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37 CHAPTER THREE INSPECTION AND MONITORING TECHNIQUES In this chapter, various inspection and monitoring techniques cables move past the inspection unit, which would need for stay cables are discussed, including their advantages and to be reversed on a cable bridge; impulse radar--good for disadvantages. Figure 53 shows survey results with respect to detecting grout defects; sonic methods--dampened to the the types of nondestructive tests performed on the cables. The point of being ineffective. most commonly used method is the vibration-based force Four single strands are to be removed (one at each measurement. However, the largest group belongs to the "not pylon) for inspection for rust every 10 years, starting in performed" category. year 2014. Figure 54 shows the survey results with respect to the types of sensor-based, long-term monitoring on the cables. SHORT-TERM EVALUATION AND MONITORING Three respondents' bridges in the United States and one in This section covers methods that can be used during limited- Canada incorporate acoustic wire break detection, whereas duration inspections of stay cables. The currently available two respondents in the United States and one in Canada methods, as well as new and promising technologies, are cat- incorporate long-term vibration monitoring. egorized and explained. The techniques that are addressed include conventional visual/manual techniques, and magnetic, Question 6 in the survey asked respondents to comment on ultrasonic, X-ray, laser, acoustic, and remote or contact vibra- the effectiveness of any nondestructive test methods for stay tion-based techniques. cables of which they are familiar. Some of their comments are given later in this chapter. Others are provided here: As discussed earlier, Mayrbaurl and Camo (2004) reported on a study of structural safety of suspension bridge parallel- Several nondestructive tests were run after an extreme wire cables. They concluded that there were currently (as of oscillation event. Practically all of the methods cited in 2004) no effective NDE methods for the condition assess- Question 6 were performed to determine if there was ment of parallel wire main cables of suspension bridges. any loss of force in the stays. Geometric, physical, and Instead, they focused their efforts on manual unwrapping and visual tests were performed. The only discoveries were opening of cables for their evaluations. deficiencies in the original construction, which were corrected. The fundamental frequency of the cables was recorded. Visual Inspections Alaska DOT&PF (Department of Transportation and Public Facilities) will monitor the cables and attempt to Visual inspections are the most common approach used on determine if the fundamental frequency of the cables has stay cables. Surveys completed by a number of respondents changed. indicated a preference for visual inspections (when feasible) The presence of the steel protective pipe limits the effec- and a desire to see stay cable designs that can be visually tiveness of many available testing methods, particularly inspected. magnetic-based methods. Implementation of laser-based cable stay force measurements are being considered by Some bridges have dual inspection schedules, a routine the department to establish baseline force data for the inspection at 2-year (or less) intervals, and more detailed cable stays. inspections at longer intervals. In the case of the Faroe Bridge Nondestructive testing is needed to determine the condi- between Sealand and Falster in Denmark, a three-step inspec- tion of tension bars inside the steel casing of the cables. tion process is used (Bloomstine and Stoltzner 1999). The The only problem is the anchorage area. So far, no bridge master performs a drive-through inspection every day. method is available for inspection. I see additional prob- Various bridge components are inspected at yearly intervals, lems with inspection of the grout-filled cables. so that the inspection of the entire bridge is completed in a Vibration-based cable load determination--effective and 5-year cycle. Special inspections are done if damage is noted. inexpensive; X-ray--expensive, slow, very questionable ability to detect wire defects; magnetic inspection-- During typical inspections of stay cables, the entire sur- used to rapidly, effectively inspect mine cables--but the face of the cable is visually inspected at close range, followed

