Creep Analysis of Small Eccentrically Compressed Steel Tube Concrete Members

Literature Overview

This paper by Han Bing and Wang Yuanfeng from Beijing Jiaotong University, published in the Journal of Engineering Mechanics in 2001, addresses a fundamental yet often overlooked issue in steel tube concrete (SRC) structural design: the long-term creep deformation of eccentrically loaded members. The authors employed the successive flow theory combined with multiaxial stress-state creep models for concrete to develop an iterative calculation method that accounts for steel ratio, material grade, and stress level. The work was supported by the Ministry of Railways Science and Technology Development Fund (Project J99Z071), reflecting the practical importance of this research for heavy infrastructure applications such as railway bridges and viaducts.

Core Technical Approach

The central methodology rests on three pillars: the successive flow theory for time-dependent deformation, multiaxial stress-state creep constitutive models for the confined core concrete, and an iterative procedure to converge on the final creep displacement. Unlike uniaxial creep models commonly used in conventional reinforced concrete design, the multiaxial approach captures the interaction between axial compression and bending moments that arise under eccentric loading. The successive flow theory allows the time-varying stress field to be decomposed into incremental stages, with each stage's creep contribution computed under the current stress state before advancing to the next.

The iterative framework is essential because creep deformation itself alters the internal force distribution—eccentrically compressed members develop additional secondary moments as the core concrete creeps unevenly across the section. The authors structured the iteration so that at each cycle, the creep displacement is computed, the internal forces are updated, and the stress state is recalculated until convergence is achieved. This approach is conceptually similar to the incremental-iterative methods used in nonlinear structural analysis, but tailored specifically to the time-dependent behavior of confined concrete.

Key Technical Parameters and Influencing Factors

The paper identifies several parameters that govern the magnitude of creep in SRC eccentrically loaded members:

Engineering Practice Connection

From a pipe and fitting engineering perspective, this work has direct relevance to the design of steel tube concrete columns used in industrial plants, pipelines support structures, and offshore platforms. In pipeline engineering, long-term creep of concrete-filled steel pipe columns can lead to progressive deflection that affects the alignment of connected piping systems, potentially inducing additional stresses at weld joints and flanged connections. The iterative method proposed here provides a more accurate prediction of long-term deflection than simplified elastic analysis, which is critical for ensuring that pipeline supports maintain their design alignment over the service life of the structure.

The confinement effect of the steel tube on the core concrete is analogous to the interaction between the pipe wall and internal contents in pressure vessels. Just as the steel tube provides lateral confinement that improves the compressive strength and ductility of the concrete, the creep behavior is fundamentally altered by this confinement. The authors' findings confirm that higher steel ratios not only increase load capacity but also significantly reduce long-term deformation—a principle that should guide the selection of steel tube wall thickness in SRC column design for pipeline support applications.

Key Questions and Reflections

One important question arises from this study: how does the creep behavior of the concrete change when the steel tube is not a seamless pipe but rather a welded pipe with a longitudinal weld seam? The presence of a weld seam introduces a local heterogeneity in the steel tube's mechanical properties, which could potentially affect the uniformity of the confinement pressure on the core concrete and thus influence the creep distribution. While this is a secondary effect, it merits consideration in high-precision applications where differential creep could lead to local stress concentrations.

Another reflection concerns the validation methodology. The authors compared their numerical results with experimental data and found good agreement, which strengthens confidence in the proposed method. However, the experimental specimens were likely short columns with controlled boundary conditions. In practical pipeline support structures, SRC columns often have complex boundary conditions with partial fixity at the base and lateral restraint from connected piping. The applicability of the method to such real-world boundary conditions would benefit from further experimental validation.

Study Insights and Implications

This paper contributes a rigorous theoretical framework for predicting the long-term behavior of SRC eccentrically loaded members. For engineers involved in the design of pipeline support structures, the key takeaway is that elastic analysis alone is insufficient for long-term performance assessment. The proposed iterative method, while computationally more demanding, provides significantly improved accuracy in predicting creep-induced deflections that could compromise the integrity of connected piping systems. The emphasis on multiaxial stress-state creep models reflects a mature understanding that confined concrete does not behave as a simple uniaxial material, and future design codes for steel tube concrete structures should incorporate such refined models.