Seismic Performance of Rectangular Steel Tube Concrete Column-Concrete Beam Lap-Type Joints

Literature Overview

The paper by Liang Shuting, Yang Jian, Zhu Xiaojun, and Dang Longji (2021), published in the Journal of Architecture and Civil Engineering (Vol. 38, No. 3, pp. 13-20), presents the development and experimental investigation of a novel lap-type joint connecting rectangular steel tube concrete (SRC) columns to reinforced concrete (RC) beams. The research, supported by the National Natural Science Foundation of China Youth Science Fund Project (Grant No. 51908336), addresses the practical challenge of connecting dissimilar structural members—steel-concrete composite columns and reinforced concrete beams—while maintaining adequate seismic performance. Low-cycle reversed loading tests and finite element analysis were employed to evaluate the joint's seismic behavior.

Core Technical Findings

The lap-type joint demonstrates favorable seismic characteristics, with the following key performance indicators:

The most critical finding is that the lap-type joint achieves the desired "strong joint, weak member" seismic design philosophy, with the plastic hinge forming in the beam end rather than at the joint itself. This is a fundamental requirement for ductile seismic design, ensuring that the joint maintains its integrity while the beam provides the necessary ductile deformation capacity.

Technical Points and Joint Design Analysis

The lap-type joint design incorporates several innovative features that enable effective force transfer between the SRC column and RC beam:

• Lap connection mechanism: The beam longitudinal reinforcement is connected to the column through a lap connection using corbels (牛腿) at the column end. This mechanism transfers both bending moment and shear force from the beam to the column through the lap splice.
• Strain distribution: The experimental results reveal that the maximum strain occurs at the end section of the lap corbel, indicating that this is the critical region for anchorage performance. The strain distribution along the beam longitudinal reinforcement and corbel flange provides valuable data for understanding the force transfer mechanism.
• Anchorage length requirement: The study demonstrates that insufficient lap corbel length leads to anchorage failure at the joint, compromising the seismic performance. Adequate lap length is essential to ensure reliable anchorage of the beam longitudinal reinforcement under combined bending and shear loading.
• Composite action: The rectangular steel tube column provides lateral confinement to the concrete core, enhancing the column's ductility and load-carrying capacity. The joint design must ensure that this composite action is not compromised at the connection region.

The finite element model validated against experimental results demonstrates good agreement, confirming the model's ability to accurately capture the joint's mechanical behavior under cyclic loading. This validated model can be used for parametric studies and design optimization.

Seismic Design Considerations

The research provides several important design guidelines for the lap-type SRC column-RC beam joint:

• Lap length adequacy: The lap corbel length must be sufficient to ensure reliable anchorage of the beam longitudinal reinforcement under seismic loading. The minimum lap length should be determined based on the combined bending and shear demand, considering the cyclic loading conditions and the potential for strain accumulation.
• Corbel design: The corbel flange and web dimensions must be designed to resist the forces transferred through the lap connection. The corbel end section, identified as the critical region for strain concentration, requires particular attention in the design.
• Joint confinement: Adequate confinement reinforcement at the joint region is necessary to maintain the integrity of the concrete and ensure the ductile behavior of the joint. The confinement reinforcement should be designed to prevent concrete spalling and reinforcement buckling under cyclic loading.
• Beam-column stiffness ratio: The relative stiffness of the beam and column influences the location of the plastic hinge and the joint's deformation capacity. The design should ensure that the beam is sufficiently flexible to form the plastic hinge before the joint or column reaches its capacity.
• Ductility requirement: The joint must provide adequate ductility to accommodate the inelastic deformations expected under seismic loading. The design should consider the required ductility ratio and ensure that the joint can achieve this without premature failure.

Integration with Engineering Practice

The lap-type joint offers practical advantages for structural engineering applications involving SRC columns and RC beams:

1. Constructability: The lap connection simplifies the construction sequence by allowing the beam to be connected to the column through a relatively straightforward lap splice, without requiring complex embedded connections or specialized welding at the column end.
2. Material efficiency: The joint design optimizes the use of both steel and concrete materials, leveraging the high strength of the steel tube column and the ductility of the RC beam.
3. Seismic retrofitting: The lap-type joint concept can be adapted for retrofitting existing SRC columns with RC beam connections, providing a practical solution for seismic upgrading of existing structures.
4. Design standardization: The validated finite element model and experimental data provide a basis for developing design guidelines and standard details for the lap-type joint, facilitating consistent and reliable design practice.

The research also highlights the importance of considering the interaction between the joint and the connected members in seismic design. The joint's performance is influenced by the stiffness and strength of both the SRC column and the RC beam, and the design must ensure that the joint performs adequately under the expected loading conditions.

Study Insights and Reflections

This research contributes to the growing body of knowledge on steel-concrete composite structural connections, which are increasingly important in modern structural engineering due to the advantages of composite construction. The lap-type joint represents a practical solution to the challenge of connecting dissimilar structural members while maintaining seismic ductility. The successful demonstration of the "strong joint, weak member" behavior validates the joint design concept and provides confidence for its application in seismic design.

The identification of the lap corbel end section as the critical region for strain concentration provides valuable insight for design optimization. This finding suggests that the corbel design should focus on ensuring adequate capacity at the end section, potentially through increased reinforcement, larger cross-section dimensions, or improved concrete confinement at this location.

The research also underscores the importance of experimental validation in structural engineering research. The low-cycle reversed loading tests provide essential data for validating the finite element model and confirming the joint's seismic performance. This experimental validation is critical for building confidence in the design concept and providing reliable data for design code development.

For practitioners involved in the design of composite structures, this paper offers a well-substantiated joint design concept that balances constructability, material efficiency, and seismic performance. The validated finite element model and experimental data provide a solid foundation for further design optimization and code development, contributing to the advancement of composite structural engineering practice.