Testing, Inspection and Maintenance of Multistrand Cables
As stay cable are exposed, they are subject to both constant static loads and fluctuating fatigue loads. Consequently, the testing, inspection and maintenance of stay cable systems is of decisive importance.
The following text presents a testing concept that was used for the DYNA Grip® multistrand cables of the Schoenebeck Bridge over the Elbe River in Germany.
Vibration measurements that were evaluated while specifically taking into account bending stiffness were used to determine stay cable forces. The vibration measurements were also evaluated with regard to the damping behavior of the stays. Elasto-magnetic force sensors were also installed to permit easy cable force measurement.
Acceleration sensors and a recently developed camera system were used for determining the first two eigenfrequencies and the dampening of the cables as well as the eigenfrequencies and the deflection of the superstructure. The measurements relating to the stay cables were carried out near the anchorages using a compact tri-axial accelerometer.
Ambient measurement data were compiled in order to measure the natural vibration behavior of the stay cables, and the measurements were carried out using manual excitation. With the help of this test series, the eigenfrequencies, natural frequencies, free vibration lengths of the stay cables, damping values and stay cable forces of all 36 stay cables were determined.
Measurements before and after the activation of the guide deviator were carried out on several stay cables in order to determine how well they functioned and their degree of efficiency in terms of damping. The performance and functionality of the installed guide deviators was demonstrated using this technique.
In addition, DYNA Force® Sensors were installed on individual strands of 6 cables on the Schoenebeck Elbe Bridge in order to measure the stay cable forces. For comparison, lift-off tests were carried out on the strands with sensors, and the DYNA Force® Sensor readings were subsequently collected. Deviations had a mean value of +0,5%, confirming that DYNA Force® Sensors are a very efficient and simple way of monitoring stay forces. Since the sensors are installed during stay cable installation, the access to individual strands that is necessary for subsequent lift-off tests can be omitted.
An electromagnetic testing device that had been specially designed for strand bundle cables was used to detect possible wire fractures outside of the anchorages. Thanks to magnetizing coils that are placed around the stay cable circumference, this modular testing device permits full magnetization and thus also ensures a detection of prestressing steel fractures in the cable. With this testing device, the 36 stay cables protected by HDPE ducts were magnetized until saturation had been obtained and thus tested with regards to possible damaged areas such as wire fractures and significant section attenuations.
Visual inspection forms the basis of all further inspections. Automatic camera systems are used for this purpose: they can create high resolution panoramic pictures using the original length of the construction component that is to be inspected. The pictures can then be evaluated and compared at any time. Data collection only requires a fraction of the previous inspection time, thus saving time, minimizing traffic disruptions and costs.
Ultrasonic techniques are particularly suitable for inspecting anchorage areas. When ultrasonically tested, wire fractures or cracks and major corrosion damage generate signals that can be evaluated by examining the corresponding amplitude and form.
Endoscopy is another method that can be used for cable evaluation. Large sections of exterior wires around the cable perimeter can be inspected optically along the entire free length using an endoscope.
As the strands in this bridge are galvanized, waxed and inserted in PE ducts at the factory, the free cable length can be considered as permanently corrosion protected and thus completely maintenance free.
Special attention is paid to the leak tightness of the anchorages in order to ensure permanent corrosion protection of the strands near the anchorages and anchorage wedges. Leak tightness was proven during the verification tests by a leak tightness test in accordance with fib Bulletin 30. If any leaks are detected during the major tests, the special design of the DYNA Grip® anchorages allows the reactivation of the squeeze-type gasket by means of a retightening of the threadbars.
The inspection methods that have been presented using the example of the Schoenebeck Elbe Bridge are product related and state of the art. The condition of the stay cables can be determined reliably and nondestructively.
Winner of the Structural Award 2014