Temperature Distribution in Rectangular Concrete Filled Steel Tube Sections in Severe Cold Regions

Literature Overview

This paper by Tian Zhijuan and colleagues from Chang'an University investigates the thermal behavior of rectangular concrete-filled steel tube (CFST) sections under solar radiation in severe cold regions, using Xining City as the representative site. The study combines field measurement with finite element thermal analysis using ABAQUS, simplifying the three-dimensional temperature field to a two-dimensional model. The research is supported by the Qinghai Provincial Key R&D Program (2017-SF-137) and was published in the Journal of Architecture and Civil Engineering in 2018.

From a steel pipe engineering perspective, this work is significant because it addresses thermal loading on steel tube structures in harsh environments, which directly impacts weld integrity, dimensional stability, and long-term structural performance. The temperature gradients induced by solar radiation create differential thermal expansion between the steel shell and the inner concrete core, generating internal stresses that can compromise joint connections and weld HAZ integrity over time.

Core Technical Points and Thermal Behavior Analysis

The experimental setup involves a horizontally placed rectangular CFST section with longitudinal stiffening ribs, instrumented with temperature sensors at both the steel tube surface and within the concrete core. The key findings can be summarized as follows:

The measured temperature values at both steel tube and concrete monitoring points show good agreement with the ABAQUS finite element calculation results, validating the simplified two-dimensional thermal model approach. The non-uniform temperature field across the section creates a pronounced temperature gradient, which is particularly critical for weld quality assessment in severe cold regions where base metal toughness can be significantly reduced at low temperatures.

Implications for Welding and Steel Pipe Fabrication

For engineers involved in steel pipe fabrication and welding, the thermal gradients identified in this study have direct practical consequences. The temperature differential between the outer steel surface and the inner concrete can reach magnitudes that induce significant residual stresses at welded joints, particularly at longitudinal weld seams and stiffener attachment welds. In severe cold regions where base metal may be specified at low-temperature grades (such as API 5L X65 with impact testing at minus 40 degrees Celsius), the combined effect of service temperature extremes and thermal cycling from solar radiation can accelerate fatigue crack initiation at weld toes.

The finding that stiffening ribs reduce the thermal lag and lower the gradient temperature difference is particularly relevant to pipe fitting design. Stiffener ribs, whether integral or welded, create additional heat conduction paths that moderate the thermal gradient. This has implications for the design of stiffened pipe sections used in cold-region bridges, where the choice between smooth-walled and stiffened CFST sections should consider not only structural capacity but also thermal performance.

Finite Element Modeling Approach and Process Considerations

The authors simplify the three-dimensional temperature field to a two-dimensional model, which is a practical engineering approximation. The ABAQUS thermal analysis incorporates solar radiation, wind speed, and ambient temperature as boundary conditions. For engineers familiar with welding process simulation, this thermal modeling approach is analogous to the heat transfer models used in welding thermal cycle prediction, where boundary conditions and material property temperature dependence play critical roles.

The key modeling parameters include:

• Solar radiation intensity as a function of time and angle of incidence
• Convective heat transfer coefficient dependent on wind speed
• Thermal conductivity and specific heat of both steel and concrete as temperature-dependent properties
• Contact resistance at the steel-concrete interface, which is influenced by the quality of concrete placement and the presence of stiffening ribs

For steel pipe engineers, the validation of the simplified two-dimensional model against field measurements provides confidence in using similar approaches for thermal stress analysis at welded joints. The model can be extended to predict thermal stresses at circumferential and longitudinal weld seams, which is essential for fatigue life assessment in cold-region applications.

Engineering Practice Integration and Quality Control Implications

In engineering practice, the thermal loading effects described in this study must be considered during the design and fabrication stages of steel pipe structures intended for severe cold regions. The following quality control measures are recommended:

1. Weld procedure qualification should account for the thermal cycling effects identified in the study, with particular attention to the heat-affected zone (HAZ) toughness at the minimum service temperature.
2. Pre-weld heating and interpass temperature control during field welding should be adjusted based on the expected thermal environment, not just ambient temperature.
3. Non-destructive testing (NDT) protocols should include evaluation of weld quality at stiffener attachment points, where the thermal gradient effects are most pronounced.
4. Residual stress measurement, preferably using the hole-drilling method or ultrasonic techniques, should be performed at representative welded joints after fabrication to assess the combined effect of welding residual stress and thermal residual stress.

The study's finding that the steel tube temperature slightly lags behind ambient temperature while exhibiting larger amplitude extremes is consistent with the thermal mass behavior of steel, which has relatively low specific heat compared to concrete but high thermal conductivity. This means that during rapid temperature changes, such as those common in severe cold regions with frequent weather fronts, the steel tube can experience thermal shock that may be detrimental to weld integrity if the weld metal or HAZ has reduced ductility.

Key Reflections and Study Insights

The most valuable insight from this literature is the recognition that thermal loading in severe cold regions is not merely a structural design consideration but also a material and welding quality consideration. The temperature gradients across the CFST section create differential thermal expansion that can either relieve or amplify welding residual stresses, depending on the phase of the thermal cycle. During heating, the outer steel expands more than the inner concrete, potentially opening micro-cracks in the weld zone if they exist. During cooling, the reverse occurs, and compressive stresses may be induced.

From a standards perspective, current design codes such as GB 51247 (Code for Design of Concrete-Filled Steel Tubular Structures) address thermal effects primarily through expansion joints and thermal movement accommodation. However, the micro-scale thermal gradients within the section, as quantified in this study, are not explicitly addressed in most codes. This represents a gap in standards interpretation that welding engineers should be aware of when performing fitness-for-service assessments of CFST structures in cold regions.

The practical recommendation is to incorporate thermal gradient effects into the welding quality assurance plan for CFST structures in severe cold regions, particularly at critical welded connections. This includes enhanced NDT coverage, more stringent acceptance criteria for weld defects at stiffener attachments, and consideration of thermal stress relief treatments where feasible. The study provides a solid foundation for developing these enhanced practices.