Initial Rotational Stiffness of End-Plate Bolted Connections Between H-Beams and Rectangular Steel Tube Columns

Overview and Research Significance

The paper by Li, Duan, Lu, and Zhang from Tongji University, published in the Journal of Tongji University (Natural Science) in 2018, addresses a critical yet often underappreciated aspect of steel structure design: the initial rotational stiffness of end-plate bolted connections between H-beams and rectangular hollow section (RHS) columns. Funded by the National Key R&D Program of China (2016YFC0701200), this research is particularly significant given the increasing adoption of RHS columns in modern structural systems, where their aesthetic appeal, torsional rigidity, and efficient use of material make them an attractive alternative to traditional wide-flange columns.

As a welding and metallurgical engineer with extensive experience in pipe fitting fabrication, I recognize that the performance of these connections is fundamentally dependent on the quality of the RHS column manufacturing, the welding integrity of the connection details, and the mechanical properties of the connection components. The initial rotational stiffness directly affects the overall structural behavior, including natural frequencies, lateral deflections, and the distribution of internal forces throughout the frame.

Experimental Program and Component Method

The study conducted tests on six groups of end-plate bolted connections, covering both flush (flush-type) and extended (outstanding-type) end-plate configurations. The component method, a well-established approach in connection design based on the European standard EN 1993-1-8, was employed to decompose the complex connection behavior into individual component actions.

The component method identifies five distinct stiffness contributions that collectively determine the overall connection rotational stiffness. Each component represents a specific deformation mechanism within the connection, and their individual stiffnesses are combined through series and parallel spring models to obtain the total connection stiffness.

Derivation of Component Stiffness Formulas

Bolt Tensile Stiffness

The tensile stiffness of the bolts represents the resistance of the bolt group to elongation under the tensile force induced by the applied moment. The stiffness depends on the bolt diameter, the number of bolts, the bolt material grade, and the effective bolt length. For high-strength bolts conforming to GB/T 1228 or ASTM A325, the tensile stiffness is typically in the range of 500 to 800 kN/mm per bolt, depending on the specific geometry and preload conditions.

Tension End-Plate Bending Stiffness

The bending stiffness of the end-plate in tension is governed by the flexural rigidity of the end-plate material acting as a cantilever beam between the bolt lines and the beam flange. This stiffness is highly sensitive to the end-plate thickness and the geometry of the bolt pattern. Thicker end-plates provide higher stiffness but may introduce manufacturing challenges in terms of plate flatness and bolt hole alignment.

Column Wall Tensile Stiffness

The tensile stiffness of the column wall (the RHS tube wall behind the end-plate) is a critical component that is often neglected in preliminary design but becomes governing in detailed analysis. The RHS column wall acts as a deep beam resisting the tensile force transmitted through the bolts. The stiffness depends on the wall thickness, the column depth, and the distance between the bolt line and the column flange.

Column Wall Compressive Stiffness

The compressive stiffness of the column wall represents the resistance of the RHS tube wall to local deformation (bearing) under the compressive force from the beam flange. This is essentially a bearing stiffness problem, and it is influenced by the wall thickness, the end-plate thickness, and the column material properties.

Compressive Extended End-Plate Stiffness

For extended end-plate connections, the portion of the end-plate extending beyond the beam flange provides additional compressive resistance through bending. This component is unique to extended configurations and contributes significantly to the overall stiffness of such connections.

Comparison of Flush and Extended Configurations

The study provides valuable comparative data between flush and extended end-plate configurations. The following table summarizes the key differences:

The extended end-plate configuration generally provides higher initial rotational stiffness due to the additional bending resistance offered by the extended plate portion. However, this comes at the cost of increased material usage and manufacturing complexity. The choice between configurations should be guided by the specific moment demand, available fabrication capacity, and economic considerations.

Engineering Practice Implications

Welding Considerations for Connection Fabrication

The fabrication of end-plate bolted connections involves several critical welding operations that directly affect the connection performance:

• End-plate to column weld: This weld transfers the reaction forces from the connection to the column. It should be designed as a full-penetration groove weld with complete radiographic examination. The heat-affected zone (HAZ) of this weld can be a critical region for fatigue initiation under cyclic loading.
• End-plate to beam weld: This weld must accommodate the full beam end reaction. For thick end-plates, multi-pass welding with appropriate interpass temperature control is essential to prevent cracking in the HAZ.
• Bolt hole preparation: The accuracy of bolt hole drilling or machining directly affects the preload distribution among bolts. Misaligned holes can lead to uneven bolt loading and reduced connection stiffness.

Material Selection and Heat Treatment

The material properties of the connection components significantly influence the rotational stiffness:

• End-plate steel: Q345B or S355JR is commonly specified, providing a good balance of strength and weldability. For higher-strength applications, Q390 or S420 may be used but require careful welding procedure qualification.
• Bolt grade: 10.9 grade bolts (equivalent to ASTM A325) are standard for structural connections. The bolt pretension force should be controlled within ±10% of the specified value to ensure consistent stiffness.
• Column material: The RHS column material should have adequate toughness to withstand the bearing stresses at the connection. Charpy V-notch impact testing at the service temperature is recommended.

Quality Control and Inspection

The following quality control measures are essential for ensuring the designed connection stiffness is achieved in practice:

Study Insights and Reflections

The component method employed in this study provides a rational and physically meaningful framework for understanding connection behavior. The decomposition of the overall connection stiffness into five distinct components allows engineers to identify the governing mechanism and optimize the design accordingly. For example, if the column wall tensile stiffness is the governing component, increasing the wall thickness of the RHS column may be more effective than increasing the end-plate thickness.

From a manufacturing perspective, the study highlights the importance of the RHS column wall thickness in connection performance. This has direct implications for pipe and tube manufacturing, where wall thickness uniformity is a critical quality parameter. Variations in wall thickness of even a few percent can lead to significant variations in connection stiffness, affecting the overall structural behavior.

The study also underscores the value of the component method as a design tool that bridges the gap between theoretical analysis and practical fabrication. By understanding the individual component contributions, engineers can make informed decisions about material selection, geometric proportions, and fabrication tolerances that collectively ensure the connection performs as intended.

In conclusion, this research by Li et al. provides a rigorous analytical framework for predicting the initial rotational stiffness of end-plate bolted connections between H-beams and RHS columns. The derived component stiffness formulas, validated against experimental data, offer practical tools for engineers to design connections with predictable stiffness characteristics. The emphasis on the component method as a systematic design approach is particularly valuable for the growing use of RHS columns in modern structural systems, where connection performance is critical to overall structural efficiency and safety.