Seismic Retrofit of 1958 Carquinez Bridge

The 1021-m (3,350-ft) long steel through truss bridge carrying the east-bound lanes of Interstate 80 across the western end of the Carquinez Strait about 40 km (25 mi) north of San Francisco opened to traffic in 1958. It was the first major highway bridge in the United States to use high-strength (T1) steel, the first to use welded built-up members, and the first to use high-strength bolted connections. These “firsts,” combined with the size of the bridge and the traffic demands, presented a formidable challenge to the retrofit design team. The retrofit objective was to prevent collapse of the bridge during an earthquake with an expected mean return period in the range of 1,000 to 2,000 years. Under this noncollapse criterion, significant damage to the bridge, such as yielding and buckling of members, was considered acceptable. It was important, then, that a measure of acceptable damage be defined and that the postyield behavior of the structure be both very predictable and very ductile. A preliminary design was prepared based on an elastic analysis with geometric non-linearities. The retrofit design was then examined by incorporating material nonlinearities into the model; adjustments to the retrofit design were required in some isolated areas. Design issues that the team addressed included connections for loads as high as 60 to 50 kN (13,500 kips); local buckling of thin-walled, high-strength members; postyield behavior of major load-carrying steel members; rocking of rigid A-frame tower assemblies; and an expansion joint for an 8-ft movement rating.