Compression-Bending Performance of Mega CFST Bifurcated Column Joints

Literature Overview

Yang Guang et al. (2018), published in the Journal of Beijing University of Technology, present experimental research on the compression-bending performance of mega concrete-filled steel tube (CFST) bifurcated column joints. The study was conducted with reference to the Beijing "China Zun" project, one of the most challenging CFST structures in the world. Funded by the National Natural Science Foundation of China, this research addresses a critical structural engineering challenge: the behavior of massive CFST columns where a single column bifurcates into multiple branches, creating a complex joint region subject to combined compression and bending.

Experimental Configuration and Test Results

Two specimens with different structural configurations were tested under eccentric compression cyclic loading. The specimens represent the mega CFST bifurcated column joints found in the China Zun tower, where the massive steel tubes have diameters and wall thicknesses that present unique fabrication and welding challenges. The test results provide valuable data on the failure modes, bearing capacity, lateral bending stiffness, lateral deformation, and strain distribution in these complex joints.

Failure Characteristics

The specimens exhibited predominantly bending failure, with the failure location correlated with the stiffness distribution of the bifurcated column joint. The lateral deformation was primarily concentrated in the column branch with larger lateral displacement, indicating that the stiffer branch attracts less deformation while the more flexible branch accommodates the majority of the lateral deflection. This finding is critical for understanding the load distribution mechanism in bifurcated column joints and has direct implications for the design of weld connections at these critical regions.

Plane Section Assumption Verification

An important finding is the verification of the plane section assumption (Bernoulli hypothesis) for the irregular cross-sections of the column branches. Before steel yielding, the irregular cross-sections of the column branches conform to the plane section assumption, meaning that the strain distribution across the section is linear. After steel yielding, the assumption is approximately satisfied, though deviations occur due to local buckling and non-uniform plastic strain distribution. This finding supports the use of simplified elastic-plastic analysis methods for the design of mega CFST bifurcated column joints, provided that the analysis accounts for post-yield behavior.

Critical Finding on Constraint Effects

A particularly significant finding is that when only the constraint effect of the steel tube is considered in the bearing capacity calculation, ignoring the contribution of transverse structural measures, the calculated capacity differs substantially from the measured values. This indicates that the combined constraint effect of the steel tube and transverse diaphragms (or internal braces) is significantly stronger than the steel tube constraint alone. This finding has profound implications for the design of mega CFST bifurcated column joints, as it demonstrates that the internal structural measures play a crucial role in enhancing the composite action and bearing capacity.

From a welding engineering perspective, this finding highlights the importance of the weld quality at the junction between the steel tube and the internal diaphragms. These welds must be designed and executed to full structural capacity, as the diaphragm contribution to the overall bearing capacity is substantial. Inadequate weld quality at these internal connections would not only reduce the diaphragm's effectiveness but could also initiate local buckling of the steel tube wall, leading to premature failure.

Welding Challenges in Mega CFST Bifurcated Columns

The fabrication of mega CFST bifurcated column joints presents extraordinary welding challenges:

• The large diameter and wall thickness of the steel tubes require extensive preheating and controlled cooling to prevent hydrogen-induced cracking and maintain weld ductility.
• The bifurcation geometry creates complex stress states at the weld joints, with combined axial compression, bending, and torsion acting simultaneously.
• Internal diaphragm welds are difficult to access and inspect, requiring advanced non-destructive testing methods such as phased array ultrasonic testing (PAUT) or electromagnetic acoustic transducers (EMAT).
• The residual stresses from welding massive sections can be significant and may affect the post-weld fatigue performance under cyclic loading.

Welding Procedure Considerations

For mega CFST bifurcated column joints, the following welding procedure considerations should be implemented:

• Multi-pass welding with controlled interpass temperature to manage heat input and prevent excessive HAZ softening.
• Post-weld heat treatment (PWHT) for critical joints to relieve residual stresses and restore HAZ toughness.
• Weld sequencing designed to minimize angular distortion and out-of-plane warping of the massive steel tube sections.
• Full radiographic testing (RT) or ultrasonic testing (UT) of all full-penetration welds, with acceptance criteria aligned with ASME B31.3 or equivalent standards.
• Magnetic particle testing (MT) or penetrant testing (PT) of all weld toes and surface-breaking defects.

Engineering Practice Integration

The findings of this study should be integrated into the design and fabrication specifications for mega CFST bifurcated column joints. The combined constraint effect of the steel tube and internal diaphragms must be explicitly modeled in structural analysis, rather than relying on simplified steel-only constraint models. The welding specifications must be developed in close coordination with the structural design team to ensure that the weld details can achieve the assumed capacity contributions from the internal diaphragms.

Quality assurance programs for mega CFST fabrication should include:

• Welding procedure qualification (WPQ) and welder performance qualification (WPQ) for the specific joint configurations and steel grades used.
• In-process monitoring of welding parameters including heat input, interpass temperature, and cooling rate.
• Post-fabrication dimensional inspection to verify that weld distortion has been controlled within acceptable tolerances.
• Documentation of all weld inspections and test results for traceability and long-term asset management.

Study Insights and Reflections

This research makes a significant contribution to the understanding of mega CFST bifurcated column joint behavior under combined compression and bending. The experimental validation of the plane section assumption for irregular cross-sections provides confidence in the use of standard analysis methods for design. However, the study primarily focuses on monotonic and low-cycle fatigue behavior. In practical engineering, these joints are subjected to wind-induced vibrations and potential seismic loading, which may cause high-cycle fatigue damage at weld locations. The interaction between the combined constraint effect and fatigue crack propagation deserves further investigation. Additionally, the study does not address the effect of manufacturing tolerances on the initial geometry of the mega steel tubes, such as ovality and straightness, which can affect the stress distribution at the bifurcation region. Future research should incorporate these practical considerations to provide more comprehensive design guidance for engineers working with mega CFST structures.