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 (2020), published in Journal of Vibration and Shock, presents a comprehensive investigation into the progressive collapse resistance of square steel tube concrete (STC) column-composite beam frames under middle column failure scenarios. This work combines theoretical modeling with experimental validation, providing both analytical formulas and empirical evidence for understanding the structural response of such frames to localized member loss. The significance of this study lies in its direct relevance to the design and fabrication of steel tube concrete structural systems, where the welding quality of column-to-beam connections and the integrity of the steel tube itself are critical to the frame's ability to redistribute loads after a local failure.

Resistance Mechanisms and Theoretical Model

The authors developed an analytical model that accounts for several important structural behaviors that are often simplified or neglected in conventional design:

1. The constraint effect of the column at the beam end, which provides rotational and translational restraint that influences the development of plastic hinges.
2. The difference in flexural stiffness of the composite beam under positive and negative bending moments, reflecting the asymmetric reinforcement layout typical of composite construction.
3. The bond-slip effect between the concrete slab and the steel beam, which affects the effective composite action and the moment redistribution capacity.

The resistance mechanism evolution during progressive collapse follows a well-documented sequence: the beam mechanism activates first, followed by the transition to the catenary mechanism as large deflections develop. The compressive arch mechanism and the catenary mechanism are identified as the primary contributors to enhanced collapse resistance beyond the initial beam mechanism capacity.

Experimental Program and Results

A one-bay, two-span, one-third scale model of a square STC column-composite beam frame was designed and subjected to quasi-static loading to simulate middle column removal. The experimental results confirmed the theoretical predictions regarding the resistance mechanism transition and provided valuable data on the following aspects:

Welding Quality Implications for Collapse Resistance

The progressive collapse resistance of steel tube concrete frames depends heavily on the integrity of welded connections at the column-beam joints. In the context of middle column failure, the remaining columns must sustain increased axial loads and bending moments through their connections to the composite beams. The welded joints, whether full-penetration groove welds or fillet welds on connection plates, must maintain their load-carrying capacity under conditions of large inelastic deformation and potential impact loading.

From a welding engineering perspective, the following quality considerations are critical:

• Weld defects such as lack of fusion, undercut, and porosity at the column-beam connection welds can significantly reduce the connection's ductility and ultimate load capacity, potentially triggering premature joint failure that accelerates progressive collapse.
• The heat-affected zone (HAZ) of the welds in the steel tube walls, particularly for thicker-walled columns typical of STC construction, may exhibit reduced toughness due to microstructural coarsening, which is detrimental to the frame's ability to undergo large deformations without brittle fracture.
• Residual stresses from welding can interact with the increased bending moments imposed on the connections during progressive collapse scenarios, potentially initiating cracks at weld toes.

Theoretical Model Correction and Validation

The authors proposed corrections to the theoretical formulas based on the comparison between analytical predictions and experimental results. The corrections primarily address the overestimation of the beam mechanism capacity and the underestimation of the catenary mechanism contribution at large deflections. The corrected formulas provide better agreement with experimental data and can be used for preliminary design evaluation of the collapse resistance of square STC column-composite beam frames.

Engineering Practice Integration

For engineers involved in the fabrication and quality control of steel tube concrete structural components, this study provides the following actionable guidance:

1. Welding procedure specifications for column-beam connections in STC frames should be qualified for the expected ductility demands, including the ability to sustain large rotations without fracture. This may require the use of low-hydrogen welding consumables with high elongation and the implementation of post-weld heat treatment to reduce residual stresses.
2. Non-destructive testing (NDT) requirements for connection welds should be enhanced, with particular attention to volumetric examination (UT or RT) of full-penetration welds to detect internal defects that could compromise ductility.
3. The steel tube wall thickness and material grade should be selected to ensure adequate HAZ toughness, with Charpy V-notch impact testing of weld metal and HAZ recommended for connections in regions where progressive collapse resistance is a design consideration.

Study Insights and Conclusions

The research demonstrates that square STC column-composite beam frames possess inherent progressive collapse resistance through the sequential activation of beam, arch, and catenary mechanisms. The experimental and theoretical work provides a solid foundation for design guidelines, but the practical realization of this collapse resistance depends on maintaining the structural integrity of welded connections throughout the loading history. Engineers must recognize that progressive collapse resistance is not merely a matter of global structural configuration but is intimately tied to the local quality of welded joints, the material properties of the steel tubes, and the composite action between the steel tube, concrete infill, and the composite beams. The corrected theoretical formulas offer a practical tool for preliminary design evaluation, but detailed finite element analysis incorporating realistic material models and welding-induced residual stresses is recommended for critical structures where progressive collapse is a credible threat.