Anchorage Performance of Prefabricated Connections in Composite Steel Tube Concrete Columns

Literature Overview

The study by Fan Guangming, Wang Weihua, Xu Zheren, and Cai Minwei, published in the Journal of Disaster Prevention and Mitigation Engineering in 2023, addresses a critical challenge in prefabricated construction: the reliability of connections in composite steel tube concrete (CFTC) columns. The research was supported by the National Natural Science Foundation (51208217), the Fujian Provincial Natural Science Foundation (2014J01195), and the Quanzhou Science and Technology Plan (2017T002). The authors propose an innovative connection method that reverses the traditional sequence: instead of the conventional assemble-then-grout approach, they advocate for a grout-then-insert method, where reinforcement is inserted into pre-grouted holes.

Connection Methodology and Test Program

The traditional prefabricated connection method involves assembling components first and then grouting the joint. This approach carries a significant risk: if the grout is not properly consolidated, hidden voids and weak interfaces can compromise the structural integrity of the connection. The proposed grout-then-insert method addresses this concern by ensuring that the grout is placed and consolidated before the reinforcement is inserted, creating a more reliable bond between the reinforcement and the surrounding grout.

The experimental program was comprehensive, involving 36 test specimens that examined seven influential parameters: reinforcement diameter, reinforcement position, anchorage length, preformed hole shape and diameter, grout strength, and concrete strength. The pull-out test method was used to evaluate the anchorage performance, which is a direct and reliable measure of the bond strength between the reinforcement and the surrounding material.

Key Findings and Failure Modes

Three typical failure modes were observed: reinforcement pulled out without yielding, reinforcement yielded before pull-out, and reinforcement fracture. The predominant failure mode was reinforcement yielding followed by pull-out, indicating good anchorage performance in most cases. The loading process of the grouted reinforcement was divided into four stages: elastic stage, yielding stage, strain hardening stage, and failure stage.

The following table summarizes the parametric effects on anchorage performance:

A notable finding is that corner-positioned grouted reinforcement exhibited higher ultimate load capacity and ultimate bond stress compared to center-positioned reinforcement. This is attributed to the greater confinement provided by the corner geometry, which restricts the radial expansion of the grout under pull-out loading and thereby enhances the frictional bond.

Engineering Practice Connections

For the steel pipe manufacturing and welding industry, this study has several direct implications. The composite steel tube concrete column system involves the integration of steel tubes with concrete, and the connection details between prefabricated segments are critical to the overall structural performance. The grout-then-insert method requires careful control of the grouting process to ensure complete filling of the preformed holes before reinforcement insertion. This means that the preformed holes in the steel tube segments must be precisely located and dimensioned, which places demands on the fabrication accuracy of the steel tube components.

The welding of connection plates, anchor bolts, and other connection hardware to the steel tube segments must meet high quality standards. Any welding defects at these locations could lead to premature failure of the connection under seismic loading. The recommended minimum preformed hole diameter of 2d and not less than 32 mm should be incorporated into fabrication drawings, and the hole diameter tolerance should be controlled to ensure proper reinforcement insertion and grout consolidation.

Study Insights and Reflections

This paper addresses a practical and important problem in the growing field of prefabricated construction. The grout-then-insert method is a logical improvement over the traditional assemble-then-grout approach, as it eliminates the risk of incomplete grout consolidation. The comprehensive parametric study provides engineers with clear design guidance on the relative importance of different parameters. The finding that corner reinforcement provides superior anchorage is particularly useful for detailed design, as it suggests that reinforcement should be preferentially placed at corner positions where possible.

The recommendation to determine anchorage length based on the Concrete Structure Design Code (GB 50010) is practical, as it leverages existing code provisions rather than requiring new design procedures. However, engineers should be aware that the bond characteristics of grouted reinforcement in a composite steel tube concrete environment may differ from those in conventional concrete, and the code provisions may need adjustment for this specific application. This study provides the experimental data needed to develop such code provisions in the future. The research is a significant contribution to the advancement of prefabricated composite steel tube concrete structures, and its findings should be considered in the design of future projects in this field.