Pervasiveness of Excessive Segmental Bridge Deflections: Wake-Up Call for Creep

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Title: Pervasiveness of Excessive Segmental Bridge Deflections: Wake-Up Call for Creep

Author(s): Zdenek P. Bazant, Mija H. Hubler, and Qiang Yu

Publication: Structural Journal

Volume: 108

Issue: 6

Appears on pages(s): 766-774

Keywords: box girders; creep prediction models; design code; multidecade creep; shrinkage

Date: 11/1/2011

Abstract:
A recent study revealed that the main cause of the gross underestimation of the observed 18-year deflections of the Koror-Babeldaob (KB) Bridge in Palau was the use of an obsolete standard recommendation for creep design. Motivated by this finding, a search for data on similar bridges was undertaken. The results amount to a wake-up call: 56 other large-span, prestressed concrete, segmentally erected box girders (66 by the time of proof) have already been found to exhibit excessive long-time deflections and it is likely that many more such girders exist. The observed deflections give no sign of approaching a finite bound, as implied in the empirical ACI Committee 209, CEB-fib, and GL models for creep, and are found to evolve approximately logarithmically beginning at approximately 1000 days after span closing. Whereas sufficient data for the finite element (FE) creep analysis of these deflections proved impossible to obtain, it is demonstrated by comparisons with accurate deflection solutions for the KB Bridge that the terminal logarithmic deflection trend can be predicted quite well by a simple extrapolation of the measured 1000-day deflection under the hypothesis of proportionality to the compliance function increment since the time of span closing. Comparisons of the extrapolations according to various creep models show that, for the theoretically based Model B3, which is a 1995 RILEM recommendation, the underestimation of long-time deflections is much less severe than it is for the ACI Committee 209, CEB-fib, and GL models, and that the terminal trend is correct—that is, logarithmic. Because Model B3’s terminal trend can be separately controlled, a simple update of this model that gives the same mean terminal trend as the 56 bridges is devised. The use of this updated Model B3 should allow for the improvement of the durability of segmental bridges and other structures that are highly sensitive to creep.