Anti-Progressive Collapse Performance of Square Steel Tube Concrete Column-Composite Beam Frames Under Middle Column Failure

Literature Overview

The research by Xuan Wei, Wang Lai, Liu Changjiang, Xing Guoqi, and Yang Ning, published in Vibration and Shock (2020, Vol. 39, No. 3), investigates the progressive collapse resistance of square CFT column-composite beam frames when subjected to middle column removal. The study combines theoretical modeling with a 1/3-scale two-span frame test, providing comprehensive insights into the load redistribution mechanisms and resistance development during progressive collapse scenarios.

Core Technical Points

Resistance Mechanisms and Their Transition

The study identifies a clear transition in resistance mechanisms during the collapse process:

1. Beam mechanism (initial stage): The composite beams resist the redistributed loads through flexural bending, with plastic hinge formation at beam ends and midspan.
2. Arch mechanism (intermediate stage): As beam deflections increase, axial compression develops in the beams, creating a compressive arch action that transfers loads to adjacent columns.
3. Catenary mechanism (final stage): At large displacements, tensile membrane action develops in the reinforcement and slab, providing additional resistance through cable-like behavior.

Key Influencing Factors

Theoretical Model Development

The authors developed a resistance-deformation analytical model that accounts for:

• The constraint effect of adjacent columns on beam rotation
• The asymmetry between positive and negative moment resistance of composite beams
• The bond-slip degradation between the concrete slab and steel beam at large deformations

The theoretical formulas were subsequently corrected based on comparison with experimental results, yielding improved accuracy for practical design applications.

Interpretation from a Steel Tube and Welding Perspective

The square CFT columns in this study represent a critical structural element whose performance depends fundamentally on the quality of steel tube fabrication and concrete infill. From a manufacturing standpoint, several aspects deserve attention:

Steel Tube Requirements

The square tubes must maintain dimensional accuracy and straightness to ensure uniform concrete confinement. Deviations in wall thickness or squareness can lead to non-uniform confinement pressure distribution, which directly affects the column's ductility and ultimate load capacity. For the collapse resistance application, the tube's strain-hardening behavior in the HAZ of longitudinal welds becomes critical, as the column must sustain large inelastic deformations without premature weld failure.

Concrete Infill Quality

The concrete infill must be properly compacted to avoid voids, particularly near the tube walls and at the column-beam junction regions. Incomplete compaction leads to reduced confinement effectiveness and potential early crushing of the concrete core. The use of self-consolidating concrete (SCC) is recommended for square tubes with larger cross-sectional dimensions where vibrator access is limited.

Welding Considerations

The connection details between the square CFT columns and composite beams involve critical welds that must perform under large cyclic and monotonic deformations. The study's finding that the arch mechanism and catenary mechanism significantly enhance collapse resistance implies that the beam-column connections must maintain integrity under large rotations (potentially exceeding 0.05 rad). Welded connections with adequate ductility, such as extended end plate connections with thickened stiffeners, are essential.

Connection with Engineering Practice

In progressive collapse design, the square CFT column serves as the primary vertical load path element. The column's performance under both gravity loads and the dynamic effects of column removal must be evaluated. Practical considerations include:

1. Impact of tube-to-concrete interaction: The confinement provided by the square steel tube enhances the concrete's compressive strength and ductility, which is crucial for the column's ability to redistribute loads after the middle column fails.
2. Weld integrity at column-beam junctions: The high stress concentrations at the beam-column connections require careful weld design, including adequate weld size, proper weld sequencing to minimize residual stresses, and post-weld heat treatment where necessary.
3. Slab-beam composite action: The shear connectors (studs) between the steel beam and concrete slab must be designed for the large deformations associated with the catenary mechanism, requiring consideration of stud pullout resistance at large displacements.

Key Questions and Reflections

The study raises important questions about the scalability of the 1/3-scale test results to full-scale structures. The size effect on concrete fracture, the geometric nonlinearity at large deformations, and the potential for different failure modes in full-scale structures all require careful consideration. Additionally, the dynamic effects of sudden column removal (as opposed to the quasi-static loading used in the test) may alter the resistance mechanism transition sequence and the ultimate collapse resistance.

Study Insights and Implications

This research provides valuable guidance for the design of square CFT column-composite beam frames in progressive collapse-prone scenarios. The identification of the beam-arch-catenary resistance transition and the development of a practical analytical model represent significant contributions to the field. For steel pipe manufacturers and welding engineers, the implications are clear: the quality and design of square CFT columns and their connections must be optimized not only for gravity and seismic loads but also for the extreme deformation demands imposed by progressive collapse scenarios. The confinement effectiveness of the steel tube and the ductility of the welded connections are the two most critical factors governing the overall collapse resistance of the frame system.