to previously measured forces or designer’s prediction to detect patterns associated with damage to the cable itself and/or changes to the structure elsewhere. eTh estimated damping ratios are also compared against predefined criteria to infer about susceptibility against wind-induced vibrations and other vibrations. The technique provides rapid, effective, and accurate means for health monitoring of cable-supported structures. It determines the locations and elements with potential damage and the need for detailed and hands on inspection. To date, the technique has been used successfully for evaluation of twenty-five major bridges in the US and abroad. o Th ugh originally devised for condition assessment of stay cables, it has been developed further to include a variety of systems and conditions among them structural hanger ropes in suspension, truss, and arch supported bridges, ungrouted stay cables, cables with cross-ties, and external posttensioning tendons in segmental bridge construction. It has also found a valuable place in construction-phase activities for verification of forces in tension elements with minimal efforts. Future endeavors for automation and aerial delivery are being considered for this technique. 1. Introduction cable-supported structures. Incidents of major damage to cables in some cable-supported bridges [1–4] have created Despite all the advantages associated with structural cables enough concern for the owners to initiate a series of investi- and ropes, their use has not been without concerns and gation resulting in development of health monitoring systems questions. These include consequences of high stress con- and preventive measures. centrations in anchorage zones, corrosion, and susceptibility General inspection of cable structures provides use- ful information but can only reveal visible and/or highly to wind-induced vibrations and other vibrations. Because of these concerns, structural cable systems are being con- advanced damage that normally necessitates extensive repairs tinuously modiefi d and standard cable systems have yet to and remedies. This is because, normally, main tension ele- be established. Monitoring the health and performance of ments of structural cables, especially in the case of stay existing structural cable systems can help in determina- cables of cable-stayed bridges, are covered by layers of tion of advantages and shortfalls of various systems and corrosion protection elements and their condition is hidden identification of proper remedies for problems observed for from inspectors using traditional methods of inspection. 2 Advances in Acoustics and Vibration Therefore, availability of nondestructive testing methods for frequencies and damping ratios are calculated. Using this detection of damage early in the service life of the structural information and a formulation specifically developed for cables is essential for health monitoring of structures that rely structural cables, tension forces in the cables are calculated. heavily on these elements for their integrity and load carrying The resulting force distribution array in cables is used to capacity. Owing to the unique structural system in cable- make much more ren fi ed judgments about the condition, supported constructions, cables and tension members are aging, reliability, and maintenance of critical facilities than very suitable as surrogate sensors for reliable structural health previously possible. eTh innovation was so practical and monitoring adaptation for detecting damage and deteriora- effective that it was put to work right away as the NDT (Non- tion. Variation of forces and other dynamic characteristics of destructive Testing) tool of choice for assisting evaluation cables not only are indicative of their condition but also can of cable-supported bridges nationwide and abroad. Since its be indication of damage at any other location in the structure. development, this technique has been used successfully for Damage and changes at any location on the cable-supported condition evaluation of twenty-five landmark bridges in the structure do induce a change in the force distribution of all US, including thirteen cable-stayed bridges, vfi e suspension members, including the cables. It is also understood that bridges, and seven arch bridge. the intrinsic damping of structural slender elements such as Furthermore, one of the primary parameters which affect cables is normally very low [5], making them vulnerable to the susceptibility of the cables to aerodynamic vibration prob- large oscillations in presence of wind-induced vibrations and lems is the cable damping. Structural cables have minimal other source of excitations. intrinsic damping that is not normally adequate to suppress A rapid, laser-based vibration technique for force and wind-induced vibration in the cables. eTh y can potentially be damping measurement, along with numerical algorithms driven to large amplitude oscillations by a variety of wind- developed through federally and privately funded research, related mechanisms. Of the various wind-related mecha- has provided a practical, cost-effective tool to address imme- nisms, the structural cables are generally susceptible to three diate concerns and determine the need for action in relation main phenomena, namely, rain-wind-induced vibrations, to structural cables and cable-supported bridges [6, 7]. As galloping of various types, and vortex excitation [1, 5]. Of described