National Academies Press: OpenBook

Inspection and Maintenance of Bridge Stay Cable Systems (2005)

Chapter: Chapter Three - Inspection and Monitoring Techniques

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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
×
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Suggested Citation:"Chapter Three - Inspection and Monitoring Techniques." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Maintenance of Bridge Stay Cable Systems. Washington, DC: The National Academies Press. doi: 10.17226/13689.
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37 In this chapter, various inspection and monitoring techniques for stay cables are discussed, including their advantages and disadvantages. Figure 53 shows survey results with respect to the types of nondestructive tests performed on the cables. The most commonly used method is the vibration-based force measurement. However, the largest group belongs to the “not performed” category. Figure 54 shows the survey results with respect to the types of sensor-based, long-term monitoring on the cables. Three respondents’ bridges in the United States and one in Canada incorporate acoustic wire break detection, whereas two respondents in the United States and one in Canada incorporate long-term vibration monitoring. Question 6 in the survey asked respondents to comment on the effectiveness of any nondestructive test methods for stay cables of which they are familiar. Some of their comments are given later in this chapter. Others are provided here: • Several nondestructive tests were run after an extreme oscillation event. Practically all of the methods cited in Question 6 were performed to determine if there was any loss of force in the stays. Geometric, physical, and visual tests were performed. The only discoveries were deficiencies in the original construction, which were corrected. • The fundamental frequency of the cables was recorded. Alaska DOT&PF (Department of Transportation and Public Facilities) will monitor the cables and attempt to determine if the fundamental frequency of the cables has changed. • The presence of the steel protective pipe limits the effec- tiveness of many available testing methods, particularly magnetic-based methods. Implementation of laser-based cable stay force measurements are being considered by the department to establish baseline force data for the cable stays. • Nondestructive testing is needed to determine the condi- tion of tension bars inside the steel casing of the cables. • The only problem is the anchorage area. So far, no method is available for inspection. I see additional prob- lems with inspection of the grout-filled cables. • Vibration-based cable load determination—effective and inexpensive; X-ray—expensive, slow, very questionable ability to detect wire defects; magnetic inspection— used to rapidly, effectively inspect mine cables—but the cables move past the inspection unit, which would need to be reversed on a cable bridge; impulse radar—good for detecting grout defects; sonic methods—dampened to the point of being ineffective. • Four single strands are to be removed (one at each pylon) for inspection for rust every 10 years, starting in year 2014. SHORT-TERM EVALUATION AND MONITORING This section covers methods that can be used during limited- duration inspections of stay cables. The currently available methods, as well as new and promising technologies, are cat- egorized and explained. The techniques that are addressed include conventional visual/manual techniques, and magnetic, ultrasonic, X-ray, laser, acoustic, and remote or contact vibra- tion-based techniques. As discussed earlier, Mayrbaurl and Camo (2004) reported on a study of structural safety of suspension bridge parallel- wire cables. They concluded that there were currently (as of 2004) no effective NDE methods for the condition assess- ment of parallel wire main cables of suspension bridges. Instead, they focused their efforts on manual unwrapping and opening of cables for their evaluations. Visual Inspections Visual inspections are the most common approach used on stay cables. Surveys completed by a number of respondents indicated a preference for visual inspections (when feasible) and a desire to see stay cable designs that can be visually inspected. Some bridges have dual inspection schedules, a routine inspection at 2-year (or less) intervals, and more detailed inspections at longer intervals. In the case of the Faroe Bridge between Sealand and Falster in Denmark, a three-step inspec- tion process is used (Bloomstine and Stoltzner 1999). The bridge master performs a drive-through inspection every day. Various bridge components are inspected at yearly intervals, so that the inspection of the entire bridge is completed in a 5-year cycle. Special inspections are done if damage is noted. During typical inspections of stay cables, the entire sur- face of the cable is visually inspected at close range, followed CHAPTER THREE INSPECTION AND MONITORING TECHNIQUES

