Shear Capacity of Double Steel Tube Concrete External Rib Ring Plate Joints

Literature Overview

This paper by Zhang Yufen, Wang Fu, and Zhu Ge from North China University of Technology investigates the shear bearing capacity of double steel tube concrete (DSTC) external rib ring plate joints. The study builds upon existing theoretical and experimental research to analyze the shear behavior of the joint core zone. A shear force model is established for the joint core, dividing the shear contribution into three components: the shear contribution of the inner and outer steel tube webs, the shear contribution of the vertical ribs and anchor webs of the main joint connectors, and the shear contribution of the concrete in the joint zone. Formulas for the yielding and ultimate shear bearing capacity are derived, and a new indicator, the shear capacity reserve coefficient, is proposed to quantify the safety reserve of the joint after beam hinge formation. The research is funded by the National Natural Science Foundation of China (51478004) and published in the World Earthquake Engineering journal in 2018.

Core Technical Findings

Shear Force Model Development

The shear force model developed in this research provides a comprehensive framework for calculating the shear capacity of DSTC external rib ring plate joints. The model divides the total shear capacity into three distinct components, each contributing to the overall shear resistance of the joint core zone.

The total shear capacity is the sum of these three components, with each component calculated using appropriate material models and geometric parameters.

Yielding and Ultimate Shear Capacity

The study derives formulas for both the yielding shear capacity and the ultimate shear capacity of the joint. The yielding shear capacity represents the point at which the joint begins to exhibit plastic deformation, while the ultimate shear capacity represents the maximum shear resistance before failure.

The yielding shear capacity is primarily governed by the steel components, as the steel tubes and connecting ribs yield before the concrete reaches its ultimate strength. The ultimate shear capacity includes the full contribution of all components, including the concrete's compressive and aggregate interlock resistance.

Shear Capacity Reserve Coefficient

A key innovation of this research is the introduction of the shear capacity reserve coefficient, which quantifies the safety reserve of the joint after beam hinge formation. This coefficient is defined as the ratio of the joint's ultimate shear capacity to the shear demand after the beam has formed a plastic hinge.

The research demonstrates that the DSTC external rib ring plate joints have sufficient shear capacity reserve, ensuring that the joint maintains adequate shear resistance even after the beam has yielded and formed a plastic hinge. This confirms that the joint design satisfies the "strong shear, weak bending" seismic design principle, which is essential for ductile seismic performance.

Engineering Practice Implications

Design Methodology

The research provides a practical design methodology for DSTC external rib ring plate joints, which can be applied in engineering practice as follows:

1. Shear Capacity Calculation: The total shear capacity should be calculated as the sum of the three components identified in the shear force model. Each component should be calculated using the derived formulas, with appropriate safety factors applied.
2. Shear Demand Estimation: The shear demand on the joint should be estimated based on the seismic design forces, considering the formation of plastic hinges in the beams. The design should ensure that the joint shear capacity exceeds the shear demand by a sufficient margin.
3. Shear Capacity Reserve Verification: The shear capacity reserve coefficient should be calculated to verify that the joint has adequate safety reserve after beam hinge formation. A minimum reserve coefficient of 1.2 is recommended to ensure ductile seismic performance.
4. Detailing Requirements: The design should include appropriate detailing requirements for the vertical ribs, anchor webs, and concrete confinement to ensure the calculated shear capacity is achieved in practice.

Design Recommendations

Based on the research findings, the following design recommendations are proposed for DSTC external rib ring plate joints:

• The vertical rib thickness should be at least equal to the thicker of the inner or outer steel tube wall thickness to ensure adequate shear contribution.
• The anchor web length should be sufficient to develop the full shear resistance of the connecting elements, with a minimum embedment length of 12 times the web thickness.
• The concrete in the joint zone should be confined with transverse reinforcement to enhance the concrete shear contribution and provide ductility.
• The welding quality of the vertical ribs and anchor webs should be carefully controlled, as weld defects can significantly reduce the shear capacity of the joint.

Key Technical Questions and Reflections

The study raises important questions about the design of DSTC joints in seismic regions. The traditional design approach often focuses on the bending capacity of the joint, neglecting the shear capacity verification. This research demonstrates that the shear capacity of the joint is a critical design consideration, and the "strong shear, weak bending" principle must be explicitly satisfied to ensure ductile seismic performance.

Another important consideration is the interaction between the three shear contribution components. The steel tube web shear, vertical rib shear, and concrete shear do not act independently, but rather interact in a complex manner that depends on the load level, deformation stage, and material properties. The shear force model developed in this research provides a practical framework for accounting for these interactions, but further research is needed to refine the model and improve its accuracy.

The research also highlights the importance of the shear capacity reserve coefficient as a design indicator. This coefficient provides a quantitative measure of the joint's safety reserve after beam hinge formation, which is critical for ensuring ductile seismic performance. The adoption of this coefficient in design codes would provide a more explicit and reliable method for verifying the seismic performance of DSTC joints.

Study Insights and Implications

This research provides valuable insights for the design of DSTC external rib ring plate joints in seismic regions. The findings demonstrate that the shear capacity of the joint is a critical design consideration, and the "strong shear, weak bending" principle must be explicitly satisfied to ensure ductile seismic performance. The shear force model developed in this research provides a practical framework for calculating the shear capacity of the joint, and the shear capacity reserve coefficient provides a quantitative measure of the joint's safety reserve.

For engineers involved in the design of DSTC structures, this research underscores the importance of shear capacity verification in joint design. The research also highlights the need for careful attention to the detailing of the vertical ribs, anchor webs, and concrete confinement, as these details directly affect the shear capacity and ductility of the joint.

Future research should investigate the cyclic shear behavior of DSTC joints, including the effects of load reversal, fatigue, and low-cycle fatigue on the shear capacity and ductility. The research should also explore the development of simplified design methods that account for the interaction between the three shear contribution components, making the design methodology more accessible to practicing engineers.