US bridge engineers and highway administrators are gaining confidence in the performance of cable-stayed bridges. As a result, more and more of these elegant bridges are being built, with the number expanding rapidly over the last decade. The US inventory of 24 bridges that existed in 1992 is expected to nearly double by 2005. Information gathered from evaluating stay cable systems for nearly 25 long-span bridges worldwide, plus the instrumentation, health monitoring, and inspection of eight cable-stayed bridges in the US, have combined to create a comprehensive approach to cable-stayed bridge inspection, condition assessment, and maintenance of these aesthetically-refined structures.
These developments are welcome because they help overcome several concerns about future bridge performance. When designing bridges, engineers run the risk of focusing so closely on solving the complex near-term problems of design, construction methods, and costs that they neglect to consider the effects of design and detailing choices on the long-term durability, maintenance needs, and behaviour of structural elements. Furthermore, the proliferation of untested proprietary systems, and the lack of any formal means by which the original designer can be kept informed of a structure's long-term performance, could impede the evolution of design improvements based on feedback from the real-world performance.
US laboratory tests of prototype cable systems have helped to further the understanding of their behaviour, in some cases raising concerns about certain characteristics. These studies have shown that cable service life can be compromised by corrosion fatigue caused by the presence of grout bleed water, pre-existing pitting in cable tension elements, fatigue-sensitive anchorage details, faulty coatings, and fretting fatigue in saddle-type supports. These laboratory results indicate some potential for the existence of harmful conditions in the stay cables of bridges that have already been built. Observations of unpredicted cable behaviour, including high-amplitude wind-induced oscillations and corrosion, have increased the emphasis on evaluation and inspection of in-service cable-stayed bridges. However, traditional bridge inspection practices are inadequate for quantitative stay cable assessments, because cables not only have many unique aspects, but their condition is usually hidden from view by permanent protective barriers. Hence the development of a unified, comprehensive approach using new methodologies and new evaluation tools was necessary.
Stay cables and their connections are undoubtedly the most critical elements of cable-stayed bridges. Efforts to develop an evaluation approach, therefore, have focused on assessing the condition of stay cables to provide an overall safety and integrity check for the bridge. Experience gained through investigations of cable-stayed bridge condition, stay cable performance testing findings, and familiarity with inspection and advanced non-destructive testing methods helped development of a unified approach for stay cable evaluation. The methodology was evolved during evaluation of eight cable-stayed bridges in the USA, and will be complemented in future with new non-destructive evaluation techniques that are presently under consideration. The overall evaluation method includes a global integrity and vulnerability check using stay cable forces and damping measurements, and localised damage detection.
A traditional visual inspection along with cable force and damping measurements can be used to evaluate the global integrity and structural safety of a bridge with minimal effort. A snapshot of the stay cable force distribution, when combined with an analytical interpretation of the force changes or comparison with baseline force measurements, provides a clear indication of the location, type, and intensity of any significant damage. Damping measurements can be used to identify the vulnerability of stay cables to wind-induced vibrations and the resulting damage. A rapid, laser-based force and damping measurement technique, along with numerical algorithms developed through federally and privately funded research, has provided a practical, cost-effective tool to address immediate concerns and determine the need for action. To date, one quarter of existing US cable stayed bridges have been evaluated using this technique.
Global evaluations that uncover existing damage and unwanted stay cable behaviour may point to a need for further investigation at a local level. Local damage detection methods are frequently used to focus on the anchorage zone or the free cable length. The removal of anchorage caps, the use of visual aids such as video-borescopes, cable dissection, and material sampling and testing are some of the means of detecting damage in stay cables. The non-destructive ultrasound technique has been successful in detecting hidden flaws such as wire breaks and grout voids in anchorage zones, and other methods for damage detection along the free length of the cables are currently under assessment. Among these, thermography and impulse radar have been validated in the laboratory and proven to be economically and technically viable for cable cover pipe defects and grout damage detection. NDE techniques such as magnetic flux leakage and radiography have shown promise in the laboratory for detection of existing wire breaks in stay cables; however, these methods are not yet economically or operationally feasible.
Retrofitting and mitigation can be designed to address the problems identified during the evaluation. Techniques include replacement of severely damaged cables or strands, repair of guide pipes and neoprene washers, concrete encasement, cable cover pipe, grouting of voids, and design and installation of vibration suppression measures.
Once the condition of the cables has been determined, sensor systems can be designed and installed to continuously monitor the health of the cables and the bridge. This includes installation and operation of integrated sensor/data acquisition/communication systems designed to monitor and warn of damage, including wire breaks, or changes in the overall performance of the bridge structure, cable forces, aerodynamic effects, material behaviour, or environmental effects.
Bridge inspections over the past decade have uncovered conditions that appear to be consistent across families of cable-stayed bridges and stay cable designs - these include problems at cable exit locations, anchorages, sheathing and grouting of cables, and stay cable corrosion and vibration issues.
In the USA, many cable-stayed bridges incorporate a steel guide pipe where cables exit the deck or pylon. In almost all cases, it was observed that the guide pipes were eccentric and misaligned with respect to the stay cables. This non-uniformity affected the function of the neoprene washer and caused damage to the guide pipe and the surface of the cable sheathing when excessive cable vibrations occurred. Such damage includes gouges on the cable surface, rupture of the seal between cable and guide pipe, cracks on the guide pipes, and spalling of the concrete deck at guide pipe exit locations.
On many bridges, a significant amount of water was found in the deck-level anchorage end caps or end sockets. This water is believed to be either grout bleed-water or moisture from precipitation that had seeped into the guide pipes through broken or ageing seals. Whatever its source, water in the end caps and sockets promotes corrosion of the strands and anchorages. The corrosion of main tension elements is a chronic problem and a source of significant concern.
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