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60 CHAPTER SIX CONCLUSIONS AND FUTURE RESEARCH NEEDS For this synthesis effort, a worldwide search was undertaken inspection and maintenance of stay cables confront challenges for information on the inspection, repair, testing, and design for which proven and accepted methodologies and tools are of stay cable. On-line sources of information as well as engi- limited and, in many cases, very costly. The main tension ele- neering databases were examined. A number of knowledge- ments (MTEs) within a cable bundle are, in most cases, hid- able individuals were contacted for additional information. A den from the view of inspectors. Access to cables for visual questionnaire was distributed among all state departments of inspections or nondestructive testing (NDT) is often difficult transportation in the United States and provincial depart- and, in the case of the anchorage zones, almost impossible. ments of transportation in Canada. Based on this informa- tion, various methods, approaches, and practices have been At the time of this study there were 36 cable-stayed bridges explained in detail and their strengths and weaknesses iden- in the United States and 16 in Canada. As of 2005, the average tified. Specific inspection and repair approaches are presented age of such bridges in the United States was 11.4 years. As and discussed. these bridges become older, the need for effective inspection and maintenance methods and tools becomes more impor- The challenges in inspection and maintenance of cable- tant. Because no one method is sufficient, a combination of stayed bridges are substantial. Those who are tasked with methods is necessary. The following table lists some of the issues involved in the inspection, maintenance, and repair of stay cables. Methods identi- fied in the literature to address those issues and their known advantages and disadvantages, as well as other factors are discussed. Issue Method Comments General inspections Visual Visual inspections are, in most cases, the only method used for cable-stayed bridges. These inspections typically include surveys of the exterior surfaces of cables, exposed surfaces of the anchorages, cross cables and their connections (if available), dampers (if available), and condi- tions of neoprene boots and guide pipes. However, inspections of neoprene rings inside guide pipes are also done in some inspections. Boroscopes or videoscopes can be used to check the exterior condition of the cable and presence of moisture inside the guide pipe. Anchorage caps can sometimes be removed to check the condition of wedges or button heads. Visual inspec- tions could potentially involve measurements of cable sag and the inclination angle by the inspector from the deck level using simple photogrammetric or optical devices. Assessment of MTE Magnetic This system has a long history in the inspection of industrial cables and ropes. Systems that can condition in free flux travel along the stay cable are commercially available. This method does not work well with length leakage cables that have steel sheathing. The cable size cannot be too large in diameter. Depending on the size of the magnet or electromagnet used, the range of detection would be limited to a thick- ness around the perimeter. The system can identify the extent of damage and its location along the length of cable, but cannot identify location of damage within the cross section. In its cur- rent state of development, this method cannot be used for inspections in the anchorage zones and in the vicinity of the anchorage zones. Unfortunately, those are the areas where most seri- ous problems have occurred. Assessment of MTE Cable force This approach is the most widely used, and sometimes misunderstood, nondestructive evaluation condition measurements (NDE) method. Measurement of cable forces (through the methods described) can help determine if global stiffness changes have occurred in cables. Methods such as the Precursor Transforma- tion Method (discussed in chapter four) can then be used to identify which cables are affected. However, damage in the form of MTE section loss does not necessarily translate into global stiff- ness change. This is particularly true in grouted cables, where a broken wire can redevelop its stress a short distance away. Therefore, when cable forces in a grouted cable array do not change over time, it cannot necessarily be concluded that there is no loss of MTE. Assessment of MTE Ultrasonic This method has been used on a few bridges to evaluate the condition of MTEs in Hi-Am-type condition testing) anchorages. A stress wave is sent into the exposed end of a wire or strand at the anchorage and the results are displayed. There has been no been systematic and controlled evaluation of this method to determine its degree of effectiveness for stay cable anchorages. Therefore, ultrasonic
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61 Issue Method Comments tests on cable anchorages for any bridge should ideally first be calibrated with a mock-up of the same anchorage with known defects. Seven-wire strands pose a larger challenge than individual wires because of the complexities of transmission of stress waves in them. The operator must be highly qualified in such tests, because the judgment of the operator is crucial, and the answers in many cases would not be clearly evident. Problems arise owing to the anchorage materials sur- rounding the MTEs. The stress wave attenuates significantly in a wire embedded in grout or epoxy compared with a wire in air. This method is not applied to the MTEs in the free length of cable. Unfortunately, this is the only known and practical test that is currently available for the assessment of MTEs in the anchorage zones of typical U.S. stay cables. Assessment of MTE Radiography Theoretically, this method has the potential to successfully assess conditions of cable anchorages condition where access to the perimeter can be achieved. This method was used on the anchorages of the Meridian Bridge in California. However, safety issues, cost, and the typically large and heavy equipment have significantly limited their use. Detection of wire Acoustic The test laboratories performing qualification fatigue tests of stay cables have long used this breaks as they monitoring method to detect wire breaks in the cable specimens as they happen. As a wire breaks, a happen stress wave travels along the length of the cable in both directions. Accelerometers attached at the anchorage can detect the event and determine its location along the cable. The recorded response or "signature" can be analyzed to determine if the event is actually a wire break. A commercial system is currently available and has been installed on a number of bridges. This system has proprietary software to identify wire breaks. In grouted cables, the attenuation of the wave is