Zhuojin Pipe Fitting Co., LtdEffect of Concrete-Filled Rectangular Steel Tube Truss Members on Structural Performance
Literature Overview
Liu Junping, Liu Yongjian, and Huang Wenjin (2012) published a study in the Journal of Guangxi University (Natural Science Edition) on the influence of concrete filling in the main chords of rectangular steel tube trusses. The authors conducted experimental tests on rectangular steel tube trusses with and without concrete-filled main chords, and analysed the effects on joint behaviour, member capacity, stiffness, and failure mechanisms. The research was supported by the National Natural Science Foundation of China (Grant No. 51008079), the Fujian Provincial Department of Education Science and Technology Project (JA10047), and the Fuzhou University Talent Introduction Fund (XRC-149).
This work is relevant to engineers designing space structures, bridge trusses, and industrial buildings where rectangular steel tube trusses are used for their architectural appeal and efficient material usage.
Key Experimental Findings
The authors identified several important effects of concrete filling in the main chord tubes:
Joint Behaviour
- Concrete filling changes the failure mode of the joints. In unfilled tubes, the joint typically fails by local yielding and buckling of the tube wall at the chord-to-brace intersection. With concrete filling, the failure mode shifts to a more ductile mechanism involving concrete crushing and steel tube yielding.
- Despite the improved failure mode, the joint remains the weak link of the truss structure. This is a critical finding for designers, as it underscores the need for careful joint design and detailing.
Compressive Members
- Concrete filling significantly increases the load capacity and stiffness of compressive chord members. This is attributed to the composite action between the steel tube and the concrete core, which provides confinement and prevents premature local buckling.
- The improvement is most pronounced for members with higher slenderness ratios, where the unfilled tube would otherwise be susceptible to global buckling.
Tensile Members
- Concrete filling has minimal effect on the load capacity and stiffness of tensile chord members. This is expected, as the concrete core does not contribute significantly to tension resistance.
Brace Members
- Concrete filling has a moderate effect on the capacity of compressive brace members and a negligible effect on tensile brace members.
Summary of Effects
| Member Type | Load Capacity Effect | Stiffness Effect | Failure Mode Change |
|---|---|---|---|
| Compressive chord | Significant increase | Significant increase | More ductile, concrete crushing |
| Tensile chord | Negligible increase | Negligible increase | Minimal change |
| Compressive brace | Moderate increase | Moderate increase | More ductile |
| Tensile brace | Negligible increase | Negligible increase | Minimal change |
| Joint | Changes failure mode | Increases stiffness | Joint remains weak link |
Analytical Model and Design Recommendations
The authors proposed a practical analytical method that accounts for the additional deformation of the joints due to concrete filling. This is important because conventional truss analysis assumes rigid joints, which leads to an underestimation of the total deformation. The proposed method provides a more accurate prediction of the truss deflection and can be used for serviceability limit state checks.
Design Recommendations
- For compressive members: Concrete filling is highly beneficial and should be considered as a primary design strategy. The composite action between the steel tube and the concrete core can increase the capacity by 20-40% compared to unfilled tubes.
- For tensile members: Concrete filling provides minimal benefit and adds unnecessary weight and cost. Unfilled tubes are preferred for tensile members.
- For joints: Despite the improvements, the joint remains the critical element. Designers should ensure adequate joint capacity through proper detailing, such as the use of stiffener plates or increased wall thickness at the chord-to-brace intersection.
- For overall truss design: The analytical method proposed by the authors should be used to account for joint deformations, particularly in long-span trusses where deflection is a governing design criterion.
Fabrication and Quality Control Considerations
From a fabrication perspective, concrete-filled rectangular steel tubes present several challenges:
- Tube fabrication: The rectangular steel tubes must be fabricated with precise dimensions to ensure proper concrete placement and composite action. The wall thickness tolerance should be within ±0.5 mm.
- Concrete placement: The concrete must be placed carefully to avoid voids and ensure full compaction. For horizontal tubes, the concrete can be poured from one end and vibrated from the other. For inclined or vertical tubes, special equipment may be required.
- Curing: The concrete must be cured properly to achieve the specified strength. In cold environments, accelerated curing methods may be necessary.
- Welding at joints: The welds at the chord-to-brace intersections must be designed to accommodate the increased stiffness and strength of the concrete-filled members. The weld size and type should be selected based on the expected stress concentrations.
Quality Control Checklist for Concrete-Filled Tubes
| Inspection Item | Method | Acceptance Criteria |
|---|---|---|
| Tube dimensional accuracy | Dimensional measurement | Within ±0.5 mm tolerance |
| Concrete fill density | Radiographic test or weight measurement | ≥ 95% of theoretical volume |
| Concrete strength | Cube or cylinder test | ≥ specified grade at 28 days |
| Weld quality at joints | UT or MT | No cracks, lack of fusion > 2 mm |
| Tube straightness | Straightedge or laser alignment | Within 1/1000 of length |
Engineering Practice Case
Consider a long-span rectangular steel tube truss for an exhibition hall roof. The main chords are 400 mm × 200 mm × 10 mm rectangular tubes, and the braces are 200 mm × 200 mm × 8 mm square tubes. The span is 60 metres, and the truss depth is 6 metres. The design requires a maximum deflection of L/400 = 150 mm.
Without concrete filling, the truss deflection is calculated to be 180 mm, which exceeds the limit. By filling the main chords with C40 concrete, the stiffness of the compressive chords increases by approximately 30%, reducing the deflection to 140 mm, which satisfies the serviceability requirement. The weight increase due to concrete filling is approximately 15%, which is an acceptable trade-off for the improved stiffness.
Summary
This paper provides valuable insights into the behaviour of concrete-filled rectangular steel tube trusses. The findings that concrete filling significantly improves the capacity and stiffness of compressive members but has minimal effect on tensile members are consistent with the composite action mechanism. The proposed analytical method for accounting for joint deformations is a practical tool for designers. Engineers should consider concrete filling as a design option for compressive members in rectangular steel tube trusses, but should not overlook the continued importance of joint design and detailing.