above, force estimation in time provides an excel- these, the rain-wind phenomenon is a widespread problem lent means for detection of damage in structural cables and for stay cables resulting in large amplitude vibration of the cable-supported bridges. Figure 1 shows the laser vibrometer cables under moderate wind and light rain and has been targeting cables and a sample comparison of estimated forces. reported on several bridges around the world. Formation To also address vulnerability of structural slender elements of water rivulets under such conditions is related to this against wind-induced vibrations, damping ratios can be phenomenon. estimatedusing thelaser-based techniqueandcompared In recent years, there has been new developments on to predefined thresholds for verification of susceptibility application of various types of noncontact sensors to vibra- to excessive vibration. eTh damping measurement is also tion measurement. One such promising method is holo- employed to verify the damping provided by external devices graphic interferometry with ability to target several moving designed to suppress large oscillations. In all, the laser-based objectsfromasingleviewpoint[8,9].Vision-basedmon- vibration technique provides an excellent tool for damage itoring devices have also been used with some success for detection, determination of susceptibility to damage, and vibration measurement applications [10, 11]. Regardless of verification of design of mitigation methods. the type of technology, it is essential ensure assure adequate To date, this technique has been used successfully for accuracy and to avoid object identification errors during eld fi twenty-five cable-supported bridges and has continuously measurement. A comprehensive QC/QA for field measure- been adapted and evolved for new applications. With the mentprocessesaswellasfor postprocessingis critical for advent of new wave of technological advances in delivery of viable data. Noncontact sensor devices are only the tool for testing and inspection methods in the form of automated collecting the vibration data from slender tension elements. aerial and ground vehicles, an entirely new endeavor has The major task for force and damping estimation still remains opened for novel application of this already tested technique. with postprocessing and employment of simple yet accurate formulation. 2. Background 3. Field Implementation and Procedure As part of a research project sponsored by the Federal Highway Administration to develop a quantitative condition- Field application of vibration-based methods normally poses assessment technique for cable-stayed bridges, an innovative problems related to attachment of sensors (accelerometers) noncontacting, laser-based vibration technique was devel- to measurement points and interruption of bridge function oped to measure vibration, forces, and damping in structural and traffic flow. For periodic cable force measurements, the slender elements [6]. With this technique, cable vibration manual method of sensor attachment involves additional from ambient sources can be recorded from a distance by cost and difficulty. The use of laser-based sensors therefore targeting the cable with the laser beam. The laser can then oer ff s many advantages. The laser technique includes the turn to the next cable and continue measurement for all cables use of a noncontact laser vibrometer aimed at cable from a in direct view. By analyzing the vibration record, the cable distance. It eliminates the need for accessing cables for sensor vibration characteristics including fundamental vibration installation and normally avoids major interruption to traffic. Advances in Acoustics and Vibration 3 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 Span 1 Center Span Stay Cable Number Designer’s (Minimum Force) Designer’s (Maximum Force) 2009 North Measured Forces 2009 South Measured Forces Figure 1: Laser vibrometer targeting cables (left), sample force comparison (right). eTh laser-based measurement technique provides a noncon- location is not available on the bridge, the laser vibrometer tact remote sensing ability, and for periodic measurements canbestationed offthebridgewithintheeeff ctive range. andforcableswithamoredicffi ultaccessibility,thetechnique The effective range of laser vibrometer, depending on the has proven to be an extremely useful tool in reducing the type of the equipment, can be from 100 to 300 ft (30 to time and eo ff rts required for measurements. In most cases, 100 m). Usually, several cables are targeted from one laser ambient excitations sources such as traffic and wind induce station. Although the high sensitivity of the laser vibrometer enough vibration and there is no need for manual excitation. allows flexibility in laser positioning and targeting, the ideal However, if these sources are not present or not adequate, is to have the laser beam as perpendicular to the cable chord cables are excited manually. Depending on accessibility and as possible. Also, targeting the 1/3 of the cable length from cable stiffness and size, manual excitation can be performed the deck provides better clarity and at the same time better with slight impacting or the use of ropes, where a rope is participation of rst fi and second mode of vibration in the passed around the cable and pulled repeatedly with a rhythm recorded response. in concert with cable’s natural vibration frequencies [2]. The vibration time history of each cable is recorded in this manner which contains data showing the amplitude of velocity with respect to time for a predefined time interval. 