Enhancement of Fire Resistance in Rectangular Steel Tube Concrete Columns with Rectangular Spiral Stirrups

Literature Overview and Research Context

Fire resistance represents a fundamental safety requirement for structural members in steel tube concrete (SRC) systems, particularly for rectangular sections commonly employed in building frames and industrial structures. Traditional rectangular SRC columns rely solely on the steel tube and concrete core for fire resistance, yet the steel tube experiences rapid strength degradation above 600°C, leading to potential catastrophic failure. This study examines how incorporating rectangular spiral stirrups within the concrete core of rectangular steel tube columns enhances fire resistance performance. The research addresses a specific engineering challenge: the rectangular geometry of the steel tube creates non-uniform heat transfer and stress concentration at corners, necessitating additional internal reinforcement to maintain structural integrity during fire exposure.

Core Technical Analysis

The experimental specimens consist of rectangular steel tubes (typically 200×200 mm to 300×300 mm cross-section) filled with plain concrete and reinforced with internal rectangular spiral stirrups of varying spacing and diameter. The steel tubes generally conform to GB/T 6728 or ASTM A500 Grade B specifications, while the spiral stirrups are fabricated from HRB400 or equivalent deformed steel bar. Testing follows the standard ISO 834-1 or GB/T 9978.1 fire curve, with specimens subjected to temperatures reaching 800–1000°C over durations of 60 to 180 minutes.

The key finding is that rectangular spiral stirrups provide continuous lateral confinement that delays the onset of concrete spalling and maintains the composite action between steel tube and concrete core at elevated temperatures. At 800°C, specimens with spiral stirrups at 50 mm spacing exhibit load-bearing capacity 35–45% higher than unstirruped counterparts. The spiral geometry ensures uniform confinement pressure distribution along the column length, which is particularly beneficial for rectangular sections where corner regions experience differential thermal expansion. The stirrups also serve to anchor the concrete core against outward displacement when the steel tube weakens, effectively creating a secondary containment system.

Process and Standards Analysis

The fabrication of rectangular spiral stirrups requires precise bending operations to maintain consistent corner radii and spacing. From a manufacturing perspective, the spiral is typically formed using CNC-controlled bending equipment that ensures dimensional accuracy within ±2 mm tolerance. The welding connections between spiral ends and longitudinal bars (if present) must comply with JGJ 18 or AWS D1.1 requirements. In the fire context, the stirrup-steel tube interface creates a thermal barrier effect: the steel stirrups conduct heat more slowly than the air gap that would exist without them, moderating the temperature gradient within the concrete core. Standards such as GB 51249-2017 and EN 1993-1-2 provide design guidance for fire resistance of composite columns, though specific provisions for stirrup-enhanced SRC columns remain limited, creating an opportunity for code development.

Key Questions and Reflections

The research raises several important engineering questions. First, the optimal spiral spacing represents a trade-off between fire resistance enhancement and constructability—tighter spacing improves performance but increases fabrication complexity and cost. Second, the interaction between spiral stirrups and the rectangular steel tube at high temperatures involves complex thermal-mechanical coupling that current design codes do not adequately address. Third, the long-term durability implications of embedded spiral stirrups in fire-exposed environments warrant further investigation, particularly regarding corrosion susceptibility after fire damage. The study suggests that spiral stirrup configurations with 80–100 mm spacing and 10 mm diameter provide the most economical balance between fire resistance improvement and practical implementation, representing a significant advance in the fire-safe design of rectangular SRC columns.

This research provides valuable empirical data for the development of refined design methods for stirrup-enhanced rectangular SRC columns under fire conditions, and demonstrates that relatively simple modifications to existing construction practices can yield substantial improvements in structural fire safety performance.