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.

Ultrasonic testing of strands on some stay cable anchorages has revealed wire breakage or voids in the epoxy grouts. This information is helpful in identifying those anchorages that will need to be monitored more closely in the future.

Several grouting-related problems have been observed on stay cables. These problems typically include voids in grout, regions that have been only partially grouted, splits in polyethylene sheathing pipes due to grouting pressure, and broken or split pipes due to mechanical or thermal causes. Split pipes and insufficient or deteriorated grout reduce the corrosion protection for the steel tension elements.

The monitoring of changes in stay cable forces over time provides an indirect method of assessing the overall health of both the cables and the bridges. Any reduction in load-carrying capacity of cables due to loss of stiffness or cross-section, as well as damage elsewhere in the structure, would result in force changes in the stay cable array. Using available analytical tools, patterns of force change in the stay cable array can be related to the location, type, and intensity of damage. To this end, monitoring on many bridges has revealed force change patterns associated with the effects of long-term bridge response, the application of additional dead load, or the adjustment of stay cable forces to control bridge geometry or deck profiles.

Field measurements have verified that the intrinsic damping of stay cables is very low and in most cases it is inadequate to suppress wind-induced stay cable vibrations. Excessive vibrations should be prevented, because they not only result in damage to guide pipes, decks, and anchorage components, but will also increase the accumulation of fatigue damage in the stay cables due to stress variation.

The old proverb 'the devil is in the detail' aptly summarises the possible causes for many of the problems that stay cables suffer.

The current design philosophy is based on providing multiple barriers, creating cable elements that encapsulate the tension element as watertightly as technology allows. But experience in the field indicates that in the case of the measures observed to date, it is nearly impossible to achieve this goal. It may be more prudent to assume that water will get to the guide pipe areas and to provide for proper drainage. Performance criteria for corrosion barriers should take into account the potential for cable vibration and cable bending effects.

Misalignment of guide pipes and cables should be avoided by stricter construction tolerances and/or adjustable collars at cable exit points.

When designing saddle-type supports, refined details should be provided to prevent damage resulting from fretting of strands or wires and stress concentration in the strand bundle and cover pipe. Recent full-scale laboratory tests conducted on specimens representing cables to be installed on the Maumee River Crossing enhances cable durability, in contrast to standard saddle design details (Bd&e issue no 23). Unfortunately, this detail has not been incorporated in any other US bridges built to date that include saddles, so these structures may prove more susceptible to fretting fatigue issues in the future.

However, some positive progress has been made - the recent introduction of new corrosion barriers such as galvanising, greasing, sheathing, and epoxy coating, in combination with effective external cover-pipe barriers and epoxy or wax injection in transition zones, has provided favourable alternatives to cement grout.

External dampers help reduce the amplitude of cable vibrations, and hence improve the long-term performance of the cables by preventing fatigue and other problems. Providing dampers also can prevent vibration-related damage in guide pipes and deck blisters. Failure of the boot, washer, keeper ring, and other components can also be reduced. Other measures, such as cable surface modifications and cable cross ties, have also been used to increase damping of cables and prevent excessive vibration.

The laser-based vibration technique for measuring cable forces has now become an accepted tool for fast verification of the health of stay cable arrays. However, for maximum efficiency, force measurements should be carried out immediately after bridge completion. This will furnish a reliable baseline with which the results of periodic force measurement can be compared.

Until cable system designs are further refined and the necessary improvements are verified by test, the need to objectively detect and assess changes in stay cable condition can be fulfilled using this comprehensive assessment approach, including nondestructive testing, continuous health monitoring, and global integrity and vulnerability checks.

Armin Mehrabi is president of Bridge Engineering Solutions, Adrian Ciolko is vice president of Construction Technology Laboratories

Non-contact solution

The non-contacting laser vibrometer technique was conceived and developed in an attempt to improve the vibration techniques that had been used to assess bridge cables on the Tacoma Narrows and Roebling Bridges. These methods required cable contact. Since development in 1997, the authors have adopted the non-contacting laser-based method for condition assessment of more than a quarter of the US inventory of cable-stayed bridges.

Surprisingly, however, this technique is now being selected more and more to analyse the condition of cables on both suspension and tied arch bridges. This trend recognises the method's ability to provide a quantitative basis for cable condition and quality decisions, and its practicality and low cost when compared to traditional mechanical lift-off techniques. Equally important is the fact that the laser vibrometer gives bridge engineers more confidence in the reliability of cable arrays than they can gain through traditional inspection methods.

* The 1,074m span Bosporus suspension bridge connects Asia and Europe across the Bosporus Strait in Istanbul, Turkey. Concerns over fatigue endurance of deck-level wire rope hanger connections caused the owner, Turkish Highway General Directorate, to conduct a comprehensive condition and service life assessment of critical connections. The solution consisted of laser-based force measurements, stress range measurements, and fatigue life analyses. Knowledge of hanger dead load forces was critical to accurately establish stress state and traffic-induced stress changes in the hanger connections, and to confidently forecast service life.

* Pennsylvania DOT's Belle Vernon Bridge carries the I-70 over the Monongahela River; it sustained fire damage when a tractor-trailer crashed against the barrier wall, exposing arch hangers to very high temperatures. Use of the laser vibrometer technique made it possible to identify the two cables that had become weakened, causing redistribution of structural component forces. Tests of thirty-six hangers were accomplished in a matter of several hours.

* Designers and contractors sought a fast way to measure tension forces in tie-down ropes of the 728m span Carquinez Suspension Bridge in California. The bridge was in the final stages of construction, with almost all dead loads in place. The designer wanted to know the force measurement in order to verify the tie-down system's range of forces with respect to specified values. Selection of the technique eliminated any need for staging of hydraulic cylinders and under-deck equipment, saving money for the contractor.

* The Trans-Alaska Pipeline System crosses the Tazlina River via a suspension bridging system. The bridge is inspected on a five-year cycle, and this year the owner once again chose the laser vibrometer technique to assess cable force variations attributable to cable deterioration and foundation changes.

* The Wisconsin Department of Transportation chose the laser vibrometer technique as a quality assurance tool for measurement of hanger forces on the new, 145m-span Cass Street tied arch bridge, presently under construction over the Mississippi River in La Crosse. Results indicated some imbalance among forces in adjacent cables in several of the hangers. Measurements were repeated after the forces were adjusted. Since the bridge deck was not in place when the first round of measurements was made, force measurements were conducted by positioning the laser measurement equipment and team on an existing parallel truss span, located 12.2m downstream.

The laser-based cable assessment technique is expected to continue making contributions to bridge building and maintenance in future, for cable-stayed, suspension and other bridges that rely on supporting cables. Wider acceptance of the technique could provide additional benefits for the construction industry. Project delivery times could be reduced through the elimination of cable anchorage staging cycles for force readjustment; operations and maintenance costs for bridges could be reduced through greater use of predictive maintenance strategies that rely on quantitative condition evaluation techniques; and in addition, construction safety could improve, reducing the exposure of bridge inspectors to traffic hazards, the number of staging-related fall injuries, and the risks to public safety from unknown structural defects.

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