3.1. Procedure. Prior to field measurements, physical prop- Using Fast Fourier Transform (FFT) analysis, the frequency erties of the cables including length, cross-sectional area, spectrum for each cable vibration is calculated and the mass per unit length, and mechanical properties are sought fundamental vibration frequencies are identified. Sampling andgatheredfromdesignoras-builtdrawingsandother rate and duration of vibration measurement are selected to sources. This information is utilized for sensor selection and provide the required frequency resolution and preventing the utilization planning, and for the selection of cable excitation aliasing eeff cts. For example, a 100 second recording duration methods and means of safe access to the cables. Normally, will offer 0.01 Hz resolution in the calculated frequency minimum traffic interruption is desired by the owners. spectrum. Also, to avoid the aliasing eects ff the data needs eTh refore, employing lift equipment to install contact sensors to be sampled at a rate at least twice the expected frequency (e.g., accelerometers) needs to be avoided, and the use of (Nyquist Theorem). Figure 2 shows a sample time history laser vibrometer is definitely preferred. Depending on the record and results in frequency domain. eTh force versus roadway shoulder width and other access limitations, the eld fi frequency relationship described later in this paper is then measurement can be performed with only one shoulder or used to calculate the force corresponding to each frequency one shoulder and a lane closure. In some instances, for cable- for cables. eTh accuracy of the force estimation using this stayed bridges with one plane of stay cables, the roadway median has been used for measurement operation without formulation is dependent on the accuracy of geometric any lane closures. For cables with customary mechanical and and physical/mechanical properties of cables provided for measurement process, as well as on the consideration and geometric properties, e.g., those with first-mode frequencies of up to 2 Hz, ambient sources normally provide adequate knowledge of the actual boundary conditions for cables. Although higher accuracies are achievable, the sampling rate excitation and there is no need for external sources. For stiffer cables that either have large cross-section or shorter lengths, and recording duration are normally selected to limit the manual excitation may be required. For measurements, the maximum error in frequency measurement to one percent, equivalent to about two percent error in the force mea- laservibrometerisstationedon thebridgedeckandcablesare targeted. For construction-phase measurement, when proper surement. For damping measurement, an excitation source Force (kips) 4 Advances in Acoustics and Vibration 1.464844 −1 −2 2.919922 −3 −4 0 10 20 30 40 50 0123456789 10 Time (Sec) Frequency (Hz) Figure 2: A vibration time history (left) and result in frequency domain (right). is required to generate a “considerable” movement in the (vi) All datasets for cables tested are stored and identified cables. It is realized that the damping ratio calculated for a uniquely with cable numbering. Other relevant infor- cable may be dependent on the initial excitation amplitude; mation such as date/time, wind, and temperature is however, to this end there is no guideline or standard as to also recorded. what this amplitude should be. The common way of exciting (vii) The time domain vibration data are transformed the structural cables for damping measurement is manual to a record of amplitude versus frequency by Fast excitation such as impacting with rubber mallet or using rope Fourier Transform processing. Dominant (funda- to apply resonating motion in the cable. Mechanical shakers mental) modes of frequency are identified and have also been used as source of excitation, but because of recorded for use in computation of tension force for concerns for damaging the cables, their application has not each individual cable. been favored by the owners. In any case, the damping ratios (viii) Individual cable forces are computed based on the test are calculated from the free vibration time history recorded data analysis protocol and mathematical formulation aer ft releasing cable using available methods. developed specicfi ally for cables of various character- Following steps are normally ensued for a typical force istics. measurement program: (ix) Time history records are also used for estimation of (i) To properly characterize physical properties of the cable damping ratios and compared to preden fi ed thresholds for vericfi ation of susceptibility to wind- cable, data related to its mass, length, geometry, stiffness, and component materials are sought and induced vibration. gathered from the client. The structural significance of cable array force distributions is studied through comparison of measurements with design- (ii) The attachment condition of the cable to other com- estimated cable forces, construction records, and/or previous ponents of the structure is obtained and studied, as measurement results. Comparative assessment seeks cursors are results of previous measurement programs for for cable force redistribution from the prior measurement or comparison. from baseline data. Signicfi ant changes of forces, e.g., lower- (iii) The data from steps one and two above are reviewed than-anticipated forces in a cable element, can be indicative of to assess their specific relevance to force measurement potential deterioration or damage. Additionally, the pattern procedures and accuracy, and the test data analysis of force changes in cable array can also indicate damage or protocol is defined for each individual cable variant changes in other elements of the bridge structure. Estimation on a structure. of damping ratios and comparison to predefined thresholds also determine the need for additional damping or adequacy (iv) The structure’s configuration is reviewed, permitting of existing damper devices. sensor selection and utilization planning and selec- tion of cable excitation techniques and safe access methods. 