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

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

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

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

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 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 on wires with and without notches and grooves of various depths. The first known application of ultrasonic testing for assessments of wire conditions in stay cable anchorages was by Suzuki et al. (1988). The cable anchorage in that case was a Hi-Am-type socket (a steel socket filled with an epoxy–steel ball compound) containing steel wires terminating at button heads. In this method, an ultrasonic transducer is coupled to the end of each wire or button head and a high-frequency stress wave is sent into the wire. The reflections are monitored by the same sensor and displayed. A trained technician can view the record and decide if a wire break has occurred. It should be noted that ultrasonic pulses could travel a long distance along a wire if that wire was free in air. However, as the wire is enclosed by grout and/or anchorage epoxy, a significant attenuation of the pulse reduces the effective length over which this method can be used. Suzuki et al. (1988) reported that the depth of wire-break detection for a Hi-Am-type anchor- age was a few meters. However, a few meters would theo- retically be sufficient for inspection of most anchorages. Following the failure of a cable on the Zarate–Brazo Largo Bridges in Argentina, a series of ultrasonic tests was per- 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- sion through a seven-wire strand. Typical ultrasonic transduc- 42 ers must be properly coupled to the cut-and-ground ends of the strands. Because the six perimeter wires wrap around the cen- ter wire and are in contact with each other, wave transmission is more complicated than in single straight wires. It is very important that an existing anchorage of an identi- cal or similar type be made available (or a mock-up made) before field testing to calibrate the results for known defects and their locations. The operator’s experience and ability is crucial, as judgment is required when interpreting results. There are however no known systematic and rigorous research programs performed to date that are aimed at quantifying the degree of accuracy of this method for various anchorages, and ways of improving the interpretation of results. Magnetic Methods When a magnetic field moves along the length of a cable con- taining steel MTEs, presence of corrosion or fracture in the wires changes the magnetic field. Sensors can detect such changes and produce electrical output as a result. Figure 61 shows a magnetic flux leakage signature, with the characteris- tic shape representing the flaw. The horizontal axis is the posi- FIGURE 59 Typical ultrasonic test record of broken wire (Prato et al. 1997). FIGURE 60 Ultrasonic testing of cable anchorage (Ciolko and Yen 1999).

