Dynamic Response of Concrete-Filled Steel Tube Structures Under Lateral Impact Loading

Literature Overview

This study by Jiang Shan, Lu Guoyun, and Yang Huiwei from Taiyuan University of Technology, published in "Explosion and Shock Waves" (2023, Vol. 43, No. 11), investigates the plastic dynamic response of circular concrete-filled steel tube (CFST) structures subjected to lateral impact loading. The work combines rigid-plastic structural modal analysis with ABAQUS/Explicit finite element simulation, providing both analytical and numerical predictions validated against experimental data. The research is supported by the National Natural Science Foundation of China (Grant No. 12172244).

Analytical Modeling Approach

The authors developed a rigid-plastic modal analysis framework for predicting the plastic dynamic response of CFST members under lateral impact:

• The CFST member was idealized as a rigid-plastic foundation beam model, where the concrete core provides distributed lateral support to the steel tube.
• Modal analysis was applied to derive an analytical solution for the mid-span lateral deformation under impact loading.
• Dimensionless parameters were identified that govern the final lateral deformation of the structure, enabling generalized predictions across different geometries and material combinations.

The analytical solution provides the following key relationships:

Numerical Simulation and Validation

The ABAQUS/Explicit model incorporated the following elements:

• Steel tube: Johnson-Cook constitutive model accounting for strain rate effects, strain hardening, and thermal softening under high-strain-rate impact conditions.
• Concrete core: Concrete Damaged Plasticity model with rate-dependent strength enhancement based on the dynamic increase factor (DIF) from ACI 544.3R.
• Contact formulation: Generalized contact with penalty method to capture tube-concrete interaction and impactor-structure contact.
• Mesh sensitivity: Element size was verified through convergence studies, with final mesh size selected to balance computational efficiency and accuracy (typically 10-15 mm for tube wall elements).

The cross-validation between analytical predictions, numerical simulations, and experimental results demonstrated good agreement, with deviations in final lateral deformation generally within 10-15%.

Parametric Study Results

The comprehensive parametric analysis revealed the following key findings:

1. Geometric parameters: The slenderness ratio (L/D) and wall thickness ratio (D/t) were identified as the dominant geometric parameters influencing final lateral deformation. Increasing L/D from 3 to 6 increased final deformation by approximately 40-50%, while increasing D/t from 20 to 40 reduced deformation by approximately 25-30%.
2. Impactor geometry: The relative width of the impact head significantly affected the deformation mode. Narrow impactors (width/diameter < 0.2) induced localized denting, while wide impactors (width/diameter > 0.5) produced global bending deformation.
3. Material parameters: Compared to geometric parameters, the physical properties of the steel tube (yield strength, elastic modulus) and concrete core (compressive strength, confinement effect) had relatively minor influence on mid-span deflection, suggesting that structural geometry is the primary design variable for impact resistance.
4. Initial impulse relationship: The lateral deformation exhibited a quadratic relationship with initial impulse (I₀), consistent with rigid-plastic theory predictions for impact loading.
5. Plastic hinge distribution: The observed plastic hinge locations in the numerical and experimental results were consistent with theoretical predictions, confirming the validity of the rigid-plastic modal analysis approach.

Impact Loading and Structural Integrity from a Metallurgical Perspective

From a materials and welding quality standpoint, impact loading introduces several critical considerations:

• Strain rate sensitivity: Under impact conditions, the effective strain rate may reach 10³-10⁴ s⁻¹, significantly exceeding the quasi-static strain rates (10⁻²-10⁻¹ s⁻¹) assumed in conventional design. The dynamic increase factor (DIF) for steel typically ranges from 1.0 to 1.5 depending on strain rate, while concrete DIF can reach 2.0-3.0 for high compressive strengths.
• Weld HAZ vulnerability: The heat-affected zone (HAZ) of welds in the steel tube may exhibit reduced ductility and increased susceptibility to brittle fracture under impact loading. The Charpy V-notch impact energy of the HAZ should be verified to meet minimum requirements (typically ≥27 J at service temperature per API 5L or relevant standards).
• Fracture mechanics: Under high-energy impact, pre-existing weld defects (porosity, lack of fusion, undercut) may serve as crack initiation sites. The fracture toughness (KIC) of the base metal and HAZ should be evaluated, with minimum values of 100 MPa·m^0.5 recommended for impact-loaded applications.
• Adiabatic heating: At very high strain rates, localized adiabatic heating in the deformation zone may cause thermal softening of the steel, reducing flow stress and potentially leading to adiabatic shear band formation. This phenomenon is particularly relevant for high-strength steels with low ductility.

Quality Control for Impact-Loaded CFST Structures

For structures designed to withstand impact loading, the following quality control measures are essential:

1. Material certification: Verify impact toughness (Charpy V-notch) of both steel tube and concrete materials at expected service and impact temperatures.
2. Weld inspection: Implement 100% ultrasonic testing (UT) or phased array ultrasonic testing (PAUT) of all welds, with zero tolerance for volumetric defects exceeding 3 mm equivalent diameter.
3. Hydrostatic testing: Perform hydrostatic pressure testing at 1.5 times design pressure after fabrication to verify structural integrity before installation.
4. Strain rate testing: Conduct dynamic mechanical testing (Split Hopkinson Pressure Bar or drop weight) to characterize material response at impact-relevant strain rates.
5. Impact testing: Where feasible, perform full-scale or component-scale impact testing to validate the analytical and numerical predictions for specific structural configurations.

Study Insights and Implications

This research provides a rigorous analytical framework for predicting the plastic dynamic response of CFST structures under lateral impact, validated through numerical simulation and experimental data. The identification of slenderness ratio and wall thickness ratio as dominant geometric parameters provides clear design guidance for impact-resistant CFST structures. The quadratic relationship between final deformation and initial impulse enables straightforward scaling of impact energy requirements for different threat scenarios. The applicability range of the analytical solution should be carefully considered, as the rigid-plastic assumptions become less accurate for very low strain rate or very high energy impact events where elastic wave propagation effects become significant.