4. Theoretical Background (v) Laser vibrometer is stationed in appropriate location Duetothecomplexnatureofthe problemoffreevibration on the deck (or off the bridge) and the cables are tar- of structural cables with vibration characteristics that are sig- geted under ambient (wind or traffic), or if necessary nificantly different from strings, accurate yet simple analytical manual excitation, and time history of cable vibration relationships that take into account all pertinent parameters is recorded. are needed. In many cases (such as stay cables), the use of Amplitude Amplitude Advances in Acoustics and Vibration 5 the string equation leads to excessive oversimplification and L =L(1 +(mgLcos𝜃/H) /8), unacceptable increased errors in force predictions. I= equivalent moment of inertia, g= acceleration of gravity, 4.1. Force Measurement. eTh laser-based technique devel- 𝜃= inclination angle of cable chord, oped for health monitoring of structural cables is a vibration- L= chord length, based method. It has been completed and enhanced by development of mathematical formulation and simplified m= mass per unit length of cable, relationships that take into account the effect of parameters H= tension force, not considered in the taut string equation [6]. This technique E = equivalent modulus of elasticity of cable cross- has been validated by extensive laboratory experimentation section, andfieldvericfi ation,andhasbeenusedsuccessfullyfor evaluation of dozens of bridges. A= equivalent cable cross-sectional area. Formulation—For the tension members that deviate from a taut string by their different dynamic characteristics, 4.2. Damping Measurement. Normally, for measuring damp- the taut string theory needs to be augmented to be able ing, cables are excited manually and let free to allow recording to account for the differing characteristics of cables from of the time history. Figure 4 shows manual excitation of cables those of theoretical taut string. Mehrabi and Tabatabai [6] using a rope for stay cables and impacting with hammer introduced a n fi ite dieff rence formulation for vibration anal- for hanger ropes. With the vibration time history, various ysis of structural cables. This formulation incorporated the methods can be used to calculate the damping including effects of bending stiffness of cable and its sag-extensibility decay method and half-power bandwidth method [13]. If characteristics into a unified solution and provided a tool the measurement is performed in windy condition, wind for accurate determination of vibration mode shapes and speed and direction is also measured for calculating the frequencies. Various cable end conditions, variable cross aerodynamic damping that is used in calculation of eeff ctive sections, and intermediate springs and/or dampers were damping for cables [5]. taken into account. Using a nondimensional form of this formulation, a parametric study was conducted on the effects 5. Health Monitoring and Damage Detection of sag-extensibility and bending stiffness. Figure 3 shows Using Laser-Based Vibration Technique variation of the rfi st-mode inplane vibration frequency of a cable with respect to sag-extensibility and bending stiffness eTh laser-based noncontact vibration technique described parameters. This formulation was veriefi d with available abovehas theability todetectdamageinindividualcables theoretical solutions and compared with n fi ite element anal- and elsewhere in the structure that would aeff ct the cable yses. A simple relationship among nondimensional cable response. It can also be employed to determine the vulner- parameters was also introduced for the range of parameters ability of the cables against wind-induced excessive vibration applicable to stay cables in cable-stayed bridges. This simple that has the potential for damaging the cable and the structure relationship provides an accurate tool for measurement of by overstress, fatigue, and collision. tensionforcesinstaycablesusing thevibration method. eTh simple equation introduced by Mehrabi and 5.1. VulnerabilitytoWind-InducedVibration. One of the Tabatabai [6] is expressed as primary parameters which affect the susceptibility of cables to aerodynamic vibration problems is the cable damping. =𝛼𝛽 −0.24 (1) Cables normally have minimal intrinsic damping that is 𝜔 𝜁 ns not normally adequate to suppress wind-induced vibration. where Cables can potentially be driven to large amplitude oscilla- tions by a variety of wind-related mechanisms. Of the various 2 2 2 𝛽 =1+2/𝜁+(4+ n 𝜋 /2)/𝜁 [12], wind-related mechanisms, the structural cables and stays are generally susceptible to three main phenomena, namely, rain- 𝜔 = n𝜔 = n𝜋/𝐿 𝐻/𝑚, ns 1s wind-induced vibrations, galloping (dry and wake gallop- 2 2 𝜆 = L((WLcos𝜃/𝐻) /(HL/𝐸𝐴)), ing), and vortex excitation. Of these three, the rain-wind 1/2 phenomenon is a widespread problem resulting in large 𝜁= L(𝐻/𝐸𝐼) , amplitude vibration of the cables under moderate wind and W= cable weight per unit length, lightrainandhasbeenreportedonseveralbridgesaroundthe world. Formation of water rivulets during light rainfall with 𝜔 = First mode frequency as a taut string, 1s 2 moderate winds is associated with this phenomenon [1]. Dry 𝜇=𝜆 for n=1(in− plane), and wake galloping normally occur at higher wind speeds and 𝜇=0 for n>1(in− plane), require certain angles of attack with respect to the cable axis and position of adjacent cables with respect to each other. 