Fire Resistance Performance Analysis of Square Steel Tube Concrete Column-Steel Beam Frames

Literature Overview

This paper by Xiang Kai, Pan Yanchong, Diao Xiaoliang, and Song Tianyi from the Tianjin Fire Research Institute (Ministry of Emergency Management) and Beijing University of Technology was published in "Fire Science and Technology" (2022, Vol. 41, No. 8, pp. 1023-1029). The study investigates the fire resistance performance of square steel tube concrete (STC) column-steel beam frame structures with concrete floor slabs, connected by one-way bolt connections, under ISO 834 (GB/T 9978) standard fire conditions.

Core Technical Content

Structural System Configuration

The studied frame system comprises:

• Columns: Square hollow steel tubes filled with concrete (STC columns)
• Beams: H-shaped steel beams
• Floor: Reinforced concrete slab
• Connection: One-way bolt connection at beam-column joints

Simulation and Experimental Approach

The research employed a combined approach:

1. Finite element analysis (FEA) for temperature distribution, fire resistance time, and failure mode prediction
2. Experimental validation of FEA results
3. Comparative analysis of STC column components versus STC frame columns

Technical Analysis

Temperature Distribution Characteristics

Failure Mode Analysis

The study identifies two primary failure modes:

1. Column failure mode: Progressive failure initiated at the column due to strength degradation of both steel tube and concrete fill at elevated temperatures
2. Beam failure mode: Failure initiated at the beam, typically through excessive deflection or local buckling of the web/flange

The failure mode transition depends on the load ratio between column and beam, as well as the connection reliability.

Fire Resistance Limit Comparison

The key finding is that the STC frame column's fire resistance exceeds that of a simply supported column member under identical conditions, demonstrating the beneficial effect of structural continuity. However, it remains below the performance of a column with one fixed and one pinned support, indicating that full moment continuity provides superior fire resistance.

Standards and Code Context

The standard fire curve (ISO 834) defines the temperature-time relationship: T = 20 + 345 × log₁₀(8t + 1), where T is temperature in °C and t is time in minutes. This curve represents a standard compartment fire condition and is the basis for fire resistance rating of structural members.

Engineering Practice Integration

Design Implications for STC Frame Structures

1. Connection design: The one-way bolt connection must be designed to maintain structural integrity throughout the fire exposure period. Connection reliability directly influences whether the beneficial frame action can be realized.
2. Thermal protection strategy: The node region's relatively lower temperature suggests that targeted thermal protection at the column mid-span (the highest temperature zone) provides optimal fire resistance improvement per unit of protection material.
3. Floor slab contribution: The reinforced concrete slab provides significant thermal mass and lateral restraint to the beams, contributing to overall frame fire resistance. This contribution should be quantified in design rather than treated as a safety margin.
4. Load ratio consideration: The column-to-beam load ratio determines the governing failure mode. In design, this ratio should be evaluated under fire conditions (reduced material strengths) rather than ambient temperature conditions.

Practical Design Recommendations

• Ensure minimum bolt connection capacity under fire temperature conditions
• Consider thermal expansion effects on connection clearance and bolt preload
• Evaluate progressive collapse potential under fire-induced member failure
• Account for the beneficial restraint effect of floor slabs on beam fire resistance

Key Reflections and Insights

This study addresses an increasingly important topic as steel-concrete composite structures become more prevalent in modern construction. The finding that STC frame columns outperform isolated STC column members demonstrates the value of structural continuity for fire resistance—a principle analogous to its well-established role in seismic design.

The practical significance of the one-way bolt connection finding is substantial. In many practical designs, connection details are optimized for ambient-temperature performance and economic efficiency, with fire resistance treated as an afterthought. This research provides evidence that even relatively simple bolt connections, when properly designed, can maintain the beneficial frame action during fire exposure.

However, several limitations warrant consideration. The study focuses on a specific connection type and structural configuration. The fire resistance performance of STC frames with welded connections, moment-resisting connections, or different beam sections may differ significantly. Additionally, the long-term fire resistance behavior—particularly the effects of sustained high temperatures on concrete strength loss and steel tube oxidation—is critical for real fire scenarios that may exceed the standard test duration.

The research also raises important questions about the interaction between fire and structural performance. In real fires, the temperature distribution is rarely uniform, and the fire duration is unpredictable. The standard fire curve provides a benchmark but does not capture the variability of actual fire conditions. Engineers should use this research as a foundation for understanding STC frame fire behavior while applying appropriate safety factors and performance-based design considerations for specific projects.