Performance of Rectangular Concrete-Filled Steel Tube Columns Under Reciprocating Eccentric Tension-Compression Loads

Literature Overview

This paper by Cao Wanlin, Duan Xiubin, Zhang Jingjie, Dong Hongying, and Zhang Jianwei, published in Structural Engineer (2012, Vol. 28, Issue 4), investigates the seismic performance of rectangular concrete-filled steel tube (CFST) columns under reciprocating eccentric tension-compression loads. Funded by the National Natural Science Foundation of China and Beijing Science and Technology Program, the study conducted tests on four rectangular CFST column specimens with different steel plate thicknesses and with/without shear stud connections. The research addresses a critical gap in understanding the behavior of rectangular CFST members under combined axial and bending loads that simulate seismic conditions.

Specimen Configuration and Test Parameters

The test matrix design effectively isolates two key variables: steel plate thickness (affecting steel ratio and confinement) and shear stud presence (affecting composite action).

Key Experimental Findings

Effect of Shear Studs

The presence of shear studs within the rectangular steel tube cavity significantly improved both tensile and compressive performance. The shear studs serve multiple functions:

1. Enhancing composite action: Shear studs transfer shear forces between the steel tube and concrete core, ensuring both materials deform together
2. Preventing concrete spalling: The studs anchor the concrete core to the steel tube walls, reducing the tendency for concrete to detach under cyclic loading
3. Improving energy dissipation: The shear stud-concrete interaction provides additional inelastic deformation capacity

Effect of Steel Plate Thickness

The finding that thicker steel plates enhance the constraining ability of shear studs is particularly important. In thick-walled rectangular tubes, the steel plate provides lateral restraint to the shear studs, preventing stud pullout and ensuring full shear transfer capacity. In thin-walled tubes, the steel plate itself may deform excessively, reducing the effective anchorage of the studs.

Engineering Practice Integration

Welding Considerations for Rectangular CFST

Rectangular CFST columns present unique welding challenges compared to circular CFST:

1. Corner welding: The corners of rectangular tubes create complex stress concentrations. Welding at corners requires careful heat input control to avoid cracking in the HAZ, particularly under cyclic loading conditions.
2. Shear stud welding: The attachment of shear studs to the inner steel surface requires either:

• Field welding inside the tube (difficult access)
• Pre-welded studs on the steel plate before tube assembly
• Post-fabrication welding through access holes

1. Residual stress effects: The welding residual stresses in rectangular CFST members are more complex than in circular members due to the non-uniform geometry. Under reciprocating loads, these residual stresses interact with the applied stresses, potentially accelerating fatigue damage.

Seismic Performance Implications

The reciprocating eccentric tension-compression loading simulates the combined effects of:

• Seismic lateral forces (causing bending)
• Gravity loads (causing axial compression)
• P-Δ effects (amplifying bending moments)
• Tension-compression reversal (fatigue-like cycling)

Connection Design Recommendations

Based on the experimental results, the following design recommendations emerge:

1. Always use shear studs for rectangular CFST columns in seismic zones. The improvement in ductility and energy dissipation is substantial.
2. Prefer thicker steel plates when seismic performance is critical. The 3 mm plate specimens outperformed the 1.5 mm specimens across all metrics.
3. Steel ratio optimization: The 8.14% steel ratio (thick plate) provided excellent performance, but the 4.07% steel ratio (thin plate) with studs also showed acceptable behavior. Economic optimization should balance steel cost against performance requirements.

Study Insights and Critical Analysis

The study provides valuable experimental data on rectangular CFST seismic performance, but several aspects warrant further investigation:

• Scale effects: The specimens are relatively small (130 × 170 mm). Full-scale members may exhibit different failure modes due to scale effects, particularly regarding concrete crushing and steel buckling.
• Loading protocol: The reciprocating eccentric tension-compression loading is somewhat idealized. Real seismic loading is more complex, involving multi-directional forces and varying load paths.
• Shear stud configuration: The study does not specify the stud diameter, height, or spacing. These parameters significantly affect the shear transfer capacity and should be optimized.
• Long-term behavior: The study focuses on short-term cyclic loading. The long-term behavior under sustained loads combined with occasional seismic events (as in real structures) remains uncertain.

The finding that thicker plates enhance stud effectiveness has direct implications for steel pipe fabrication. When shear studs are used, the steel plate thickness must be sufficient to provide adequate anchorage. For 3 mm plates with typical stud diameters of 10-13 mm, the plate-to-stud diameter ratio is approximately 0.23-0.30, which provides reasonable anchorage. For 1.5 mm plates, this ratio drops to 0.11-0.15, which is marginal.

This research contributes to the understanding of rectangular CFST seismic behavior and provides practical guidance for connection design, particularly emphasizing the importance of shear studs and adequate steel plate thickness in seismic applications.