Zhuojin Pipe Fitting Co., LtdEffective Length Analysis of Compression Webs in Circular Steel Tube Concrete Trusses Considering Joint Stiffness
Literature Overview
The paper by Wang Xinyi and Tong Lewei, published in the Journal of Harbin Engineering University in 2016 (Volume 37, Issue 2, pages 182-186), addresses a fundamental design parameter in steel tube concrete (SRC) truss structures: the effective length of compression web members when joint rotational stiffness is considered. The research was funded by the National Natural Science Foundation of China (Grant No. 50478108). This study is particularly significant because conventional design codes often assume either fully pinned or fully fixed joint conditions, which can lead to either overly conservative or unsafe designs.
Core Technical Content
The authors conducted numerical investigations on the bending stiffness of circular steel tube concrete T-joints and Y-joints under in-plane moment loading. The study compared the behavior of empty steel tube joints with concrete-filled steel tube joints to quantify the contribution of the concrete core to joint rotational stiffness.
Joint Stiffness Parameters and Their Influencing Factors
| Parameter | Definition | Effect on Joint Stiffness |
|---|---|---|
| β (beta) | Chord-to-web diameter ratio | Stiffness increases with increasing β |
| τ (tau) | Chord wall thickness-to-diameter ratio | Stiffness increases with increasing τ |
| γ (gamma) | Web wall thickness-to-diameter ratio | Stiffness decreases with increasing γ |
| θ (theta) | Angle between web and chord axis | Stiffness decreases with increasing θ |
The research established a parametric formula for the bending stiffness of SRC joints, which can be expressed as a function of the geometric parameters listed above. The key finding was that filling the chord tube with concrete significantly increases the joint bending stiffness compared to an empty tube joint. This is because the concrete core provides additional radial stiffness to the chord tube, reducing its tendency to deform under the concentrated load from the web member.
Effective Length Derivation and Design Recommendations
Based on the derived joint stiffness parameters, the authors calculated the effective length of compression web members in SRC trusses. The effective length factor is a critical parameter that directly affects the buckling load capacity of compression members.
The study concluded that:
- Using the effective length factors specified in the current Steel Structure Design Code (GB 50017) may be overly conservative for SRC truss compression webs, particularly when both chord members are concrete-filled steel tubes.
- For trusses where both upper and lower chord members are circular steel tube concrete members, the effective length of compression web members can be approximately taken as 0.7l, where l is the unsupported length of the web member.
- The joint stiffness depends primarily on the radial stiffness of the chord tube and the surface area of the intersection between the chord and web tubes.
Engineering Practice Implications
From a welding and fabrication standpoint, this research has important implications:
- The quality of the chord-web intersection weld directly affects the joint stiffness and therefore the effective length of compression members. Welders must ensure full penetration at the saddle weld with proper root preparation to achieve the design joint stiffness.
- For T-joints and Y-joints, the weld profile should be ground flush with the chord tube surface to avoid stress concentrations that could reduce the effective joint stiffness.
- When designing SRC trusses for critical applications such as offshore platforms or long-span bridges, engineers should consider the actual joint stiffness rather than relying solely on code-specified effective length factors.
- The concrete placement inside the chord tubes must be dense and free of voids, as voids would reduce the radial stiffness contribution and compromise the joint performance.
Key Reflections and Study Insights
This research bridges the gap between theoretical joint stiffness analysis and practical structural design. The finding that 0.7l is a reasonable approximation for the effective length in SRC trusses with concrete-filled chords provides a valuable design shortcut that can lead to more economical member sizing without compromising safety. However, engineers must be cautious in applying this recommendation outside the specific geometric parameter ranges investigated in the study. The parametric formula for joint stiffness should be validated against experimental data for each specific project geometry before being used in design calculations. The study also underscores the importance of weld quality in determining actual joint performance, reinforcing the need for rigorous welding procedure qualification and weld inspection in SRC truss fabrication.