Bearing Capacity of Compressive Joints in Rectangular Steel Tube Concrete Trusses

Literature Overview

The paper by Liu Yongjian, Zhou Xuhong, and Xiao Long, published in "Building Structures" (Vol. 34, No. 1, 2004), presents experimental and analytical research on the bearing capacity of compressive joints in rectangular steel tube concrete (STC) trusses. Supported by the Hunan Provincial Natural Science Foundation, the study investigates the mechanical behavior of STC truss joints under compression, proposes a bearing capacity calculation formula, and validates it against experimental results. The findings are intended to support engineering design practice and contribute to the development of the Chinese technical code for rectangular steel tube concrete structures.

Background and Engineering Significance

Rectangular steel tube concrete structures combine the advantages of steel tube confinement on concrete with the structural efficiency of steel tubes as load-bearing elements. In truss configurations, the joints represent critical load-transfer zones where multiple members connect, and the failure of a joint can lead to progressive collapse of the entire structure. Understanding the bearing capacity of compressive joints is therefore fundamental to ensuring structural safety.

The compressive joint in an STC truss differs significantly from a conventional steel tube joint or a reinforced concrete joint. The concrete infill within the steel tube provides confinement and bearing area, but the interaction between the steel tube wall and the concrete under joint compression creates complex stress states involving both axial and transverse components.

Experimental Program and Key Findings

The experimental study involved fabricating and testing full-scale rectangular STC truss joint specimens under compressive loading. The specimens incorporated various geometric parameters including steel tube dimensions, wall thickness, concrete strength grade, and joint configuration. The following table summarizes the typical parameters investigated:

The experimental results demonstrated that the bearing capacity of STC compressive joints is significantly higher than that of empty rectangular steel tube joints of equivalent dimensions. The concrete infill increases the effective bearing area and provides confinement that enhances the local buckling resistance of the steel tube walls. The failure mode typically involves local yielding and buckling of the steel tube wall at the joint, accompanied by crushing of the concrete near the loaded region.

Proposed Bearing Capacity Formula

Based on the experimental data and analysis of the failure mechanism, the authors propose a bearing capacity calculation formula that accounts for the contributions of both the steel tube and the concrete infill. The formula incorporates geometric parameters such as the width and depth of the rectangular tube, the wall thickness, the concrete compressive strength, and the steel yield strength. The formula demonstrates good agreement with experimental results, with deviations generally within acceptable engineering tolerance.

The proposed formula can be expressed conceptually as the sum of two components: the bearing capacity contribution from the steel tube walls (based on plastic hinge or local buckling criteria) and the bearing capacity contribution from the confined concrete (based on confined concrete strength models). This additive approach is consistent with the composite action principle underlying steel tube concrete design philosophy.

Implications for Structural Design Codes

The paper explicitly states that its findings are intended to provide reference for engineering designers and contribute to the development of the Chinese technical code for rectangular steel tube concrete structures. The proposed formula represents a practical design tool that bridges the gap between experimental research and code-based design methodology. For engineers practicing in steel tube concrete structures, the study underscores the importance of considering composite action in joint design rather than treating the steel tube and concrete as separate elements.

Study Insights and Reflections

This paper exemplifies the rigorous approach required in composite structure research: combining physical experimentation with analytical modeling to develop design tools that are both physically grounded and practically applicable. The emphasis on joint behavior is particularly important because, as experience in steel tube concrete engineering has shown, joints often govern the ultimate capacity of the structure rather than the individual members. For practitioners, the key takeaway is that the bearing capacity of STC joints cannot be accurately predicted by simple summation of steel and concrete capacities; the interaction effects, particularly the confinement mechanism, must be properly accounted for. The study also highlights the value of experimental validation in building confidence in analytical models, a principle that remains central to structural engineering research.