Bar Tendons Ensure a Long Service Life: The Zilwaukee Bridge
One and a half hours north-west of Detroit, Michigan’s Zilwaukee Bridge crosses the Saginaw River with two separate bridge structures, one each for north and south bound traffic. The single cell, segmental post tensioned box girder bridges accommodating four lanes each were originally constructed between 1979 and 1988 replacing an obsolete bascule draw bridge.
Each bridge is approximately 2,414m (1.5mi.) long and consists of 1,592 precast concrete elements. Each of the segments weighs 145t on average. The main river spans have a length of 120m (393ft), and the structures’ highest point above river level is at 38m (125ft). Traffic volume on this part of I-75 averages about 60,000 vehicles per day.
Prior to 2012, the Zilwaukee Bridge had already undergone several repair projects. In order to sustainably extend the service life of the bridge, the owner decided to replace all of the bearings. In total, 34 expansion bearings, 106 pier bearings and 10 abutment bearings were replaced on the bridge. Each pier consists of two columns linked by a crossbeam. At each pier, up to eight 600t capacity jacks were required to lift the superstructure for bearing replacement.
To strengthen the piers, DYWIDAG supplied Grade 150 DYWIDAG Post-Tensioning Bars in diameters of 36mm, 46mm and 65mm. The post-tensioning bars were installed as external transverse tendons in the hollow box girders near the piers.
High-strength DYWIDAG Bars were also used as tendons for replacing the bearings. The post-tensioning bars were first installed in the girders of the lifting system and stressed. Afterwards, the girders were hydraulically lifted.
DSI USA not only provided the necessary post-tensioning bars and the complete stressing equipment, but also supported the General Contractor in properly sizing the stressing equipment and selecting the materials that were to be used. Matching stressing jacks were provided for each bar diameter.
DYWIDAG also helped the General Contractor plan the complicated access to the bearings from a suspended work platform which was lifted from grade level.