Skeleton Curve Determination Method for Internal Diaphragm Joints of Square Steel Tube Concrete Columns

Literature Overview

This paper by Nie Jianguo and Qin Kai, published in the China Civil Engineering Journal (Volume 41, Issue 4, 2008, pp. 1-7), presents a study on the skeleton curve determination method for internal diaphragm joints of square steel tube concrete (SRC) columns. Based on low-cycle reverse loading tests, the moment-story drift angle skeleton curve is analyzed, and a three-segment linear model is proposed. The model decomposes the joint story drift angle into three components: beam deformation, column deformation, and joint deformation, each calculated separately. The joint deformation is calculated using an improved component method.

Core Technical Points

The proposed three-segment linear model divides the skeleton curve into three regions:

The improved component method for joint deformation considers:

• Steel tube bending: Flexural deformation of the steel tube at the joint
• Concrete compression: Shortening of the concrete core under compression
• Shear deformation: Shear distortion of the joint panel zone
• Interface slip: Slip at the steel-concrete interface

Technical Interpretation from a Steel Pipe and Welding Perspective

Joint Configuration and Steel Tube Details

The internal diaphragm joint is a common connection type for SRC columns, where an internal steel diaphragm is welded to the inside of the square steel tube column. The beam is then connected to this diaphragm by welding or bolting.

Welding Details and Quality Requirements

The welding of the internal diaphragm to the column steel tube is critical for joint performance:

• Diaphragm-to-tube welds: Full-penetration groove welds on all four sides of the diaphragm, inspected by UT or RT.
• Beam-to-diaphragm welds: Full-penetration or fillet welds, depending on design.
• Weld quality: 100% NDT inspection required, with acceptance criteria per GB/T 11345 (UT) or GB/T 3323 (RT).

The welding procedure must be qualified per GB/T 9942, with attention to:

• Pre-heat temperature (≥ 100°C for Q345B with thickness > 20 mm)
• Interpass temperature (≤ 250°C)
• Post-weld heat treatment (PWHT) for thick sections
• Residual stress control to prevent distortion

Skeleton Curve Model and Engineering Application

The three-segment linear model provides a simplified approach for predicting the moment-story drift angle behavior of SRC joints. This model is suitable for:

• Pushover analysis: Determining the capacity curve for performance-based seismic design.
• Nonlinear time-history analysis: Providing input for component-level analysis.
• Capacity design: Ensuring that plastic hinges form in beams rather than joints.

The model's accuracy in the elastic stage (deviation < 10%) makes it suitable for serviceability limit state checks, while the underestimation of ultimate moment (20-30% deviation) suggests that the model is conservative for ultimate limit state design.

Standards and Design Codes

The design of SRC joints should follow the capacity design principle, ensuring that the joint has sufficient strength and ductility to accommodate plastic hinge formation in beams. The internal diaphragm joint provides a stiff and strong connection, but the weld quality is critical for seismic performance.

Engineering Practice Implications

The proposed skeleton curve model has several implications for engineering practice:

• Pushover analysis: The three-segment linear model can be used to generate the component capacity curve for pushover analysis of SRC frames.
• Weld quality control: The joint performance depends heavily on weld quality. All critical welds should be inspected by 100% NDT, with acceptance criteria per GB/T 11345 or GB/T 3323.
• Capacity design: The joint should be designed to be stronger than the connected members, ensuring that plastic hinges form in beams rather than joints.
• Ductility assessment: The model can be used to estimate the joint ductility, which is critical for seismic design.

Key Questions and Reflections

The study does not address the effect of welding defects on joint performance. In practice, weld defects can significantly reduce the joint strength and ductility. Future research should investigate the effect of simulated weld defects on the skeleton curve behavior of SRC joints.

Another question is the effect of concrete strength on joint performance. The study uses normal-strength concrete, but ultra-high strength concrete may be used in practice. The confinement efficiency and joint behavior with ultra-high strength concrete require further investigation.

Study Insights and Conclusions

This research provides a practical method for predicting the moment-story drift angle behavior of internal diaphragm joints of SRC columns. The three-segment linear model, with its decomposition of deformation components, offers a clear physical interpretation and is suitable for engineering applications. From a steel pipe and welding quality perspective, the key requirements are proper steel tube selection, qualified welding procedures, thorough non-destructive testing, and capacity design principles. Engineers should ensure that the design, fabrication, and construction processes meet the relevant standards and specifications, with particular attention to weld quality and concrete placement control. The long-term seismic performance of SRC joints depends on proper maintenance and inspection practices throughout the service life, especially for weld joints and steel tube surfaces.