Calculation of Load-Bearing Capacity of Concrete-Filled Steel Tube Flexural Members Under Long-Term Loading

Literature Overview

The paper by Han Bing and Wang Yuanfeng (Beijing Jiaotong University, 2005), published in the Journal of Highway and Transportation Research (Vol. 22, No. 5), addresses an important but often overlooked aspect of concrete-filled steel tube (CFST) structural design: the effect of long-term loading (creep) on the load-bearing capacity of CFST flexural members. The research was supported by the Ministry of Education Key Science and Technology Research Project (03040), the University Doctoral Point Fund (20030004002), and the Beijing Jiaotong University Foundation (TJJ02012). The authors develop a calculation method for the long-term load-bearing capacity of CFST beams and demonstrate through numerical examples that creep reduces the capacity, necessitating its consideration in practical design.

Technical Background

In conventional reinforced concrete beams, long-term effects such as creep and shrinkage significantly affect the deflection and, to a lesser extent, the load-bearing capacity. For CFST members, the concrete core is confined by the steel tube, which influences both the short-term and long-term mechanical behavior. The confinement effect generally reduces the creep of concrete under compression, but the interaction between the steel tube and the concrete core under sustained loading introduces complex stress redistribution phenomena that affect the overall member capacity.

Creep Mechanism in CFST Members

Under sustained axial or bending loads, the concrete core in a CFST member undergoes time-dependent deformation (creep). This creep deformation:

• Redistributes stresses between the steel tube and the concrete core
• Increases the stress in the steel tube as it compensates for the concrete's time-dependent deformation
• May lead to earlier yielding of the steel tube in compression
• Affects the overall deflection and potentially the load-bearing capacity

Methodology and Calculation Approach

Creep Analysis Framework

The authors establish a creep analysis framework for CFST flexural members that accounts for:

1. The nonlinear stress-strain behavior of concrete under sustained loading
2. The elastic-plastic behavior of the steel tube
3. The composite action between steel and concrete
4. The time-dependent stress redistribution

Long-Term Capacity Calculation Method

The proposed calculation method integrates the short-term mechanical properties with long-term creep effects. The key steps include:

1. Short-term capacity determination: Calculate the initial load-bearing capacity using conventional CFST flexural member design methods
2. Creep coefficient estimation: Determine the creep coefficient based on concrete age, loading duration, environmental conditions, and member geometry
3. Stress redistribution analysis: Calculate the time-dependent stress redistribution between steel and concrete
4. Long-term capacity adjustment: Modify the short-term capacity to account for the effects of stress redistribution and potential steel yielding

Numerical Example

The paper applies the method to a specific CFST flexural member and demonstrates that the long-term capacity is lower than the short-term capacity. The degree of reduction depends on:

• The magnitude and duration of the sustained load
• The concrete strength and age at loading
• The steel tube thickness and grade
• The environmental conditions (humidity, temperature)

Engineering Practice Implications

Design Code Considerations

Practical Design Recommendations

1. Sustained load assessment: Engineers should identify the proportion of total load that is sustained (dead load) versus transient (live load). The sustained load component is the primary driver of long-term effects.
2. Creep coefficient selection: The creep coefficient should be estimated using appropriate models such as those provided in ACI 209, Eurocode 2, or national standards. For CFST members, the confinement effect should be considered in the creep coefficient estimation.
3. Capacity reduction factor: Based on the proposed method, a capacity reduction factor should be applied to the short-term capacity when long-term loading effects are significant. The magnitude of this factor depends on the specific member geometry, material properties, and loading conditions.
4. Serviceability considerations: Even if the ultimate load-bearing capacity is not critically affected, the long-term deflection due to creep may exceed serviceability limits. Deflection checks should be performed for both immediate and long-term conditions.

Welding and Fabrication Considerations

For CFST flexural members, the fabrication process often involves welding the steel tube to end plates, stiffeners, or connection elements. The welding residual stresses in the steel tube can interact with the sustained loads to affect the long-term behavior:

• Residual stress superposition: Welding residual stresses superimpose with service stresses, potentially accelerating the onset of yielding in the steel tube under sustained loading.
• Stress relaxation: Under sustained loading at elevated temperatures, welding residual stresses may partially relax, which could be beneficial or detrimental depending on the stress state.
• Weld quality: High-quality welds with minimal defects reduce stress concentrations and provide more predictable long-term behavior.

The welding procedure specification (WPS) for CFST member fabrication should include provisions for post-weld stress relief if the member is subjected to significant sustained loads. Post-weld heat treatment (PWHT) can reduce welding residual stresses and improve the long-term performance of the welded connections.

Key Questions and Reflections

The paper raises an important issue that is often neglected in CFST structural design: the time-dependent reduction of load-bearing capacity. Several questions merit further investigation:

1. Quantification of capacity reduction: The paper demonstrates that creep reduces capacity, but the magnitude of this reduction should be quantified for common design scenarios. Engineers need clear guidance on when long-term effects are significant enough to warrant special consideration.
2. Interaction with fatigue: In structures subjected to both sustained loads and cyclic loading (such as bridges or industrial equipment), the interaction between creep and fatigue may accelerate damage accumulation. This combined effect is not addressed in the paper.
3. Temperature effects: For CFST members in high-temperature environments (such as industrial furnaces or nuclear facilities), the creep behavior of both concrete and steel is significantly enhanced. The proposed method should be validated for elevated temperature applications.
4. Concrete quality and placement: The long-term behavior of CFST members is sensitive to the quality of concrete placement and compaction within the steel tube. Poorly compacted concrete may contain voids that increase local stresses and accelerate creep. Quality control during concrete filling is essential.
5. Steel tube imperfections: Initial geometric imperfections in the steel tube (such as out-of-roundness or waviness) can affect the stress distribution and, consequently, the long-term behavior. The manufacturing tolerances specified in relevant standards (such as GB/T 8163 or API 5L) should be considered in the long-term capacity assessment.

Summary

This paper addresses a critical gap in CFST structural design by demonstrating that long-term loading effects (creep) reduce the load-bearing capacity of CFST flexural members and should be considered in practical design. The proposed calculation method provides a framework for evaluating long-term capacity, and the numerical example illustrates the practical significance of these effects. Engineers designing CFST structures should incorporate long-term loading considerations into their design process, particularly for members subjected to high sustained load ratios or in applications where long-term deflection and capacity are critical. The welding quality of connections and the concrete placement quality are also important factors that influence the long-term performance of CFST members and should be rigorously controlled during fabrication and construction.