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

Literature Overview

The paper by Xiang Kai, Pan Yanchong, Diao Xiaoliang, and Song Tianyi, published in Fire Science and Technology in 2022, investigates the fire resistance performance of square steel tube concrete (CFT) column and steel beam frames with unidirectional bolt connections. Funded by the Emergency Management Department Fire and Rescue Bureau Key Project (2020XFZD02), this study combines finite element simulation with experimental verification to evaluate temperature distribution, fire resistance duration, and failure modes under the standard ISO 834 heating regime.

Core Technical Content

The research focuses on a composite structural system comprising square CFT columns, H-section steel beams, and reinforced concrete floor slabs, connected via unidirectional bolted nodes. The finite element model was calibrated against physical test results, providing a validated analytical framework for assessing the fire behavior of this specific structural configuration.

Temperature Distribution and Node Behavior

A key finding is that the joint region of the square CFT frame column exhibits relatively lower temperatures compared to other zones of the column. This observation is consistent with the thermal mass effect of the concrete infill and the geometric complexity of the node region, which creates thermal shielding and delays heat penetration into the critical load-bearing cross-section.

Failure Mode Classification

The study proposes a frame structure failure criterion and identifies two primary failure modes:

• Column failure mode: Governed by loss of load-bearing capacity in the CFT column under elevated temperature conditions
• Beam failure mode: Governed by excessive deflection and potential instability of the steel beam spanning between columns

Comparative Fire Resistance Limit Analysis

The following table summarizes the comparative findings regarding fire resistance limits under different boundary conditions:

When the frame beam is not subjected to additional load and serves only as a lateral constraint on the column, the square CFT frame column achieves a fire resistance limit exceeding that of a two-end hinged column member, yet remains below that of a one-end fixed, one-end hinged column member. This ranking reflects the well-established principle that rotational restraint at column ends reduces buckling length and thereby enhances fire resistance.

Engineering Practice Implications

From a design perspective, several practical insights emerge:

1. The unidirectional bolt connection must be verified for reliability under thermal expansion conditions, as the differential expansion between steel and concrete can impose significant shear forces on bolt groups at elevated temperatures.
2. The relatively low temperature at the node region should not be taken as a guarantee of safe performance, since local yielding and creep at the connection may precede global column failure.
3. Engineers should consider the interaction between floor slab restraint and column fire resistance when specifying fire protection measures, as the slab contribution can significantly influence the effective boundary conditions.

Key Reflections

This study reinforces the understanding that fire resistance design cannot be treated as a purely component-level exercise. The interaction between columns, beams, and slabs under fire conditions creates complex load redistribution patterns that must be captured through system-level analysis. The validated finite element approach offers a practical tool for parametric studies that would be prohibitively expensive through physical testing alone. However, engineers should remain cautious when extrapolating results from square CFT columns to circular or rectangular sections, as the thermal gradients and concrete confinement mechanisms differ substantially with cross-sectional geometry.