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38 90.0 80.0 U.S. Canada Percent of Bridges 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 magnetic x-ray ultrasonic vibration- other not not known no answer based force performed measurements NDT Methods FIGURE 53 Types and levels of nondestructive testing on stay cables. by an inspection of neoprene boots and neoprene rings (by Inspection for damage or dislocation of neoprene rings removing neoprene boots), visible surfaces of guide pipes, and and keeper rings, if applicable. accessible anchorage surfaces. General visual inspections of Identification of gaps between the neoprene rings and stay cables typically involve the following: the sheathing. Examination of sheathing surface inside the guide pipe Identification of longitudinal or transverse cracking or through a boroscope or other means, looking for dam- excessive bulging in the sheathing, as well as damage at age or deformation to the sheathing near the anchorage. connections to dampers or cross cables, if any. Review of cracking or damage to guide pipes or evidence Inspection for cable alignment irregularities including of the impact of cable components on guide pipes. waviness or excessive sag. Cable sag can be estimated Examination of surface conditions on the visible anchor- (measured) using optical devices or through video or age components including ring nuts, end caps, and bear- photo image processing. Cable angle can be measured ing plates. with an inclinometer at specific points. Examination of visible parts of saddles for damage, Identification of changes to bridge deck elevations. corrosion, and cracking, if applicable. Examine damage to protective tape wrapping (tears, Review of evidence of moisture or fillers (such as cracks, and delaminations). grease) exiting the anchorage components. If there is an Examine damage to sheathing, especially when PVF access port at the end cap (ideally at the lowest point), tape is not used. Attention should be paid to cracking in it can be opened and examined for moisture or moisture- the sheathing, especially at high stress areas. contaminated grease. Identification of damage to connections between anchor- Removal, in some cases, of the end caps on the sockets age pipes and cable sheathing. to allow for visual inspection of the anchorage plate and Inspection for damage, loosening, lack of water tightness, anchorage devices and to see if there is moisture or cor- and deterioration of neoprene boots and band clamps. rosion inside. 90.0 Percent of Bridges 80.0 70.0 U.S. Canada 60.0 50.0 40.0 30.0 20.0 10.0 0.0 acoustic vibration force other not not known no answer wire break monitoring measure- performed detection ments Monitoring FIGURE 54 Types and levels of sensor-based, long-term monitoring.

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39 Inspection of the cross tie cables for sagging (losing For the inspection of the Dame Point Bridge in Florida their force and need to be retensioned). ("B&N Creates Custom Device . . ." 2005), the inspection Inspection of damage or cracking on components of team custom designed a rolling device (Figure 56). The cross tie cables. Evidence of fretting and fatigue, espe- weight of the device was an important factor in the design, cially at connections, are of particular interest. because the inspector has to carry several hundred feet of Examination of dampers, if any, as per recommenda- rope and other inspection equipment ("B&N Creates Custom tions of manufacturer. Device . . ." 2005). Some maintenance manuals recommend inspection of stay In Denmark, a carrier for inspection of the main cables of cable surfaces using binoculars during routine inspections and a suspension bridge has been developed (Figure 57). close viewing during detailed inspections. However, it should be noted that the bridge maintenance community in general In their written comments, many survey respondents emphasized the desirability of finding effective ways to does not view the use of binoculars for bridge inspections pos- inspect cables visually. itively, as it may discourage the preferred method of close inspection. Vibration-Based Cable Force Measurements The access to cable components can be gained through "reach-all" trucks, and lifts or cranes with "baskets." In some The vibrating chord theory presents a simple relationship bridges, special inspection vehicles for stay cables have been between the tension in a string (T ) with its mass per unit length designed to allow for easier access to cables. For example, (m), its length (L), and its natural frequency (f) as follows: the Luling Bridge in Louisiana has two trolleys designed for T = 4 L2 f 2 m Eq. 2 inspections of cables (Elliott and Heymsfield 2003). They were reportedly built in 1985 at a cost of $3,000. Only the In its simplest form, a stay cable can also be approximated maintenance lane and one traffic lane need to be closed dur- as a vibrating string. If its natural frequency could be deter- ing inspection. The trolley is a steel frame carriage with a mined then, knowing all other parameters, the cable force detached basket (see Figure 55). Two inspectors and equip- could be determined. A number of researchers have used ment totaling 1780 N (400 lb) can be used. A wire rope is accelerometers installed on cables to measure the cable's nat- used to pull the trolley up the cables. However, there are indi- ural frequency and estimate the cable force (Casas 1994). cations that changes to the design of the trolley are recom- However, in some cases, measurement of cable frequencies mended by DOT personnel to increase redundancy and on a large number of stay cables on a major bridge can be time provide a braking system. This system is reportedly suitable consuming. The assumptions inherent in Eq. 2 are also not for larger diameter cables only. strictly valid in stay cables. Stay cables have bending stiffness, FIGURE 55 Trolley used for inspection of Luling Bridge cables (Elliott and Heymsfield 2003).

