Seismic Resilience Improvement of Underground Metro Station Using Steel Tube Concrete Composite Columns

Overview and Research Context

The study by Bao Hua, published in the Journal of Civil Engineering and Management, Volume 42, Issue 4, 2025, pages 18–29, investigates the seismic resilience improvement of underground metro station structures through the use of steel tube concrete composite columns. This research is supported by the National Key Research and Development Program of China (Project 2022YFC3800900) and addresses a critical infrastructure challenge: enhancing the seismic performance of underground metro stations, which are among the most vulnerable urban infrastructure components during earthquakes.

Composite Column Design and Material Configuration

Two composite column designs were investigated, both incorporating a steel tube, a concrete core, and an outer concrete jacket. The first composite column, designated as Composite Column 1, consists of a circular steel tube filled with C40 concrete and externally encased in ultra-high performance concrete (UHPC) cast monolithically with the surrounding structure. The second composite column, Composite Column 2, uses a steel tube filled with C60 concrete and externally encased in conventional C40 concrete.

The following table compares the material properties and design configurations:

The UHPC used in Composite Column 1 is characterized by compressive strengths exceeding 120 MPa, tensile strengths above 5 MPa, and exceptional ductility due to the inclusion of steel fibers. These properties enable the outer jacket to remain crack-free under significant deformation, providing sustained confinement to the steel tube and core concrete.

Finite Element Analysis and Seismic Performance

ABAQUS finite element models were developed to compare the mechanical performance of the two composite columns against conventional reinforced concrete columns. The models incorporated material nonlinearities, including the confined concrete constitutive model, steel tube plasticity, and UHPC tensile and compressive behavior.

Under vertical compressive loading, Composite Column 1 demonstrated superior load-carrying capacity and lateral stiffness compared to both Composite Column 2 and the conventional RC column. A notable finding is that Composite Column 1 maintained approximately elastic behavior even at a vertical compressive deformation of 30 mm, indicating a very high ductility margin. This elastic behavior under significant deformation is attributed to the combined effect of the UHPC jacket's high tensile strength and the steel tube's confinement action.

In the context of a complete metro station structure subjected to seismic loading with a peak ground acceleration of 0.3 g, Composite Column 1 exhibited smaller displacement responses and more favorable force distribution compared to the other column types. The structure incorporating Composite Column 1 remained closer to an elastic state throughout the seismic event, indicating that the composite column effectively limited the propagation of damage throughout the station structure.

The soil-metro station structure interaction was also considered in the analysis, recognizing that underground structures are significantly influenced by the surrounding soil stiffness and damping characteristics. The interaction effects were found to be beneficial for the composite column performance, as the soil confinement provided additional restraint against lateral deformation.

Welding and Fabrication Considerations for Steel Tubes

The steel tube component of the composite column requires careful attention to fabrication and welding quality. For underground metro station applications, the steel tubes are typically fabricated from hot-rolled seamless steel pipes or hot-dip galvanized welded pipes conforming to GB/T 8162 or GB/T 8163 for seamless pipes, and GB/T 3091 for welded pipes. The steel grade is typically Q345B or Q355B per GB/T 1591, with a minimum yield strength of 345 MPa and elongation of at least 21%.

The longitudinal and circumferential welds of the steel tube must be full-penetration butt welds, inspected by ultrasonic testing (UT) per GB/T 11345 or radiographic testing (RT) per GB/T 3323, with acceptance criteria meeting at least Level II quality. The weld between the steel tube and the column base plate, as well as any internal transverse stiffeners, must also be full-penetration welds with equivalent inspection requirements.

The interface between the steel tube and the UHPC jacket requires careful consideration of bond strength. While the steel tube provides confinement to the core concrete, the UHPC jacket must also be effectively connected to the steel tube to prevent delamination under seismic loading. Mechanical interlock devices, such as headed studs or corrugated steel tube surfaces, may be employed to enhance the bond between the steel tube and the UHPC.

Engineering Practice Implications

The use of steel tube concrete composite columns in underground metro stations offers several practical advantages. The steel tube acts as permanent formwork, eliminating the need for removable formwork and reducing construction time. The composite action between the steel tube and concrete provides higher strength-to-weight ratio compared to conventional reinforced concrete columns, which is particularly beneficial in underground structures where space is limited.

The UHPC jacket in Composite Column 1 provides exceptional durability and corrosion resistance, which is advantageous for underground environments where the structure may be exposed to groundwater and corrosive chemicals. The high tensile strength of UHPC also reduces the likelihood of cracking, maintaining the structural integrity and watertightness of the underground station.

However, the cost of UHPC is significantly higher than conventional concrete, and its application requires specialized mixing, placing, and curing procedures. The steel fiber content in UHPC can cause pumping difficulties during concrete placement, and the high early-age strength development may lead to thermal cracking if not properly managed. Engineers must carefully evaluate the cost-benefit ratio of using UHPC versus conventional concrete for the outer jacket.

The research findings suggest that Composite Column 1, with its UHPC jacket, offers the best seismic performance and is particularly suitable for critical metro stations located in high seismic hazard areas. For stations in moderate seismic zones or where cost is a primary constraint, Composite Column 2 with conventional C40 concrete may provide an acceptable balance between seismic performance and economic feasibility.

In conclusion, steel tube concrete composite columns represent a promising solution for enhancing the seismic resilience of underground metro station structures. The combination of steel tube confinement, high-strength concrete core, and UHPC jacket provides superior load-carrying capacity, lateral stiffness, and ductility compared to conventional reinforced concrete columns. The finite element analysis, validated against structural behavior under 0.3 g seismic loading, demonstrates that Composite Column 1 can keep the entire station structure closer to an elastic state during earthquakes. Future research should focus on full-scale testing of composite columns under cyclic loading, long-term durability assessment in underground environments, and development of cost-effective UHPC mix designs that maintain the required performance characteristics.