𝜇=0 for all n(out− of− plane), The amplitude of vibration for ordinary structural cables (e.g., n= mode number, stay cables and hanger ropes) subject to vortex excitation is 𝛼=1+0.039𝜇, normally small and structurally insignica fi nt. 6 Advances in Acoustics and Vibration showsanoticeable drop duetoapotentiallossofcross- section, slippage, or similar anomalies, the cables adjacent to it should takethemajorityofthedroppedloadandtherefore wouldshowanincreaseintheirforces.This(local) pattern / offorcechanges wouldbeindicativeofapermanentforce variation and a potential damage in the cable showing force 2 drop. Accordingly, inspection of pattern for force variation can lead to detection of potential damage to an individual cable. Figure 6 shows a sample force comparison with a pattern indicating potential damage in a cable. This, at a minimum, will warrant a special inspection of suspect cable with the use of various available NDT methods [2]. 410 0.1  0.01 0.001 5.3. Global Damage Detection. Structures when exposed to Figure 3: Parametric study for first-mode inplane vibration fre- varying environmental conditions can undergo changes in quency of cables with respect to sag and bending stiffness. stiffness, material properties, and boundary conditions over time. Damage can initiate from various sources. With a sys- tematic and rational method, in general, measured changes in structural parameters reflected by individual sensor outputs In addition to the external sources such as wind, live load, canberelatedto damagesourcesand locations. eTh se andgroundmotion, theinteraction betweenthe structure parameters may include member forces, geometry profiles and supporting cables can generate excitation in one caused by vibration of the other. Cables may experience noticeable (deflections), support reaction forces, structural strains, and oscillation by “motion-induced” and/or “parametric excita- support settlements. This forms a general framework for most tion” [14, 15]. This phenomenon is characterized by motion global damage detection techniques. of the cables induced by motion of the structure, e.g., deck Several investigations have been performed on damage and towers of a bridge, during operational response of the detection of structures including vibration-based modal structure (i.e., wind, tracffi , pedestrian, excitation, etc.). analysis [16], dissipated energy density method [17], and To investigate the vulnerability of cables to wind-induced parameter estimation methods [18, 19]. These methods usu- and other types of excitations, various measures and criteria ally require a relatively large computational eor ff t and a have been introduced. Comparison of dynamic characteris- knowledge of exact loading configuration. An analytical tics of cables measured with the laser-based method with procedure, Precursor Transformation Method (PTM), was the respective criteria determines the adequacy of existing proposed by Mehrabi et al. [20] identifying the location(s) measures or the need for additional means for preventing and relative significance of possible damage sources based excessive and damaging oscillations. For example, in the case on measured changes in structural response parameters over of stay cables of cable-stayed bridges, Scruton Number, Sc time. This method oers ff advantages in sensitivity and cost (shown in (2)) is a nondimensional parameter calculated efficiency when compared to other available methods. Devel- based on the measured damping ratio, air density, and cable oped originally for cable-supported structures, this method diameter, and mass and is used as a measure of susceptibility takes advantage of the fact that cables and tension members of cables to rain-wind-induced vibrations. areverysuitablefor reliablestructural health monitoring adaptation as surrogate sensors for detecting damage and 𝑆𝑐= deterioration. (2) Variation of forces and other dynamic characteristics of cables not only are indicative of their own condition but In thisequation,ScistheScrutonNumber,𝜉 is the damping also can be indication of damage at any other location in ratio,𝜌 is the air density, and D is the cable outer diameter. thestructure.Damageandchangesatanylocation onthe Lower Scruton Numbers correspond to higher susceptibility cable-supported structure do induce a change in the force to wind-induced oscillations. Normally, a minimum Scruton distribution of all members, including the cables. Therefore, Number of 10 is recommended for stay cables to avoid large- in this method, changes in the state of the structure are amplitude rain-wind-induced vibration [5]. If this number experimentally assessed through measurement of structural cannot be achieved for a stay