Simplified Creep Calculation Method for Axially Compressed Steel Tube Concrete Members

Literature Overview

Published in Journal of Highway and Transportation Research (2016, Vol. 33, No. 1), this study proposes a simplified creep analysis method for axially compressed steel tube concrete (SRC) members based on the age-adjusted effective modulus method and step-by-step integration. The research addresses the challenge of accurately and efficiently calculating long-term deformation of SRC members, which is critical for bridge engineering applications where time-dependent behavior significantly influences structural performance over the service life.

Core Technical Content

Methodology Development

The proposed method combines two fundamental approaches: (1) the age-adjusted effective modulus method, which accounts for the time-dependent reduction in concrete stiffness due to creep, and (2) the step-by-step integration method, which divides the loading history into discrete time intervals and calculates the incremental deformation at each step. This combination allows for the capture of load-history-dependent creep behavior while maintaining computational simplicity.

Creep Model and Humidity Influence

The study systematically evaluates the influence of concrete creep prediction models and relative humidity values on the accuracy of long-term deformation calculations. The CEB90 and EC2 models, when applied with relative humidity values in the range of 90%–98%, provide the most accurate predictions of SRC member long-term deformation. This humidity range reflects the typical internal humidity conditions within steel tube concrete members, where the steel tube provides partial moisture retention while allowing some vapor transmission.

Comparison with Traditional Methods

The simplified method demonstrates comparable accuracy to traditional detailed finite element methods while requiring significantly less computational effort. The trade-off between accuracy and computational efficiency makes this method suitable for routine engineering design applications where multiple design scenarios need to be evaluated rapidly.

Engineering Practice Implications

For bridge engineers designing SRC members, the simplified creep method provides a practical tool for predicting long-term deflection and settlement that directly affects:

• Serviceability limit state verification: Long-term creep deformation can cause excessive deflection that violates serviceability requirements, particularly in long-span bridges.
• Prestress loss estimation: In prestressed SRC bridge members, creep-induced shortening reduces the effective prestress, affecting structural capacity over time.
• Connection design: Differential deformation between SRC members and connected elements (bearings, expansion joints, secondary structures) due to creep must be accommodated in the connection design.

From a steel tube fabrication and welding perspective, the creep behavior of the concrete core influences the long-term stress state in the steel tube. As the concrete creeps and shortens, the steel tube experiences increased compressive stress due to the load redistribution from the concrete to the steel. This progressive load transfer means that the steel tube may experience higher stresses than initially calculated at the time of construction, which has implications for:

• Steel tube material selection: The long-term stress level in the steel tube should be considered in material grade selection to ensure adequate fatigue and fracture toughness margins.
• Weld quality requirements: Welds in the steel tube that are subjected to progressively increasing compressive stresses over the service life must maintain their integrity under sustained loading, making weld quality particularly important for long-term structural performance.

Key Technical Points for Practitioners

• The age-adjusted effective modulus method combined with step-by-step integration provides an accurate and computationally efficient approach for SRC creep analysis.
• The CEB90 and EC2 creep models with relative humidity values of 90%–98% yield the most accurate long-term deformation predictions for SRC members.
• The simplified method achieves accuracy comparable to detailed FEM methods while requiring significantly less computational resources.
• Long-term creep causes progressive load redistribution from concrete to steel tube, increasing steel tube stress over time.
• The method is validated against experimental data, confirming its reliability for engineering applications.

Study Insights and Reflections

This research addresses a practical engineering need — the accurate prediction of long-term deformation in SRC bridge members without requiring computationally intensive finite element analyses for every design iteration. The finding that internal relative humidity values of 90%–98% provide the best creep predictions is particularly useful, as it reflects the moisture-retaining effect of the steel tube confinement on the concrete core. For steel pipe manufacturers and welding engineers, the implications of progressive load redistribution due to creep highlight the importance of selecting appropriate steel grades with adequate long-term strength properties and ensuring weld quality that can withstand sustained increasing compressive stresses. The simplified method represents a practical bridge between academic research on concrete creep and the computational constraints of routine engineering design practice.