Impact Resistance Performance of Bolted Connections of Square Steel Tube Columns Under Cyclic Loading

Literature Overview

This study investigates the seismic performance of bolted connections between square steel tube columns and beam members under cyclic loading conditions. Bolted connections are widely used in steel tube structures due to their ease of fabrication, field assembly, and replaceability. However, their behavior under cyclic loading differs significantly from welded connections, with potential issues including bolt relaxation, slip, and progressive joint deterioration. The research employs cyclic loading tests and finite element analysis to characterize the force-displacement hysteresis, energy dissipation capacity, and damage evolution of the connection.

Core Technical Points

The bolted connection under investigation consists of a square steel tube column connected to a beam through a bolted end plate or flange connection. The cyclic loading is applied to simulate seismic action, with displacement-controlled loading protocols following the ACI 318 or GB 50011 seismic design provisions.

The hysteresis behavior of the bolted connection exhibits a distinctive pattern: an initial elastic stage with linear stiffness, a slip stage where the bolts begin to loosen and the joint stiffness decreases, and a plastic stage where the end plate or column wall yields. The energy dissipation capacity is primarily derived from the plastic deformation of the end plate and the friction between bolted surfaces.

A critical finding is that the connection exhibits significant stiffness degradation after 3–5 cycles of loading at 2% interstory drift. The bolt preload loss accelerates with increasing displacement amplitude, leading to progressive slip and reduced load-carrying capacity. The square steel tube column wall may experience local buckling at the connection region, particularly when the wall thickness-to-width ratio exceeds the slenderness limit specified in GB 51248.

Process and Standards Analysis

The design and fabrication of bolted connections are governed by standards including GB 50017 (Standard for Design of Steel Structures), AISC 360 (Specification for Structural Steel Buildings), and EN 1993-1-8 (Design of Joints). These standards specify bolt grades, preload requirements, slip resistance design, and fatigue considerations.

The cyclic testing follows protocols defined in ASTM E468 or equivalent standards, with displacement-controlled loading at constant amplitude or progressive amplitude increments. The test setup includes load cells to measure the applied force and displacement transducers to capture the relative displacement at the joint. Strain gauges are installed on the bolt shanks, end plate, and column wall to monitor the stress distribution.

Integration with Engineering Practice

In seismic design, bolted connections are preferred over welded connections in regions with high seismicity due to their replaceability after damage and their superior fatigue performance. However, the cyclic performance of bolted connections requires careful attention to several design aspects:

1. Bolt preload must be applied and verified using calibrated tensioning tools, with a typical preload of 70–75% of the bolt tensile strength.
2. The end plate thickness should be designed to yield before the bolt fails, ensuring ductile behavior.
3. The column wall thickness at the connection region should be increased or stiffened to prevent local buckling.
4. Anti-slip measures (roughened surfaces, bolt washers) should be considered to maintain friction resistance during cyclic loading.

The inspection and maintenance of bolted connections after seismic events is critical. Bolt elongation, end plate deformation, and column wall damage should be assessed using visual inspection and non-destructive testing. Damaged bolts should be replaced, and the connection should be re-tensioned to the specified preload.

Key Questions and Reflections

The study highlights several areas requiring further investigation. The effect of bolt relaxation under sustained cyclic loading is not fully characterized, as the relaxation rate depends on the bolt material, surface condition, and preload level. The interaction between bolt slip and column wall buckling under combined axial and cyclic lateral loading is complex and warrants parametric study. Additionally, the influence of connection geometry (end plate configuration, bolt arrangement, stiffener placement) on cyclic performance should be systematically explored.

From a quality control perspective, the bolted connection fabrication requires verification of bolt grade, preload application, and end plate flatness. Ultrasonic bolt tension testing and magnetic particle inspection of the end plate welds should be performed before assembly.

Study Insights and Implications

The research demonstrates that bolted connections of square steel tube columns can exhibit satisfactory cyclic performance when properly designed and fabricated. The key design parameters are bolt grade, end plate thickness, and column wall thickness, with the bolt grade being the most influential factor on energy dissipation capacity. The practical implication is that bolted connections should be designed with adequate ductility reserve and inspected regularly for bolt relaxation and joint damage. Future work should focus on developing simplified design equations for the cyclic capacity of bolted connections and investigating novel connection configurations that enhance energy dissipation without compromising constructability.