Evaluating Suspension Bridge Cables on North America's Busiest International Crossing

The main cables of the Ambassador Bridge—carrying four lanes of traffic over the Detroit River between Detroit, Michigan, and Windsor, Ontario, Canada—have recently been inspected internally for the first time, after 78 years of service. The bridge has a suspended main span of 1,850 ft, with back spans consisting of trusses supported on steel bents. The main cables comprise 37 strands of 218 wires each, protected with red lead paste. The wires are No. 6 gauge, cold drawn and double galvanized, with a specified minimum tensile strength of 215 ksi. Cable strength was calculated by means of the procedures of NCHRP Report 534: Guidelines for Inspection and Strength Evaluation of Suspension Bridge Parallel Wire Cables. Four locations were inspected along the length of each cable, and wire samples removed for testing. All three models presented in NCHRP Report 534 were used to estimate cable strength: the simplified strength model, the brittle wire model, and the limited ductility model. Modifications to the NCHRP procedure were required for the unloaded backstay locations. The results of the evaluation showed the cables to be in very good condition, with a calculated factor of safety of 2.70, which was based on the limited ductility model. The reduction in strength from the as-built condition was less than 10% at the worst location.