Quasi-Static-Dynamic Transition Method for Fire-Induced Progressive Collapse of Concrete-Filled Steel Tube Frames

Literature Overview

The paper by Wang, Wang, and Li (2017), published in Engineering Science and Technology (Vol. 49, No. 4, pp. 53-60), presents a nonlinear analysis of fire-induced progressive collapse in a nine-storey CFST spatial frame using the quasi-static-dynamic transition method. Funded by the National Natural Science Foundation of China (Grant No. 51268038), the research employs ABAQUS predefined field and restart functionality to simulate the full process of fire-induced collapse, from the initial thermal expansion to the final dynamic failure. The study is significant because it bridges the gap between quasi-static fire analysis and dynamic collapse analysis, providing a computationally efficient yet physically accurate approach to progressive collapse assessment.

Core Technical Points

The quasi-static-dynamic transition method recognises that fire-induced structural collapse is fundamentally different from instantaneous collapse mechanisms such as those caused by earthquakes, impacts, or explosions. In a fire scenario, the structure undergoes a prolonged period of quasi-static thermal loading, during which material properties degrade, thermal expansion occurs, and internal forces redistribute. Only at a critical point, when a key member fails, does the response transition to a dynamic collapse phase. The method exploits this physical reality by performing a quasi-static analysis for the initial thermal loading phase and switching to a dynamic analysis only when the collapse is initiated.

The analysis modelled a nine-storey CFST spatial frame with the following key modelling features:

The five stages of fire-induced collapse identified in this study are: (1) initial thermal expansion phase, (2) local buckling phase, (3) brief equilibrium phase, (4) unloading phase, and (5) local collapse failure phase. Stages 1 through 4 are quasi-static in nature, while stage 5 is a nonlinear dynamic problem. This staged approach is physically sound and computationally efficient, as it avoids the unnecessary computational cost of performing a full dynamic analysis throughout the entire fire exposure period.

Material Degradation and Welding Implications

The fire-induced degradation of CFST members involves complex interactions between the steel tube, the concrete core, and the interface bond. At elevated temperatures, the steel tube loses both strength and stiffness, while the concrete core experiences thermal cracking, spalling, and strength reduction. The interface bond between the steel tube and concrete is also affected, as the differential thermal expansion coefficients of steel and concrete can lead to interface debonding at high temperatures.

From a pipe manufacturing and welding perspective, the fire performance of CFST members is influenced by several fabrication-related factors:

1. The steel grade of the tube affects the temperature at which significant strength loss occurs. Higher-grade steels (e.g., Q460 or Q550 per GB/T 1591) may lose strength more rapidly at elevated temperatures compared to lower-grade steels (e.g., Q235 or Q345 per GB/T 1591).
2. The weld quality of the tube's longitudinal or spiral weld affects the structural integrity of the member under fire exposure. Welds with defects such as incomplete fusion, porosity, or lack of penetration are more susceptible to failure at elevated temperatures.
3. The presence of welding residual stress in the tube wall can influence the buckling behaviour of the member under fire loading. Residual stresses from the welding process are superimposed on thermal stresses, potentially reducing the buckling resistance.
4. The concrete fill density and quality affect the fire resistance of the CFST member. Poorly compacted concrete with voids or honeycombing will have reduced fire resistance compared to well-compacted concrete.

Progressive Collapse Mechanism and Load Path Redundancy

A key finding of this study is that after the failure of the corner column at the 6th floor, the load redistribution follows a "nearest neighbour" principle. This means that the load from the failed column is transferred to the adjacent columns rather than being redistributed globally across the entire frame. This finding has important implications for the design of CFST frames against progressive collapse, as it suggests that local load path redundancy is more critical than global structural redundancy.

The collapse critical time of 253 minutes (4.2 hours) is significantly longer than the typical fire resistance requirement for individual structural members (typically 1 to 2.5 hours depending on the occupancy classification). This indicates that the overall fire safety of the structure is governed by the progressive collapse mechanism rather than by the fire resistance of individual members. This is a critical insight for structural engineers, as it implies that designing individual members to meet standard fire resistance ratings may not be sufficient to prevent progressive collapse.

Engineering Practice Implications

For engineers involved in the design and fabrication of CFST structures in fire-prone environments, this study offers several practical recommendations:

1. The quasi-static-dynamic transition method is a recommended approach for progressive collapse analysis, as it provides a good balance between computational efficiency and physical accuracy.
2. The fire resistance of CFST members should be assessed considering the full range of thermal exposure, not just the standard fire curve up to 2.5 hours, as progressive collapse may occur at later times.
3. The load path redundancy of the structure should be verified through progressive collapse analysis, with particular attention to the local load redistribution mechanism identified in this study.
4. Welding quality assurance for CFST tubes is essential, as weld defects can reduce the fire resistance of the member and potentially initiate progressive collapse at temperatures lower than predicted.

Study Insights and Outlook

This study provides a rigorous and physically grounded framework for analysing fire-induced progressive collapse in CFST structures. The identification of five distinct collapse stages and the demonstration that the quasi-static-dynamic transition method can accurately capture the full collapse process are significant contributions to the field. For pipe fabrication engineers, the key takeaway is that the welding quality and material properties of the steel tube directly influence the fire performance of the CFST member, and therefore the overall fire safety of the structure. Future research should investigate the effect of different welding processes (e.g., SMAW versus GMAW versus SAW) on the fire-induced collapse behaviour of CFST frames, as well as the influence of welding residual stress on the thermal buckling of CFST columns. The integration of fire engineering and welding engineering disciplines is essential for developing comprehensive fire safety strategies for CFST structures.