Optimization of Bolted External Strengthening Ring Connections for Concrete-Filled Steel Tube Beam-Column Joints

Literature Overview

This research by Shi Ruoli, Pan Zhicheng, Xiao Gongjie, Zhang Jun, Xiong Jianli, and Sun Zhiying, published in the Journal of Vibration Engineering (2022, Vol. 35, No. 1), investigates the mechanical performance of bolted external strengthening ring connections for circular concrete-filled steel tube (CFST) column-steel beam rigid joints. The study is supported by the National Natural Science Foundation of China and involves collaboration among Yunnan University, Central South University, Shenzhen Tianjian Real Estate Group, and Chongqing Jiaotong University. The research addresses the challenge of achieving rigid joint behavior in CFST structures without resorting to full welding, which can be difficult and costly to inspect.

Core Technical Points

Connection Configuration and Design Objectives

The external strengthening ring bolted connection consists of an external steel ring welded to the CFST column, with the steel beam bolted to the ring. This configuration aims to achieve rigid joint behavior comparable to fully welded connections while offering the advantages of field-assemblability and reduced welding inspection requirements. The key design parameters include the number of bolt rows, the presence and configuration of web stiffeners and ring stiffeners, and the ring thickness.

Finite Element Modeling and Validation

The authors developed a three-dimensional refined finite element model using ABAQUS, incorporating appropriate constitutive models for both concrete and steel materials. The concrete model must capture the nonlinear compressive behavior, including the confinement effect from the steel tube. The steel model should include strain hardening and plasticity to accurately represent the post-yield behavior.

The model was validated against monotonic loading test results, with good agreement in load-displacement curves and failure modes. This validation is critical because the parametric study conclusions depend on the accuracy of the numerical model. The failure mode of the bolted connection typically involves yielding of the beam flanges, local buckling of the column tube wall, or bolt shear failure, depending on the relative stiffness of the components.

Parametric Study Results

The parametric study compared different structural configurations of the bolted connection, including variations in the number of bolt rows and the presence of stiffeners. The key finding is that a configuration with 4 rows of bolts, combined with both web stiffeners and ring stiffeners, achieves initial stiffness and bending capacity comparable to a fully welded strengthening ring rigid joint.

The initial stiffness of the connection is a critical parameter for seismic design, as it affects the overall lateral stiffness of the structure and the distribution of seismic forces among structural elements. A connection that achieves the same initial stiffness as a welded connection can be substituted in seismic-resistant structures without compromising the design assumptions.

Bolt Preload and Slip Resistance

The performance of bolted connections is sensitive to bolt preload and the slip resistance of the bolted interfaces. High-strength bolts (typically Grade 8.8 or 10.9) should be tensioned to the specified preload using calibrated tensioning tools. The slip resistance depends on the surface condition of the bolted interfaces, which should be prepared by sandblasting or other methods to achieve a specified slip coefficient. In seismic applications, the connections must be designed to remain elastic during design-level earthquakes, relying on the slip resistance to prevent bolt loosening.

Engineering Practice Implications

For structural engineers designing CFST structures, this study provides a practical alternative to fully welded connections that is particularly valuable for field erection. The bolted connection can be assembled on-site with standard tensioning equipment, reducing the need for extensive welding inspection in the field. The requirement for 4 rows of bolts and both web and ring stiffeners adds material and fabrication complexity, but the benefit of field-assemblability may outweigh this cost in many applications.

From a welding engineering perspective, the external strengthening ring itself is welded to the CFST column, which requires a qualified welding procedure for the circumferential weld. This weld should be inspected by radiographic testing (RT) or ultrasonic testing (UT) to ensure full penetration and absence of defects. The bolted connection then transfers the beam loads to the strengthening ring, which distributes them to the CFST column through the weld.

The use of external strengthening rings also affects the local buckling behavior of the CFST column. The ring provides additional confinement to the concrete and steel tube at the joint region, which can improve the local stability of the column under combined axial and bending loads.

Key Questions and Reflections

An important practical question is the long-term behavior of the bolted connection under cyclic loading. Monotonic loading tests and static finite element analyses provide information about the ultimate capacity, but seismic performance depends on the connection's ability to withstand repeated loading cycles without progressive damage. The bolt preload may decrease under cyclic slip, and the stiffeners may experience fatigue damage. Additional cyclic loading tests or fatigue analyses would be valuable to confirm the seismic adequacy of the proposed connection.

Another consideration is the effect of fire on the bolted connection. The external strengthening ring and bolts are exposed to fire and may lose strength at elevated temperatures. The connection should be designed to maintain adequate load-carrying capacity during the required fire endurance period, which may require fireproofing of the external ring and bolts.

Study Insights and Implications

This study demonstrates that carefully designed bolted external strengthening ring connections can achieve rigid joint behavior comparable to fully welded connections, offering a practical solution for CFST structures where field welding is impractical or undesirable. The recommended configuration of 4 bolt rows with web and ring stiffeners provides a clear design guideline for engineers. The combination of experimental validation and parametric finite element analysis gives confidence in the proposed design approach. For future work, cyclic loading tests and fatigue analyses would further strengthen the case for seismic application of this connection type.