Nonlinear Analysis of Concrete-Filled Steel Tube Columns Under Blast Loading

Literature Overview

This study by Cao Xueye, Zhao Junhai, and Li Yan from Chang'an University addresses the nonlinear dynamic response of concrete-filled steel tube (CFST) columns subjected to blast loading. Published in 2015 in the Journal of Architecture and Civil Engineering, the paper develops a theoretical framework based on unified strength theory to derive the plastic limit bending moment and ultimate displacement of simply supported CFST beams under uniformly distributed load. The authors employ an equivalent single-degree-of-freedom (SDOF) model combined with the step-by-step integration method to capture the nonlinear effects arising from mass and stiffness changes during the blast reaction process. The research is supported by multiple national and provincial funding agencies, reflecting the strategic importance of blast-resistant design in civil infrastructure.

Core Technical Methodology

The unified strength theory provides a comprehensive constitutive description that accounts for the interaction between tensile and compressive stress states in concrete and steel materials. The derivation of the plastic limit bending moment considers the confining effect of the steel tube on the concrete core, which significantly enhances the compressive strength of concrete in the confined region. The confining coefficient, defined as the ratio of the steel tube cross-sectional area to the concrete cross-sectional area, directly influences the plastic limit bending moment and ultimate displacement. As the confining coefficient increases, both the plastic limit bending moment and ultimate displacement increase proportionally, indicating that the composite action between steel and concrete becomes more effective under higher confinement levels.

The lateral pressure coefficient, which represents the degree of lateral constraint imposed by the steel tube on the concrete core, also plays a critical role in determining the structural response. With increasing lateral pressure coefficient, the plastic limit bending moment increases, demonstrating that the confinement effect is a key mechanism for improving blast resistance. When the enhancement of concrete compressive strength due to steel tube confinement is considered, the calculated plastic limit bending moment improves by 12% to 19% compared to cases where this effect is neglected. This finding underscores the importance of incorporating the confinement effect in blast-resistant design calculations.

Equivalent SDOF Model and Step-by-Step Integration

The equivalent single-degree-of-freedom model simplifies the complex dynamic response of the CFST column into a lumped mass-spring-damper system. This approach captures the essential nonlinear behavior, including mass redistribution and stiffness degradation during the blast event. The step-by-step integration method allows for the numerical solution of the governing differential equation at each time increment, enabling the tracking of displacement, velocity, and acceleration throughout the blast duration.

The blast loading is typically characterized by a triangular or modified Hopkinson-Cranz pressure-time profile, where the peak pressure and duration are the primary parameters. The nonlinear analysis accounts for the progressive yielding of the steel tube, the crushing of the concrete core, and the potential separation between steel and concrete at the interface. The theoretical results are validated against published literature data, confirming the accuracy and reliability of the proposed methodology.

Engineering Practice Implications

For practical blast-resistant design of CFST columns, the following engineering considerations emerge from this study. First, the confining coefficient should be optimized to achieve an adequate balance between structural efficiency and blast resistance. Second, the confinement effect on concrete strength must be explicitly included in design calculations, as neglecting this effect can lead to unconservative estimates of structural capacity. Third, the SDOF model with step-by-step integration provides a practical tool for engineers to evaluate the dynamic response of CFST columns under various blast scenarios without resorting to computationally intensive finite element simulations.

The study also highlights that the plastic hinge formation in CFST columns under blast loading is governed by the interaction between the steel tube yielding and the concrete crushing. Engineers should pay particular attention to the connection details between CFST columns and adjacent structural elements, as these connections must accommodate the large deformations that occur during blast events. The proposed calculation method meets the accuracy requirements for blast analysis of CFST columns and provides a valuable reference for blast-resistant design and protection of civil infrastructure.

Key Reflections and Study Insights

The integration of unified strength theory with dynamic analysis methods represents a significant advancement in the analytical capabilities for CFST structures under extreme loading. The 12%-19% improvement in plastic limit bending moment when considering the confinement effect is a practically important finding that should be incorporated into design codes and standards. Engineers working on blast-resistant design should be aware that conventional elastic-plastic analyses may underestimate the capacity of CFST columns if the confinement effect is not properly modeled. The SDOF approach, while simplified, offers a computationally efficient means of preliminary design evaluation that can be refined with more detailed numerical models when necessary. This study contributes to the growing body of knowledge on the behavior of composite steel-concrete structures under dynamic and impact loading, which is increasingly relevant for infrastructure protection against accidental and malicious threats.