cable, each external measure response parameters such as displacement, strain, or internal or a combination external measures such as cable surface cable forces for the case of cable-supported structures at modification, viscous and other type of dampers, and cross- discrete points on the structure at a reference time and ties will need to be implemented to suppress the vibration. later at any desired time. For application of this method, Figure 5 shows Scruton Numbers calculated for a cable array the external loading state at different measurement times and comparison with the criteria discussed above. should be constant. This loading could be the dead load of the structure alone or augmented with additional live 5.2. Damage to Individual Cables. In general, it is expected load. To uncouple the effects of different sources of damage that if dead load force (or a constant loading state) in a cable and to determine their locations and relative significance 𝜌𝐷 𝑚𝜉 Advances in Acoustics and Vibration 7 Figure 4: Manual excitation for damping measurement; using rope (left) and making impact with rubber mallet (right). 30.0 25.0 20.0 Pattern indicating potential damage to Cable NE55 15.0 10.0 5.0 0.0 Cable No. Cable No. Estimated Forces (Period 2) Figure 5: Sample Scruton Number calculated for cables using data Baseline Estimated Forces (Period 1) recorded with the laser technique. Figure 6: A sample of force comparison with a pattern indicating potential damage in cable. based on the experimental data, an analytically determined transformation matrix is utilized. From an analytical standpoint, the sources of damage can remaining service life in cable-supported bridges. Examples be characterized as precursor events (or damage precursors) are force measurement of hanger ropes in the Hoan Arch that precipitate changes in the state of the structure. Precur- Bridge (Figure 8) and the Bosporus Suspension Bridge sors are externally imposed and are therefore independent of (Figure 9). thestructure andthe subsequentchangesinthe stateofthe structure. Examples of damage sources that can be modeled as precursor events include loss of material or stiffness, 6. Success in Implementation joint slippage, support settlements, and loosening of bolts. For the case of cable-supported structures, the precursor eTh laser-based noncontact vibration technique for structural transformation matrix contains patterns of force changes in health monitoring was initially developed for condition cablearrayeachgeneratedanalyticallyforaspecicp fi recursor assessment of cable-stayed bridges. Its application was limited [20]. Figure 7 shows force variation calculated for cables of a to stay cables of certain congfi uration. However, the method cable-stayed bridge (Figure 7(b)) and calculated precursors as has been evolving in time to include various stay cable damage identified for three of its cables (Figure 7(c)). conditions, and its application was extended into evaluation Laser-based vibration measurement technique has been of cables and ropes in suspension and arch bridges. It can, in usedalsoforinvestigationofcauseofdamageorestimationof general, be utilized for any structure with slender structural Scruton No. 1n 2n 3n 4n 5n 6n 7n 8n 9n 10n 11n 12n 13n 14n 15n 16n 17n 18n 19n 20n 21n 22n 23n 24n 25n 26n 27n 28n 29n 30n 31n 32n 33n 34n 35n 36n 37n 38n 39n 40n 41n 42n Force (kips) NE41 NE42 NE43 NE44 NE45 NE46 NE47 NE48 NE49 NE50 NE51 NE52 NE53 NE54 NE55 NE56 NE57 NE58 NE59 NE60 NE61 NE62 NE63 NE64 NE65 NE66 NE67 NE68 NE69 NE70 8 Advances in Acoustics and Vibration Table 1: List of bridges for which the laser-based vibration technique has been employed. Cable-Stayed Bridges Location Scope Date(s) Second Vivekananda Bridge Kolkata, India Stay Cable Force Measurement 2016 Leonard Zakim Bridge Boston, MA Stay Cable Force Measurement 2015 Sunshine Skyway Bridge St. Petersburg, FL Stay Cable Force and Damping Measurement 1999, 2009,2015 Dames Point Bridge Jacksonville, FL Testing and Evaluation 2008,’10,’12,’16 Luling Bridge Luling, LA Stay Cable Force and Damping Measurement 2002-2006 Queen Elizabeth II Dartford, UK Stay Cable Force Measurement 2008 Maumee River Crossing Toledo, OH Stay Cable Force Measurement 2006 Varina-Enon Bridge Henrico, VA Stay Cable Force Measurement 1999 & 2007 C&D Canal Bridge Middletown, DE Stay Cable Force Measurement 2005 Fitchburg Bridge Fitchburg, MA Stay Cable Force Measurement/ Construction Phase 2003 Talmadge Memorial Talmadge, GA Stay Cable Force Measurement 2000 Cochrane Bridge Mobile, AL Stay Cable Force and Damping Measurement 1998 Weirton-Steubenville Weirton, WV Stay Cable Force Measurement 1997 Suspension Bridges Bosporus Bridge Istanbul, Turkey Hanger Force Measurement, Failure and Fatigue Analysis 2004 Tazlina Pipeline Bridge Glennallen, AK Hanger Ropes Force Measurement 1999 & 2004 Tanana Bridge, AK Delta Junction, AK Hanger Force Measurement 2001 Carquinez Bridge Vallejo, CA Hanger Force Measurement/ Construction Phase 2003 Paseo Bridge Kansas City, MO Hanger Force Measurement 2002 Arch Bridges Hart Bridge Jacksonville, FL Hanger Force Measurement/ Construction Phase 2016 Sherman-Minton Louisville, KY NDE Testing 2011 Troup Howell Rochester, NY Hanger Force Measurement/ Construction Phase 2006 & 2007 Telegraph Road Taylor, MI Hanger Force Measurement/ Construction Phase 2007 Cass Street Bridge La Crosse, WI Hanger Force Measurement/ Construction Phase 2004 & 2005 Belle-Vernon Bridge Belle-Vernon, PA Hanger Force Measurement/ Following Accident 2003 Hoan Bridge Milwaukee, WI Hanger Force Measurement /Failure Analysis 2001 elements. A list of twenty-five major bridges for which this structures where access to the cables is extremely limited. method hasbeenutilizedisshownin Table1. Table 1 identifies the bridges for which the technique has been used during construction to verify the forces and apply adjustments if necessary. 