Nakai, Kurita, and Ichinose 1991

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The authors performed an experimental study on creep and drying shrinkage in circular CFTs. They attempted to predict the time-dependent behavior of these structures by means of a viscoelastic model, whose parameters were evaluated experimentally. Creep is an important consideration in the design for serviceability and has a significant role in the amount of stress and deformation during the construction of a structure. To the point of the article's writing, little study of creep had been performed.

Experimental Study

Three test specimens were used to investigate each type of behavior. One of each group of three was a plain concrete specimen with no tube. The tests for all specimens were performed simultaneously in the same environment. The specimens measuring creep were loaded and the specimens measuring drying shrinkage remained unloaded. The duration of the test was 6 months, at which point the amount of creep had stabilized.

Analytical Study

The authors used a viscoelastic model -- the relatively simple Kelvin model. The element consisted of a spring and dashpot connected in parallel. The applied stress was assumed to be a function of the initial stress and experimentally-derived parameters obtained from tests of long-term axial loading of CFT columns.

Comparison of Results

The results of the experimental and theoretical methods showed good agreement except in the initial part of the strain versus time curve. The simplicity of the viscoelastic model may have accounted for this discrepancy. The authors hesitated in recommending a more complicated element because they felt it would lead to excessive computation time were this element to be employed in a finite element evaluation. The amount of creep as measured by the creep coefficient (ratio between the final strain and the elastic strain at time 0) varied from 1.44 to 1.61 for the CFTs and from 2.72 to 2.84 for the plain concrete column. Drying shrinkage of the concrete in the CFTs was very small in comparison to the plain concrete and may be neglected in practice.

References

Nakai, H., Kurita, A., and Ichinose, L. H. (1991). “An Experimental Study on Creep of Concrete Filled Steel Pipes,” Proceedings of the Third International Conference on Steel- Concrete Composite Structures, Wakabayashi, M. (ed.), Fukuoka, Japan, September 26-29, 1991, Association for International Cooperation and Research in Steel-Concrete Composite Structures, pp. 55-60.