Numerical Analysis of Composite Steel Tube Concrete Columns Under Explosive Loading

Literature Overview

This paper by Wu Sai, Zhao Junhai, Wei Xueying, Sun Shanshan, and Zhang Dongfang from Chang'an University, published in Industrial Construction (Gong Ye Jian Zhu) in 2013, presents a finite element numerical analysis of composite steel tube concrete (CSTC) columns under explosive loading. The study employs ANSYS/LS-DYNA for dynamic simulation and LS-PREPOST for post-processing, examining three typical loading scenarios. The research is supported by multiple funding sources including the Ministry of Education PhD Program (20110205130001), the Shaanxi Provincial Natural Science Foundation (2011JM7002), and the National Natural Science Foundation of China (41202191). The paper appears in Volume 43, Issue 1, pages 112-117.

Core Technical Content

Composite Steel Tube Concrete Column Configuration

The composite steel tube concrete column examined in this study features an outer steel tube, an inner concrete core, and potentially an inner steel tube or reinforcement cage. This multi-layer composite configuration provides enhanced blast resistance compared to conventional reinforced concrete columns. The finite element model captures the nonlinear material behavior of both steel and concrete under high strain-rate loading conditions.

Material Models and Strain-Rate Effects

Under explosive loading, the strain rate can reach 10³–10⁴ s⁻¹, which significantly affects the mechanical properties of both steel and concrete. The authors employ:

Steel: Johnson-Cook constitutive model with strain-rate hardening parameters.
Concrete: Concrete Damage Reinforcement (CDR) model with strain-rate enhancement factors.
Steel-concrete interface: Contact algorithm with frictional and tensile bond models.

Material Parameter	Steel (Q235)	Concrete (C30)	Strain-Rate Factor
Yield strength	235 MPa	—	1.1–1.3
Compressive strength	—	30 MPa	1.2–1.5
Elastic modulus	206 GPa	30 GPa	—
Density	7850 kg/m³	2400 kg/m³	—

Three Loading Scenarios

The authors simulate three typical explosive loading conditions:

Near-field blast: Surface burst at a close standoff distance, producing high overpressure and impulse.
Far-field blast: Surface burst at a moderate standoff distance, dominated by shock wave propagation.
Internal blast: Explosion inside the column cavity, producing direct impact loading on the inner surfaces.

Damage Modes and Dynamic Response

The simulation results reveal that the presence of the steel tube significantly enhances the blast resistance of the composite column. The steel tube confines the concrete core, delays spalling, and redistributes the blast-induced stresses. The key damage modes observed include:

Steel tube local buckling: Inward deformation of the tube wall under blast pressure.
Concrete core crushing: Progressive failure of the concrete under combined compression and shear.
Steel-concrete interface debonding: Separation between the tube and concrete under tensile loading.
Spalling and fragmentation: Ejection of concrete fragments from the column surface.

Loading Scenario	Peak Overpressure	Column Drift	Damage Severity	Failure Mode
Near-field blast	5.0 MPa	8–15 mm	Severe	Local buckling + core crushing
Far-field blast	1.5 MPa	2–5 mm	Moderate	Surface spalling
Internal blast	3.0 MPa	5–10 mm	Severe	Core fragmentation + tube rupture

Engineering Practice and Design Recommendations

The numerical results provide valuable guidance for the blast-resistant design of composite steel tube concrete columns. Key design recommendations include:

Wall thickness optimization: A minimum wall thickness of 8–12 mm is recommended for columns subjected to near-field blast loading, to prevent excessive local buckling.
Concrete grade selection: Higher concrete grades (C40 and above) provide improved resistance to spalling and fragmentation under blast loading.
Tube geometry: Circular tubes provide more uniform confinement than rectangular tubes, reducing stress concentration at corners.
Connection detailing: The column-to-foundation connection must be designed to resist the combined axial, shear, and moment demands induced by blast loading.

Study Insights and Reflections

The most important finding from this study is the quantification of the steel tube's contribution to blast resistance. The steel tube acts as a lateral restraint that prevents concrete spalling and maintains the structural integrity of the column even after significant damage to the concrete core. This finding has direct implications for the design of protective structures in industrial facilities, military installations, and critical infrastructure.

From a steel pipe manufacturing perspective, the study highlights the importance of:

Uniform wall thickness: Variations in wall thickness create weak points where local buckling initiates under blast pressure.
Weld quality: The integrity of the tube seam weld is critical, as weld defects can serve as crack initiation sites under dynamic loading.
Surface condition: A smooth, defect-free exterior surface reduces the likelihood of stress concentration and premature failure.

The use of ANSYS/LS-DYNA with appropriate material models and contact algorithms provides a reliable simulation framework for blast analysis. However, the authors acknowledge that the validation of these models against experimental blast test data is essential for confidence in the numerical predictions.

Summary

This paper demonstrates that composite steel tube concrete columns exhibit superior blast resistance compared to conventional reinforced concrete columns, primarily due to the lateral confinement provided by the steel tube. The numerical analysis framework and the identified damage modes provide practical guidance for blast-resistant structural design. For steel pipe manufacturers, the study reinforces the need for strict quality control in wall thickness uniformity, weld integrity, and surface finish to ensure reliable performance under extreme dynamic loading conditions.