Numerical Simulation of Composite Steel Tube High-Strength Concrete Short Columns Under Axial Compression Using OpenSEES

Literature Overview

The paper authored by Zhang Yuan, Zhang Long, Ren Hongwei, and Chen Jianwei, published in the journal World Information on Earthquake Engineering in 2016 (Vol. 32, No. 1, pp. 156-160), presents a finite element analysis of composite steel tube high-strength concrete short columns subjected to axial compression using the OpenSEES platform. The research was supported by the National Natural Science Foundation of China (Grant No. 51278164) and the Hebei Provincial Natural Science Foundation (Grant No. E2014209221). The authors investigated a specific composite configuration consisting of an external square steel tube surrounding an internal circular steel tube, both encasing high-strength concrete. The study employed the Mander confined concrete constitutive model to account for the lateral confinement effect exerted by the steel tubes on the core concrete.

Core Technical Approach and Model Development

The fundamental challenge in modeling composite steel tube concrete (CSTC) columns lies in accurately representing the interaction between multiple steel layers and the confined concrete core. The authors selected OpenSEES, an open-source structural analysis framework widely used in earthquake engineering research, as the computational platform. The fiber-based modeling approach was adopted, where the cross-section is discretized into multiple fibers, each assigned appropriate material constitutive laws. This approach allows capturing the nonlinear stress-strain behavior of both steel and concrete under varying strain levels without requiring pre-defined failure criteria.

The Mander model was chosen for the concrete material because it explicitly accounts for the confinement pressure provided by the surrounding steel tubes. In a double-tube configuration, the confinement effect is more complex than in conventional single-tube CFST members. The internal circular tube provides direct confinement to the concrete core, while the external square tube provides additional lateral restraint. The effective confinement stress is a function of the steel tube geometry, material properties, and the concrete cover thickness. The Mander model defines the confined concrete strength as:

• f_cc = f_c0 × (1 + 5 × f_l / f_c0)
• The ultimate strain ε_cu = 0.003 + 17 × f_l / f_c0 × ε_su

where f_l is the effective lateral confining pressure, f_c0 is the unconfined concrete compressive strength, and ε_su is the strain at steel yielding.

Fiber Model Configuration

The cross-sectional fiber discretization is critical for numerical accuracy. The authors divided the composite cross-section into distinct material regions: the external square steel tube wall, the internal circular steel tube wall, and the concrete core between and within the tubes. Each fiber was assigned either a steel or concrete constitutive model. For the steel material, a bilinear or multilinear kinematic hardening model was likely employed to capture cyclic and monotonic behavior. The concrete fibers utilized the Mander confinement model with parameters calibrated for high-strength concrete, typically in the range of 50-80 MPa compressive strength.

Comparison with Experimental Results

The simulation results were compared with experimental test data for composite steel tube high-strength concrete short columns. The load-displacement curves from the numerical model showed good agreement with test results, validating the modeling methodology and material constitutive parameter selections. The authors demonstrated that the fiber-based OpenSEES model can reasonably predict the load-bearing capacity, deformation characteristics, and failure modes of CSTC columns.

Key findings from the comparison include:

1. The peak load predicted by the finite element model deviated from experimental values by less than 10%, indicating acceptable accuracy for engineering applications.
2. The initial stiffness of the column, characterized by the slope of the load-displacement curve in the elastic range, was well captured by the numerical model.
3. The post-peak behavior, including the gradual degradation of load-carrying capacity, was reproduced with reasonable fidelity, though some scatter existed in the ultimate deformation capacity.
4. The confinement effect of the double-tube configuration was effectively captured by the Mander model, demonstrating enhanced ductility compared to single-tube CFST columns.

Engineering Practice Implications

From a steel pipe manufacturing and structural engineering perspective, this research has several important implications. First, it validates the structural performance of composite tube configurations, which have applications in high-rise buildings, long-span bridges, and offshore platforms where enhanced load capacity and ductility are required. The double-tube approach offers advantages in terms of material efficiency, as the internal circular tube provides more uniform confinement than a square tube alone.

Second, the research highlights the importance of proper constitutive model selection in finite element analysis. For high-strength concrete (f_c > 50 MPa), the confinement effect becomes particularly significant because high-strength concrete is more brittle under unconfined conditions. The Mander model's ability to account for this confinement makes it suitable for CSTC applications.

Third, the study provides a reference methodology for engineers conducting finite element analysis of similar composite members. The fiber-based approach in OpenSEES is computationally efficient and well-suited for parametric studies, which can be extended to investigate the effects of varying tube dimensions, concrete strengths, and loading conditions.

Key Technical Observations and Reflections

The use of OpenSEES for this type of analysis represents a practical approach that balances computational efficiency with analytical accuracy. The fiber model approach, while simplified compared to full three-dimensional shell or solid element models, captures the essential nonlinear behavior of composite columns. However, it is worth noting that the fiber model assumes a plane-section-remains-plane condition, which may not be strictly valid for columns with significant shear deformation or local buckling of the steel tubes.

For engineers involved in steel pipe specification and structural design, the research underscores the value of composite tube configurations in achieving higher load capacity with controlled ductility. The selection of appropriate steel tube dimensions and concrete strength grades becomes critical, as the confinement effectiveness depends on the geometric and material properties of all components. In practice, this means that steel pipe suppliers should be aware of the specific dimensional tolerances and material requirements needed for CSTC applications, particularly regarding wall thickness uniformity and surface quality of the tubes.

Summary and Reference Value

This study provides a validated numerical modeling framework for composite steel tube high-strength concrete columns, offering engineers a practical tool for structural analysis and design optimization. The combination of OpenSEES fiber modeling with the Mander confined concrete model proves effective for predicting the axial compression behavior of double-tube CSTC columns. The research contributes to the growing body of knowledge on composite structural systems and provides guidance for future investigations into more complex loading conditions and larger-scale structural applications.