6.1. New Developments. Following describes some of the later applications for this technique. 6.4. Extradosed Bridges. In 2016, the laser-based vibration technique was used to measure forces in stay cables of the 6.2. Pipeline Bridges. In 1999, 2001, and 2005, engineers were Second Vivekananda Bridge in Kolkata, India, as a part of able to stand on the spectacular Tanana and Tazlina River routine maintenance and inspection program. This bridge, banks in Alaska and measure remotely the forces of suspender shown in Figure 11, is the first extradosed bridge for which and other cables of two suspension bridges carrying Trans this technique has been utilized. One of the unique features Alaskan Pipeline System with great efficiency, accuracy, and of stay cables in an extradosed bridge is their relatively short speed. This was a part of scheduled evaluation project to length and higher bending stiffness. determine the safety and soundness of this major pipeline to continue carrying crude oil in Alaska [21]. Figure 10 shows laser being used for force measurement of hangers in one of 6.5. Ungrouted Stay Cables. Most of the newer generation these bridges. of stay cables in the US and elsewhere do not use grout filling as corrosion protection method. Having unbonded 6.3. Construction-Phase Force Verification. The exceptional and normally detached cover pipe was thought to introduce value of the laser-based vibration technique for construction- complications in vibration measurement of the stay system, phase cable force vericfi ation was quickly recognized. eTh therefore resulting in difficulties for estimating the force techniqueoeff redarapidand economic yetaccuratemethod from uncoupled vibration characteristics. The laser-based as alternative to logistically cumbersome methods such as technique adapted for taking into account the noncomposite li-ft offs. Additionally, the remote noncontact nature of the action of the cable cross-section was used successfully for technique made it the only reasonable choice for unn fi ished the rfi st time in 2015 for force measurement of ungrouted Advances in Acoustics and Vibration 9 1A 1B 17 24B 24A North Side South Side (a) 300 500 CABLE 15 CABLE 21 CABLE 3 −100 200 −200 −300 −400 NORTH SIDE NORTH SIDE −100 −500 SOUTH SIDE SOUTH SIDE (b) (c) Figure 7: Damage detection using precursor transformation method. (a) schematic of a cable-stayed bridge superstructure, (b) force variation in cables, and (c) damage detected in three cables. stay cables of the Leonard Zakim Bridge in Boston, MA inspection, and health monitoring program. The technique (Figure 12). wasalsousedin2015forforcemeasurement oftied stay cables of the Leonard Zakim Bridge in Boston, MA (Fig- ure 12). 6.6. Tied Stay Cables. Cross-ties, designed as a measure for vibration suppression of structural cables, alter dynamic characteristics of the cables, making it difficult to iden- 6.7. Structural Health Monitoring Using Periodic Force Mea- tify their individual vibration frequencies for force calcu- surement. eTh laser-based vibration technique is being used lation using the available algorithms. Dames Point Bridge by some bridge owners for periodic force measurement as in Jacksonville, FL, is a cable-stayed bridge whose stay part of bridge health monitoring and maintenance programs. cables aretiedtoeachotherusingcross-ties(Figure 13). The Sunshine Skyway Bridge in Florida shown in Figure 14 After a series of verification experiments and research in is one of these bridges. For this bridge, in addition to force 2008 to 2010, a new procedure for efi ld application was measurement, the laser technique is also used for damping developed and numerical formulation was complemented measurement of the cables with and without contribution of for taking into account the connectivity of cables. eTh laser the external damping devices. This ensures proper function- techniquehassincebeenusedfor twosuccessfulperiodic ing of the dampers and adequacy of the overall damping for cable force measurements as part of its routine maintenance, suppression of wind-induced vibration. Force Changes (kN) Precursor 10 Advances in Acoustics and Vibration Figure 8: Hoan Bridge in Milwaukee, WI. Figure 9: Bosporus Bridge in Istanbul Turkey. 