Nonlinear Dynamic Analysis of Progressive Collapse in Steel Tube Concrete Frame-Core Tube Hybrid Structures

Literature Overview

The paper by Wei Guoqiang, Zheng Long, Wang Wenda, and Song Zilong from Lanzhou University of Technology presents a nonlinear dynamic analysis of progressive collapse in a 33-story steel tube concrete (CFT) frame-core tube hybrid structure. Published in the Journal of Earthquake Engineering in 2019 (Volume 41, Issue 3, pages 581–587), the study was supported by the National Natural Science Foundation of China (Grant No. 51268038), the Gansu Science and Technology Support Program (1604FKCA107), and the Gansu Provincial Higher Education Research Project (2018A-079). The research employed ABAQUS finite element software with fiber beam elements and layered shell elements, utilizing a material constitutive subroutine (iFiberLUT) developed by the research group.

Structural Configuration and Analysis Methodology

Building Configuration

Parameter	Description
Total stories	33 floors
Structural system	CFT frame + core tube hybrid
CFT columns	Concrete-filled steel tubes as frame columns
Core tube	Reinforced concrete shear wall core
Floor system	Reinforced concrete slabs
Failure scenarios	Column failure at floors 1, 17, and 33; Core wall failure at floors 1, 17, and 33

Finite Element Modeling Approach

The analysis employed two complementary element types:

Element Type	Application	Key Features
Fiber beam element	CFT columns and beams	Captures material nonlinearity through fiber discretization
Layered shell element	Core tube walls	Models through-thickness stress distribution

The material constitutive subroutine iFiberLUT was developed to accurately represent the stress-strain behavior of steel tubes, concrete, and the interaction between steel and concrete under complex loading conditions. This includes the confinement effect of the steel tube on the internal concrete, which is critical for capturing the post-yield behavior of CFT members.

Key Analytical Results

Progressive Collapse Response by Failure Location

The nonlinear dynamic analysis revealed distinct progressive collapse behaviors depending on the failure location:

Failure Location	Upper Node Vertical Displacement	Oscillation Intensity	Relative Severity
Floor 33 (top)	Maximum	Most pronounced	Highest
Floor 17 (middle)	Moderate	Moderate	Intermediate
Floor 1 (bottom)	Minimum	Least pronounced	Lowest

This trend is counterintuitive from a static perspective, where one might expect the bottom floor failure to be most critical. However, in dynamic progressive collapse analysis, the top-floor failure results in the largest displacement response because the remaining structure above the failure point has less mass to resist the dynamic effects, and the inertia forces from the falling mass are not effectively dissipated by the lower structural elements.

Column Failure versus Core Wall Failure

Failure Type	Upper Node Vertical Displacement	Oscillation	Structural Redundancy
CFT column failure	Larger	More pronounced	Lower
Core wall failure	Smaller	Less pronounced	Higher

Column failure results in larger upper node vertical displacements and more pronounced oscillations compared to core wall failure. This is because the core tube provides significant lateral stiffness and load-bearing capacity, and its failure does not immediately compromise the global load path as severely as the failure of a critical frame column.

Role of the Core Tube in Progressive Collapse Resistance

The analysis confirmed that the core tube has a relatively small impact from the various failure scenarios and that its presence significantly enhances the membrane effect of the floor slabs. The closer the failure location is to the core tube, the more pronounced the improvement in collapse resistance. This finding highlights the importance of the core tube as a redundant load path and its role in distributing impact forces through the floor system.

Engineering Implications for CFT Column Design and Fabrication

CFT Column Steel Tube Requirements

For CFT frame-core tube hybrid structures, the steel tubes forming the CFT columns are critical structural elements. The following requirements should be considered:

Requirement	Specification	Rationale
Steel grade	Q345 or Q390 minimum	Adequate strength for seismic and progressive collapse resistance
Wall thickness	Minimum 6.0 mm for main columns	Ensure confinement and ductility
Steel tube diameter	As per structural design	Adequate moment of inertia
Concrete strength	C40 or higher	Ensure confinement effectiveness
Weld quality	Full penetration, 100% NDT	Critical load path integrity

Welding and Fabrication Quality Control

The fabrication of CFT columns for hybrid structures requires stringent quality control:

Steel tube manufacturing: seamless or longitudinally welded tubes with tight dimensional tolerances, conforming to GB/T 8162 (seamless) or GB/T 9948 (welded).
Steel tube end preparation: square cutting with deburring, bevel preparation for splicing.
Splicing welds: full-penetration butt welds using SAW or SMAW, qualified WPS, 100% RT or UT inspection.
Concrete fill: proper placement and compaction through the steel tube, with verification of fill density.
Connection welds: welded connections between CFT columns and beams must be designed and fabricated to maintain ductility.

Progressive Collapse Resistance Design Considerations

Based on the analytical findings, the following design strategies should be considered:

Ensure adequate load path redundancy by providing sufficient alternative load paths through the floor system and core tube.
Design CFT columns with sufficient ductility to accommodate large deformations during progressive collapse events.
Strengthen the connections between CFT columns and floor slabs to enhance the membrane effect of the floor system.
Consider the proximity of critical columns to the core tube when evaluating progressive collapse vulnerability.
Implement robust detail design at column bases and beam-column connections to prevent brittle failure modes.

Study Insights and Professional Reflections

This research provides valuable insights into the progressive collapse behavior of CFT frame-core tube hybrid structures, which are increasingly used in tall buildings due to their excellent seismic performance and space efficiency. The finding that top-floor failure results in the most severe dynamic response is particularly important for risk assessment and design strategy development. Engineers should not assume that bottom-floor failure is always the most critical scenario in progressive collapse analysis.

The role of the core tube in enhancing the membrane effect of floor slabs is a significant finding that has implications for the design of floor systems. The floor slab design should explicitly account for the membrane action that develops during progressive collapse events, and the connections between floor slabs and the core tube should be designed to facilitate this load transfer mechanism.

From a steel pipe manufacturing perspective, the quality of CFT columns is directly related to the progressive collapse resistance of the structure. The steel tubes must be manufactured with high dimensional accuracy, the concrete must be properly filled and compacted, and the welds must be of high quality to ensure that the CFT columns can develop their full ductile capacity during progressive collapse events.

Summary and Recommendations

The nonlinear dynamic analysis by Wei et al. provides essential insights into the progressive collapse mechanisms of CFT frame-core tube hybrid structures. The findings regarding the severity of failure at different floors, the comparative vulnerability of column versus core wall failure, and the role of the core tube in enhancing collapse resistance should inform the design and quality assurance of CFT structures. Engineers involved in the fabrication of steel tubes for CFT columns should ensure that the manufacturing quality supports the assumed ductile behavior in progressive collapse analysis, with particular attention to weld quality, dimensional accuracy, and concrete fill integrity. The integration of progressive collapse analysis results into the fabrication quality control program is essential for producing structures that can withstand local failures without catastrophic collapse.