Zhuojin Pipe Fitting Co., LtdEffect of Creep on Axial Compressive Stability Bearing Capacity of Hollow Steel Tube Concrete Members
Literature Overview
This paper by Wang Hongxin and Zha Xiaoxiong from the Department of Urban and Civil Engineering, Shenzhen Graduate School, Harbin Institute of Technology (2009), investigates the influence of concrete creep on the axial compressive stability bearing capacity of hollow steel tube concrete (HSTC) members. The research develops a theoretical framework for calculating the critical bearing capacity of HSTC columns and employs finite element analysis to quantify the creep-induced reduction in stability capacity. The study provides a practical design recommendation regarding the application of a creep reduction coefficient for members subjected to sustained loads.
Technical Framework and Methodology
The research approach combines theoretical derivation with numerical simulation to capture the time-dependent behavior of HSTC members. The methodology involves:
- Derivation of a critical bearing capacity formula specific to HSTC members
- Finite element calculation of three types of creep: free creep of plain concrete, unloading creep of core concrete, and longitudinal creep of the member
- Calculation of the stability bearing capacity reduction rate based on the change in tangent modulus of both steel and concrete after creep development
- Parametric analysis of the influence of hollow ratio, steel grade, and load proportion on the creep effect
| Creep Component | Description | Influence on Stability |
|---|---|---|
| Free creep of plain concrete | Creep of concrete without restraint | Reduces effective stiffness |
| Unloading creep of core concrete | Creep due to unloading of core concrete | Affects stress redistribution |
| Longitudinal creep of member | Overall member creep under sustained load | Reduces critical buckling load |
The hollow steel tube concrete configuration is distinct from solid STC members in that the steel tube is separated from the concrete core by a hollow space. This configuration reduces the overall stiffness compared to solid STC members and creates a more complex interaction between the steel tube and concrete core, particularly under sustained loading conditions.
Key Findings on Creep Effects
The parametric analysis reveals clear trends in the influence of various parameters on the creep-induced reduction of stability bearing capacity:
| Parameter | Direction of Effect | Magnitude of Influence |
|---|---|---|
| Hollow ratio (increasing) | Reduces creep effect | Moderate to high |
| Steel grade (higher strength) | Reduces creep effect | Moderate |
| Sustained load proportion (increasing) | Increases creep effect | High |
| Slenderness ratio (increasing) | Increases creep effect | Significant |
| Concrete age at loading (younger) | Increases creep effect | Moderate |
The most significant finding is the threshold effect related to the sustained load proportion. When the permanent load-induced axial force accounts for 30% or more of the total axial force, the stability bearing capacity should be multiplied by a creep reduction coefficient. This threshold provides a clear criterion for determining when creep effects must be considered in the design of HSTC columns.
Critical Bearing Capacity Formula Development
The derived critical bearing capacity formula accounts for the composite action of the steel tube and concrete core, considering the hollow configuration's influence on the overall member stiffness. The formula incorporates:
- The effective elastic modulus of the composite section after creep development
- The geometric properties of the hollow STC cross-section
- The boundary conditions and effective length of the column
- The interaction between the steel tube and concrete core through shear transfer
The finite element analysis validates the theoretical formula by simulating the full nonlinear behavior of HSTC columns under sustained loading, including the time-dependent stress redistribution and the progressive loss of stability capacity due to creep.
Design Recommendations and Practical Guidelines
Based on the research findings, the following design recommendations are provided for engineers working with HSTC columns:
- When the permanent load proportion exceeds 30% of the total axial force, apply a creep reduction coefficient to the stability bearing capacity
- The reduction coefficient should be determined based on the specific combination of hollow ratio, steel grade, slenderness ratio, and concrete age at loading
- Higher steel grades provide better resistance to creep-induced capacity loss due to their higher stiffness contribution
- Larger hollow ratios reduce the creep effect by decreasing the concrete volume subject to creep deformation
- Slender HSTC columns are more sensitive to creep effects and require careful stability analysis
Engineering Practice Implications
The research has direct implications for the design of HSTC columns in practical applications, including:
- Multi-story building columns where permanent loads constitute a significant portion of the total load
- Long-span bridge columns where sustained service loads develop over time
- Industrial structures with heavy permanent equipment loads
- Nuclear facility structures where long-term stability is critical
The identification of the 30% threshold for sustained load proportion provides a practical screening criterion for engineers to determine when creep effects must be included in the stability analysis. This threshold can be incorporated into design checklists and quality assurance procedures for HSTC structures.
Key Questions and Reflections
The research raises several important questions regarding the long-term performance of HSTC members. The interaction between creep and the potential for concrete cracking under sustained loading is not fully addressed. In practice, creep-induced stresses can lead to cracking, which further reduces the member's stiffness and accelerates the loss of stability capacity. This progressive degradation mechanism may result in a more severe capacity loss than predicted by the linear creep model used in the analysis.
Additionally, the research does not consider the effect of environmental factors such as temperature variations, humidity, and carbonation on the creep behavior of the concrete core. In real structures, these factors can significantly influence the creep rate and magnitude, potentially leading to unexpected capacity reductions.
The applicability of the creep reduction coefficient to different loading scenarios, including cyclic loading and impact loading, remains uncertain. The coefficient is derived for sustained axial compression, and its extension to other loading conditions requires further investigation.
Study Insights and Implications
This research provides a valuable contribution to the understanding of time-dependent behavior in HSTC columns, addressing a critical aspect of structural design that has received limited attention in existing codes. The development of a specific critical bearing capacity formula for HSTC members and the identification of the 30% sustained load threshold offer practical tools for engineers designing long-life structures. The parametric analysis clearly demonstrates the influence of key design parameters on the creep effect, enabling engineers to optimize the HSTC configuration for minimum creep sensitivity. For the structural engineering community, this research highlights the importance of considering time-dependent effects in the design of composite columns and supports the need for code provisions that explicitly address creep-induced capacity loss. The findings encourage the continued development of HSTC structural systems while emphasizing the need for comprehensive long-term performance evaluation in design practice.