Shear Lag Effect and Uncertainty in Concrete Box Girder Creep

Kř́ıstek, Vladiḿır; Bažant, Zdeněk P.

doi:10.1061/(asce)0733-9445(1987)113:3(557)

Cited by 36 publications

(10 citation statements)

References 10 publications

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“…Using the database (Bažant and Li 2008a) as the prior information, the statistics of long-term deflection can be further improved by means of Bayesian statistical analysis (Křístek and Bažant 1987). The probabilistic problem of deflections is fortunately much easier than the problem of structural safety.…”

Section: Uncertainty Of Deflection Predictions and Calculation Of Conmentioning

confidence: 99%

Excessive Long-Time Deflections of Prestressed Box Girders. I: Record-Span Bridge in Palau and Other Paradigms

2012

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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, Comité Euro-International du Béton (or Comité Euro-International du Béton-Fédération internationale de la précontrainte), 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. The detailed method of analysis and the lessons learned are presented in Part II.

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Section: Uncertainty Of Deflection Predictions and Calculation Of Conmentioning

confidence: 99%

Excessive Long-Time Deflections of Prestressed Box Girders. I: Record-Span Bridge in Palau and Other Paradigms

2012

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“…Křístek and Bažant [20] investigated the shear lag under longterm loading in concrete box girders using a visco-elastic model. They pointed out that additional shear deformability causes an increase in both the maximum values of stresses and deflections in the long term.…”

Section: Introductionmentioning

confidence: 99%

Long-term analysis of steel–concrete composite beams: FE modelling for effective width evaluation

Macorini

Fragiacomo

Amadio

et al. 2006

Engineering Structures

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“…The shear lag effect is a nonuniform stress distribution along the flanges of slender flexural members, such as box girders. The large shear force transferred from the vertical webs to the horizontal flanges of the box girder induces an in-plane shear deformation resulting in a higher longitudinal deflection at the flanges near the web [11]. In summary, the current design calculations significantly underestimate the long-term deflection, especially for large-span bridges built using a segmental construction method [2,9].…”

Section: Introductionmentioning

confidence: 90%

Long-Term Deflection of Prestressed Concrete Bridge Considering Nonuniform Shrinkage and Crack Propagation by Equivalent Load Approach

2020

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Long-span prestressed concrete (PSC) bridges often suffer excessive deflection during their service lives. The nonuniform shrinkage strains of concrete caused by uneven moisture distributions can induce significant additional deflections, when combined with the creep and cracking of the concrete. Current design practices usually overlook these factors, and the few proposed approaches to consider them are complex and computationally expensive. This study proposes a simplified approach for considering the effect of nonuniform shrinkage by using the equivalent load concept in combination with a nonlinear analysis of the creep and cracking using three-dimensional finite element models. The long-term deflections of short-, medium-, and long-span PSC bridges are calculated under the combined effects of creep, shrinkage, and cracking. The results show that the nonuniform shrinkage effect is significant in medium- to long-span bridges, and that the cracking of the concrete reduces the stiffness, thereby increasing the long-term deflection of the bridges (more severely so in combination with creep and shrinkage). The predicted long-term deflections reasonably agree with the measured data. Thus, the equivalent load approach is effective for calculating long-term deflections considering nonuniform shrinkage strains, without the complicated and expensive coupling of moisture transport and structural analyses.

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Shear Lag Effect and Uncertainty in Concrete Box Girder Creep

Cited by 36 publications

References 10 publications

Excessive Long-Time Deflections of Prestressed Box Girders. I: Record-Span Bridge in Palau and Other Paradigms

Excessive Long-Time Deflections of Prestressed Box Girders. I: Record-Span Bridge in Palau and Other Paradigms

Long-term analysis of steel–concrete composite beams: FE modelling for effective width evaluation

Long-Term Deflection of Prestressed Concrete Bridge Considering Nonuniform Shrinkage and Crack Propagation by Equivalent Load Approach

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