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40 shine Skyway Bridge in Florida. Figure 58 shows the laser measurement approach in the field. Cunha and Caetano (1999) used the developed laser mea- surement approach to measure cable frequencies on the Vasco de Gama cable-stayed bridge in Lisbon, Portugal. Also, the survey results in this study indicated that the Ministry of Transportation personnel in Quebec, Canada, have themselves measured the cable forces on the Galipeault Bridge using the same laser-based method. Yamagiwa et al. (1999) presented a method for simultane- ous identification of bending stiffness and tension in a cable using vibration measurements. Experiments on a spiral rope FIGURE 56 Rolling device for inspection of the stay cables on for a cable-stayed bridge were performed and the authors the Dame Point Bridge ("B&N Creates Custom Device . . ." 2005). reported good agreement between measured and calculated values. whereas Eq. 2 assumes zero bending stiffness. Cables also sag It should be noted that results of similar accuracy could under their own dead weight and have other complicating fac- alternatively be obtained by simply attaching an accelerom- tors such as neoprene rings, viscous dampers, and variable eter on the cables to determine frequencies, and then using stiffness along their length (e.g., anchorage sockets), that fur- the available equations to estimate forces. Whether the ther complicate the analytical relationship. To address these accelerometer or laser-based approach is selected, it is issues, FHWA funded a research project in the mid-1990s to important to reemphasize that one could not necessarily develop a laser-based noncontact method for cable vibration conclude that there has not been a section loss because measurements in the field (Angelo 1997). The effectiveness cable forces have not changed. This is especially true in of using a laser Doppler vibrometer was established for mea- grouted cables where broken wires redevelop over a short surements of ambient cable vibrations from distances of up to distance. Unless and until wire breaks result in global stiff- several hundred feet (Tabatabai et al. 1998b). More impor- ness changes in the cable, section loss could not be inferred tantly, nondimensional relationships that included the effect of from cable force measurements. cable bending stiffness, cable sag, and so forth, were developed for a more accurate estimation of cable forces (within 1% to The following comments related to vibration-based force 3% accuracy) using measured frequencies. This approach has measurements were provided by the respondents to the survey: been used on several U.S. cable-stayed bridges including the WeirtonSteubenville Bridge in West Virginia, VarinaEnon "Laser-based force measurements will give results that Bridge in Virginia, Cochrane Bridge in Alabama, and Sun- will indicate if a cable is deviating from the trending val- FIGURE 57 Carrier for inspection of main cable of suspension bridge in Denmark.

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41 FIGURE 58 Use of laser doppler vibrometer for stay cable vibration and force measurements. ues of the other cables. It may not give you an accurate axial accelerometers to determine frequency of the cable value of the force in a cable. It is relatively easy and and relate back to force." inexpensive to perform." "TxDOT has employed vibration-based force measure- ments to refine the model used for designing viscous Other Methods of Measuring Cable Forces dampers on each of the cable stay bridges. The technique seemed to give good correlation cable dimensions and Some stay cable suppliers and contractors have used mea- damping requirements. The technique requires some traf- surements of cable sag to estimate cable forces. Cable sag is fic control and depending upon the number of lanes car- defined as the maximum vertical displacement of the cable ried by the structure could produce minor-to-significant with respect to a line connecting its two ends. There is a sim- traffic disruption. At least one lane and the shoulder will ple inverse relationship between the sag of a cable and its ten- need to be closed; therefore, if the bridge is narrow with sion. However, the results of the survey in this study did not a small number of lanes carrying two-way traffic the dis- reveal any instances where inspectors have measured cable ruption could be considerable. This could last for several sag as part of their routine inspections of cable-stayed bridges. weeks if there are a large number of stays that need to be Photogrammetric or optical methods can be used to allow tested. The cost can run anywhere from $50,000 to inspectors to measure cable sag from the deck level without $75,000 per bridge per test event depending upon the size the need for specialized assistance. of the structure." "Laser-based force measurements were utilized in the Another option for cable force measurements on new cables initial in-depth inspection of this bridge in 1999. The cost would be to install low-profile load cells under the anchor- incurred was approximately $35,000, with minimum age. This could be an effective, although relatively costly impact on traffic." option. Contractors have also used a method called "liftoff" "Force measurements on selected MTEs will be per- to measure forces. In this approach, a large hydraulic jack is formed as part of the SHM system with the use of uni- used to lift the anchorage off of the anchorage plate. The

