Excessive Deflections of Record-Span Prestressed Box Girder

The Koror-Babeldaob (KB) Bridge, built in 1977, connected the islands of Koror and Babeldaob in the Republic of Palau. It had a main span of 241 m, which once set the world record for a segmentally built prestressed concrete box girder. After 18 years, deflection became so excessive that it was decided to install additional prestressing and eliminate the bridge's midspan hinge. However, 3 months after retrofitting was completed and the KB Bridge was re-opened, it suddenly collapsed under negligible traffic load and with no apparent external trigger in September 1996. In design, the final midspan deflection accumulated since the end of cantilever erection, measured from the design camber of -0.30, was expected to be in the tolerable range from 0.76 to 0.88 m, which would have led to a final sag of only 0.46 to 0.58 m compared to the design elevation. During the first 2 years, the deflections were benign but then accelerated unexpectedly. After 18 years, the deflection increase, measured since the installation of the midspan hinge, reached 1.39 m and kept growing . Compared to the design camber, additional creep deflection of 0.22 m was accumulated earlier during segmental erection, and so the total deflection was 1.61 m. This study has a two-fold objective: 1) explain the excessive deflections, and 2) compare the predictions obtained with the main existing creep and shrinkage prediction models currently used in practice. Investigation of the collapse itself will be presented in a later article. Due to length limitations, details of the creep and shrinkage analyses cannot be presented here, but can be found in a recent report available at www.civil.northwestern.edu/people/bazant/PDFs/Papers.