1-1401-1 / Bending Fatigue Response of Grouted Stay Cables

Both the Fred Hartman Bridge and the Veterans Memorial Bridge have experienced large-amplitude vibrations of the stay cables. A major concern resulting from these vibrations is the possibility of fatigue damage to the parallel,seven-wire, stressing strand in the grouted stay cables – and the overall safety of the bridges.

An experimental investigation was conducted to determine the susceptibility of grouted stay cables to fatigue damage. Two series of experiments were conducted: twelve stay-cable specimens were subjected to bending fatigue loads in the first series and three small-diameter specimens were subjected to bending fatigue loads in the second series.

The test results indicate that fatigue damage is expected to be concentrated in the regions of highest bending stress: the ends of the stays and locations where a damper or restrainer induces local bending in the stay. The risk of fatigue damage was considered to be low at the tension ring, along the free length of the stay, and in the vicinity of unintentionally crossed strands. The acoustic monitoring systems installed on the Fred Hartman Bridge and the Veterans Memorial Bridge provided a reliable means of detecting wire breaks in the laboratory specimens. However, the actual location of a wire break may be 2 to 3 ft from the location identified by the acoustic sensors. Transverse stiffness and natural frequencies of the test specimens were not sufficiently sensitive to detect the accumulation of fatigue damage.

Accumulation of fatigue damage is a slow process, and many wire fractures can be tolerated before the strength or stiffness of the grouted stay cable is compromised. However, if the number of wire breaks detected at a single location exceeds a threshold level of 10% of the total number of wires in the stay,corrective action is recommended for an existing grouted stay cable.