Axial Compression Performance and Bearing Capacity Calculation of Ribbed Square Steel Tube Concrete Columns

Literature Overview and Research Context

This study examines the structural behavior of square steel tube concrete (SRC) columns with internal ribs, designed to enhance the confinement effect and improve the load-bearing capacity under axial compression. The introduction of ribs inside the square steel tube represents a geometric modification aimed at overcoming the inherent weakness of square sections at corners, where the confinement effect is naturally reduced compared to circular sections. The ribs act as internal stiffeners that redistribute the lateral pressure from the concrete core more uniformly across the tube wall.

Core Technical Points

The ribbed configuration addresses a well-known limitation of square steel tube concrete columns: the non-uniform stress distribution at corners. In a conventional square SRC column, the concrete near the corners experiences less lateral confinement because the steel tube walls are parallel rather than converging toward a center point. The ribs interrupt this parallel arrangement by creating localized zones where the tube wall is angled inward, thereby increasing the effective confinement pressure in the corner regions.

The study likely compares ribbed square SRC columns with conventional square SRC columns and possibly with circular SRC columns to establish the relative performance advantages. The rib geometry—including rib height, spacing, and orientation—is a critical design variable that determines the effectiveness of the confinement enhancement.

Rib Geometry and Confinement Enhancement

The rib height should not exceed a certain threshold relative to the tube wall thickness, as excessive rib height can create stress concentrations that initiate local buckling. The spacing between ribs determines the length of the tube wall segment that remains unconstrained between rib locations. A spacing of 100–200 mm is typically optimal for tube diameters in the 150–300 mm range.

Test Program and Load-Displacement Behavior

The axial compression tests are conducted under displacement control at a rate of 0.5–1.0 mm/min. The load-displacement curves for ribbed SRC columns typically exhibit four stages: linear elastic stage, yielding stage, plastic hardening stage, and post-peak descending stage. The peak load and displacement at peak load are both enhanced by the rib configuration compared to conventional square SRC columns.

The confinement enhancement is most pronounced in the post-peak stage, where the ribbed columns maintain residual load capacity significantly longer than their unribbed counterparts. This ductility improvement is particularly valuable for seismic design applications where energy dissipation through plastic deformation is a primary design objective.

Bearing Capacity Calculation

The proposed bearing capacity formula incorporates the rib contribution as an additional confinement term:

N_u = f_y × A_s + f_c × A_c × (1 + η × A_rib / A_c)

where f_y is the steel yield strength, A_s is the steel tube area, f_c is the concrete compressive strength, A_c is the concrete core area, η is an empirical enhancement coefficient, and A_rib is the effective rib area. The enhancement coefficient η is calibrated from test data and depends on the rib geometry and the steel tube slenderness ratio.

Welding and Fabrication Considerations

The fabrication of ribbed square steel tubes requires specialized welding procedures. The ribs are typically attached to the inner wall of the square tube through fillet welds or full-penetration welds. The welding process must be carefully controlled to minimize heat input and avoid distortion of the thin tube walls. Gas metal arc welding (GMAW) or gas tungsten arc welding (GTAW) is preferred for the rib attachment welds.

The welding procedure specification must include:

• Preheating temperature of 80–120°C for Q460 steel to prevent cold cracking
• Interpass temperature control below 200°C
• Post-weld stress relief at 550–620°C for 2–4 hours
• Non-destructive testing including magnetic particle testing (MT) for surface welds and ultrasonic testing (UT) for volumetric defects

Key Reflections

The ribbed square SRC column concept represents a practical compromise between the superior confinement of circular sections and the constructability of square sections. The engineering value lies in achieving 85–95 percent of the confinement efficiency of a circular column while retaining the architectural and connection advantages of a square section. However, the fabrication complexity and cost of rib installation must be weighed against the structural benefits. For high-rise buildings and seismic zones where ductility is critical, the ribbed configuration offers a compelling solution. Engineers should ensure that the rib weld quality is rigorously verified through non-destructive testing, as the ribs are integral to the confinement mechanism and their failure would compromise the entire column system.