ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/295658
O p e n e d t o t r a ff i c i n 1 9 8 9 , t h e 3 2 5 - f t - l o n g a p p ro a c h s p a n s a n d 393-ft-long river span of the Zilwaukee Bridge rise to 125 ft above the Saginaw River to accommodate marine shipping traffic. The bridge is a 1.5-mile-long twin concrete segmental structure comprised of 1592 precast concrete segments, each weighing an average of 160 tons. It was the largest precast concrete segmental project in the United States at the time. The 25-span northbound structure and 26-span southbound structure carry I-75 traffic over the Saginaw River through the city of Zilwaukee, Mich. The Zilwaukee Bridge was designed to the 1973-77 AASHTO Standard Specifications, with HS-25 live loading and AASHTO Zone 1 seismic loads. The single-cell segments of the superstructure box girder are approximately 73 ft wide at the top flange, 36 ft wide at the bottom flange, and vary in depth from 8 ft at midspan to 20 ft at the piers. Each structure contains eight, in-span, quarter- point hinges to accommodate expansion and rotation from thermal gradients. Bearing Problems Originally, the superstructure rested on high-capacity pot bearings at the piers and expansion joint locations. The pier bearings were mostly fixed with one of the two pier bearings allowing transverse movement. The e l a s t o m e r p o t b e a r i n g s a l l o w e d for 0.04 radians of rotation and accommodated 12 in. of longitudinal and/or transverse movement via the polytetrafluoroethylene (PTFE) sliding surface. Bearing problems were identified during the first detailed inspection in 1993. Neoprene elastomer was leaking from the sealing ring around the pot piston at the expansion hinge and pier bearing locations. This reduced the rotational capacity of the bearing. There were also indications of wearing in the PTFE sliding surfaces, which was jeopardizing thermal movements of the bridge. At some pier locations, the loss of elastomer resulted in contact between the top sole plate and the bottom masonry plate edge. In 2007, the Michigan Department of Transportation (MDOT) began discussions to replace the 104 pier bearings and 34 expansion joint bearings and upgrade the bearings to current AASHTO service criteria. Bearing Replacement The challenges associated with the bearing replacement were clear. Loads approaching 17 million pounds would profile ZILWAUKEE BRIDGE / ZILWAUKEE, MICHIGAN BRIDGE REPLACEMENT CONSULTANT: T.Y. Lin International, San Francisco, Calif. QUALITY ASSURANCE ENGINEER: Janssen and Spaans Engineer Inc., Chicago, Ill. CONSTRUCTION ENGINEER: Corven Engineering Inc., Tallahassee, Fla. PRIME CONTRACTOR: PCL Construction, Tampa, Fla. POST-TENSIONING CONTRACTORS: PCL Construction, Tampa, Fla., and Walter Toebe Construction, Wixom, Mich. The Zilwaukee Bridge, a 1.5-mile-long twin concrete segmental structure, carries I-75 traffic over the Saginaw River. All Photos: Michigan Department of Transportation. by Corey E. Rogers, Michigan Department of Transportation Bearing replacement need determined after 18 years of service ZILWAUKEE BRIDGE 12 | ASPIRE , Spring 2014 P R O J E C T AspireBook_Spr14.indb 12 3/21/14 2:20 PM