significantly higher than in the ungrouted cables. Detection of grout Impulse Hand-held impulse radar equipment can be placed over the cable and moved longitudinally to voids inside high- radar identify potential grout voids inside the cable sheathing. This method has been tested on density polyethylene mock-up specimens involving HDPE sheathing. A field application of this method is planned. pipe (HDPE) sheathing Repair of large Vacuum This method has long been used in post-tensioning tendon applications. A vacuum is used to grout voids grouting ensure that the grout would fill all the voids in the affected area. Specialty post-tensioning con- tractors can perform these tasks. Cable force Vibration-based In this method, a laser vibrometer is used to measure small vibrations of the cable from a large measurements using laser distance. No special targets need to be placed on the cable. A low-power laser beam (class 2 vibrometer laser) is used. The measured response is then used to determine the frequencies of vibration. The measured frequencies are then used to calculate force. One should note that to improve accuracy the effects of bending stiffness, neoprene rings, and socket stiffness changes should be considered. When cross cables or dampers are used, the calculation process becomes more difficult. Using an accelerometer in lieu of a laser vibrometer is expected to provide simi- larly accurate results. However, in some cases, the laser vibrometer could speed up the mea- surement process. Cable force Vibration-based Similar to the laser-based method described previously. measurements using accel- erometer Cable force Based on Although the tension in a cable is related to the square of the fundamental frequency, it is also measurements measurement inversely proportional to the cable sag. Therefore, measurements of the cable sag can also be of cable sag used to estimate cable tension. It is expected that the cable sag could be measured by inspec- tors from the deck level using simple photogrammetric or optical methods. This approach has apparently not yet been used for inspections; however, contractors have reportedly used it dur- ing construction. Detection of Infrared Hand-held infrared thermography equipment can be used to detect splits in HDPE pipes that hidden splits thermography are hidden under the protective tape. This method has been tested on mock-up specimens in HDPE involving HDPE sheathing. A field application of this method is planned. However, tests indi- cated potential problems in using ambient temperature changes to monitor such defects. The test report suggested that external thermal applications may be required. Detection of Infrared Similar to the method discussed previously. damage to poly- thermography vinyl fluoride tape Assessment of Long-term When cable vibration problems are suspected, sensors (accelerometers) can be mounted on select cable vibrations monitoring using cables to monitor vibrations over a period of several weeks, months, or years. Typically, a accelerometers weather station is also installed on the bridge to obtain local wind and rain information. The data are collected through a high-speed data acquisition system, and typically transmitted to the engi- neer by means of conventional or wireless communications. The vibration amplitudes and asso- ciated frequencies are then studied in conjunction with rain and wind data to assess vibration conditions. (continued)
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62 Issue Method Comments Assessment of Video cameras There are no known instances of using video cameras to monitor vibrations on cable-stayed cable vibrations on bridge bridges. However, this option was discussed for two bridges in the path of hurricanes, but was not implemented. There is research (discussed in chapter three) on using photogrammetric techniques to measure structural vibrations. Assessment of Vibration There are different approaches to measuring cable damping. In one, an accelerometer is first cable damping decay method attached on the cable. Then, a rope is placed around the cable and around an individual standing on the deck next to the cable. The individual pushes back on the rope in a rhythmic fashion try- ing to match the frequency of the cable. At the same time, the individual would pull the rope higher along the cable. When the cable achieves sufficiently high-vibration amplitudes, the cable is allowed to slow down while the accelerometer is monitored. The time it takes to decay the signal is used to calculate the damping ratio. There is no single method that would answer all of the tions in the cables (especially anchorages) for incorpo- questions regarding the condition of stay cables. In most ration into new cable designs. cases, it is the combination of NDT techniques together with · Undertake the identification of the appropriate choice the experience, knowledge, and judgment of engineers, inspec- and use of combinations of NDE methods based on the tors, and technicians that can possibly lead to the appropri- conditions at hand. ate answer. The effectiveness and accuracy of many of the · Study the feasibility of global, three-dimensional, phys- methods described here would be significantly enhanced if ical mapping of stay cables and the entire cable-stayed baseline comparative measurements were available when the bridge. bridge is known to be defect free. · Study the development of safe and effective cable inspec- tion vehicles that can travel along cable length and carry Based on the results of this effort, it is recommended that NDE test hardware. It may be possible that different states the following research be undertaken: with similar cable systems can share the equipment. · Undertake the development of a national resource for · Consider the possibility of establishing minimum require- information on stay cables and cable-stayed bridges ments for the information that must be included in the to assist bridge owners and others with information on inspection and maintenance manuals for cable-stayed inspections, maintenance, and testing of stay cables; new bridges. A list of possible topics is included in this report. trends and methods; and a statistical database of cable · Study the strengths and limitations of ultrasonic testing information. Creation of a database of information on of MTEs in various types of anchorages. stay cables was strongly supported (90%) by the respon- · Study the effectiveness of visual inspection techniques, dents to the questionnaire. and the development of visually inspectable stay cable · Study the development of national or regional periodic systems. training programs for state engineers and inspectors · Review the feasibility of built-in or remote inspection responsible for cable-stayed bridges to learn about new and imaging systems for the monitoring of MTE condi- developments and to share their experiences.