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

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

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

Dr. Hegeon Kwun of SwRI indicated that they have tested anchorage areas of main suspension cables where wires are separated. SwRI researchers believe that the MsS technology can be applied to stay cables; however, they as yet have not had an opportunity to test stay cables. Dr. Kwun believes that, in the anchorage zone, small defects (some broken wires) would likely not be detectable using these guided waves. Laser Ultrasound A guided ultrasonic wave for NDT and evaluation can be applied to a test structure (e.g., a strand) in different ways, such as coupled (contact) ultrasonic transducer, MsS, or laser ultrasound. In ultrasonic testing of strands in stay cable an- chorages, the ends of seven-wire strands are typically ground smooth to allow perfect coupling with the ultrasonic trans- ducer. However, typically larger transducers that are used cannot be practically coupled to individual wires. The same applies to the MsS technique. However, the laser ultrasound can be applied as a point load anywhere at the end of the strand or wire. It can also be applied eccentrically to gener- ate both longitudinal and flexural modes (Rizzo and Lanza di Scalea 2004). In this article the authors discuss the dispersive and attenuating behavior of guided ultrasonic waves in multi- wire strands. The use of laser ultrasound may potentially offer a way to improve the basic ultrasonic technique for inspec- tion of stay cable anchorage, either on its own or in combi- nation with the MsS technique. Other Methods Telang et al. (2004) performed a number of tests on two mock- up stay cable specimens to evaluate various NDT techniques including impulse response, impulse radar, infrared thermo- graphy, and radiography. These specimens contained parallel steel wires enclosed within PE sheathing and grouted. The objective was to find methods that could be used to identify deficiencies in PE sheathing (cracking and previously epoxy- repaired cracks), damage to ultraviolet (UV)-resistant wrap- ping over the sheathing, and grout defects. The sheathing defects were hidden under a UV-resistant wrapping tape. The sheathing was cut in different directions to represent cracks before wrapping. Telang et al. (2004) reported that the impulse response method was found not to be effective. The impulse radar method (involving high-frequency EM energy) was reportedly successful in detecting grout voids or damage. Fig- ure 64 shows a radar survey identifying grout voids. Telang et al. (2004) summarized their results as shown in Table 8. They suggested that splits in PE sheathing (under the tape) can best be identified with infrared thermography or low- energy X-rays. The authors explained that the thermographic method was not able to discern filled voids or voids in the shade on the bottom of the specimen. In addition, the method was not able to see defects beneath areas with damage to the UV tape. The solar heating of the black pipe in areas where it 46 was exposed masked any potential defects in the PE. It should be noted that the best results occurred immediately after the specimens were moved from the climate-controlled laboratory to the outside in a warm and sunny environment. This thermal gradient may not be representative to normal environmental heating and cooling except in extreme conditions. It is likely that the effectiveness of the thermography would be limited to early morning or late evening. They also noted that: The infrared thermography was very effective in detecting unfilled splits in the HDPE under certain environmental conditions. This condition requires sudden variation in the ambient temperature to result in temperature gradient in the cable material. It is believed that to keep the effectiveness of thermography for unfilled split detection, perhaps also for filled split detection, thermography should be combined with heat generation source. Figure 65 shows a thermographic image from this test series. Finally, another method that has been discussed in the liter- ature for detection of corrosion in steel cables is Time Domain Reflectometry (TDR). This method has been referred to as “closed-loop” radar (Ciolko and Tabatabai 1999). It has been widely used in identifying problems in transmission lines. The process involves sending a high-frequency signal through the sensing cable and monitoring the reflections. The reflections come about as a result of impendence changes along the length of the cable. There have been a number of research efforts aimed at using strands as sensing wires in the TDR setup. Ciolko and Tabatabai (1999) reported that the results of labo- ratory and field studies on this method were not encouraging. Liu et al. (2002) discussed using TDR in a manner slightly different from the earlier studies. In this research, an external wire is used in conjunction with the strand to form the “trans- mission line” for TDR tests. This method is sensitive to the presence of or variations in moisture. At the present time, the available data do not indicate a potential for successful field applications to stay cables. LONG-TERM EVALUATION AND MONITORING This section includes methods that could be used for long-term monitoring and inspections of stay cables. Acoustic Monitoring Acoustic monitoring is a passive method for detection of wire breaks in stay cables. It “listens” for shock waves emanating from wire breaks. It is called “passive” because it cannot detect existing wire breaks. It has to be there and be “on” if it is to detect a break. Acoustic monitoring for stay cables probably began when a method for detecting wire breaks during qualification tests of stay cables was needed. Various test laboratories that per- formed such tests needed to count the number of wire breaks

47 FIGURE 64 Sample impulse radar survey (Telang et al. 2004). Source: Telang et al. (2004). Impulse response Relative Cost Relative Inspection Rate Adaptability to Environmental Conditions Unfilled Split in Sheathing Epoxy- Filled Split in Sheathing Damage to UV Tape Grout Void or Damage Effectiveness low high high none none none none low medium high none none none good low medium low good none good none high low high good fair good good NDT Method Impulse radar Infrared thermography Low energy X-ray TABLE 8 COMPARISONS OF SOME NDT METHODS FOR DEFECTS IN PE SHEATHING, GROUT, AND TAPE

