Futuristic construction site with translucent CFSTs

Angle-Encased CFST Under Axial Tension: The Load-Transfer Revolution

Avery Sinclair in Tech & Innovation September 2025 • 4 min read.

"Discover how innovative composite designs are enhancing structural integrity and transforming construction practices."

In the world of construction, engineers constantly seek innovative methods to enhance the strength and durability of structures. Concrete-filled steel tubes (CFSTs) have emerged as a popular choice, offering excellent structural performance under various loading conditions. However, challenges arise when dealing with large-scale members where increasing the tube-wall thickness becomes impractical and uneconomical.

To address this issue, a solution lies in incorporating encased built-up angles within the CFSTs. This method provides a construction advantage by serving as scaffolding before the concrete is poured. These members are pivotal in applications such as electric transmission towers, demonstrating their potential in large-scale projects.

Recent studies have highlighted the enhanced compressive behavior of reinforced CFST members, yet limited research exists on their tensile behavior, leaving a knowledge gap regarding the behavior of encased angles during loading and the load-transfer mechanism between components. Addressing this gap is vital for optimizing the design and application of angle-encased CFST structures.

Unlocking the Secrets of Angle-Encased CFSTs: How Do They Work?

Futuristic construction site with translucent CFSTs

Recent studies have explored the mechanics of concrete-filled steel tubes (CFSTs) with encased built-up angles under concentric tension, combining experimental and numerical methods to assess their performance. Researchers tested seven full-scale specimens, each four meters long, varying parameters such as the diameter-to-thickness ratio (D/T), connecting type, and encased-angle configuration. These tests aimed to understand how each element contributes to the overall structural integrity.

Finite Element Analysis (FEA) was employed to deeply examine the tensile behavior of these composite members, focusing on load distribution, load-transfer mechanisms, and interactions between the steel tube, inner concrete, and encased angles. This approach allowed for a detailed look at how each component behaves under stress.

Load Distribution: Understanding how loads are distributed among the different components.
Load-Transfer Mechanism: Examining how loads are transferred from one component to another.
Component Interactions: Investigating how the steel tube, inner concrete, and encased angles interact with each other under tension.

The study revealed that all components work together synergistically under axial tension, even when the encased angles are disconnected from the endplates. The strength of the angles is effectively utilized through load transfer from the concrete, facilitated by the interactions between components. Connecting types had a minimal effect on ultimate tensile strength, despite an insufficient confining region within the connecting area.

Designing for Success: A Reliable Method for Tensile Strength

Based on the insights gained from experimental and numerical analyses, a tensile strength design method was proposed for angle-encased CFST members. Comparisons between calculated predictions and test results demonstrated that the proposed method is accurate and reliable, offering a robust approach for designing these advanced structural elements. This research contributes to optimizing the use of composite structures in construction, enhancing safety, and structural integrity.

About this Article -

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This article is based on research published under:

DOI-LINK: 10.1016/j.engstruct.2018.10.015, Alternate LINK

Title: Load-Transfer Mechanism In Angle-Encased Cfst Members Under Axial Tension

Subject: Civil and Structural Engineering

Journal: Engineering Structures

Publisher: Elsevier BV

Authors: Fei Xu, Jun Wang, Ju Chen, Yu-Hang Wang

Published: 2019-01-01

Everything You Need To Know

What is a Concrete-Filled Steel Tube (CFST) and why are they used?

A Concrete-Filled Steel Tube (CFST) is a construction element comprising a steel tube filled with concrete. They are favored in construction for their excellent structural performance under diverse loads. However, when dealing with large-scale members, increasing the tube-wall thickness of CFSTs becomes impractical and uneconomical. This led to the development of angle-encased CFSTs.

How do angle-encased CFSTs work, and what are the key components?

Angle-encased CFSTs incorporate built-up angles inside a CFST. The key components are the steel tube, the inner concrete, and the encased angles. Finite Element Analysis (FEA) is used to examine the behavior of these components under tension, focusing on load distribution, load-transfer mechanisms, and the interactions between these elements. The encased angles serve as scaffolding before the concrete is poured, offering a construction advantage.

What is the role of encased angles within a CFST, and how do they contribute to structural integrity?

Encased angles in CFSTs enhance structural integrity by working synergistically with other components under axial tension. Load is transferred from the concrete to the encased angles. Studies have shown that even when the angles are disconnected from endplates, their strength is effectively utilized through this load transfer mechanism, which relies on component interactions. They also serve as scaffolding during construction.

What factors influence the tensile behavior of angle-encased CFSTs, and how were they studied?

The tensile behavior of angle-encased CFSTs is influenced by parameters such as the diameter-to-thickness ratio (D/T), the connecting type, and the encased-angle configuration. Researchers used experimental and numerical methods, including testing full-scale specimens and employing Finite Element Analysis (FEA), to understand the tensile behavior, load distribution, and component interactions under axial tension. These tests assessed how each element contributes to the overall structural integrity.

What is the significance of the proposed tensile strength design method for angle-encased CFST members?

The proposed tensile strength design method is a crucial step in optimizing the design and application of angle-encased CFST structures. Comparisons between calculated predictions and test results have shown that the proposed method is accurate and reliable. This design method ensures engineers can accurately predict the tensile strength, which is vital for the structures safety and durability. It allows for the use of these advanced structural elements, improving safety and structural integrity in construction.