Nonlinear Analysis of Steel Tube Concrete Frame-Central Braced Structures Against Progressive Collapse

Literature Overview

This paper by Wang Jingxuan, Zhang Yufeng, and Shen Yajun, published in 2023 in the Journal of Earthquake Engineering, Volume 45, Issue 6, presents a comprehensive nonlinear numerical investigation into the progressive collapse resistance of steel tube concrete (STC) frame structures with various central bracing configurations. Supported by the National Natural Science Foundation of China (Grants 52068047 and 51708270) and the Gansu Provincial Youth Science and Technology Fund (20JR5RA437), the research was conducted at the School of Civil Engineering, Lanzhou University of Technology. The study employs fiber beam modeling to evaluate five distinct STC frame-central braced structural configurations under different column failure scenarios, providing valuable quantitative data for progressive collapse-resistant design.

Progressive Collapse and Structural Redundancy

Progressive collapse, also known as disproportionate collapse, refers to the phenomenon where the initial failure of a single structural element triggers a chain reaction of failures that extends far beyond the local damage zone. This failure mode has become a critical concern in modern structural engineering, particularly following high-profile incidents such as the 1995 Alfred P. Murrah Federal Building bombing in Oklahoma City and the 2001 collapse of the World Trade Center towers.

The concept of structural redundancy is central to progressive collapse prevention. A redundant structure possesses multiple load paths that allow the redistribution of loads following the loss of a primary load-bearing element. Central bracing systems enhance structural redundancy by providing alternative load transfer mechanisms that bypass the failed element and redistribute forces to adjacent structural members.

Fiber Beam Modeling Methodology

The fiber beam model is a sophisticated numerical technique that discretizes the cross-section of each structural member into numerous small rectangular elements (fibers), each assigned its own material constitutive law. This approach captures the nonlinear stress-strain behavior of both steel and concrete at every point within the cross-section, enabling accurate simulation of local yielding, concrete crushing, steel rupture, and their interaction under complex loading conditions.

For the STC members in this study, the fiber model incorporates:

• Steel tube material model: A kinematic hardening model (e.g., Chaboche model) that captures cyclic plasticity and Bauschinger effects during the dynamic loading phase.
• Concrete core material model: A confined concrete model (e.g., Mander model or Kent-Park model) that accounts for the lateral confinement effect of the steel tube on the concrete compressive strength and strain capacity.
• Interface behavior: Contact elements between the steel tube and concrete core that simulate the bond-slip mechanism and potential separation under large deformations.

The nonlinear dynamic analysis simulates the structural response following a sudden column removal, capturing the inertial effects, geometric nonlinearities, and material nonlinearities that govern the progressive collapse mechanism. The analysis proceeds through the following phases:

1. Pre-failure phase: Elastic response under gravity and lateral loads.
2. Column removal phase: Sudden removal of the target column, inducing dynamic oscillation.
3. Redistribution phase: Load redistribution through alternative load paths, including catenary action and membrane action in floor slabs.
4. Stabilization or collapse phase: The structure either stabilizes at a new equilibrium position or continues to fail progressively.

Key Findings and Engineering Implications

The research reveals several important findings that have direct implications for progressive collapse-resistant design:

1. Central bracing consistently improves structural performance: All bracing configurations examined demonstrate significant improvements in both global stiffness and ultimate load capacity compared to the unbraced baseline. The improvement is more pronounced for edge column failure scenarios than for middle column failure scenarios, which can be attributed to the greater load redistribution demand in edge column failures.
2. X-bracing provides the highest stiffness and capacity: The X-bracing configuration offers the greatest enhancement in structural stiffness and load-carrying capacity across all failure scenarios. However, its ductility is moderate, and the compression diagonal may buckle prematurely under large deformation conditions.
3. Inverted V-bracing exhibits superior ductility: The inverted V-bracing configuration demonstrates the best combination of ductility and ultimate load capacity. The tensile diagonal in the inverted V-brace remains effective throughout the deformation process, providing a reliable load path even at large displacements.
4. New load transfer paths are established: The central bracing system creates alternative load paths that effectively reduce the internal forces assigned to adjacent members of the failed column. This load redistribution mechanism is the primary reason for the improved progressive collapse resistance.

Welding and Fabrication Considerations for Braced STC Structures

From a fabrication and welding perspective, the implementation of central bracing systems in STC structures requires careful attention to several technical aspects:

• Brace-to-column connections: The connection between the bracing members and the STC columns must be designed to develop the full plastic capacity of the braces. Bolted end-plate connections with high-strength bolts are commonly used, but welded connections may be preferred for higher capacity requirements.
• Brace-to-beam connections: Similar to column connections, the beam-brace connections must be robust and ductile. The connection design should accommodate the large deformation demands during progressive collapse events without brittle fracture.
• Steel tube fabrication quality: The STC columns and beams must be manufactured to high dimensional tolerances to ensure proper fit-up at connection interfaces. Welding of connection plates to the steel tubes should be performed using qualified welding procedures, with 100% NDT inspection of all structural welds.
• Residual stress management: The welding of connection plates and stiffeners to the steel tubes introduces residual stresses that may affect the member's buckling resistance and fatigue performance. Preheating, interpass temperature control, and post-weld stress relief should be implemented according to the applicable welding code requirements.

Study Insights and Reflections

This paper contributes significantly to the progressive collapse research community by providing detailed nonlinear analysis results for STC frame-central braced structures. The systematic comparison of five bracing configurations under multiple failure scenarios offers engineers a practical basis for selecting optimal bracing systems based on their specific structural requirements and design priorities.

The findings align with the performance-based design philosophy advocated by modern building codes such as ASCE 37 (Seismic Evaluation of Existing Buildings), GSA (General Services Administration) Technical Security Guide, and BS 8118-1 (Steel, Composite and Timber Bridges: Progressive Collapse). These codes emphasize the importance of alternative load paths, member ductility, and structural redundancy as key mechanisms for progressive collapse prevention.

For engineers involved in the design of critical infrastructure, such as nuclear facilities, transportation hubs, and high-rise commercial buildings, the results of this study provide quantitative data that can inform the selection of bracing systems and the verification of progressive collapse resistance. The integration of STC construction with central bracing represents a promising strategy for achieving high progressive collapse resistance while maintaining structural efficiency and constructability.

In conclusion, this research demonstrates that central bracing is an effective and practical strategy for enhancing the progressive collapse resistance of STC frame structures. The choice of bracing configuration should be guided by the specific failure scenarios of concern, the desired balance between stiffness and ductility, and the fabrication and welding constraints of the project. Engineers are encouraged to adopt performance-based design approaches that explicitly evaluate progressive collapse resistance through nonlinear analysis, as demonstrated in this study.