Creep Design Formula for Concrete-Filled Steel Tube Flexural Members

Literature Overview

This paper by Wang Yuanfeng, Zhu Haibin, and Han Bing from Beijing Jiaotong University addresses a critical but often underappreciated aspect of concrete-filled steel tube (CFST) structural design: time-dependent creep deformation in flexural members. Published in 2007 in Highway and Transportation Research, the work was supported by the Ministry of Education Key Research Project (03040) and the Doctoral Point Fund (20030004002). The authors build upon the hereditary flow theory framework to develop a practical regression-based design formula for creep in CFST bending members, targeting direct application in bridge engineering design codes.

Core Technical Approach

The fundamental challenge in CFST creep analysis lies in the composite action between the steel tube and the core concrete. Unlike plain concrete members, the steel tube imposes lateral confinement on the core concrete, which fundamentally alters the creep behavior. The authors adopt the hereditary flow theory as their theoretical basis, which expresses creep strain as a convolution integral of the stress history with a time-dependent creep compliance function. This approach is physically rigorous but computationally demanding for routine design purposes.

The key innovation is the application of regression analysis to distill the complex theoretical framework into a compact design formula. The authors systematically consider three primary influencing parameters:

The regression methodology extracts the dominant functional dependencies from theoretical calculations, producing a formula that is simple in form yet comprehensive in the factors considered.

Technical Points and Engineering Relevance

The distinction between theoretical calculation methods and design formulas cannot be overstated in structural engineering practice. Theoretical methods based on hereditary flow theory require iterative computation and detailed material property inputs, making them impractical for routine design work. The regression formula bridges this gap by encoding the essential physics into an algebraic expression that designers can apply directly.

From a materials engineering perspective, several observations are particularly noteworthy. The confinement effect of the steel tube on core concrete is not merely a geometric consideration—it fundamentally changes the stress state from uniaxial to triaxial compression, which is known to suppress micro-crack propagation in concrete and thereby reduce creep. However, the steel tube itself undergoes plastic deformation under sustained loading, and this steel creep contributes to the overall member deflection. The interaction between concrete creep and steel creep creates a coupled deformation mechanism that is more complex than either component alone.

The bending moment level parameter deserves special attention. At low moment levels, the steel tube primarily carries elastic stresses and the concrete creep dominates. As the moment level increases, the steel tube approaches its yield condition, and the stress redistribution between steel and concrete changes the creep trajectory significantly. The regression formula captures this non-linearity, which is a significant improvement over linearized approaches.

Verification and Comparative Analysis

The authors validate their regression formula against theoretical calculations from the literature. The comparison demonstrates good agreement between the simplified design formula and the rigorous theoretical method, confirming that the regression approach preserves the essential physics while dramatically reducing computational complexity. The validation approach, while sound, is limited to comparison with theoretical values rather than experimental data. This is a notable gap, as experimental creep data for CFST members under bending is inherently difficult to obtain due to the long time scales involved and the difficulty of maintaining controlled loading conditions.

Study Insights and Engineering Implications

The practical value of this work lies in its direct applicability to design codes. A formula that is simple, comprehensive, and validated against theory is exactly what code committees need for adoption. The progressive creep behavior of CFST flexural members has significant implications for bridge engineering, where long-term deflection limits govern serviceability. Designers who neglect creep in CFST members risk underestimating long-term deflections, potentially leading to serviceability failures.

The hereditary flow theory foundation provides a physical basis that gives confidence in the formula's extrapolation beyond the calibration range. However, engineers should exercise caution when applying the formula to members with steel ratios, moment levels, or time durations outside the calibration database. The formula represents a best-fit approximation, and its accuracy degrades as the input parameters deviate from the calibration range.

For bridge engineers working with CFST girders and piers, this formula provides a practical tool for serviceability limit state design. The integration of creep considerations into the design process is a step toward more realistic and safe structural design. Future work should focus on experimental validation under controlled laboratory conditions and extension of the formula to account for additional factors such as temperature effects, concrete maturity at loading, and cyclic loading histories typical of bridge structures.