Measurement and Evaluation of Mooring Cable Forces During Storm Events on the Evergreen Point Floating Bridge: Lake Washington, Washington State

On January 20, 1993, the Evergreen Point Floating Bridge incurred structural damage at two mooring cables and at various other locations during a storm event of approximately the 20-year return period magnitude. The two mooring cables damaged were the shorter and stiffer cables located at the ends of the bridge and were likely damaged because of load attraction issues. To improve the performance of the floating bridge during storms, special replacement cables were installed after the 1993 storm near the areas where cable distress was noted. Measurements of cable forces were made during the winter season of 2001–2002 to evaluate the effectiveness of the replacement mooring cables. From the experimental measurements, it was found that the special replacement mooring cables have reduced the load attraction at the shorter end cables with respect to cable tension values reported for the pre-retrofit analysis. However, the measurements indicate that the replacement cables continue to attract loads between 65% and 80% higher than those measured at the longer and more flexible cables located near the midspan of the floating bridge during storm events of approximately the 1-year return period magnitude.