Stiffened T-Joints of High-Strength Square Steel Tube and Lightweight Aggregate Concrete Truss Structures

Literature Overview

This paper, published in Journal of Harbin Institute of Technology (2020, Vol. 52, No. 8, pp. 103-111) by Wang Wanzhen, Li Hua, Wu Xiaocong, and Ni Weikang from Ningbo University, presents experimental investigation of T-joints in high-strength square steel tube lightweight aggregate concrete (LWAC) truss structures. The research was supported by the National Natural Science Foundation (51878360), Zhejiang Provincial Basic Public Welfare Technology Research Program (LGF18E080007), and Ningbo Leading and Top Talent Project.

Core Technical Content

The study examines the influence of stiffener plate configurations and the width ratio between branch and chord members on the failure modes, load-bearing capacity, and stress-strain distribution in the connection zone of high-strength square steel tube LWAC truss T-joints.

Failure Modes Identification

The typical failure modes observed in the tested joints are:

Load-Bearing Capacity Comparison

The conventional joint's load-bearing capacity is governed by the branch member root and weld strength, together with the local compressive strength of the chord top flange. In contrast, the stiffened joint's capacity depends on the combined effect of: (1) the branch member root and weld strength including the stiffener diffusion effect, (2) the chord top flange diffusion bearing capacity, and (3) the stiffener plate buckling strength.

Effect of Width Ratio

The load-bearing capacity of stiffened joints increases with the increase of the branch-to-chord width ratio. This is attributed to the greater diffusion area provided by wider branch members relative to the chord, which allows more effective load transfer through the stiffener plates.

Engineering Practice Insights

This research has direct implications for the design and fabrication of square steel tube truss structures used in large-span roofs, sports facilities, and industrial buildings. The finding that stiffener plates can improve yield capacity by up to 40% and ultimate capacity by up to 48.3% is particularly valuable for engineers working with high-strength steel tubes (typically Q355, Q420, or Q460 grades) combined with lightweight aggregate concrete infill.

From a welding engineering perspective, the identification of weld fracture as a potential failure mode underscores the importance of weld quality control in T-joint fabrication. The welds between stiffener plates and branch members are particularly critical, as their failure can trigger premature joint collapse. Engineers should ensure that fillet welds at these locations meet the full-penetration or enhanced-throat-thickness requirements specified in GB 50017 or EN 1993-1-8.

The observation that stiffened joints do not exhibit a yield plateau—instead showing a gradual load increase—is significant for seismic design. While the absence of a clear yield point may complicate ductility assessment, the continuous load increase suggests progressive hardening behavior that can be beneficial in quasi-static loading scenarios.

Key Reflections and Implications

The research demonstrates that lightweight aggregate concrete, despite its lower density, can function effectively as infill material in high-strength square steel tube truss joints when properly supported by stiffener plates. This combination offers an attractive solution for weight-sensitive applications where high strength-to-weight ratio is paramount. Engineers should note that the diffusion effect of stiffener plates is a key mechanism that should be explicitly considered in finite element modeling of such joints, as neglecting it may lead to non-conservative capacity predictions.