Experimental Study on New Joints for Steel Tube Concrete Trusses

Literature Overview

This paper by Cao Baozhu, Zhou Xuhong, and Liu Yongjian (Chang'an University, 2006) presents a comparative experimental study of 12 specimens: 6 hollow steel tube gusset plate joints and 6 CFT gusset plate joints. The research investigates the load-bearing capacity, local buckling behavior, and ductility of both joint types under concentrated loading at the gusset plate. The findings demonstrate that CFT gusset plate joints significantly outperform hollow tube joints in terms of capacity, with better ductility and reduced local buckling.

Core Technical Viewpoints

Joint Configuration

The gusset plate joint (also known as plate-type joint or node plate joint) consists of:

1. Gusset plate: A flat steel plate that connects multiple tube members at a node
2. Tube members: Circular hollow steel tubes or CFT tubes connected to the gusset plate via welds or bolts
3. Connection details: Welded or bolted connections between tube members and the gusset plate

The advantage of gusset plate joints over chord-cut (K-joint or T-joint) configurations is:

• Simpler fabrication (no complex tube intersections)
• Easier inspection of welds
• More flexible connection arrangements
• Better suited for large-diameter tubes where chord-cut joints become impractical

Experimental Results Summary

Failure Mechanism Analysis

The failure mechanisms differ fundamentally between the two joint types:

Hollow tube joints:

• Local buckling initiates at the tube surface directly beneath the gusset plate
• Buckling propagates rapidly, leading to sudden loss of load capacity
• The failure is brittle in nature with limited warning
• The hollow tube provides no confinement to the compressed region

CFT joints:

• The concrete core provides lateral support to the steel tube
• Local buckling is suppressed by the concrete confinement
• The steel tube yields before buckling, providing ductile behavior
• The concrete core carries compressive forces directly, reducing demands on the steel tube
• Failure occurs through gradual yielding and mild buckling, with significant energy absorption

Process and Standards Analysis

Fabrication Requirements

The fabrication of CFT gusset plate joints involves several critical steps:

1. Tube preparation:

• Cut tubes to length with square ends
• Clean internal surface (shot blast to Sa 2.5) for concrete bond
• Install end seals or plugs for concrete containment

1. Gusset plate fabrication:

• Plate cutting (plasma or oxy-fuel) to shape
• Drilling of bolt holes (if bolted connection)
• Surface preparation for welding (grind to bare metal)

1. Welding operations:

• Connect tube members to gusset plate via fillet or groove welds
• Weld size determined by design load and tube wall thickness
• All welds require visual and magnetic particle inspection
• Critical welds require UT inspection

1. Concrete filling:

• Mix concrete with appropriate workability for tube filling
• Pump or pour concrete into tubes with vibration
• Monitor fill level and ensure complete filling
• Allow concrete to cure before loading

Weld Quality Requirements

Concrete Specifications

Connection with Engineering Practice

Application in Truss Structures

The research findings have direct implications for the design of CFT truss structures:

1. Roof structures: Large-span roofs for stadiums, airports, and exhibition halls benefit from CFT trusses with gusset plate joints due to the superior capacity and ductility.
2. Bridge structures: Truss bridges with CFT members and gusset plate joints provide lightweight, high-capacity solutions for medium-span applications.
3. Industrial structures: Heavy industrial buildings with dynamic loads benefit from the energy dissipation capacity of CFT gusset plate joints.
4. Seismic applications: The ductility and energy absorption of CFT joints make them suitable for structures in high-seismic zones.

Design Recommendations

Based on the experimental results, the following design recommendations emerge:

1. Capacity design: Design gusset plate joints for 1.3-1.6 times the nominal capacity of hollow tube joints to account for the composite action
2. Ductility provisions: Ensure that the gusset plate itself is ductile (adequate thickness, material ductility) to prevent brittle plate failure
3. Concrete quality control: Implement rigorous quality control for concrete filling to ensure complete fill and adequate bond
4. Weld inspection: Mandate 100% UT inspection of all tube-to-gusset plate welds
5. Detail design: Provide adequate clear distance between the gusset plate edge and the tube end to prevent stress concentration

Comparison with Chord-Cut Joints

Key Questions and Reflections

The research raises the question of why gusset plate joints have not been more widely adopted despite their advantages. Several factors may explain this:

1. Design standards: Most existing design standards (such as GB 50017, JGJ 4) provide detailed guidance for chord-cut joints but limited guidance for gusset plate joints
2. Historical practice: Chord-cut joints have a longer history in steel tube structures, creating path dependence in design practice
3. Material efficiency: Chord-cut joints use less material for the same capacity, which is economically attractive
4. Aesthetic considerations: Chord-cut joints provide a cleaner appearance for architectural applications

However, for CFT applications specifically, gusset plate joints offer distinct advantages:

• The concrete fill cannot extend into complex chord-cut intersections
• Weld inspection is feasible for gusset plate joints but challenging for chord-cut joints
• The fabrication process is simpler and more quality-controllable

Study Insights and Implications

This research by Chang'an University provides compelling experimental evidence that CFT gusset plate joints are a superior solution for truss structures compared to hollow tube joints. The 30-60% capacity improvement and significantly enhanced ductility make CFT gusset plate joints particularly attractive for applications where structural reliability and energy dissipation are critical.

The research methodology of direct comparison testing (hollow vs. CFT under identical conditions) provides clear, unambiguous evidence of the benefits of CFT technology. This approach is valuable for convincing practicing engineers and code committees of the advantages of CFT joints.

For steel pipe manufacturers, this research highlights the growing market for CFT structural applications and the importance of providing tubes with:

• Excellent internal surface quality for concrete bond
• Precise dimensional tolerances for gusset plate alignment
• High-quality welds that can withstand the additional stresses of composite action
• Consistent material properties throughout the production run

The findings also suggest opportunities for standardization — developing design provisions for CFT gusset plate joints that capture the experimental findings in practical design formulas would facilitate wider adoption of this technology.