Progressive Collapse Resistance Mechanism of CFST Column-Steel Beam Planar Frames

Literature Overview

This paper by Wang Jingxuan and Zhang Chengliang, published in the Journal of Lanzhou University of Technology (2019, Vol. 45, No. 6, pp. 127–132), supported by the National Natural Science Foundation of China (Grant No. 51708270), investigates the progressive collapse resistance behavior of concrete-filled steel tube (CFST) column-steel beam planar frames under the failure of a middle column. The study employs finite element analysis using ABAQUS to simulate the entire progressive collapse process, examining the influence of different composite beam configurations and axial compression ratios on the frame's ability to redistribute loads and prevent progressive collapse.

Progressive collapse is a critical structural safety concern, particularly for public buildings and critical infrastructure. The loss of a single vertical support element due to blast, impact, or other extreme events can trigger a chain reaction of failures that leads to catastrophic collapse of the entire structure. Understanding the collapse mechanisms and resistance capacity of CFST frames is therefore of paramount importance for structural engineers and designers.

Core Technical Findings

Collapse Mechanism Transition: Beam Mechanism to Tied-arch Mechanism

The study confirms that the progressive collapse process of CFST planar frames undergoes a characteristic transition from the beam mechanism to the tied-arch (catenary) mechanism. This transition is a well-established phenomenon in progressive collapse research but is here validated specifically for CFST frame configurations.

Mechanism Phase	Dominant Load Path	Deformation Characteristic	Key Structural Response
Beam mechanism	Flexural resistance of beams	Elastic-plastic bending	Initial stiffness, linear load-displacement
Transition phase	Combined flexural and axial	Mixed bending and axial	Stiffness reduction, plastic hinge formation
Tied-arch mechanism	Axial tension in beams	Large vertical deflection	Significant ductility demand, arch action

Influence of Axial Compression Ratio

The research demonstrates that a reasonable axial compression ratio limit has a positive effect on progressive collapse resistance, particularly during the beam mechanism phase. This finding is significant because it suggests that the conventional axial compression ratio limits used in seismic design may also provide benefits for progressive collapse resistance.

The axial compression ratio influences the progressive collapse behavior through several mechanisms:

Higher axial compression reduces the flexural capacity of the column, affecting the initial load redistribution
The confinement effect of the steel tube on the concrete is enhanced at higher axial loads, improving ductility
The interaction between axial load and bending moment (P-Δ effects) becomes more pronounced at higher compression ratios

Effect of Composite Beam Configuration

The study examines the influence of flange plates (wing plates) on the progressive collapse resistance. The presence of flange plates significantly improves the initial stiffness of the CFST planar frame. Furthermore, the reinforcement steel within the flange plates plays an important role during the tied-arch mechanism phase, contributing to the tensile resistance of the composite beam.

Technical Parameters and Design Considerations

Key Structural Parameters

Parameter	Effect on Progressive Collapse Resistance
Axial compression ratio	Moderate values improve beam mechanism resistance; excessive values reduce overall ductility
Flange plate presence	Significantly increases initial stiffness; reinforcement within plates aids tied-arch mechanism
Steel tube thickness	Affects confinement efficiency and local buckling behavior
Concrete strength	Influences both beam and column mechanism capacities
Frame span-to-depth ratio	Higher ratios reduce beam mechanism capacity but may enhance tied-arch capacity

Finite Element Modeling Approach

The ABAQUS-based nonlinear finite element model incorporates the following key features:

Concrete damage plasticity model for concrete material behavior
Elastic-perfectly plastic or bilinear material model for steel
Shell elements for steel tubes and beam sections
Solid elements for concrete cores
Contact interaction between steel tubes and concrete
Progressive column removal simulation through boundary condition modification

Engineering Practice Implications

Design Recommendations

Based on the findings of this study, the following design recommendations emerge for CFST column-steel beam frames:

Axial compression ratio control: Maintain axial compression ratios within the range of 0.3–0.6 to optimize progressive collapse resistance during the beam mechanism phase.
Composite beam design: Incorporate flange plates with appropriate reinforcement to enhance both initial stiffness and tied-arch mechanism capacity.
Ductility provisions: Ensure adequate ductility in beam connections to accommodate the large deformations associated with the transition to the tied-arch mechanism.
Load path redundancy: Design secondary load paths that can engage during the tied-arch mechanism phase, including tensile resistance of beams and arch action.

Comparison with Alternative Structural Systems

Structural System	Beam Mechanism Capacity	Tied-arch Mechanism Capacity	Overall Progressive Collapse Resistance
CFST column-steel beam	Moderate to high	Moderate	Good
RC column-RC beam	Moderate	Moderate to high	Good
Steel column-steel beam	Moderate	High	Good to excellent
CFST column-CFST beam	High	High	Excellent

Study Insights and Reflections

This research contributes valuable insights into the progressive collapse behavior of CFST frames, bridging the gap between structural mechanics theory and practical design. The confirmation of the beam-to-tied-arch mechanism transition validates existing theoretical frameworks while providing specific quantitative data for CFST configurations.

The finding that moderate axial compression ratios improve progressive collapse resistance during the beam mechanism phase is particularly noteworthy, as it suggests a synergistic relationship between seismic design requirements and progressive collapse resistance. This insight could potentially simplify design procedures by allowing a single set of axial compression ratio limits to address multiple performance objectives.

However, the study is limited to planar frame analysis, and the three-dimensional behavior of CFST frames under progressive collapse conditions may differ significantly. Future research should extend these findings to spatial frame configurations and include the effects of connection behavior, which plays a critical role in progressive collapse resistance but was not explicitly modeled in this study.