Axial Compression Performance of Square High-Strength Steel Tube-Concrete Composite Columns in Cold Regions

Literature Overview

This paper by Yan Jiabao and colleagues from Tianjin University and the Key Laboratory of Earthquake Engineering and Engineering Vibration, published in the Journal of Tianjin University (Natural Science and Engineering) in 2022, presents a comprehensive experimental investigation of the axial compression behavior of square high-strength steel tube-concrete composite columns under low-temperature conditions. The study involves 10 test specimens utilizing Q690 and Q960 square steel tubes, addressing a critical gap in the understanding of composite column performance in cold regions where such structures are increasingly being deployed.

Experimental Program and Test Parameters

The experimental specimens incorporated square steel tubes of two high-strength grades: Q690 (yield strength approximately 690 MPa) and Q960 (yield strength approximately 960 MPa). The concrete core was designed to achieve typical high-strength grades suitable for structural applications. The specimens were tested under controlled low-temperature conditions to simulate cold region service environments.

Failure Modes and Mechanical Behavior

The study reveals distinctive failure modes for square high-strength steel tube-concrete composite columns under low-temperature axial compression:

1. Concrete crushing: Occurs at the peak load capacity, representing the initial failure event. The confined concrete reaches its ultimate compressive strain and begins to disintegrate.
2. Local buckling of steel tube: Following concrete crushing, the high-strength steel tube undergoes local buckling, particularly at locations of high compressive stress concentration.
3. Corner weld cracking: The welding joints at the corners of the square tube experience cracking during the post-peak stage. This is attributed to the high restraint at corner welds and the thermal-mechanical stresses developed during loading.

The load-displacement curves exhibit three distinct stages: linear elastic, nonlinear hardening, and post-peak degradation. The similarity between low-temperature and ambient temperature curves suggests that the fundamental mechanical behavior is preserved, but the quantitative values shift systematically with temperature.

Temperature Effects on Mechanical Properties

The improvement in ultimate capacity and stiffness with decreasing temperature is beneficial for structural design, allowing more efficient use of materials. However, the simultaneous reduction in ductility and fracture toughness is a significant concern, particularly for seismic applications where energy dissipation through ductile deformation is essential. The corner weld cracking observed in the tests is particularly worrisome, as welds are often the weakest links in structural components subjected to cyclic or impact loading.

Welding Metallurgy Considerations

The welding of high-strength square steel tubes presents unique challenges that are exacerbated at low temperatures. The welding of Q690 and Q960 steels requires careful control of:

• Preheating temperature: 100-200°C for Q690, 200-300°C for Q960 to prevent hydrogen-induced cold cracking
• Interpass temperature: Maintained between 150-250°C to control cooling rate
• Heat input: Limited to prevent excessive grain growth in the HAZ
• Post-weld heat treatment: 600-650°C for 2-4 hours to relieve residual stresses and improve toughness

The corner welds of square tubes are particularly challenging due to the geometric discontinuity and high restraint. The welding sequence must be carefully planned to minimize angular distortion and residual stresses. For seismic applications in cold regions, the weld toughness must be verified through Charpy V-notch impact testing at the minimum expected service temperature.

Standards Comparison and Design Implications

The paper compares the predicted ultimate compressive capacities using different design codes:

The finding that the Chinese code (GB 50936-2014) is conservative while the American code (AISC 360-10) provides more accurate predictions has significant implications for the design of composite columns in cold regions. The conservative predictions of the Chinese code may lead to over-design and increased material costs, while the more accurate American code predictions could enable more efficient designs. However, the conservatism of the Chinese code may provide an additional safety margin that is particularly valuable in cold regions where material properties are less well-characterized.

Study Insights and Engineering Practice

This research fills an important gap in the understanding of composite column behavior in cold regions. The construction of infrastructure in cold regions, such as high-speed railways, bridges, and industrial facilities in northern China, Russia, Scandinavia, and Canada, increasingly relies on high-strength steel tube-concrete composite columns for their excellent strength-to-weight ratio and construction efficiency.

The observed corner weld cracking under low-temperature loading highlights the need for enhanced welding procedures and quality control in cold region applications. Welders should be certified for low-temperature service, and weld procedures should be qualified through low-temperature impact testing. The use of consumables with low hydrogen content and appropriate impact toughness is essential.

The development of the finite element model that accounts for low-temperature effects provides a valuable tool for design engineers. The model can be used to analyze complex structural systems where analytical solutions are not available, enabling more efficient and accurate design of composite structures in cold regions. Future research should extend this work to include cyclic loading, impact loading, and long-term low-temperature exposure effects on material properties and weld integrity.