Thermal Field Observation and Effect Analysis of Concrete-Filled Steel Tube High Piers Under Cold Snap Conditions

Literature Overview

The paper by Liu Yang and Xiao Xinhui from Changsha University of Science and Technology, published in Highway and Transport Research in 2011, presents a comprehensive field measurement study on the temperature field distribution of concrete-filled steel tube (CFST) high piers under cold flow conditions. The research was conducted on the Labajin Special Bridge on the Yaxi-Luzhou Expressway, a project of considerable engineering significance in southwestern China. The study was supported by the National Natural Science Foundation of China (Grant No. 50608009) and multiple other funding bodies, reflecting the recognized importance of thermal effects in large-scale bridge infrastructure.

From my perspective as a metallurgist and steel pipe specialist, this study is particularly relevant because the steel tubes used in CFST piers are typically manufactured as large-diameter welded steel pipes, often produced via longitudinal submerged-arc welding (LSAW) or UOE processes. The thermal behavior of these steel tubes under environmental temperature cycling directly influences the structural integrity, residual stress state, and long-term fatigue performance of the entire pier system. Understanding the temperature gradient distribution within the steel tube wall and the interface between steel and concrete is critical for both design and quality assurance in steel pipe manufacturing for structural applications.

Core Technical Findings

Radial Temperature Distribution Pattern

The study reveals that along the radial direction of the CFST column cross-section, the temperature change becomes increasingly delayed as one moves toward the center of the steel tube. This is a direct consequence of the thermal inertia of the concrete core, which acts as a thermal mass that buffers rapid atmospheric temperature fluctuations. The steel tube wall itself, being relatively thin compared to the concrete core, responds more quickly to external temperature changes, but the thermal wave propagation through the concrete creates a significant phase lag at the center.

The researchers employed mathematical statistical methods to fit the temperature gradient patterns under cold snap conditions. They found that the radial temperature gradient can be adequately represented by a cubic polynomial, while the web temperature difference (the difference between the top and bottom surfaces of the steel tube wall) can be expressed using an exponential function. These mathematical models are valuable for structural engineers who need to predict thermal stresses without conducting extensive field measurements on every project.

Negative Temperature Difference Effects

A particularly important finding is that negative temperature differences (where the inner surface is warmer than the outer surface during cold snaps) generate substantial tensile stresses at the outer surface of the steel tube and cause measurable displacement at the pier top. The researchers emphasize that these thermal effects have a non-negligible influence on the internal forces of the completed bridge structure. This is of direct concern to steel pipe manufacturers and welding engineers, as the residual stresses introduced during the welding and forming of large-diameter steel tubes can interact with thermal stresses, potentially leading to stress concentrations that exceed design allowances.

Steel Pipe Manufacturing and Welding Implications

Material Selection for CFST Structural Steel Tubes

The thermal cycling behavior described in this study has direct implications for the selection of steel grades used in CFST applications. For large-diameter structural steel tubes used in bridge piers, common grades include Q345, Q390, and Q420 according to GB/T 1591, or A572 Gr. 50 and Gr. 65 according to ASTM A572. These grades must exhibit adequate low-temperature toughness, as the cold snap conditions can reduce ambient temperatures well below the normal operating range.

The Charpy V-notch impact energy requirement at low temperatures is critical because cold snap conditions can subject the steel tube to temperatures as low as -15°C to -25°C in northern China. Steel tubes with inadequate low-temperature toughness may exhibit brittle fracture susceptibility under combined thermal and mechanical loading.

Welding Considerations for Thermal Cycling Resistance

Large-diameter steel tubes for CFST piers are commonly manufactured using UOE (Upstream, Forming, Expansion, Welding) processes or LSAW (Longitudinal Submerged Arc Welding) methods. The welding process introduces residual stresses and microstructural variations in the heat-affected zone (HAZ) that can interact with thermal stresses from cold snap conditions. For LSAW-welded tubes, the multi-pass welding sequence and the welding parameters must be carefully controlled to minimize residual stress magnitudes.

The HAZ of low-alloy structural steels such as Q345 and Q390 can be susceptible to temper embrittlement and cold cracking under certain conditions. When these welded tubes are subsequently subjected to thermal cycling from cold snaps, the cumulative effect can lead to fatigue cracking initiation at the weld toe or in the HAZ. Post-weld heat treatment (PWHT) is often recommended for thick-walled structural tubes to relieve welding residual stresses, but this must be balanced against the risk of over-tempering the base metal.

Temperature Gradient Modeling and Engineering Practice

The cubic polynomial representation of the radial temperature gradient and the exponential function for the web temperature difference provide practical tools for engineers. In engineering practice, these models can be incorporated into finite element analysis (FEA) software to simulate thermal stress distributions in CFST piers under various cold snap scenarios. The key parameters that should be monitored include:

• Maximum radial temperature gradient (°C/m)
• Maximum web temperature difference (°C)
• Rate of temperature change (°C/hour)
• Duration of cold snap exposure (hours)

For steel pipe quality control, it is important to ensure that the steel tube material maintains adequate toughness at the minimum design temperature, which should account for the additional thermal stresses identified in this study. The Charpy impact test temperature should be set at least 10°C below the minimum expected ambient temperature to provide an adequate safety margin.

Key Questions and Reflections

One question that arises from this study is whether the thermal fatigue life of the steel tube wall has been adequately assessed. The repeated thermal cycling from seasonal cold snaps and daily temperature variations can accumulate damage over the design life of the bridge. For a 100-year design life, this could represent thousands of thermal cycles, each contributing to fatigue crack initiation and propagation. The steel tube wall thickness, welding quality, and surface condition all play roles in determining fatigue resistance.

Another consideration is the interaction between thermal stresses and the residual stresses from steel tube manufacturing and welding. If the residual stress pattern from welding happens to be in the same direction as the thermal stress pattern from cold snaps, the combined stress could exceed the yield strength locally, leading to plastic deformation and potential cracking. This interaction should be considered in the design and quality control of structural steel tubes.

The study also raises the question of whether the current design codes adequately account for thermal effects in CFST structures. Many codes specify thermal expansion coefficients and temperature loads but may not fully capture the complex temperature gradient distributions and their effects on internal forces. Engineers should ensure that thermal analysis is performed for critical CFST applications, particularly in regions prone to severe cold snaps.

Study Insights and Implications for Steel Pipe Engineers

This study provides valuable insights for steel pipe engineers involved in the manufacturing and supply of structural steel tubes for bridge applications. The findings emphasize that steel tubes for CFST piers are not merely structural elements that bear mechanical loads; they are also thermal elements that experience significant temperature variations and gradients. The material properties, welding quality, and manufacturing processes must all be optimized to ensure that the steel tubes can withstand both mechanical and thermal loading throughout their service life.

For quality control purposes, I recommend that steel pipe manufacturers for CFST applications implement additional testing protocols that include low-temperature Charpy impact testing, residual stress measurement using the hole-drilling method or X-ray diffraction, and full-penetration ultrasonic testing of welds. The welding procedure specifications should be qualified for the specific steel grade and wall thickness, with careful attention to preheat temperatures, interpass temperatures, and post-weld heat treatment parameters.

The mathematical models developed in this study for temperature gradient distribution can serve as a basis for developing thermal stress assessment procedures that steel pipe suppliers can offer to their customers. By providing customers with thermal stress analysis data based on verified temperature gradient models, steel pipe manufacturers can demonstrate value-added engineering support and help ensure the long-term performance of CFST structures. This study, while primarily focused on structural engineering, has clear and practical implications for the steel pipe industry that should not be overlooked.