Center for Sustainable Engineering of Geological and Infrastructure Materials

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Excessive long-time deflections of prestressed box girders

The segmental prestressed concrete box girder of Koror-Babeldaob (KB) Bridge in Palau, which had a record span of 241 m (791 ft), presents a striking paradigm of serviceability loss because of excessive multidecade 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 a midspan deflection of 1.61 m (5.3 ft) compared with the design camber after 18 years, and it collapsed in 1996 as a consequence of remedial prestressing, after a 3-month delay. Compared with 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 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 American Concrete Institute, Japan Society of Civil Engineers, Comite Euro-International du Beton, and Gardner and Lockman prediction models for creep and shrinkage are used. These models give 18-year deflection estimates that are 50–77% lower than measured and yield unrealistic shapes of the deflection history. They also predict the 18-year prestress loss to be 46–56% lower than the measured mean prestress loss, which was 50%. Model B3, which is the only theoretically based model, underestimates the 18-year deflection by 42% and gives a prestress loss of 40% when the default parameter values are used. However, in Model B3, several input parameters are adjustable and if they are adjusted according to the long-time laboratory 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 the cross section. The differences in temperature and possible cracking of the 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 the Czech Republic. Their excessive deflections can also be explained.

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Three-dimensionally meshed model of KB bridge.

References:

Bazant, Z.P., Hubler, M.H., and Yu, Qiang (2011). “Pervasiveness of excessive segmental bridge deflections: A wake-up call for creep.” ACI Structural Journal 108 (6, Nov.-Dec.), 766-774.

Bazant, Z.P., Yu, Qiang, and Li, Guang-Hua (2012). “Excessive long-time deflections of prestressed box girders: I. Record-span bridge in Palau and other paradigms.” ASCE J. of Structural Engrg. 138 (6), 676–686.

Bazant, Z.P., Yu, Qiang, and Li, Guang-Hua (2012). “Excessive long-time deflections of collapsed pre-stressed box girders: II. Numerical analysis and lessons learned.” ASCE J. of Structural Engrg. 138 (6), 687–696.