during fatigue tests on the cable specimens, and therefore developed their own acoustic monitoring techniques. The basic system essentially consists of accelerometers located at selected points and the anchorages. The location of a break could be determined by comparing the arrival time of the shock wave at different sensors. It should be noted that the attenuation of acoustic waves in grouted cables is much higher than in ungrouted cables. Tabatabai et al. (1995) performed tests on a one-tenth-scale model of a nuclear containment structure containing unbonded post-tensioning strands. Wires were cut, and the wire breaks were detected by accelerometers. A commercial acoustic monitoring system based on piezo- electric sensors and proprietary software is available. The sys- tem was initially developed for post-tensioned buildings and parking garages, and was then extended to bridges and other structures. According to the company, this system has been installed on the following cable-stayed bridges: • Fred Hartman Bridge (Texas)—acoustic monitoring system was installed in March 2002 on all 192 cables (grouted seven-wire strands). • Quincy Bridge (Illinois)—system was installed in June 2002 on 14 of 56 stays (grouted seven-wire strands). • Seyssel Bridge (France)—system was installed in May 2003 on 4 of 36 stays (grouted seven-wire strands). • Penang Bridge (Malaysia)—system was installed in December 2003 on 120 of 148 stay cables (grouted bars). A research program involving the acoustic monitoring sys- tem has been in progress at the University of Texas–Austin. On request, Prof. Sharon L. Wood and the research team at 48 University of Texas–Austin prepared a write-up of the test plan and a summary of their findings based on two Master’s theses. The research team’s conclusions for the acoustic mon- itoring system are given here: [The system] provides an accurate method for monitoring wire breaks due to fatigue damage in grouted stay cables. The system was able to identify the number of wire breaks accurately. The locations of the estimated wire breaks along the free length of the cable were typically within 6 in. of the actual breaks. The accuracy of the system was less near the anchor heads, but the geometry of the specimen is much more complex in this region. The locations of the estimated wire breaks near the ends of the cable tended to be within 18 in. of the actual breaks. Long-Term Sensor-Based Monitoring A number of parameters can be measured on stay cables using sensors. In previous sections of this report, examples of long- term vibration monitoring were given. Uniaxial or biaxial accelerometers are generally used for vibration monitoring. Uniaxial accelerometers are used to capture in-plane vibra- tions. Biaxial accelerometers can measure both in-plane and out-of-plane vibrations. The accelerometers are attached to the cable with suitable clamps or other hardware that could withstand long-term exposure. The positions of the sensors are selected to maximize the desired sensor response for the vibration modes of interest, and considering access limita- tions. In conjunction with acceleration measurements, weather data are also typically collected including wind speed, direc- tion, rain, and so forth. The sensors are connected to a high- speed data acquisition system at a secure location on the bridge. The system is typically powered with AC (alternat- ing current) power (if available) or solar panels. The system should be designed in such a way as to protect against dam- FIGURE 65 Infrared thermography image for detection of HDPE splits under tape (Telang et al. 2004).

49 age resulting from lightning, vandalism, moisture, extreme heat, and extreme cold. Typically, data transfer to the office can be accomplished through wireless or landline modems. Data can also be stored on-site for manual retrieval. In addition to vibration measurements, stay cable sensing could also include cable tension measurements through load cells or other force sensors. Bronnimann et al. (1998) reported on the testing of distributed fiber optic strain sensors for stay cables. Continuous (nondiscrete) strain or even acceleration sensing along the length of cable, if practical and reasonable from a cost standpoint, can be important in condition assess- ments. In such cases, the localized strain changes, or changes in mode shapes indicative of damage, could potentially be determined. A search of the literature and the survey results did not identify any bridges where moisture or humidity sensing is performed. In the James River Bridge in Richmond, Virginia, drain holes are placed in the bottom of the area between the guide pipe and the transition pipe near the threaded anchor heads to prevent accumulation of water. Considering that pen- etration of moisture is an important issue, humidity measure- ments or moisture sensing inside the guide pipe, anchorage caps, or other cable components could be made in the future. It appears that in at least two cable-stayed bridges that were in the path of hurricanes, the idea of monitoring cable vibra- tions with security cameras mounted on the tower, the deck, or on the shores was explored, but it is believed not to have been implemented. There is also no indication as to whether any monitoring of rain–wind vibrations using security cameras has occurred. In large-scale monitoring systems, an appropriate method for analyzing and interpreting the large amounts of data that are collected must be designed. This has been an important issue in all large-scale monitoring systems.

Next: Chapter Four - Maintenance and Repair of Stay Cables »
Inspection and Maintenance of Bridge Stay Cable Systems Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 353: Inspection and Maintenance of Bridge Stay Cable Systems identifies and explains various inspection and maintenance techniques for bridge stay cable systems. It discusses both short- and long-term approaches. The report information on methods for inspections and assessments, including nondestructive testing and evaluation procedures; repair and retrofit; methods for control of cable vibrations, including rain–wind vibrations; stay cable fatigue and failure; effectiveness of various inspection and repair methods; limitations of available technologies; and trends and recommendations for future study.

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