Mechanical Properties of Hollow Steel Tube Concrete Composite Short Columns Under Fire Conditions

Literature Overview

This research investigates the mechanical behavior of hollow steel tube concrete (HSTC) composite short columns subjected to elevated temperatures representative of structural fire scenarios. Hollow steel tube concrete columns represent a hybrid structural system where a hollow steel tube encases a concrete core with an internal void, creating a unique load-bearing configuration that differs from conventional concrete-filled steel tubes. The study addresses a critical gap in fire engineering design by providing experimental data and analytical models for predicting the residual bearing capacity of these composite members after fire exposure.

Core Technical Findings

The experimental program involved subjecting HSTC specimens to standard fire temperature curves (ISO 834) and measuring their mechanical response at various heating durations. The hollow interior creates a thermal gradient within the column cross-section, with the outer steel tube experiencing higher temperatures than the inner concrete surface. This temperature differential influences the stress distribution and failure mechanisms differently compared to solid CFST columns.

Key Technical Parameters

Failure Modes Under Fire

The study identifies three primary failure modes depending on temperature level and exposure duration:

1. Steel tube local buckling (dominant at 600–800°C): The steel tube walls lose stiffness and buckle inward due to thermal expansion combined with reduced yield strength.
2. Concrete core spalling (dominant at 800–1000°C): Internal void pressure from steam generation causes explosive spalling of the concrete cover.
3. Combined steel-concrete failure (dominant above 1000°C): Both materials lose their structural capacity simultaneously, leading to catastrophic collapse.

Interpretation of Technical Points

The hollow interior creates a unique thermal environment that must be understood for accurate fire design. The void acts as a thermal buffer, slowing the temperature rise in the inner concrete surface compared to solid CFST columns. However, this same void can become a pressure vessel during fire exposure, as moisture in the concrete converts to steam and cannot escape. The resulting internal pressure can reach 5–15 MPa, exceeding the tensile strength of concrete and causing spalling.

The confinement mechanism in HSTC columns operates differently under fire conditions. At ambient temperature, the steel tube provides lateral confinement to the concrete core, enhancing its compressive strength. Under elevated temperatures, the steel tube loses its yield strength and may buckle inward before the concrete reaches its peak temperature. This premature loss of confinement reduces the residual bearing capacity more significantly than predicted by simple superposition of material strength reductions.

Residual Strength Reduction Factors

Integration with Engineering Practice

Fire design of HSTC columns requires a systematic approach that considers thermal expansion, material property degradation, and confinement loss. The following design workflow is recommended:

1. Thermal analysis: Determine the temperature distribution within the column cross-section at the design fire duration using finite element thermal analysis or simplified hand calculation methods.
2. Material property evaluation: Apply temperature-dependent reduction factors to both steel and concrete properties based on the determined temperature distribution.
3. Confinement assessment: Evaluate whether the steel tube maintains sufficient stiffness to provide confinement at the elevated temperatures. If not, reduce the confined concrete strength accordingly.
4. Residual capacity calculation: Compute the residual axial bearing capacity using the modified material properties and confinement conditions.
5. Safety verification: Compare the residual capacity against the design load with appropriate safety factors (typically 1.5–2.0 for fire design).

The study recommends a minimum residual capacity ratio of 0.50 for columns supporting gravity loads and 0.30 for columns in lateral force-resisting systems, ensuring structural integrity during fire events.

Welding Considerations Under Fire Conditions

The weld quality of the steel tube construction significantly influences fire performance. Longitudinal welds in the steel tube create stress concentrations that can initiate buckling under thermal loading. The following welding practices are recommended:

• Use full-penetration welds with adequate root preparation to avoid lack of fusion defects.
• Apply post-weld heat treatment to relieve residual stresses that could accelerate buckling under thermal gradients.
• Inspect all welds using ultrasonic testing before fire exposure simulation tests to ensure defect-free joints.
• Consider using spiral-welded tubes for HSTC columns as the spiral weld pattern distributes stress more uniformly than longitudinal welds.

Key Questions and Reflections

The interaction between the hollow void and fire-induced pressure remains a complex phenomenon requiring further investigation. The void pressure depends on multiple factors including concrete moisture content, void geometry, and the rate of temperature rise. In practical applications, moisture content varies significantly depending on environmental conditions and the age of the structure. Engineers should conduct moisture measurements before fire design calculations to ensure accurate prediction of void pressure.

The long-term degradation of HSTC columns after fire exposure is another critical concern. Even if the column survives the fire event with acceptable residual capacity, the material properties may have been permanently altered. Post-fire assessment protocols should include non-destructive testing to evaluate the extent of damage and determine whether the column can be retained or must be replaced.

Study Insights and Implications

This research provides valuable experimental data and analytical models for the fire design of hollow steel tube concrete columns. The key finding is that the hollow interior creates both advantages (thermal buffering) and disadvantages (void pressure generation) that must be balanced in the design. The proposed analytical method predicts residual bearing capacity within ±15% of experimental values, which is acceptable for fire engineering design purposes.

From a materials science perspective, the microstructural changes in steel at elevated temperatures significantly influence the confinement mechanism. Above 600°C, the ferrite-austenite phase transformation in carbon steel causes a sudden loss of yield strength. Engineers should select steel grades with appropriate thermal stability for fire-critical applications. Low-carbon steels (such as Q235 or S235) exhibit more gradual strength degradation compared to high-strength steels, making them more suitable for fire-exposed structural elements.

The practical implementation of HSTC columns in fire-prone environments requires careful consideration of fire protection strategies. External fire-resistant coatings can delay the temperature rise in the steel tube, preserving confinement effectiveness for longer durations. The application of intumescent coatings with a thickness of 3–5 mm can extend the time to reach critical temperatures by 30–60 minutes, significantly improving the residual capacity of the column.