Failure Mechanism and Capacity of Variable-Section Square Steel Tube Lightweight Aggregate Concrete Column with Steel Box Beam Folded Plate Through-Node

Literature Overview

The paper by Ni Weikang, Wang Wanzhen, Jia Zhen, and Wu Xiaocong from Ningbo University and Zhongheng Zhuochuang International Engineering Design Co., Ltd. (Engineering Mechanics, Vol. 36, No. 10, 2019, pp. 122-133) presents a comprehensive study of the low-cycle cyclic loading behavior of a variable-section square steel tube lightweight aggregate concrete (LAC) edge column connected to a steel box beam through a folded plate through-node. The study examines the influence of the folded plate reinforcement configuration on failure mode, load capacity, plastic hinge rotation, and ductility. Blind hole method was used to measure welding residual stresses at critical locations in the node zone. Numerical simulations incorporating welding residual stresses, an ellipsoidal fracture model for structural steel, a coupled ellipsoidal yield model, and a quadratic failure surface model for lightweight aggregate concrete were performed to analyze the failure mechanism.

Core Technical Findings

The experimental and numerical results provide detailed insights into the failure mechanism of this complex composite node:

The folded plate reinforcement effectively redirects the failure mechanism from the column to the beam, which is the desired behavior for seismic design (strong column-weak beam principle). The residual stress distribution obtained by the blind hole method reveals significant tensile residual stresses at the weld seams connecting the folded plate to the column and beam, with values reaching up to 0.6-0.8 times the yield strength of the structural steel.

Welding Residual Stress Analysis

The blind hole method measurements provide critical data for understanding the residual stress state in the node zone:

These residual stresses have profound implications for the seismic performance of the node. The tensile residual stresses at weld seams can initiate micro-cracks under cyclic loading, leading to progressive fatigue damage. The compressive residual stresses in the column wall may slightly enhance the local buckling resistance but can also promote intergranular cracking in lightweight aggregate concrete under high confinement pressures.

Numerical Modeling Approach

The numerical model employed in this study is sophisticated and incorporates several advanced material models:

The inclusion of welding residual stresses in the numerical model is a significant advancement, as many previous studies have neglected this important factor. The results show that the residual stress field significantly affects the stress distribution and damage evolution in the node zone, particularly under cyclic loading conditions.

Welding Procedure and Quality Control

Given the critical role of welding in this complex node, the following welding and quality control measures are essential:

1. Welding sequence: The welding sequence should be optimized to minimize distortion and residual stresses. A symmetric welding pattern from the center outward is recommended for the folded plate connections.
2. Welding process selection: GTAW for root passes and FCAW for fill and cap passes is recommended for the folded plate to column and beam welds. The heat input should be controlled to prevent excessive HAZ softening.
3. Post-weld treatment: Stress relief heat treatment or mechanical stress relief (such as shot peening or vibration stress relief) should be considered for the node zone to reduce tensile residual stresses.
4. NDE requirements: 100% UT or RT examination of all full-penetration welds, 100% MT examination of all welds, and dimensional inspection of the folded plate geometry.
5. Welder qualification: Welders must be qualified on the specific joint configurations used in the node, including all-position welding on variable-thickness joints.

Lightweight Aggregate Concrete Considerations

The use of lightweight aggregate concrete (LAC) in the column introduces unique challenges compared to normal weight concrete:

The lower elastic modulus and Poisson's ratio of LAC result in reduced lateral expansion under axial compression, which in turn reduces the confining pressure exerted on the steel tube. This can be compensated by the folded plate reinforcement, which provides additional confinement and shear resistance.

Study Insights and Reflections

This study represents a significant contribution to the understanding of complex composite nodes involving variable-section steel tube LAC columns and steel box beams. The integration of experimental testing, residual stress measurement, and advanced numerical modeling provides a comprehensive picture of the failure mechanism. The finding that the folded plate reinforcement effectively improves seismic performance by redirecting the failure to the beam is highly valuable for practical design. However, the study is limited to a specific configuration, and the results may not be directly applicable to other node geometries or loading conditions. From a welding perspective, the high residual stresses measured at the weld seams highlight the critical need for careful welding procedure development, weld sequencing, and post-weld stress relief in the fabrication of such nodes. The long-term fatigue performance under cyclic loading remains a concern, and further research on the interaction between welding residual stresses, lightweight aggregate concrete properties, and cyclic loading is warranted. The practical implementation of such nodes requires close coordination between design engineers, fabrication shops, and welding engineers to ensure that the predicted performance is achieved in the as-built structure.