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42 force required for the liftoff is the cable force. This method is cumbersome and costly, especially for inspection purposes. Force measurement sensors on selected individual strands on a cable are likely to be developed based on magnetoelas- tic effect or other effect in the near future. Some cable sup- pliers are working to develop force-measuring systems for their cables. The sensors can be applied to the entire cable or to individual strands. If individual strands are instrumented, the total force is estimated based on an assumption of equal forces in all strands. Ultrasonic Assessments of MTEs in Anchorage Zones FIGURE 59 Typical ultrasonic test record of broken wire (Prato et al. 1997). Ultrasonic techniques have been used for assessment of MTEs in stay cables. Desimone et al. (2001) studied the pulse wave propagation along a bar (wire), and reported on experiments ers must be properly coupled to the cut-and-ground ends of the on wires with and without notches and grooves of various strands. Because the six perimeter wires wrap around the cen- depths. The first known application of ultrasonic testing for ter wire and are in contact with each other, wave transmission assessments of wire conditions in stay cable anchorages was is more complicated than in single straight wires. by Suzuki et al. (1988). The cable anchorage in that case was a Hi-Am-type socket (a steel socket filled with an epoxysteel It is very important that an existing anchorage of an identi- ball compound) containing steel wires terminating at button cal or similar type be made available (or a mock-up made) heads. In this method, an ultrasonic transducer is coupled to before field testing to calibrate the results for known defects the end of each wire or button head and a high-frequency stress and their locations. The operator's experience and ability is wave is sent into the wire. The reflections are monitored by crucial, as judgment is required when interpreting results. the same sensor and displayed. A trained technician can view There are however no known systematic and rigorous research the record and decide if a wire break has occurred. It should programs performed to date that are aimed at quantifying the be noted that ultrasonic pulses could travel a long distance degree of accuracy of this method for various anchorages, and along a wire if that wire was free in air. However, as the wire ways of improving the interpretation of results. is enclosed by grout and/or anchorage epoxy, a significant attenuation of the pulse reduces the effective length over which Magnetic Methods this method can be used. Suzuki et al. (1988) reported that the depth of wire-break detection for a Hi-Am-type anchor- When a magnetic field moves along the length of a cable con- age was a few meters. However, a few meters would theo- taining steel MTEs, presence of corrosion or fracture in the retically be sufficient for inspection of most anchorages. wires changes the magnetic field. Sensors can detect such changes and produce electrical output as a result. Figure 61 Following the failure of a cable on the ZarateBrazo Largo shows a magnetic flux leakage signature, with the characteris- Bridges in Argentina, a series of ultrasonic tests was per- tic shape representing the flaw. The horizontal axis is the posi- formed on the remaining anchorages. The failure was noted in the cable near the entrance to the anchorage socket (Hi- Am-type). Prato et al. (1997) reported on the ultrasonic tests undertaken in which a large number of wire breaks were detected in various cables. Figure 59 shows an ultrasonic test record indicating a wire break. However, it is not clear if the test record shown in the figure is indicative of the clarity and definiteness of a typical ultrasonic test record or perhaps a representation of one of the better results. The first application of ultrasonic testing on seven-wire strands was done on 12 anchorages of the Cochrane Bridge in Alabama (Tabatabai et al. 1998a; Ciolko and Yen 1999). Figure 60 shows testing on a tower anchorage. There are further complications with stress wave transmis- FIGURE 60 Ultrasonic testing of cable anchorage (Ciolko and sion through a seven-wire strand. Typical ultrasonic transduc- Yen 1999).

