Static Performance of CFST Column-Steel Beam Single-Sided Bolt Connections

Literature Overview

The paper by Li Deshan, Tao Zhong, and Wang Zhibin (2015, Journal of Hunan University, Vol. 42, No. 3, pp. 43-49) investigates the static performance of single-sided bolt connections between concrete-filled steel tube (CFST) columns and steel beams. Eight specimens were tested under axial tension and bending-shear loading to examine the influence of restraint tie bar configurations on joint integrity and to evaluate the working performance of single-sided bolts.

Test Configuration

Key Test Findings

The study establishes several important conclusions regarding single-sided bolt connection performance:

1. Single-sided bolts exhibit good mechanical performance and reliable working behavior under both axial tension and bending-shear loading.
2. Square CFST columns benefit significantly from restraint tie bars: The tie bars effectively constrain wall deformation, substantially improve joint core zone integrity, and reduce the damage area.
3. Circular CFST columns show limited improvement from restraint tie bars because the circular cross-section inherently provides better wall stability.
4. Through-column bolt connections provide the best joint core zone integrity, serving as a benchmark for comparison.
5. Under bending-shear loading, single-sided bolt connections demonstrate good rotational capacity.

Welding and Fabrication Perspectives

The single-sided bolt connection involves welding of the end plate to the steel beam and, in some configurations, welding of restraint tie bars to the CFST column. These welds are subject to significant stress during seismic events:

Weld Quality Requirements

Residual Stress and Distortion Control

The welding of restraint tie bars to the CFST column introduces local residual stresses that can interact with the stresses from bolt pre-tensioning. For square CFST columns, the tie bars are typically welded to the interior faces of the column walls, creating a complex stress field. The following measures are recommended:

• Use symmetrical welding sequences to minimize angular and longitudinal distortion.
• Apply back-gouging and back-welding on critical welds to relieve surface residual stresses.
• Consider post-weld stress relief heat treatment for heavily restrained joints.
• Use low-hydrogen welding consumables (E7018 or equivalent) to minimize hydrogen-induced cracking risk.

Joint Core Zone Integrity Analysis

The concept of joint core zone integrity is central to this study. In a CFST column-steel beam connection, the joint core zone is the region where the steel beam connects to the column, and it must transfer bending moments and shear forces without local buckling or yielding of the column wall.

Engineering Practice Integration

For pipe fabrication and steel erection companies, this study provides practical guidance for CFST column-steel beam connections:

1. Square CFST columns: Always install restraint tie bars to prevent local buckling of the flat walls. The tie bars should be designed to yield after the beam plastic hinges form, ensuring the "strong joint, weak member" design philosophy.
2. Circular CFST columns: The inherent stability of the circular cross-section reduces the need for tie bars, but they should still be considered for high-seismicity regions where additional robustness is desired.
3. Bolt specification: The single-sided bolts should be high-strength bolts (Grade 8.8 or 10.9 per GB/T 1228) with controlled pre-tensioning. The bolt torque should be verified during erection using calibrated torque wrenches or tension indicators.
4. End plate design: The end plate thickness and bolt layout must be designed to ensure that the end plate yields before bolt fracture. The weld between the end plate and beam flange is the critical weld detail.

Key Questions and Reflections

A significant question remains regarding the interaction between bolt pre-tensioning and weld residual stresses. When the single-sided bolts are tightened to their specified pre-tension, they introduce compressive stresses in the end plate and column wall. If the weld residual stresses are not properly managed, the superimposed stresses could exceed the yield strength of the base metal or weld metal, leading to premature failure.

Additionally, the study does not address the effect of column concrete strength on joint performance. The concrete fill in the CFST column provides confinement to the steel tube, but the concrete strength and stiffness influence the local buckling resistance of the column wall at the joint zone. Future research should parametrically study the concrete strength effect on single-sided bolt connection performance.

Study Insights and Outlook

The single-sided bolt connection offers a practical and economical solution for CFST column-steel beam connections, particularly where through-column bolts are impractical. The key insight from this study is that the effectiveness of restraint tie bars depends on the column cross-section geometry, with square CFST columns benefiting substantially and circular CFST columns showing limited improvement. For fabrication engineers, the primary challenge is ensuring that all welds in the joint zone are of high quality and that bolt pre-tensioning is accurately controlled during erection. The study provides a solid foundation for the design and construction of CFST column-steel beam connections in bridge and building structures, and the results should be incorporated into future revisions of relevant design codes such as GB 50011 and JTG/T 2211.