Experimental Study on Seismic Performance of Internal Partition Joint in Square Steel Tube Concrete Frames

Literature Overview

This paper, authored by Wang Lai, Wang Tiecheng, and Deng Peng from Tianjin University and Shandong University of Science and Technology, was published in the journal Earthquake Engineering and Engineering Dynamics, Volume 25, Issue 1, 2005, pages 76–80. The research is supported by the Tianjin Municipal Commission of Housing and Urban-Rural Development Major Science and Technology Project (No. 01-2-073). The study designs a three-story, two-bay square steel tube concrete (CFT) frame model with internal partition joints at beam-column connections, and conducts pseudo-static cyclic loading tests to evaluate the seismic performance of these joints.

Core Technical Content and Experimental Methodology

The experimental program involved fabricating a scaled model of a three-story, two-bay square CFT frame. The beam-column joints employ an internal partition (diaphragm) connection type, which is a widely used structural system for square and rectangular steel tube concrete frames. In this joint configuration, a steel plate diaphragm is inserted inside the square steel tube at the connection location, and the beam is welded or bolted to this internal plate, effectively transferring forces from the beam to the column through the diaphragm rather than through the column tube walls directly.

The pseudo-static test method was employed, applying low-cycle repeated horizontal loads to the frame model. The loading protocol typically follows a displacement-controlled pattern, with increasing displacement amplitudes corresponding to different damage levels. The test monitored the following performance indicators:

The test results demonstrate that the internal partition joint configuration exhibits good seismic performance, with satisfactory ductility, stable hysteretic loops, and a progressive damage mechanism that avoids brittle failure.

Technical Analysis of Internal Partition Joint Behavior

The internal partition joint is a critical structural element in square CFT frames because it addresses the inherent challenge of connecting beams to square steel tubes. Unlike round steel tubes, which can be connected using sleeve-type or corbel-type connections, square tubes require internal connection plates to effectively transfer beam forces into the column cross-section.

The internal partition plate serves several structural functions: it provides a large bearing area for beam end connections, it distributes forces uniformly across the column cross-section, and it prevents local buckling of the column tube walls under concentrated beam forces. The welding connections between the internal partition plate and the square tube walls are critical stress concentrations and potential weak links.

From a welding engineering perspective, the fabrication of internal partition joints requires careful attention to several aspects. The partition plate is typically inserted into the square tube through a pre-cut opening, requiring precise dimensional tolerances. The welds connecting the partition plate to the tube walls are full-penetration groove welds that must achieve complete fusion and adequate toughness. The welding sequence is important to minimize distortion of the square tube cross-section, which could affect the alignment of beam connections.

Engineering Practice Implications

The experimental results provide valuable validation data for the design of square CFT frame joints. The demonstration of good ductility and progressive damage mechanisms gives engineers confidence in using internal partition joints for seismic-resistant design. The test data can be used to calibrate finite element models and to develop simplified design equations for internal partition joints.

For welding engineers, the study highlights the importance of weld quality in joint seismic performance. The cyclic loading applied during the test subjects the welds to repeated stress reversals, which can lead to fatigue cracking if the welds contain defects such as lack of fusion, porosity, or excessive residual stress. The welding procedure specification (WPS) for internal partition joints should therefore include requirements for weld preheating, interpass temperature control, and post-weld heat treatment to minimize residual stress and ensure adequate HAZ toughness.

Key Questions and Reflections

While the study provides valuable experimental data, several aspects could be further investigated. First, the test does not address the effect of concrete fill quality on joint performance. Variations in concrete strength, compaction, and air void content can significantly affect the interaction between the internal partition plate and the concrete core. Second, the study does not examine the effect of welding residual stress on the cyclic behavior of the joint. Residual stresses from the fabrication process can accelerate crack initiation and propagation under cyclic loading, potentially reducing the effective ductility of the joint.

Additionally, the study does not discuss the practical challenges of inserting the internal partition plate into the square tube. In large-scale construction, this operation requires careful planning to ensure that the plate is positioned accurately and that the weld access is adequate for full inspection. The use of pre-fabricated joint assemblies or advanced welding technologies such as positional welding with automatic wire feeding could improve both quality and productivity.

Study Insights and Implications

This research contributes important experimental evidence supporting the use of internal partition joints in square CFT frames for seismic-resistant construction. The demonstration of good ductility and progressive damage mechanisms provides a basis for code provisions and design guidelines. For welding engineers and structural engineers, the key insight is that the seismic performance of CFT frame joints is governed by the combined behavior of the steel tube, concrete core, internal partition plate, and all connecting welds. A holistic approach to joint design and fabrication, considering both structural mechanics and welding metallurgy, is essential for achieving the desired seismic performance. The study exemplifies the value of experimental research in validating analytical models and informing practical design decisions.