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43 FIGURE 61 Signature from a flaw in a steel cable (courtesy: A. Ghorbanpoor, University of WisconsinMilwaukee). tion along the scanned length of cable. The size of flaw and that EM methods had been used in Germany for bridge stay distance from the sensor determine the signal amplitude and cables for 25 years (the publication date of the paper was shape. The method to identify location and extent of damage 1995). In addition, they referred to a device that can travel based on the above approach is variably called magnetic per- along the cable and that uses four differential sensors (LF turbation, magnetic flux leakage, or magnetic induction. type) to detect wire breaks. They also discussed the effects of trapped magnetic debris on the accuracy of LMA mea- Barton et al. (1989) developed the first prototype device surements. for inspection of the free lengths of stay cables based on the magnetic perturbation method. This device would surround In the United States, Ghorbanpoor (1999) developed a the cable and move along its accessible free length. The first MFL robotic device for NDE of strands within prestressed application of this device was on the Luling Bridge in Loui- concrete girders. This device would attach itself to the bottom siana. Teller et al. (1990) also reported on the use of this flange of typical I-girders and would automatically travel the device on the PascoKennewick Bridge in Washington State. length of the beam. This system was effective; however, because of its large size and weight, it was difficult and time consuming to position Kitagawa et al. (2001) briefly described using the magnetic and move the device from one cable to another. It was also flux method to detect corrosion in hangers of a suspension limited to the cable free length and could not access the bridge in Japan. Wichmann et al. (2003) described an EM res- anchorages. onance measurement method for identification of localized fractures in tendons. The idea is described as follows: the ten- EMPA, a materials science and research institution in don is considered as an "unshielded resonator located in a Switzerland, has developed a magneto-inductive evaluation material with electromagnetic loss (e.g., concrete). An elec- system for stay cables (Bergamini et al. 2003). This system was tromagnetic wave of variable frequency is coupled into the used to evaluate the conditions of 68 cables of the Rama IX end of the tendon." The reflection coefficient is scanned over Bridge in Thailand in 2001. EMPA's device uses an electro- a frequency spectrum to measure resonance frequencies. The magnet instead of permanent magnets to allow magnetic sat- authors suggest that the method has the advantage that only uration of large stay cables. The current system can travel one end of a tendon has to be accessed. along the cable and detect the position of flaws along the length of the cable and provide a "qualitative statement about The MFL methods described previously have not been the position and size of the flaw within the cross section." applied to stay cable anchorages because the magnet and the EMPA is trying to increase the amount of information sensors cannot physically reach around the anchorage within obtained so that additional information on the size and posi- a reasonable distance. However, if future anchorage designs tion of flaws within the cross section can be determined. allow such access, then this methodology could potentially be developed for anchorages as well. Weischedel and Hohle (1995) discussed the use of dual- function electromagnetic (EM) instruments for evaluation of stay cables. They referred to the following two different and Video Monitoring (Photogrammetry) distinct EM inspection methods: Aas-Jakobsen et al. (1995) used a video camera to measure the 1. Localized flaw inspection (LF inspection). amplitude of stay cable vibrations on the Helgeland Bridge in 2. Inspection for loss of metallic cross-sectional area Norway. Elgamal et al. (2001) considered the use of video (LMA inspection). monitoring on an FRP bridge. Video cameras with sensor data activation and target tracking software were also considered. Weischedel and Hohle suggest that the LF inspection (as used in the United States and elsewhere) is based on differ- Dr. Derek Lichti of the Curtin University of Technology ential sensors that cannot measure gradual changes in condi- (Perth, Western Australia) has used video monitoring of tion such as corrosion, wear, and so forth. They assert that an beam deflections in static tests, and reportedly plans to per- absolute sensor is required to measure such changes. A dual form dynamic measurements at 50 Hz frequency or greater. system would include the two different sensor types and Software has been developed to capture image sequences from would measure LF and LMA at the same time. They reported two video cameras at 50 Hz. Targets are imaged and, using

