Hysteretic Performance of End-Plate Bolted Connections Between Rectangular Steel Tube Columns and H-Beams

Literature Overview

This paper by Yang Xiaojie, Zhang Long, Li Guoqiang, Liu Yushu, Lu Ye, and Sun Jianyun, published in Progress in Steel Building Structures (2013, Vol. 15, No. 4, pp. 16-23), investigates the hysteretic behavior of end-plate bolted connections between rectangular hollow section (RHS) columns and H-beams. The research was supported by the National Science and Technology Support Program (2012BAJBB02) and the National Natural Science Foundation of China (Grant No. 51108340). The study addresses a significant gap in the structural engineering literature, as most connection research has focused on H-beam to H-column connections, while RHS columns are increasingly used in modern steel structures due to their efficient closed cross-section.

Technical Background and Connection Design

Rectangular steel tube columns present unique challenges for connection design because their closed cross-section prevents the use of conventional bolted end-plate connections that rely on access to the column flanges. The authors propose a through-bolted connection as a practical solution, in which bolts pass through the column wall to connect the beam end plate to the column.

The connection was studied through a combination of theoretical modeling, experimental testing, and finite element analysis, providing a comprehensive understanding of its hysteretic behavior under cyclic loading.

Hysteretic Behavior and Restoring Force Model

The experimental hysteretic curves revealed the characteristic behavior of bolted connections under cyclic loading:

• Initial loading: The connection behaves elastically until the first bolt begins to slip or the end plate begins to yield.
• Post-slip behavior: After bolt slip, the connection exhibits a stable hysteretic loop with a characteristic flag-shaped or bilinear response.
• Cyclic degradation: With increasing number of cycles, the hysteretic loops show progressive degradation, indicating accumulating damage in the connection components.

Based on the experimental hysteretic curves, the authors proposed a restoring force model for the connection. This model is of practical value for seismic design of steel frame structures, as it can be incorporated into nonlinear time-history analysis to predict the seismic response of frames with RHS column connections.

Welding Quality and Connection Performance

From a welding and fabrication perspective, the end-plate bolted connection involves several critical welds:

1. Beam-to-end plate welds: These are typically full-penetration groove welds that must be designed and inspected to full-quality standards. The welds are subjected to complex stress states including direct shear, transverse shear, and bending moments from the beam.
2. Column wall preparation: The through-bolt holes in the column wall create stress concentrations that can initiate fatigue cracks under cyclic loading. The hole edges should be deburred and the surrounding material should be inspected for any existing defects.
3. Weld residual stress interaction: The residual stresses from the beam-to-end plate welds interact with the bolt preload and the cyclic loading. This interaction can reduce the effective tensile capacity of the connection and accelerate fatigue damage at the weld toes.

The welding procedure for the end plate welds should be qualified in accordance with ASME Section IX or GB/T 19866, with attention to:

• Preheat temperature to prevent cold cracking in high-strength steels
• Interpass temperature control to limit grain growth in the HAZ
• Post-weld heat treatment for thick sections to relieve residual stresses

Non-Destructive Testing Requirements

Given the critical role of the connection in seismic load paths, comprehensive NDT of all welds is essential:

Engineering Practice and Seismic Design Implications

The proposed restoring force model provides a practical tool for seismic analysis of steel frames with RHS column connections. Engineers should note the following:

• The hysteretic behavior of the connection must be considered in the overall frame behavior. Connections that are too rigid may concentrate damage in the column, while connections that are too flexible may not provide adequate lateral resistance.
• The bolt preload is a critical design parameter that affects the slip capacity and the post-slip stiffness of the connection. Bolt tension should be verified after installation and periodically during service.
• The connection should be designed to fail in a ductile manner, with yielding occurring in the beam web or flange rather than in the bolted connection itself. This ensures that the connection remains functional even after significant inelastic deformation.

Study Insights and Conclusions

This study addresses an important and under-researched topic in steel structure engineering: the seismic performance of connections between H-beams and rectangular steel tube columns. The combination of theoretical analysis, experimental testing, and finite element modeling provides a comprehensive understanding of the connection behavior. The proposed restoring force model is a practical tool for seismic design, and the experimental data provide a valuable benchmark for validating analytical and numerical models. For engineers practicing in steel structure design, this study highlights the importance of connection design in the overall seismic performance of structures with non-conventional column sections. The welding quality of the end plate connections is a critical factor in ensuring the designed connection performance, and rigorous welding procedure qualification and NDT inspection protocols are essential.