Mechanical Properties of Rectangular Steel Tube Concrete Short Columns with Internal Steel Sections Throughout the Service Lifecycle

Literature Overview

This 2020 study by Shi Yanli, Zhang Wenxu, Jia Zhilu, and Wang Wenda, published in Journal of Architecture and Civil Engineering, investigates the mechanical behavior of rectangular steel tube concrete (STC) short columns with internal steel sections under axial compression throughout their entire service lifecycle. Using ABAQUS finite element analysis validated against existing experimental data, the authors examined the effects of initial stress in the steel tube and long-term loading on load-deformation behavior, stress distribution, and interaction forces between steel and concrete components.

Core Technical Content

Lifecycle Modeling Approach

The study takes a comprehensive lifecycle perspective by incorporating two key factors that evolve over time: the initial stress state in the steel tube (resulting from construction sequencing and pre-loading effects) and the long-term sustained load (representing dead load and service loads applied over the column's service life). This approach is more realistic than conventional single-loading analyses that consider only the ultimate load stage.

Key Findings on Load-Deformation Behavior

The research reveals that while the ultimate load-bearing capacity of the STC short columns with internal steel sections shows minimal change compared to single-loading conditions, the longitudinal strain at ultimate capacity increases by 84.2%. This significant ductility enhancement is an important finding, as it indicates that lifecycle effects primarily affect the deformation capacity rather than the strength of these composite columns.

Stress Distribution and Component Interaction

During the long-term loading phase, the core concrete experiences an unloading phenomenon, with its load share decreasing by approximately 30% before resuming load-bearing during the reloading phase. This behavior is attributed to the viscoelastic and creep characteristics of concrete under sustained loading. The contact stress between the steel tube and concrete is maximum at the mid-section and gradually decreases toward both ends of the column, which has implications for the design of end connections and the potential for local buckling near the column ends.

Parametric Influence Analysis

The steel tube steel ratio has a pronounced effect on both capacity and deformation, with higher ratios yielding greater capacity and reduced deformation. In contrast, the internal steel section ratio has a limited influence on deformation, suggesting that the steel tube provides the primary confinement effect while the internal steel sections contribute mainly to axial load capacity. As the initial stress coefficient and long-term load ratio increase, the longitudinal deformation of the column increases proportionally.

Engineering Practice Implications

This study has direct relevance to the design and assessment of existing structures where long-term loading effects must be considered. For steel pipe manufacturers, the findings highlight the importance of ensuring high-quality steel tube fabrication, as the steel tube's performance under combined initial stress and long-term loading is critical to the overall column behavior. The 84.2% increase in strain at ultimate capacity suggests that lifecycle effects significantly enhance ductility, which is beneficial for seismic resilience but may require consideration in displacement-based design approaches. The observed unloading-reloading behavior of the core concrete under long-term loading has implications for the assessment of existing columns, as the actual stress state may differ from predictions based on elastic analysis alone.

Study Insights and Reflections

The lifecycle perspective adopted in this study represents a significant advancement in the understanding of STC column behavior, moving beyond the traditional single-loading paradigm. The finding that ultimate capacity remains largely unchanged while ductility increases substantially is counterintuitive but well-supported by the detailed finite element analysis. This suggests that the initial stress and long-term loading effects primarily redistribute internal forces among the component materials rather than fundamentally altering the composite action mechanism. From a quality control standpoint, ensuring consistent steel tube dimensions and material properties is essential, as variations in steel tube thickness or yield strength could amplify the effects of initial stress on the overall column performance. The study also raises important questions about the applicability of current design codes, which may not adequately account for lifecycle effects in STC column design. Engineers should consider these findings when assessing the long-term performance of existing STC structures and when designing new structures that must accommodate both construction-phase and service-phase loading conditions.