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44 photogrammetric algorithms, "sub-pixel target measurements" The following is a survey comment received regarding the are obtained and transformed into three-dimensional coordi- Meridian Bridge in California: nates. At least one cable supplier plans to investigate and incorporate some form of video monitoring for stay cables. Used radiographic testing once on this structure. It was costly In addition to dynamic measurement from a distance through and impractical, but did appear to give satisfactory results. Would not use this method for routine inspections on this bridge. a camera, photogrammetric techniques can also be used for static measurements such as cable sag. Telang et al. (2004) performed tests on cable mock-ups This synthesis effort did not identify methods to obtain a to determine whether a low-energy X-ray method could be three-dimensional image of the entire stay cable for compar- effective in identifying splits in PE sheathing, previously isons with future such images. However, some forms of scan- repaired splits in PE sheathing, damage to external tape, and ning (perhaps laser-based) may eventually become available. grout void or damage. They made the following overall assessment: Radiography The low-energy, X-ray radiography was effective for almost all types of flaws in the cable specimen. However, the use of radiog- Nondestructive test methods based on radiography have been raphy is associated with higher cost and slower process, and the results require expert interpretation. used in civil structures and, in limited cases, on stay cables. The radiation source in radiography is either X-rays or gamma A number of manufacturers produce portable radiographic rays. There are safety hazards associated with both of them. systems for field applications, especially for grouted post- Special high-voltage machines (X-ray tubes) produce X-rays, tensioned tendon applications (Brown and St Leger 2003). and gamma rays are produced from radioactive isotopes. Pla- Keating et al. (2000) reported on advances in industrial com- Rucki and Eberhard (1995) presented a summary of various puted tomography applications. imaging technologies for reinforced concrete, including radi- ography. General radiography produces two-dimensional In 2004, Akers and Rideout discussed a new Photon/ images, whereas computed tomography can produce cross- sectional images of the three-dimensional object. Neutron Induced Positron Annihilation method for detecting corrosion and fatigue in bridge structures and cables. This The anchorage sockets of the Sacramento River Bridge method was developed at the Idaho National Engineering (Meridian) cables (wire rope cables) were inspected in 1988 and Environmental Laboratory. According to the authors, using a 6.0 MeV portable linear accelerator. The inspected positrons, which are anti-particles of electrons, are sensitive sockets were 203 mm (8 in.) in diameter, and a length of to change in a material's atomic structure. The authors stated 150 mm (6 in) was inspected. According to California DOT that the method can detect damage at the atomic level before personnel, the testing was successful and clear images were overt manifestation of damage. In response to an inquiry, one obtained. However, the process was considered lengthy and of the authors indicated that they have not yet performed tests costly. There were no indications of distress detected. on wire bundles, and hope to conduct research on cables in the future. FHWA has constructed a mock-up of a stay cable compo- nent for the C&D Canal Bridge in Delaware. This mock-up Magnetostrictive Sensors included wire and strand breaks and grout voids. The mock-up was tested by a company that specialized in radiographic test- The magnetostrictive sensor (MsS) technology was developed ing. Field testing on this bridge saddle has not been done. The in the early 1990s at the Southwest Research Institute (SwRI) Delaware DOT has investigated this method and offered the (Bartels et al. 1996). This technology is based on the concept following observations in response to the survey: that magnetic fields produce small changes in the physical X-ray imaging of the cable stays was considered and dismissed. dimensions of a ferromagnetic material (such as steel), and Several concerns were encountered with this method including material strains produce changes in magnetization. Therefore, protection of public and working personnel during the exposure, if the magnetic field around a bar is changed, an elastic wave access and holding the equipment at the higher elevations of the (guided wave) would be generated, which would travel in both cable stay, and scheduling of the equipment. Interpretation of the image was also a concern. It is believed that the multiple materi- directions along the length of wire. The stress wave would als (steel, grout, steel strand) which comprise the cable stays com- change the magnetic induction of the material, thus generat- bined with the changing geometry would make interpretation of ing voltage in the receiving coil, which can be monitored. the image difficult and would not allow for an accurate under- standing of the conditions. Our understanding is that the X-ray The transmitting and receiving coils can be identical. This imaging would only be able to detect gross section loss of the stay approach is a form of ultrasonic testing. Figure 62 shows the and is not precise enough to discern the onset or early stages of basic MsS concept. This approach was used on the hanger corrosion. Finally, when the X-ray imaging method was consid- cables of the George Washington Bridge in New York City. ered, it only allowed a view of a discrete section of the cable stay as opposed to a global or `traveling' operation, which would allow Figure 63 shows the trace of the results as well as the attach- an investigation of the entire length of the cable stay. ment of sensors on the hanger.

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FIGURE 62 Schematic diagram of MsS sensors (Bartels et al. 1996). FIGURE 63 Application of MsS technology to inspection of hanger cables (Kwun 2003).