Anti-Collapse Mechanism and Residual Load-Bearing Capacity of Steel Tube Concrete Joints After High Temperature Exposure

Literature Overview

This paper examines the post-fire behavior of steel tube concrete (STC) joints, focusing on the anti-collapse mechanism and residual load-bearing capacity following high-temperature exposure. The research is motivated by the increasing vulnerability of steel-concrete composite structures to fire hazards, particularly in multi-story buildings and industrial facilities where fire resistance requirements are stringent. The authors conduct experimental investigations combined with numerical simulations to characterize the damage evolution and residual strength of STC joints after exposure to elevated temperatures ranging from ambient conditions to 1000°C.

Core Technical Points

Damage Mechanism Under High Temperature

The anti-collapse behavior of STC joints after fire exposure is governed by the differential thermal expansion between the steel tube and the concrete core, coupled with the progressive degradation of material properties. At temperatures below 400°C, both steel and concrete retain most of their mechanical properties, and the joint maintains full load-bearing capacity. Between 400°C and 600°C, concrete experiences significant strength loss due to the dehydration of calcium silicate hydrate (C-S-H) gel, while steel begins to lose yield strength. Above 600°C, the steel tube undergoes severe strength degradation, and the concrete may spall due to internal steam pressure buildup.

Anti-Collapse Mechanism Analysis

The anti-collapse mechanism of STC joints relies on three primary load transfer paths: axial compression through the steel tube, shear transfer through the bond interface between steel and concrete, and confinement pressure from the steel tube on the concrete core. After fire exposure, the integrity of these load paths determines the residual capacity. The study identifies that joints with intact steel tubes but degraded concrete retain approximately 15-20% of their original capacity through steel tube action alone. However, if the steel tube experiences local buckling or significant ovalization, the residual capacity drops precipitously.

The confinement effect, which normally enhances concrete strength by 20-40% in STC members, becomes ineffective after severe fire exposure because the steel tube loses its confining capacity at temperatures above 600°C. This transition from confined to unconfined behavior represents a critical threshold in the joint's load-bearing performance.

Engineering Practice Integration

Welding Quality and Fire Resistance

From a welding and fabrication standpoint, the quality of welds connecting STC joints to adjacent structural members becomes paramount for post-fire performance. Welds that experience thermal cracking or excessive heat-affected zone (HAZ) softening during a fire event create preferential failure locations. The following welding considerations are critical:

1. Weld procedure qualification: Welding procedures for STC joints should be qualified for both ambient and elevated temperature conditions, with particular attention to HAZ toughness at temperatures up to 400°C.
2. Weld geometry optimization: Fillet welds at joint connections should have adequate throat thickness to maintain structural continuity even if the base metal experiences some strength loss.
3. Post-weld treatment: Where feasible, post-weld heat treatment or controlled cooling should be applied to minimize residual stresses that could be exacerbated during fire exposure.

Residual Capacity Assessment Methodology

The study proposes a residual capacity assessment methodology that incorporates temperature-dependent material properties, damage indices for both steel and concrete, and a reduction factor accounting for the loss of confinement. The methodology can be summarized as:

• Step 1: Determine the maximum temperature experienced by the joint during the fire event using fire engineering calculations or fire test data.
• Step 2: Apply temperature-dependent reduction factors to the steel and concrete properties based on established material degradation curves.
• Step 3: Evaluate the integrity of the steel tube for local buckling, ovalization, or cracking.
• Step 4: Calculate the residual axial capacity using the modified section properties and material strengths.
• Step 5: Apply a safety factor appropriate for the post-fire assessment level (Level 1, 2, or 3 per BS 7974 or equivalent standards).

Key Questions and Reflections

A critical question for engineering practice is the threshold temperature beyond which STC joints are deemed unserviceable and require replacement rather than repair. The study suggests that joints exposed to temperatures exceeding 600°C for extended durations generally require structural replacement, while those exposed to temperatures below 400°C may be suitable for repair and continued service with appropriate load reduction.

The study also raises important considerations regarding the interaction between fire exposure and pre-existing damage. In aging infrastructure, joints may already contain fatigue cracks, corrosion damage, or fabrication defects that compound the fire-induced degradation. A comprehensive assessment must account for the combined effects of aging and fire exposure rather than evaluating each damage mechanism in isolation.

Study Insights and Implications

This research provides essential guidance for the post-fire assessment and repair of STC structures. The identified anti-collapse mechanisms and residual capacity values enable engineers to make informed decisions about structural repair versus replacement, directly impacting life-safety outcomes and economic considerations. For welding engineers, the findings emphasize the importance of weld quality as a determinant of post-fire structural performance, reinforcing the need for rigorous welding procedure qualification and in-process quality control. The proposed assessment methodology offers a systematic approach that can be integrated into existing fire engineering practice, providing quantitative residual capacity values that support decision-making in post-fire structural evaluation.