Excessive Long-Time Deflections of Prestressed Box Girders. Structural Engineering Report No. 09-12/ITIPublic Deposited
The segmental prestressed concrete box girder of Koror-Babeldaob (KB) Bridge in Palau, which had the record span of 241 m (791 ft.), presents a striking paradigm of serviceability loss due to excessive multi-decade deflections. The data required for analysis have recently been released and are here exploited to show how the analysis and design could be improved. Erected segmentally in 1977, this girder developed after 18 years the mid-span deflection of 1.61 m (5.3 ft.), compared with the design camber, and collapsed in 1996 in consequence of remedial prestressing, delayed by 3 months. Compared to three-dimensional analysis, the traditional beam-type analysis of box girder deflections is found to have errors up to 20% (although greater errors are likely for bridges with a higher box width-to-span ratios than the KB Bridge). However, even three-dimensional finite element analysis with step-by-step time integration cannot explain the observed deflections when the current ACI, JSCE, CEB (or CEB-FIP) and GL prediction models for creep and shrinkage are used. These models underestimate the 18-year deflection by 50% to 70% and yield an unrealistic shape of deflection history. They also predict the 18-year prestress loss to be between 22% and 29%, while the measured mean prestress loss was about 50%. Model B3, which is the only theoretically based model, underestimates the 18-year deflection by 42% and gives the prestress loss of 40% when the default parameter values are used. In model B3, however, several input parameters are adjustable and, if they are adjusted according to the long-time tests of Brooks, a close fit of all the measurements is obtained. For early deflections and their extrapolation, it is important that model B3 can capture realistically the differences in the rates of shrinkage and drying creep caused by the differences in the thickness of the walls of cross section. The differences in temperature and possible cracking of top slab also need to be taken into account. Other paradigms on which data have recently been released are four bridges in Japan and one in Czech Republic. Their deflections can also be explained. The detailed method of analysis and the lessons learned are left for part II which follows.