Seismic Performance Test of Steel Tube Concrete Column-Hybrid Beam Joints

Literature Overview

Published in the Journal of Zhejiang University (Engineering Science) in 2021 by Feng Shuaike, Guo Zhengxing, Ni Luyao, Li Guojian, Gong Changyi, Xie Chao, and Man Jianzheng from Southeast University and Zhongyifeng Construction Group, this paper investigates the seismic performance of steel tube concrete (SRC) column-hybrid beam joints designed for prefabricated long-span composite frame structures. Supported by the National Thirteenth Five-Year Key R&D Program (2016YFC0701703), the study tests two full-scale interior column joint specimens under low-cycle reversed loading. The two specimens differ in the beam-end connection detail: one uses a reduced beam section (RBS) joint and the other uses a conventional beam-end connection. The tests examine failure modes, energy dissipation capacity, load-bearing capacity, ductility, and strain distribution in the hybrid beam.

Core Technical Points and Interpretation

Joint Configuration and Design Philosophy

The proposed joint is designed for prefabricated long-span composite frame structures, where the SRC column serves as the vertical load-bearing element and the hybrid beam (a composite beam combining steel and concrete) serves as the horizontal member. The hybrid beam typically consists of a steel I-section with a concrete slab on top, connected by shear connectors. The joint design must accommodate the following requirements:

• Prefabrication: The joint must be suitable for factory fabrication and site assembly, minimizing on-site welding and bolted connections.
• Seismic performance: The joint must exhibit adequate ductility, energy dissipation, and strength under cyclic seismic loading.
• Long-span capability: The joint must transfer large bending moments and shear forces from the long-span beam.

Reduced Beam Section (RBS) Joint

The RBS joint is a well-established seismic design strategy in steel structures, where the flanges of the beam end are locally reduced (flared or cut) to create a plastic hinge zone away from the connection. This approach has several advantages:

• Plastic hinge relocation: The reduction in flange section modulus forces the plastic hinge to form in the reduced zone rather than at the weld, avoiding brittle weld fracture.
• Improved ductility: The plastic hinge in the reduced zone can undergo larger inelastic deformations without fracture, improving the overall ductility of the joint.
• Weld protection: By relocating the plastic hinge away from the weld, the weld is subjected to elastic or mildly inelastic strains, reducing the risk of weld cracking.

The test results confirm that the RBS treatment effectively promotes plastic hinge formation in the reduced zone, avoiding brittle failure at the beam-end weld. Compared to the conventional joint, the RBS joint exhibits superior ductility and energy dissipation capacity.

Conventional Beam-End Joint

The conventional joint uses a full-strength beam-end connection without flange reduction. The test results show that this joint is prone to brittle weld failure at the beam-end weld, which severely limits the ductility and energy dissipation capacity. The weld fracture occurs before the plastic hinge can develop in the beam, leading to a sudden loss of strength.

Hybrid Beam Strain Distribution

The strain distribution in the hybrid beam reveals important behavior characteristics:

• Flange strain concentration: The steel flanges of the hybrid beam experience the highest strains, particularly at the plastic hinge location.
• Concrete slab contribution: The concrete slab on top of the steel beam contributes to the compressive force in the beam, reducing the net tensile force in the bottom flange.
• Shear connector slip: The shear connectors between the steel beam and concrete slab may slip under large deformations, affecting the composite action and the overall joint performance.

Additional Reinforcement Bond Slip Issue

A critical finding of the study is that the bond slip of additional reinforcement at the beam bottom after yielding affects the energy dissipation capacity of the joint. The additional reinforcement (typically deformed bars placed at the beam bottom to enhance the tensile capacity) can experience bond slip after yielding, which reduces the stiffness and energy dissipation of the joint. The authors recommend that, while the anchorage length should meet code requirements, the reinforcement ratio should be appropriately increased to prevent premature bond slip after yielding.

Engineering Practice Implications

Welding Quality Requirements for RBS Joints

The RBS joint involves several critical welds that must meet high quality standards:

The weld quality is critical because any defect in the weld can initiate brittle fracture under cyclic loading, leading to catastrophic joint failure. The weld procedure qualification (WPQ) must include cyclic loading verification for the specific joint configuration.

Material Selection for Seismic SRC Joints

The material selection for seismic SRC joints requires careful consideration of the following factors:

• Steel grade: Low-yield-strength steel (e.g., Q235 or Q345) is preferred for the plastic hinge zone to ensure adequate ductility. Higher-grade steel (Q390, Q420) may be used for the beam outside the plastic hinge zone.
• Welding consumables: The welding consumables must match the base steel grade and provide adequate toughness (impact energy) at the service temperature. Low-hydrogen electrodes or wires are recommended to prevent hydrogen-induced cracking.
• Concrete grade: The concrete in the SRC column should have adequate strength (C40 or higher) and ductility to accommodate the large deformations induced by the plastic hinge in the beam.

Prefabrication Considerations

The prefabricated nature of the joint introduces additional challenges:

1. Field welding: If field welding is required, the weld quality must be ensured under less controlled conditions. Preheating, wind protection, and post-weld heat treatment may be necessary.
2. Bolted connections: Bolted connections may be used for field assembly, but they must be designed to accommodate the inelastic deformations of the plastic hinge without bearing failure or bolt fracture.
3. Transportation: The prefabricated joint components must be designed for safe transportation, with temporary bracing and protective packaging for critical welds.

Key Questions and Reflections

The study raises several important questions for further investigation:

• How does the RBS geometry (reduction depth, flare angle, transition length) affect the ductility and energy dissipation of the joint?
• What is the effect of the SRC column confinement on the joint performance? Does the concrete core in the column provide additional ductility to the joint?
• How does the hybrid beam shear connector layout affect the joint ductility and energy dissipation?
• Can the bond slip issue of additional reinforcement be mitigated by using higher-strength reinforcement or improved anchorage details?

The finding that bond slip of additional reinforcement affects energy dissipation is particularly important for practical design. It suggests that the reinforcement detail must be carefully designed to maintain composite action throughout the full range of seismic deformations.

Study Insights and Implications

This paper provides valuable experimental data on the seismic performance of SRC column-hybrid beam joints, which are increasingly used in prefabricated long-span frame structures. The confirmation that RBS treatment effectively improves ductility and energy dissipation validates the use of this well-established steel structure design strategy in SRC applications. For steel pipe manufacturers, the key insight is that the SRC column material and fabrication quality directly influence the joint performance. The concrete in the SRC column provides confinement that enhances the column's ductility, but the joint performance is primarily governed by the beam-end connection detail. The RBS joint is a practical and effective solution that can be implemented with minimal changes to existing fabrication and construction practices. The study also highlights the importance of reinforcement detailing in the hybrid beam — the bond slip issue of additional reinforcement is a practical concern that must be addressed in the design to ensure the full seismic performance of the joint.