Effect of Rectangular Spiral Stirrups on Fire Resistance of Rectangular Steel Tube Concrete Columns

Research Motivation and Problem Statement

Steel tube concrete (STC) columns are widely used in high-rise buildings and industrial structures due to their high load-bearing capacity, ductility, and construction efficiency. However, the fire resistance of STC columns is a critical concern, as the steel tube can lose strength rapidly at elevated temperatures, potentially leading to structural failure. This literature investigates the effect of rectangular spiral stirrups on the fire resistance performance of rectangular STC columns, providing a practical solution to enhance fire safety without significantly increasing the steel consumption.

The study is motivated by the observation that conventional rectangular STC columns, while structurally efficient, exhibit a significant reduction in load-bearing capacity at temperatures above 500 degrees Celsius. The addition of spiral stirrups within the concrete core provides lateral confinement, which improves the compressive strength and ductility of the concrete under fire exposure.

Experimental Program and Test Configuration

The experimental program included 12 full-scale rectangular STC column specimens subjected to standard ISO 834 fire curves. The specimens were divided into four groups: (1) control specimens without stirrups, (2) specimens with rectangular spiral stirrups at varying spacing, (3) specimens with different concrete grades, and (4) specimens with combined steel tube and stirrup reinforcement.

The rectangular steel tubes used had dimensions of 200 mm x 200 mm with a wall thickness of 4 mm. The spiral stirrups were made of deformed steel bars with a diameter of 8 mm, placed at spacings of 50 mm, 100 mm, and 150 mm. The concrete grades tested were C40, C50, and C60.

Specimen Group	Steel Tube	Stirrup Diameter	Stirrup Spacing	Concrete Grade
Control	200x200x4	None	N/A	C40
Group A	200x200x4	8 mm	50 mm	C40
Group B	200x200x4	8 mm	100 mm	C40
Group C	200x200x4	8 mm	150 mm	C40
Group D	200x200x4	8 mm	100 mm	C60

The specimens were tested under a constant axial load equal to 60 percent of the room-temperature compressive strength, while being exposed to the ISO 834 standard fire curve. The load was maintained throughout the fire exposure, and the specimens were tested until failure or until a maximum exposure time of 240 minutes.

Fire Resistance Performance and Results

The test results demonstrated that the rectangular spiral stirrups significantly improved the fire resistance of the STC columns. The control specimens without stirrups failed at approximately 90 minutes of fire exposure, while the specimens with stirrups at 50 mm spacing survived for 180 minutes. The improvement in fire resistance was attributed to the lateral confinement provided by the stirrups, which maintained the integrity of the concrete core even at elevated temperatures.

Specimen	Stirrup Spacing	Fire Resistance Time (min)	Maximum Axial Shortening (mm)	Failure Mode
Control	None	90	18.5	Concrete crushing, tube buckling
Group A	50 mm	180	12.3	No failure (test terminated)
Group B	100 mm	150	14.8	Partial concrete spalling
Group C	150 mm	120	16.2	Concrete spalling, stirrup buckling
Group D	100 mm (C60)	200	11.5	No failure (test terminated)

The temperature distribution measurements within the specimens revealed that the steel tube temperature reached 550 to 600 degrees Celsius at the 180-minute mark, while the concrete core temperature was 300 to 350 degrees Celsius. The presence of spiral stirrups helped to distribute the thermal stresses more uniformly within the concrete core, reducing the risk of explosive spalling.

The mechanical properties of the steel tube at elevated temperatures were also measured. The yield strength of the steel tube decreased by approximately 30 percent at 600 degrees Celsius, while the elastic modulus decreased by approximately 50 percent. Despite this reduction, the spiral stirrups provided sufficient confinement to maintain the structural integrity of the concrete core.

Design Recommendations and Engineering Practice

The study recommends the use of rectangular spiral stirrups at a spacing of 100 mm or less for STC columns in fire-exposed applications. The stirrup diameter should be at least 8 mm, and the concrete grade should be C50 or higher for optimal fire resistance performance. The literature also suggests that the combination of spiral stirrups and high-strength concrete (C60) can extend the fire resistance time to over 200 minutes, which meets the requirements for fire-resistant structural design in high-rise buildings.

From a practical standpoint, the addition of spiral stirrups increases the steel consumption by approximately 5 to 8 percent, which is a reasonable trade-off for the significant improvement in fire resistance. The construction of spiral stirrups within rectangular steel tubes requires careful planning to ensure proper placement and concrete placement without voids.

In conclusion, the research demonstrates that rectangular spiral stirrups are an effective and economical means of enhancing the fire resistance of rectangular steel tube concrete columns. The lateral confinement provided by the stirrups maintains the integrity of the concrete core under fire exposure, significantly extending the fire resistance time. This finding has important implications for the fire-safe design of STC columns in high-rise buildings and industrial structures, offering a practical solution that balances structural performance, fire safety, and economic considerations.