Mechanical Performance of Novel-Shaped CFST Internal Joints

Literature Overview

Chen Qian et al. (2016), published in the Journal of Henan University of Science and Technology, investigates the seismic performance of a novel internally partitioned CFST joint. Five half-scale specimens were tested under low-cycle reversed loading to evaluate hysteresis behavior, load-bearing capacity, and energy dissipation. Finite element analysis using ANSYS was conducted to validate experimental results and investigate the damage evolution of concrete and the influence of steel tube confining pressure on the joint zone.

Specimen Configuration and Test Setup

The novel joint incorporates an internal partition plate within the CFST section, which divides the joint zone into separate cells. This configuration is designed to address the common weakness of conventional CFST joints, where the joint zone often becomes the critical failure location under seismic loading due to the complex stress state involving bending, shear, and axial forces.

Hysteresis and Energy Dissipation Analysis

The test results demonstrate that the internally partitioned CFST joint exhibits high load-bearing capacity and good seismic performance. The hysteresis loops are full and stable, indicating effective energy dissipation through inelastic deformation of both the steel tube and the confined concrete.

The internal partition plate plays a crucial role in distributing the shear forces across the joint zone. In a conventional CFST joint without internal partitioning, the shear forces are concentrated in the core concrete, leading to diagonal cracking and progressive degradation of the joint capacity. The partition plate redistributes these forces to the steel tube walls and the surrounding concrete, creating a more uniform stress distribution.

Finite Element Analysis Insights

The finite element analysis provides valuable insights into the damage evolution of the joint zone that cannot be directly observed in physical tests. The analysis reveals:

1. Concrete damage initiates at the corners of the joint zone where the stress concentration is highest.
2. The steel tube confining pressure significantly improves the compressive strength and ductility of the core concrete, particularly in the later stages of loading.
3. The partition plate experiences plastic deformation at its edges, contributing to the overall energy dissipation capacity.

Welding Quality and Joint Integrity

From a welding perspective, the internal partition plate must be welded to the steel tube walls with high quality to ensure the intended load distribution. The welds at the partition-tube junction are subjected to complex multiaxial stress states during seismic loading, making them susceptible to fatigue cracking and brittle fracture.

Key welding considerations include:

1. The weld type should be selected based on the partition plate thickness and the expected stress levels. For thick partition plates, groove welds with full penetration are preferred.
2. The weld metal should have mechanical properties compatible with the base metal to avoid mismatch in the HAZ.
3. Post-weld stress relief treatment may be necessary to reduce residual stresses that could initiate cracks under cyclic loading.
4. Non-destructive testing (NDT) of all critical welds, including ultrasonic testing (UT) for internal defects and magnetic particle testing (MT) for surface cracks, should be performed before concrete placement.

Engineering Implications

The internally partitioned CFST joint represents a promising solution for seismic-resistant construction in high-rise buildings and long-span structures. The combination of high strength, good ductility, and effective energy dissipation makes it suitable for applications where both gravity and lateral load resistance are critical. However, the fabrication complexity and welding requirements must be carefully managed to ensure that the theoretical performance advantages are realized in practice.