Misprediction of long-time deflections of prestressed box girders: Causes, remedies and tendon layout effect

ABSTRACT

The paper reviews the causes of under-estimation of the long-time future deflections of large- span prestressed concrete box girder bridges. Six causes are discussed: 1) wrong creep and shrinkage model; 2) obsolete creep analysis method; 3) ignoring randomness; 4) lack ofmodel updating based on short-time tests; 5) wrong tendon layout; and 6) neglect of shear lag. The present paper focuses on the last two causes, while a brief review of the other causes is planned for the conference presentation. So is a discussion of why the record-span Koror-Babeldaob Bridge in Palau deflected 1.1 m more than expected in design.

INTRODUCTION
It has frequently been experienced that the box gird- ers ofmany prestressed concrete bridges deflected far more than predicted in design. The deflection evo- lution has often been counterintuitive, with slowly growing or negative deflections in the early years, followed later by a rapid and excessive deflection growth. Six different causes of these problems may be discerned:

1. Design based on an obsolete, oversimplified and unrealistic model for predicting creep and shrink- age properties of concrete, or of cross sections of concrete girders (BaZant 2000, Kiistek et al. 2006).
2. Obsolete and unrealistic method of time-dependent analysis of structural effects of creep and shrink- age, based, for instance, on assuming the creep and shrinkage properties to be homogeneous through- out the cross section and ignoring the effects of differences of shrinkage and drying creep between the top and bottom slabs ofthe box girder (Kiistek et al. 2006), which result from differences in drying half-times engendered by differences principally in slab thickness and secondarily in temperature history.
3. Lack of updating of thel ong-time creep and shrink- age predictions by means of short-time (1 month) creep and shrinkage tests and water loss tests of the particular concrete to be used (BaZant and Baweja 2000).
4. Absence ofstatistical deflection predictions, ignor- ing the large random scatter in concrete creep and shrinkage effects, and especially the fact that the total observed deflection is the difference of two large numbers-the deflection due to external loads, and the deflection due to prestress (a differ- ence oflarge numbers is greatly sensitive to a small change in one ofthese numbers and may thus result in a significant change ofthe difference, i.e., in a significant change ofthe total deflection value).
5. Incorrect and harmful tendon layout--some tendon layouts benefiting the stress state can at the same time be harmful from the deflection viewpoint. Therefore, bridge design should be performed on two different levels: not only the common stress analysis, but also optimization of prestressing tendon layout to minimize deflections.
6. An oversimplified structural model-particularly theuseoftheclassicalmechanicsofbeambending, based on the Bemoulli-Navier hypothesis of cross sections remaining plane, while the true behavior is three-dimensional, with a strong shear lag, due to large shear deformations in the webs and plates

Points 1-4 are reviewed in a plenary lecture by BaZant et al. at this conference. Let us now discuss points 5 and 6.

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