Fatigue Performance of Steel Tube Concrete Truss Rib Inner Stud Joint

Research Background and Significance

Steel tube concrete truss ribs represent an advanced structural system that combines the advantages of steel tube concrete columns with the efficiency of truss action. The inner stud joint is a critical connection detail that transfers forces between the truss members and the steel tube concrete web element. Under cyclic loading conditions typical of seismic and wind environments, the fatigue performance of this joint directly determines the service life and safety of the entire structural system. This study systematically investigates the fatigue behavior through both experimental testing and analytical modeling.

Experimental Program and Test Results

The experimental program included 12 joint specimens with varying geometric and material parameters. The test matrix was designed to cover the range of practical design variables:

The fatigue tests were conducted under displacement-controlled cyclic loading with a frequency of 0.5 Hz. The S-N curves obtained from the experiments show that the fatigue strength of the stud joint at 2 million cycles ranges from 120 to 185 MPa depending on the parameter combination. The primary failure mode observed was stud fracture initiated at the weld root, followed by progressive cracking in the heat-affected zone of the steel tube wall.

Fatigue Performance Analysis

The study identifies three critical factors that govern the fatigue life of the inner stud joint:

1. Stress concentration at the stud-to-tube weld: The geometric discontinuity at the stud base creates a stress concentration factor of 2.1 to 3.4, which is the dominant factor in fatigue crack initiation.
2. Concrete confinement effect: Higher concrete strength provides improved lateral confinement to the steel tube, delaying local buckling and extending the fatigue life by 15 to 30 percent.
3. Stud spacing effect: Closer stud spacing increases the interaction between adjacent stud stress fields, which can either improve or degrade fatigue performance depending on the loading mode.

The modified S-N relationship proposed by the authors accounts for the concrete confinement factor and the stud geometry factor:

The fatigue strength at N million cycles is expressed as: sigma_N = C × (f_cu / f_cu,ref)^alpha × (d_stud / d_ref)^beta × (1 + gamma × rho_concrete)

where C, alpha, beta, and gamma are empirical constants calibrated from the test data.

Welding Process and Defect Control

The welding quality of the stud-to-tube connection is paramount for fatigue performance. The recommended welding process parameters are:

Common defects that significantly degrade fatigue life include incomplete fusion at the stud root, porosity clusters in the weld cap, and microcracks in the HAZ. The study recommends that all stud welds be inspected by penetrant testing, and critical joints should undergo ultrasonic testing to detect internal discontinuities.

Engineering Recommendations

Based on the research findings, the following design recommendations are proposed for practical applications. The stud diameter should be selected to ensure that the nominal stress at the weld root does not exceed 120 MPa for 2 million cycles in typical structural applications. The stud spacing should be maintained between 100 and 160 millimeters to balance load transfer efficiency with fatigue performance. The steel tube wall thickness should be at least 1.5 times the stud diameter to provide adequate bearing support and reduce stress concentration.

This study provides valuable guidance for the design of steel tube concrete truss structures in seismically active regions. The combination of experimental data and analytical models enables more accurate fatigue life prediction, which is essential for ensuring long-term structural reliability. The emphasis on welding quality control and defect prevention highlights the importance of manufacturing precision in achieving the full fatigue potential of the joint.