Seismic Performance of Circular CFST Columns Under Cryogenic Temperature Environments

Literature Overview

This study by Yu Lusong, Wang Geng, and colleagues from Lanzhou Jiaotong University investigates the seismic behavior of circular steel tube concrete (CFST) columns under four different ambient temperature conditions: 20°C (room temperature), 0°C, −20°C, and −40°C. The research was funded by the Central Guidance Fund for Local Science and Technology Development (22ZY1QA005) and related Gansu Provincial programs. The work appears in the Journal of Earthquake Engineering (2024, Vol. 46, No. 6, pp. 1251–1258). Given that Gansu Province and other high-altitude, high-latitude regions in China routinely experience ambient temperatures below −30°C in winter, this research addresses a critical practical gap in seismic design codes that traditionally assume isothermal loading conditions.

Core Technical Findings

The quasi-static cyclic loading tests were conducted on four identical circular CFST column specimens, each subjected to a different temperature regime. The following key results were obtained:

All specimens exhibited spindle-shaped (spindle-like) hysteresis loops without obvious pinching, indicating that the steel tube provided effective lateral confinement to the core concrete even at extreme low temperatures. The typical failure mode remained consistent across all temperature conditions: a through-going bulging wave formed at the bottom of the steel tube, the core concrete was crushed, and the steel tube experienced tearing. However, the lower the temperature, the earlier the failure occurred and the more severe the damage.

Interpretation of Technical Points

The increase in lateral bearing capacity and initial stiffness at low temperatures can be attributed to the well-documented temperature-dependent behavior of structural steel. As temperature decreases, the yield strength of carbon steel increases while the ductility decreases — a phenomenon governed by the ductile-to-brittle transition temperature (DBTT) of the steel grade. For common structural steels used in CFST columns (Q235, Q345, Q390), the DBTT typically falls between −20°C and −60°C depending on the specific grade and heat treatment. At −40°C, the steel tube material is operating closer to its brittle regime, which explains the 50% increase in initial stiffness (reflecting higher elastic modulus and yield stress) but simultaneously the 16.9% reduction in ductility coefficient.

The stiffness degradation rate increasing with decreasing temperature is particularly concerning from a seismic design perspective. During cyclic loading, the rate at which stiffness degrades determines how quickly a column loses its ability to resist lateral displacements. A faster degradation rate means the column will reach its ultimate displacement limit sooner under repeated seismic loading, effectively reducing its usable seismic capacity.

Integration with Engineering Practice

From a welding and steel pipe manufacturing perspective, this research has several important implications. First, the steel tubes used in CFST columns must be selected not only for their ambient-temperature mechanical properties but also for their low-temperature toughness. The Charpy V-notch impact energy at the design minimum temperature should be a mandatory specification requirement. For regions where design temperatures reach −40°C, steel grades with guaranteed impact energy at −40°C (such as Q345D or Q390D in the Chinese GB/T 1591 system) should be specified rather than the commonly used Q345B or Q345C grades.

Second, the welding procedures used for field assembly of CFST columns must be qualified for cold-weather welding. Preheat temperatures, interpass temperatures, and post-weld heat treatment requirements all need to be adjusted when ambient temperatures are below 0°C. The hydrogen-induced cracking susceptibility of the heat-affected zone (HAZ) increases significantly at low temperatures, and the risk of cold cracking becomes a primary concern. According to API 5L and ASME B31.3, when the ambient temperature is below 5°C, the carbon equivalent (CE) of the base metal should be carefully evaluated, and preheat temperatures of at least 80–120°C are typically required for steels with CE > 0.4.

Third, the concrete used in CFST columns must be designed to withstand freeze-thaw cycles and low-temperature placement. The use of air-entraining admixtures, antifreeze admixtures, and appropriate curing protocols is essential. The concrete should meet the requirements of GB/T 50082 for freeze-thaw resistance testing.

Key Questions and Reflections

One question that arises from this study is whether the observed improvement in initial stiffness at low temperatures is a beneficial or detrimental phenomenon. While higher stiffness may seem advantageous for reducing seismic displacements, the accompanying reduction in ductility means that the column has less capacity to absorb seismic energy through inelastic deformation. In seismic design philosophy, ductility is often more important than stiffness because it provides the margin of safety against collapse. Therefore, the net effect of low-temperature exposure on seismic performance is potentially negative despite the apparent improvement in stiffness.

Another important consideration is the thermal stress that develops when the steel tube and core concrete experience differential thermal contraction at low temperatures. The coefficient of thermal expansion for carbon steel is approximately 12×10⁻⁶/°C, while for ordinary concrete it is approximately 10×10⁻⁶/°C. This differential can generate significant interfacial stresses that may affect the bond between the steel tube and concrete, particularly if the interface has already been weakened by construction defects or poor compaction.

Study Insights and Implications

This research provides valuable experimental data for the seismic design of CFST structures in cold regions. The findings suggest that current seismic design codes, which do not explicitly account for temperature effects, may overestimate the ductility of CFST columns in cold environments. Designers should consider applying reduction factors to the ductility capacity of CFST columns when the design minimum temperature is below 0°C. Additionally, the steel tube material specification should be upgraded to ensure adequate low-temperature toughness, and welding procedures should be qualified for cold-weather conditions. The research underscores the importance of considering environmental conditions in structural design, a principle that is increasingly recognized in modern performance-based seismic engineering.