Improved algorithm for efficient and realistic creep analysis of large creep-sensitive concrete structures

Recent compilation of data on numerous large-span prestressed segmentally erected box girder bridges revealed gross underestimation of their multi-decade deflections. The main cause has been identified as incorrect and obsolete creep prediction models in various existing standard recommendations and is being addressed in a separate study. However, previous analyses of the excessive deflections of the Koror-Babeldaob (KB) Bridge in Palau and of four Japanese bridges have shown that a more accurate method of multi-decade creep analysis is required. The objective of this paper is to provide a systematic and comprehensive presentation, appropriate not only for bridges but also for any large creepsensitive structure. For each time step, the solution is reduced to an elastic structural analysis with generally orthotropic elastic moduli and eigenstrains. This analysis should normally be threedimensional (3-D). It can be accomplished with a commercial finite element code such as ABAQUS. Based on the Kelvin chain model, the integral-type creep law is converted to a rate-type form with internal variables, which account for the previous history. For time steps short enough to render aging during each step to be negligible, a unique continuous retardation spectrum for each step is obtained by Laplace transform inversion using simple Widder's formula. Discretization of the spectrum then yields the current Kelvin chain moduli. The rate-type creep analysis is computationally more efficient than the classical integral-type analysis. More importantly, though, it makes it possible to take into account the evolution of various inelastic and nonlinear phenomena such as tensile cracking, cyclic creep, and stress relaxation in prestressing tendons at variable strain, as well as the effects of humidity and temperature variations, and the effect of wall thickness variation on drying creep and shrinkage. Finally, the advantages compared to the existing commercial programs, based on step-by-step integration of memory integrals, are pointed out and illustrated by a simple example.