Bending Capacity of ACSDST Composite Sections Using Plastic Stress Distribution Method

Literature Overview

This paper by Zeng Weizhe, Ning Chunzhen, and Zeng Xiang from Hainan University (2025) presents a novel calculation methodology for the flexural capacity of circular hollow double-skin aluminum-concrete-steel tube composite sections (ACSDST). The research is supported by the Hainan Provincial Natural Science Foundation (521RC504), the National Natural Science Foundation of China (52268024), and the Hainan Graduate Innovation Research Program (Qhys2022-171). Published in the Journal of Hainan University (Natural Science Edition), Volume 43, Issue 1, pages 41-49, this work addresses a significant gap in the structural engineering literature regarding hybrid composite sections combining aluminum alloy tubes, concrete cores, and steel tubes.

Core Technical Approach

The authors adopt the Plastic Stress Distribution Method (PSDM) as the theoretical foundation for deriving closed-form equations for the bending capacity of ACSDST sections. The PSDM approach has been previously validated for various composite cross-sections, but its application to this specific three-material hybrid configuration represents a meaningful extension. The method partitions the cross-section into distinct stress zones—tension zone, compression zone, neutral axis transition region, and shear-critical zones—each governed by different material constitutive laws.

The key innovation lies in the derivation of a set of formulas that precisely account for the varying areas of different stress regions under bending. Rather than relying on simplified assumptions such as a single equivalent modulus or a uniform strain distribution across all materials, the authors develop zone-specific stress-strain relationships that capture the distinct mechanical behavior of aluminum alloy, concrete, and steel at each depth of the section.

Comparison of Calculation Methodologies

Parameter	Traditional Method	PSDM-Based Method (This Study)
Material interaction	Simplified equivalent section	Explicit zone-by-zone analysis
Neutral axis location	Iterative or assumed	Derived analytically
Concrete contribution	Linear elastic assumption	Full plastic zone modeling
Aluminum-steel bond	Perfect bond assumed	Accounted for differential slip
Average relative error	8-12% (reported in literature)	≤5.5% (168 specimens)
Validation basis	Limited experimental data	168 test + FEA specimens

Key Technical Findings

The validation against 168 ACSDST specimens—comprising both experimental test data and finite element analysis results—demonstrates an average relative error not exceeding 5.5%. This level of accuracy is comparable to or better than established design formulas for conventional composite sections such as concrete-filled steel tubes (CFST) and steel-concrete composite beams. The error distribution analysis suggests that the formula performs well across the full range of slenderness ratios, concrete strengths, and steel grades examined.

From a practical engineering perspective, the ACSDST configuration offers several advantages over conventional sections:

The outer steel tube provides superior fire resistance and impact protection
The intermediate concrete layer contributes compressive strength and stiffness
The inner aluminum alloy tube reduces overall weight while maintaining structural integrity
The hollow core permits utility routing and future inspection access

Engineering Practice Implications

For engineers designing structures incorporating ACSDST members, this study provides a reliable analytical tool for preliminary design and code compliance verification. The PSDM-based formulas can be directly integrated into structural analysis software or used for hand calculations during the conceptual design phase.

However, several practical considerations remain:

The formula assumes full composite action between the three materials, which requires proper mechanical interlock or chemical bonding at the interfaces. In practice, the quality of the aluminum-steel bond is critical and may require surface preparation or adhesive layers.
The method does not explicitly address buckling behavior of the aluminum tube under compression, which becomes critical for slender sections. Engineers must supplement the bending capacity calculation with stability checks.
Fatigue performance under cyclic loading is not addressed, which is essential for bridge and wind turbine applications where ACSDST members may be deployed.

Study Insights and Reflections

This work represents a commendable step toward rational design of multi-material composite sections. The extension of PSDM to a three-material system demonstrates the method's versatility and robustness. The 5.5% average error is engineeringly acceptable, particularly when considering that design codes typically incorporate safety factors of 1.5 to 2.0 on nominal strengths.

A notable limitation is the absence of discussion regarding connection details—how ACSDST members join to other structural elements remains an open challenge. Additionally, the long-term behavior under sustained loads (creep of concrete, stress relaxation in aluminum) could influence the accuracy of the plastic stress distribution assumptions over time. Future research should address these aspects to enable comprehensive design guidance for ACSDST members in real-world applications.