Seismic Performance of Prefabricated Double-Tube Concrete-Filled Joints

Literature Overview

This paper, published in the Journal of Harbin Institute of Technology (Vol. 55, No. 10, 2023, pp. 49-62), proposes a novel prefabricated double-tube concrete-filled joint designed to facilitate post-disaster repair and replacement. The research was supported by the National Natural Science Foundation of China (Project No. 51878056). Six scaled specimens (1:2 scale) were subjected to quasi-static cyclic loading to evaluate seismic performance, with a complementary nonlinear finite element analysis performed to validate and extend the experimental findings.

Design Concept and Specimen Configuration

The prefabricated double-tube concrete-filled joint concept incorporates an end plate connection system that allows for disassembly and reassembly, which is particularly advantageous for post-earthquake inspection and repair. The key design variables investigated include:

Variable	Purpose
End plate thickness	Influence on joint strength and stiffness
Column axial compression ratio	Effect on joint behavior under combined loading
Bolt diameter	Influence on connection strength
Concrete filling ratio	Effect on energy dissipation and stiffness
Inner steel tube cross-section shape (circular vs. square)	Influence on overall joint performance

Failure Modes and Damage Patterns

The quasi-static testing revealed four primary failure modes:

Failure Mode	Description	Severity
End plate bending	Plastic deformation of end plate	Moderate to severe
Steel beam flange buckling	Local buckling of beam flange near joint	Moderate
End plate-flange weld cracking	Fracture at weld interface between end plate and flange	Critical
Bolt warp fracture	Deformation and fracture of bolts	Critical

The identification of weld cracking as a critical failure mode has direct implications for welding quality control in prefabricated joint fabrication. The end plate-flange weld is the primary load transfer mechanism, and any weld defects (porosity, incomplete fusion, undercut) would significantly reduce the joint's seismic performance.

Seismic Performance Evaluation

The study evaluated seismic performance through multiple quantitative indicators:

Performance Indicator	Result	Assessment
Hysteresis loop fullness	Full and well-shaped	Strong energy dissipation capacity
Displacement ductility coefficient	All > 4.89	Excellent plastic deformation capacity
Strength degradation coefficient	0.9 to 1.0	Good load-bearing stability
Stiffness degradation	Gradual	Acceptable for seismic design

The displacement ductility coefficient exceeding 4.89 for all specimens is a particularly favorable result, indicating that the joint can undergo significant inelastic deformation without catastrophic failure. This is essential for seismic design where the structure must survive large inter-story drifts.

Parametric Study Results

The parametric analysis revealed several important trends:

End plate thickness: Increasing end plate thickness significantly improves all seismic performance indicators. This is the most effective design parameter for enhancing joint performance.
Column axial compression ratio: Increasing axial compression ratio increases joint load-bearing capacity but has minimal effect on energy dissipation.
Concrete filling ratio: Higher concrete filling ratio increases load-bearing capacity but significantly reduces energy dissipation capacity.
Inner tube shape (square vs. circular): Replacing the inner circular tube with a square tube increases load-bearing capacity but reduces energy dissipation.
Bolt diameter: Bolt diameter has almost no effect on joint load-bearing capacity or energy dissipation.

Finite Element Model Validation

The nonlinear finite element model developed in the study achieved good agreement with experimental results in terms of failure modes and load-bearing capacity. This validates the modeling approach and provides a reliable tool for parametric studies beyond the physical testing scope.

Engineering Practice Implications

From a fabrication and welding quality control perspective:

End plate-flange weld: This is the critical weld in the joint. Full-penetration GTAW or SAW welds should be specified with 100% UT inspection to ensure complete fusion and absence of defects.
Bolt hole precision: The bolt diameter finding suggests that bolt quality is less critical than weld quality, but bolt hole alignment and bolt pretension must be maintained for proper load transfer.
Prefabrication quality: The prefabricated nature of the joint means that field welding is minimized, but factory welding quality becomes paramount. Any weld defects present at fabrication will directly compromise seismic performance.
Concrete placement in double tubes: The concrete filling ratio finding suggests that partial filling may be acceptable from a seismic standpoint, which could simplify construction.

Study Insights and Reflections

The concept of prefabricated joints for post-disaster repair is highly innovative and addresses a critical practical need. The finding that bolt diameter has minimal influence on performance is somewhat surprising and suggests that the joint behavior is dominated by the end plate and weld performance rather than the bolt connection. This has cost implications, as lower-grade bolts may be acceptable. However, the reduction in energy dissipation with higher concrete filling ratios is a trade-off that engineers must carefully evaluate: while higher filling ratios increase strength, they reduce the ductile energy absorption capacity that is essential for seismic resilience. The balance between strength and ductility in prefabricated joints remains an important design consideration that this study helps to clarify.