Axial Compression Performance of High-Strength Steel Strip Confined External UHPC Steel Tube Concrete Composite Columns

Literature Overview and Research Motivation

This study investigates the axial compression behavior of composite columns that combine a steel tube concrete (STC) core with an external layer of ultra-high performance concrete (UHPC) confined by high-strength steel strips. The composite column concept addresses a fundamental limitation of conventional STC columns: the relatively low compressive strength of normal-weight concrete, which typically ranges from 30 to 60 MPa. By incorporating UHPC with compressive strengths exceeding 120 MPa as an external strengthening layer, the overall load-bearing capacity and deformation characteristics of the column are significantly enhanced. The high-strength steel strips, typically with yield strengths of 800-1200 MPa, provide additional lateral confinement to the UHPC layer, creating a multi-layer confinement system that maximizes the triaxial compression state of the concrete materials.

Experimental Configuration and Material Properties

The specimens tested in this research are designed with a steel tube inner core filled with ordinary concrete, surrounded by a UHPC layer that is externally confined by high-strength steel strips. The steel tube is typically a seamless or ERW welded pipe with wall thicknesses of 6-12 mm and outer diameters of 200-300 mm. The UHPC layer thickness ranges from 20 to 50 mm, providing sufficient material volume to achieve the desired confinement effect without excessive weight increase.

Core Findings and Load-Bearing Mechanism

The research demonstrates that the composite column exhibits a distinct multi-stage load-bearing mechanism. In the initial elastic stage, all components share the axial load proportionally based on their respective stiffness contributions. As the core concrete begins to crack at approximately 60-70% of its uniaxial compressive strength, the UHPC layer and steel strips progressively take on a larger share of the load. The high-strength steel strips create a hoop confinement pressure on the UHPC layer, which in turn provides additional confinement to the steel tube and core concrete through the interface friction and mechanical interlock.

The ultimate load capacity of the composite columns is 40-65% higher than that of equivalent conventional STC columns. The ductility improvement is particularly notable, with ductility ratios reaching 4.5-6.0 compared to 2.5-3.5 for unstrengthened STC columns. The failure mode is characterized by a progressive yielding of the steel strips followed by concrete crushing, which is significantly more ductile than the sudden buckling failure typical of unstrengthened columns.

Welding and Fabrication Considerations

From a welding and fabrication standpoint, the attachment of high-strength steel strips to the exterior of the steel tube presents several technical challenges. The significant difference in yield strength between the Q345/Q460 steel tube and the Q890/Q1100 steel strips creates a mismatch in the heat-affected zone (HAZ) properties. The high-carbon equivalent of the strip material increases susceptibility to cold cracking during welding, particularly in cold weather conditions. Preheating to 100-150°C is recommended for strip attachment welds, and low-hydrogen electrode consumables (E7018 equivalent or higher) should be used to minimize hydrogen-induced cracking risk.

The welding sequence for strip attachment should follow a symmetric pattern to minimize distortion of the steel tube. For large diameter tubes, the strips should be welded in segments rather than as continuous full-length welds to accommodate thermal expansion during the welding process. The UHPC layer is then cast around the tube-strip assembly, and the mechanical bond between the UHPC and the steel strips is achieved through surface roughening of the strip exterior and the use of shear keys or dowel bars.

Engineering Practice Integration and Standards Reference

This composite column concept has direct applicability to high-rise building core walls, bridge piers, and industrial equipment supports where high compressive loads and limited space are design constraints. The design approach aligns with the general principles of ASCE 317 (Commentary on the Structural Concrete Reinforcing Steel Institute's Guide) for externally bonded reinforcement, though the specific combination of UHPC and high-strength steel strips is not explicitly covered in current standards.

In the context of pipe fitting and welding quality control, the fabrication of such composite columns requires adherence to stringent welding quality standards. The strip attachment welds should be inspected using ultrasonic testing (UT) in accordance with ASTM E164 or equivalent, with acceptance criteria following AWS D1.1 for structural steel welds. The UHPC casting process must ensure proper compaction to avoid voids at the strip-tube interface, which could serve as stress concentrators under cyclic loading.

Key Insights and Implications

The research highlights an important principle in composite structural design: the effectiveness of confinement is not solely determined by the confinement pressure magnitude but also by the compatibility of deformation between the confining elements and the confined material. The high-strength steel strips, despite their high yield strength, must have sufficient ductility to accommodate the large plastic strains in the UHPC layer without premature fracture. The research confirms that strips with elongation values exceeding 12% at fracture perform adequately in this application.

For engineers involved in pipe rack and support structure design, this composite column concept offers a pathway to reduce column cross-sectional dimensions while maintaining or increasing load capacity. This is particularly valuable in congested industrial facilities where every square meter of footprint is at a premium. The combination of UHPC and high-strength steel strips represents a material optimization strategy that leverages the superior compressive strength of UHPC and the high confinement efficiency of high-strength steel to create a synergistic composite system.

This study provides a solid foundation for the development of design guidelines for multi-layer confined composite columns. The key insight is that the external UHPC-steel strip system does not merely add to the load capacity of the existing STC core but fundamentally alters the failure mechanism from a brittle to a ductile mode. Future research should investigate the long-term creep and fatigue behavior of these composite columns under sustained loading, as well as the seismic performance under cyclic lateral loading, to enable their broader application in critical infrastructure projects.