Simplified Creep Calculation Method for CFST Axially Compressed Members

Literature Overview

The research by Wang Yongbao, Zhao Renda, Xu Tengfei, and Zhan Yulin from Southwest Jiaotong University presents a simplified analytical method for calculating the long-term creep deformation of concrete-filled steel tube (CFST) axially compressed members. Published in Journal of Highway and Transportation Research and Development, Volume 33, Issue 1, 2016, pages 57-63, and funded by the National Natural Science Foundation (grant 51208431) and Central University Basic Scientific Research Funds (SWJTU12CX064), the study develops a practical approach based on the age-adjusted effective modulus method and step-by-step integration.

Technical Background

Creep deformation is a critical long-term behavior consideration for CFST members in bridge structures, where sustained compressive loads from dead weight, prestress, and permanent traffic loads act over decades of service life. The interaction between the concrete core and the steel tube modifies the creep behavior significantly compared to plain concrete columns. The steel tube constrains the concrete laterally, which reduces creep strain through the confinement effect, while the steel tube itself does not exhibit creep.

Traditional methods for calculating CFST creep deformation include:

• Direct application of plain concrete creep models, which overestimate deformation by ignoring the steel tube restraint.
• Full finite element analysis with time-dependent material models, which is computationally expensive and requires complex input data.
• Empirical approaches based on test data, which lack theoretical rigor and have limited extrapolation capability.

The proposed simplified method bridges the gap between computational simplicity and analytical accuracy.

Methodology and Key Findings

The simplified method is based on two fundamental approaches:

1. Age-adjusted effective modulus method: This accounts for the time-dependent stiffness reduction of concrete by using an effective modulus that incorporates the creep coefficient and the age factor. The method is well-established in the European fib Model Code and CEB-FIP approaches.
2. Step-by-step integration: The loading history is divided into discrete time intervals, and the creep deformation is accumulated incrementally at each step. This approach captures the nonlinear interaction between the steel tube and concrete core as the concrete ages and creeps.

The study systematically evaluates the influence of concrete creep prediction models and relative humidity values on the accuracy of the simplified method. The key finding is that using the CEB90 model or the EC2 model with relative humidity values in the range of 90-98% yields results that closely match experimental data.

The high relative humidity values (90-98%) are physically justified because the concrete core in a CFST member is enclosed by a sealed steel tube, creating a near-saturated environment that inhibits moisture loss. This is fundamentally different from plain concrete members exposed to ambient conditions, where relative humidity typically ranges from 40-70%.

Engineering Practice Implications

For bridge engineers designing CFST members, the simplified method provides a practical tool for long-term deformation prediction:

• The method can be implemented in standard structural analysis software or even in spreadsheet calculations, making it accessible for routine design checks.
• The step-by-step integration approach allows for accurate modeling of complex loading histories, including staged construction sequences common in bridge erection.
• The recommended humidity range of 90-98% should be used for all CFST members where the steel tube is continuous and watertight. For members with potential moisture pathways (e.g., incomplete concrete filling, open ends), a lower humidity value should be considered.

From a steel pipe manufacturing perspective, the watertight integrity of the steel tube is critical for ensuring the assumed humidity conditions. Any leak in the steel tube weld, particularly at field-welded splices, can allow moisture to escape and accelerate concrete drying, leading to increased creep deformation beyond the predicted values. Therefore:

• Field-welded splices in CFST bridge members must achieve full-penetration welds with 100% UT inspection.
• The internal surface of the steel tube must be free of pinholes or cracks that could allow concrete leakage during casting.
• Post-casting inspection should verify complete concrete filling, as voids within the concrete core can create internal moisture pathways.

Study Insights

This research contributes a practical and accurate method for predicting long-term creep deformation of CFST axially compressed members. The key insight is that the enclosed nature of the concrete core in CFST members fundamentally changes the creep environment compared to plain concrete, and this must be accounted for in design calculations. The recommendation to use CEB90 or EC2 creep models with humidity values of 90-98% provides clear guidance for design engineers. The simplified method's accuracy, validated against experimental data, gives confidence in its application to bridge design. For the steel pipe industry, the study reinforces the importance of weld quality and dimensional accuracy, as the structural performance of CFST members depends critically on the integrity of the steel tube as a confinement element. The method should be incorporated into Chinese design codes for CFST bridge members to replace the current conservative approaches that do not adequately account for the steel-concrete interaction effects on long-term behavior.