Effective Length of Compression Web Members in Circular Steel Tube Concrete Trusses Considering Joint Stiffness

Literature Overview

The paper by Wang Xinyi and Tong Lewei (2016), published in the Journal of Harbin Engineering University (Vol. 37, No. 2, pp. 182-186), addresses an important structural engineering issue: the effective length of compression web members in circular steel tube concrete trusses, taking into account the joint stiffness. Funded by the National Natural Science Foundation of China (50478108), this research numerically investigated the flexural stiffness of circular steel tube concrete T/Y-type joints under in-plane bending moment, compared the flexural stiffness of empty steel tube joints with concrete-filled steel tube joints, established a parameterized formula for joint flexural stiffness, and provided the effective length of compression web members considering joint stiffness.

Core Technical Content

Background and Motivation

In traditional steel truss design, the effective length of compression members is determined based on the assumption of pinned or fixed joints, as specified in design codes such as the Chinese Code for Design of Steel Structures (GB 50017). However, in practice, joints are neither perfectly pinned nor perfectly fixed, and their actual stiffness depends on the joint configuration, member properties, and material behavior. For circular steel tube concrete trusses, the presence of concrete filling in the chord tubes significantly affects the joint stiffness, which in turn influences the buckling behavior and effective length of compression web members.

Numerical Investigation of Joint Flexural Stiffness

The authors conducted numerical studies on circular steel tube concrete T-type and Y-type joints under in-plane bending moment conditions. The key findings included:

• Comparison of empty tube joints and concrete-filled tube joints: When concrete is filled inside the chord tubes, the joint flexural stiffness increases significantly compared to empty steel tube joints. This is attributed to the concrete's contribution to the radial stiffness of the chord tube.
• Factors affecting joint stiffness: The joint stiffness depends on the radial stiffness of the chord tube and the surface area of the intersection between the chord tube and web member.
• Parametric relationships: Joint stiffness increases with increasing beta (β, the ratio of web member width to chord tube diameter) and tau (τ, the ratio of web member wall thickness to chord tube wall thickness), and decreases with increasing gamma (γ, the ratio of chord tube diameter to wall thickness) and theta (θ, the angle between the web member and the chord tube).

Effective Length Formula

Based on the numerical results, the authors established a parameterized formula for the flexural stiffness of steel tube concrete joints and derived the effective length of compression web members considering joint stiffness. The study concluded that 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 actual length of the web member.

Technical Points and Engineering Implications

Parametric Analysis Results

Comparison with Design Code Provisions

The study found that using the effective length provisions from the Code for Design of Steel Structures (GB 50017) for steel tube concrete trusses may be conservative. The code typically assumes pinned joints for compression member effective length calculations, which results in a longer effective length and therefore lower buckling resistance. For steel tube concrete trusses with concrete-filled chord tubes, the actual joint stiffness is higher than assumed, leading to a shorter effective length and higher buckling resistance.

Practical Design Recommendations

For engineers designing circular steel tube concrete trusses:

1. The effective length of compression web members can be taken as approximately 0.7l when both upper and lower chord members are concrete-filled steel tubes.
2. The joint stiffness should be evaluated based on the specific geometry and material properties of the truss members.
3. Parametric studies should be conducted for critical truss configurations to verify the adequacy of the simplified effective length approach.
4. The interaction between joint stiffness and member buckling should be considered in the overall structural analysis.

Key Questions and Reflections

Several aspects of this research merit further consideration:

• The study focuses on T-type and Y-type joints, but K-type and N-type joints are also commonly used in steel tube concrete trusses. How do the findings extend to these other joint configurations?
• The numerical analysis likely assumes elastic or elasto-plastic behavior, but what is the influence of material nonlinearity on the joint stiffness under service and ultimate limit states?
• The recommendation of 0.7l as an approximate effective length factor is a simplification. What is the margin of safety associated with this approximation, and under what conditions might it be insufficient?

From a practical standpoint, the conservative nature of code-based effective length calculations means that using the code provisions will not compromise safety, but may result in heavier and more expensive truss members. The findings of this study provide a basis for more rational and economical design, but engineers should exercise caution when applying these results outside the specific conditions studied.

Study Insights and Implications

This paper makes a valuable contribution to the rational design of circular steel tube concrete trusses by quantifying the effect of joint stiffness on compression member effective length. The parametric analysis provides clear guidance on how geometric parameters influence joint stiffness, which is useful for optimizing truss geometry. The recommendation of 0.7l as an approximate effective length factor for concrete-filled chord trusses is a practical simplification that can be applied in preliminary design stages. However, for critical structures or unusual configurations, a more detailed analysis considering the actual joint stiffness is recommended. The study also highlights the importance of considering the concrete filling effect on joint behavior, which is often overlooked in preliminary design calculations.