Seismic Performance of Double CFST External Stiffening Ring Plate Joints

Literature Overview

This study by Zhang Yufen et al. (2019) from Hebei University of Technology and North China University of Technology investigates the seismic performance of external stiffening ring plate joints connecting double CFST (concrete-filled steel tube) columns with H-section steel beams. The research is supported by the National Natural Science Foundation of China (51478004) and was published in the journal "Industrial Construction." The study conducted low-cycle reverse loading tests on 9 double CFST external ring plate joint specimens and 1 single CFST comparison joint specimen, varying the axial compression ratio, external ring plate width, beam-to-column line stiffness ratio, and the presence of ribs on the anchorage web plate.

Joint Configuration and Test Parameters

The external stiffening ring plate joint is a widely used connection type for CFST structures because of its simple construction and clear load transfer path. In this study, the joint connects a double CFST column (which provides increased axial capacity) with an H-section steel beam. The external ring plate is welded to the outer surface of the column tube and provides a bearing surface for the beam connection.

The test specimens were designed with the following variable parameters:

The low-cycle reverse loading test simulates the cyclic loading that joints experience during earthquakes. The loading was applied to the beam end while the column was subjected to a constant axial compression. The test monitored the load-displacement response, hysteresis curves, skeleton curves, ductility, energy dissipation capacity, and strength and stiffness degradation.

Failure Modes and Performance Characteristics

The study identified two primary failure modes depending on the beam-to-column line stiffness ratio. When the beam-to-column line stiffness ratio is small, the joint tends to develop a flexural failure mode at the beam end, which is the desirable failure mode because it provides good ductility and energy dissipation. When the beam-to-column line stiffness ratio is large, the joint tends to develop a flexural-compressive failure mode at the column end, which is less desirable because it can lead to a more brittle failure.

The double CFST external ring plate joint exhibited uniform, continuous, and stable strength and stiffness degradation under cyclic loading. This is a significant advantage over the single CFST joint, which showed more abrupt degradation. The double CFST joint demonstrated good energy dissipation capacity and deformation capacity, indicating satisfactory seismic performance.

Detailed Performance Analysis

The axial compression ratio has a notable effect on the initial stiffness of the joint. As the axial compression ratio increases, the initial stiffness of the joint increases because the compressive stress in the concrete core enhances the bearing capacity of the joint. However, a higher axial compression ratio may also reduce the ductility of the joint because the concrete core becomes more brittle.

The external ring plate width has a significant effect on the ductility of the joint. A wider ring plate provides a larger bearing area and allows for greater deformation before failure. The study showed that increasing the ring plate width from 100 mm to 200 mm significantly improved the ductility of the joint.

The anchorage web plate with ribs showed a higher load capacity than the plate without ribs. The ribs provide additional bending resistance and prevent local buckling of the web plate, which improves the overall strength of the joint.

Engineering Practice Implications

For the design of double CFST external ring plate joints in seismic regions, the following guidelines can be derived from the study. First, the beam-to-column line stiffness ratio should be kept below 1.0 to ensure that the beam end yields before the column end, providing the desired ductile failure mode. Second, the axial compression ratio should be limited to below 0.5 to maintain adequate ductility while providing sufficient initial stiffness. Third, the external ring plate width should be designed to be at least 1.5 times the beam flange width to ensure adequate bearing capacity and ductility. Fourth, the anchorage web plate should be provided with ribs to increase the load capacity and prevent local buckling.

Key Questions and Reflections

The study provides valuable experimental data on the seismic performance of double CFST external ring plate joints, but several questions remain. First, the test specimens are relatively small-scale, and the behavior of full-scale joints may differ due to size effects. Second, the study does not consider the effect of cyclic loading on the concrete core inside the column, which may degrade over time. Third, the study does not evaluate the fatigue performance of the joint under repeated loading, which is important for structures subjected to frequent seismic events. Future research should address these limitations through full-scale testing, fatigue testing, and numerical simulation.

Study Insights and Outlook

This research makes a significant contribution to the understanding of the seismic performance of double CFST external stiffening ring plate joints. The systematic evaluation of design parameters provides practical guidelines for the design of these joints in seismic regions. The finding that the double CFST joint exhibits more uniform and stable degradation than the single CFST joint is particularly important because it indicates that the double CFST configuration provides better seismic resilience. The study should be used as a reference for the design of CFST structures in seismic regions, but engineers should be aware of the specific test conditions and limitations when applying the results to their own projects. The findings support the use of double CFST columns with external ring plate joints as a viable seismic design solution for industrial and civil structures.