Creep Analysis of Square Steel Tube Concrete Axially Compressed Members

Literature Overview

The paper by Liang Yaping and Wang Yuanfeng, published in the Journal of Beijing Jiaotong University in 2003 (Volume 27, Issue 1, pages 59-62), presents a theoretical analysis of the creep behavior of square steel tube concrete (SRC) axially compressed members. The authors, affiliated with the School of Civil Engineering and Architecture at Beijing Jiaotong University, developed creep calculation formulas based on the successive flow theory of concrete creep, combined with the specific loading characteristics of SRC axially compressed members.

This work addresses an important aspect of the long-term behavior of SRC structures that has been insufficiently studied in the literature. The creep of the concrete core in SRC members leads to time-dependent deformations that affect the serviceability of the structure, including excessive deflections, cracking, and redistribution of internal forces. Understanding and predicting the creep behavior is essential for the reliable design of SRC structures subjected to sustained loading.

Theoretical Framework: Successive Flow Theory

The successive flow theory, also known as the age-adjusted effective modulus method, is a well-established approach for analyzing the time-dependent behavior of concrete structures. The theory considers the effects of creep and shrinkage on the stress-strain relationship of concrete, accounting for the age-dependent properties and the nonlinearity of the creep behavior.

In the context of SRC axially compressed members, the successive flow theory is applied to the concrete core, considering the confinement effect of the steel tube on the concrete properties. The steel tube provides lateral confinement that increases the compressive strength of the concrete and affects the creep behavior by reducing the tensile stresses that drive the creep deformation.

The theoretical analysis involves deriving the creep calculation formulas for SRC axially compressed members by considering the equilibrium, compatibility, and constitutive relationships of the composite section. The formulas account for the interaction between the steel tube and the concrete core, including the effect of the confinement on the creep behavior of the concrete.

Creep Calculation Formulas

The authors derived a set of creep calculation formulas that consider the specific characteristics of SRC axially compressed members. The formulas are based on the successive flow theory and account for the following factors:

1. The age-dependent creep properties of the concrete core.
2. The confinement effect of the steel tube on the concrete creep behavior.
3. The time-dependent redistribution of stresses between the steel tube and the concrete core.
4. The effect of sustained loading duration on the creep deformation.

The iterative calculation procedure involves solving the equilibrium and compatibility equations at each time step, updating the stresses and strains based on the creep and shrinkage properties of the concrete. The iteration continues until the solution converges, providing the time-dependent stress and strain distributions in the composite section.

Validation with Experimental Data

The authors validated the derived formulas by comparing the calculated creep deformations with experimental data from SRC column tests. The experimental data included measurements of the axial strain in both the steel tube and the concrete core at different time intervals under sustained loading.

The comparison showed good agreement between the calculated and experimental results, confirming the accuracy of the derived formulas. The iterative calculation procedure effectively captured the time-dependent behavior of the SRC members, including the initial rapid creep deformation and the subsequent gradual increase in deformation over time.

Engineering Practice Implications

The creep analysis of SRC axially compressed members has important implications for the design and serviceability assessment of SRC structures. The time-dependent deformations due to creep can lead to excessive deflections, cracking, and redistribution of internal forces, which may affect the structural performance and durability.

In the design of SRC columns, the creep deformation must be considered in the serviceability limit state analysis. The deflection limits specified in design codes, such as GB 50017, Eurocode 4, and AISI, must be checked against the predicted creep deformations to ensure that the structure meets the serviceability requirements.

The creep analysis also provides guidance for the design of SRC structures subjected to long-term loading, such as those in buildings, bridges, and industrial structures. The time-dependent behavior of the concrete core must be considered in the design of connections, foundations, and other structural components to ensure that the structure performs adequately over its design life.

Key Questions and Reflections

Several important questions arise from this study that deserve further investigation. How does the confinement effect of the steel tube vary with the concrete strength and the steel tube geometry? The confinement effect is a critical factor in the creep behavior of the concrete core, and its variation with different parameters must be understood for accurate predictions.

How does the creep behavior change under cyclic loading conditions, such as those encountered in structures subjected to traffic or seismic loads? The combination of sustained loading and cyclic loading may lead to accelerated creep deformation and premature failure, which must be considered in the design of dynamic structures.

Furthermore, the long-term durability of SRC members under environmental exposure conditions needs to be investigated. The creep behavior of the concrete core may be affected by the ingress of aggressive agents, such as chlorides and sulfates, which can accelerate the degradation of the concrete and affect the structural performance.

Study Insights and Conclusions

This paper provides a rigorous theoretical framework for the creep analysis of square steel tube concrete axially compressed members. The derivation of creep calculation formulas based on the successive flow theory offers a rational basis for predicting the time-dependent behavior of SRC structures under sustained loading.

The validation with experimental data confirms the accuracy of the proposed formulas and demonstrates their applicability to practical engineering problems. The iterative calculation procedure provides a practical tool for engineers to evaluate the creep deformations of SRC columns and ensure compliance with serviceability requirements.

The work contributes to the advancement of SRC structural engineering by addressing an important aspect of the long-term behavior that has been insufficiently studied. The derived formulas provide a foundation for the development of design provisions that incorporate creep effects in the analysis and design of SRC structures. Future research should focus on extending the analysis to more complex loading conditions, including cyclic loading and combined loading, and on investigating the long-term durability of SRC members under environmental exposure.