6.8. External Posttensioning Tendons. Currently, the laser- methods for structural health monitoring. Undoubtedly, the based method is being considered for a new application to laser vibration technique discussed in this paper has a great external posttensioning tendons of a segmental concrete box potentialasatriedandveriefi dmethodtobeimplemented girder bridge. Most external posttensioning tendons utilize with an automated delivery, let it be aerial or ground vehicle. a system very similar to grouted stay cables; therefore, their force estimation can be performed using the same eld fi and 7. Summary and Conclusion analysis procedure described earlier. Tendons however are normally stressed to much higher stress levels than stay cables A laser-based vibration technique for health monitoring and hence possess dynamic characteristics that are different of cable-supported structures was discussed in this paper. fromthoseofstaycables.Tendonsareexpectedtohavehigher The technique includes field implementation of noncontact frequencies owing to their shorter lengths and higher forces remote sensing laser vibrometer to record the vibration in comparison with stay cables. and calculate the dynamic properties and a formulation developed specicfi ally for structural cables for calculation 6.9. Future Applications. Technological advances in the appli- of their tension forces. eTh comparison between estimated cation of automated unmanned vehicles and robotics have cable forces using this technique and previously measured created new opportunities for application of nondestructive or expected forces can be used to establish a pattern of Advances in Acoustics and Vibration 11 Figure 10: eTh use of laser-based vibration technique for pipeline bridges. Figure 11: Extradosed cable-stayed bridge for which the laser-based technique was used. Figure 12: Laser technique was used for evaluation of ungrouted stay cables of the Leonard Zakim Bridge. 12 Advances in Acoustics and Vibration Figure 13: eTh forces of tied stay cables of the Dames Point Bridge are periodically evaluated using the laser technique. Figure 14: eTh Sunshine Skyway Bridge in Florida. changes indicative of location, type, and intensity of the segmental bridge construction and force estimation of exter- nal posttensioning tendons. Future endeavors for automation potential damage to the cable and the structure elsewhere. and aerial delivery are being considered for this technique. eTh damping measured for cables with this technique is used for comparison against predefined thresholds and to determine their vulnerability to various types of wind- Data Availability induced and other oscillations. Though developed originally Data underlying the findings of the research work described for condition assessment of cable-stayed bridges, the laser- here is available through the references cited in this paper. based vibration method was adapted for use in other types Availability of data generated during bridge evaluation of bridges. The technique has been used successfully for projects referenced in this paper is with discretion of the evaluation of twenty-five major bridges in the US and funding authorities and bridge owners. abroad that include cable-stayed, extradosed, suspension, pipeline, and arch supported bridges. In recent years, this vibration method has been complemented with operational Disclosure and formulation features to apply to differing conditions of cable systems including ungrouted stay cables and cables Opinions expressed in this paper are those of the authors and do not necessarily represent those of the sponsors. connected to each other with cross-ties. It has also found a valuable place in construction-phase activities for verification of forces in tension elements with minimal efforts. eTh Conflicts of Interest technique has proven to provide a rapid, cost-effective, and accurate method for evaluation and health monitoring of eTh authors declare that there are no conflicts of interest cable-supported bridges. Its application is also extending into regarding the publication of this paper. Advances in Acoustics and Vibration 13 Acknowledgments [15] E. Caetano, Dynamics of Cable-stayed Bridges: Experimental Assessment of Cable-Structure Interaction [Ph.D. thesis],Univer- The original development of the technique presented in sity of Porto, Porto, Portugal, 2000. this paper was performed in the Construction Technology [16] E. Aktan, D. Brown, C. Farrar, A. Helmicki, V. Hunt, and J. Yao, Laboratories (CTL Group), Skokie, IL, and was supported by “Objective Global Condition Assessment,” in Proceedings of the Federal Highway Administration under Contract no. DTFH- 115thInternationalModalAnalysisConference,vol.1,pp.364– 373, Orlando, FL, USA, February 1997. 61-96-C-00029. eTh technique was further developed in the course of several bridge evaluation projects. [17] P.W. Mast,J.G.Michopoulos,R.Badaliance, H.H.Chaskelis, and J. S. Sirkis, “Dissipated energy as the means for health monitoring of smart structures,” in Proceedings of the 1994 North References American Conference on Smart Structures and Materials,pp. 199–207, Orlando, FL. [1] N. Telang, C. Minervino, and P. Norton, “Retrofit of Aerody- [18] M. R. Banan, M. R. Banan, and K. D. Hjelmstad, “Parameter namic Cable Instability on a Cable-Stayed Bridge: Case Study,” estimation of structures from static response. 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Tabatabai, “Evaluation of stay cable tension using a non-destructive vibration technique,” in Proceedings of the 15th Structures Congress, pp. 503–507, ASCE, New York, NY, USA, April 1997. [8] R.Kulkarni andP.Rastogi,“Simultaneousestimationofmul- tiple phases in digital holographic interferometry using state space analysis,” Optics and Lasers in Engineering,vol.104,pp. 109–116, 2018. [9] A. M. Beigzadeh, M. R. R. Vaziri, and F. Ziaie, “Modelling of a holographic interferometry based calorimeter for radia- tion dosimetry,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,vol.864,pp. 40–49,2017. [10] D.Feng,T.Scarangello,M.Q.Feng, andQ.Ye,“Cabletension force estimate using novel noncontact vision-based sensor,” Measurement,vol.99, pp.44–52,2017. [11] Y. Xu, J. Brownjohn, and D. 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