Steel pole bending under pressure, showing plastic deformation.

Bending Under Pressure: Unlocking the Secrets of Steel Pole Design

Q: Why are polygonal hollow sections preferred over circular hollow sections in steel pole design?

Polygonal hollow sections are favored over circular hollow sections primarily due to their economic advantages and aesthetic appeal. In applications like lighting towers and transmission line pylons, they offer a more cost-effective solution. Additionally, their design allows for minimizing land usage and achieving a visually subtle profile, crucial for modern infrastructure projects.

Q: How does the c/t ratio influence the design of steel poles according to Eurocode 3?

The c/t ratio (width-to-thickness ratio) of the individual plate strips in polygonal hollow sections is critical in Eurocode 3 (EC3). This ratio determines the cross-section's classification (Class 1, 2, 3, or 4), which then dictates whether plastic design is permissible. The classification, as per EC3-1-1, depends on the c/t ratio and the yield strength, influencing the section's ability to utilize its plastic capacity and, consequently, its load-bearing potential. Understanding and correctly classifying sections based on their c/t ratio is fundamental to ensuring accurate and efficient steel pole design.

Q: What are the key differences between Class 1, 2, 3, and 4 sections in the context of EC3, and how do they affect steel pole design?

In EC3, the classification of sections significantly impacts design considerations. Class 1 and 2 sections can develop a plastic moment, allowing for plastic design, which fully utilizes the material's capacity. Class 3 sections can only reach the yield limit at the extreme fiber, necessitating elastic design methods, thus limiting the full plastic capacity. Class 4 sections experience buckling before reaching the yield limit, requiring the use of effective cross-section properties. The transition between these classes, particularly from Class 2 to Class 3, can result in a significant, and sometimes unjustified, drop in resistance, which research aims to address.

Q: Why is understanding plastic behavior crucial in the design of steel poles?

Understanding plastic behavior is crucial because it allows engineers to design more efficient and cost-effective structures. By leveraging plastic reserves, engineers can design steel poles that can withstand higher loads before failure. This is particularly relevant for polygonal sections where current design standards may underestimate the actual load-bearing capacity. Refining design standards to better reflect plastic behavior ensures more resilient and reliable steel pole structures, optimizing both material usage and safety margins.

Q: How does the research presented aim to improve current standards like Eurocode 3 for steel pole design?

The research focuses on refining current design standards, particularly Eurocode 3 (EC3), to better account for the plastic behavior of steel sections in polygonal hollow sections. It addresses the discontinuity in bending moment capacity that occurs when sections transition from Class 2 to Class 3. By investigating methods to refine these classifications, the research aims to bridge the gap between theoretical design and real-world performance. This involves a deeper understanding of how steel poles behave under stress, leading to more accurate designs and potentially more efficient use of materials, ultimately improving the safety and reliability of steel pole structures.

Lena Kashyap in Tech & Innovation September 2025 • 4 min read.

"Exploring the Plastic Behavior of Polygonal Hollow Sections in Structural Engineering"

Steel poles with polygonal sections are increasingly favored over circular hollow sections, particularly for lighting towers and transmission line pylons. This preference stems from their economic advantages and aesthetic appeal. In the design of transmission lines, the focus is on minimizing land usage and achieving a visually subtle profile.

However, reducing the pole diameter increases the slenderness of the sections, necessitating thicker walls to maintain the required structural resistance. This increase in wall thickness can compromise the cost-effectiveness of the structure. Current European design rules for overhead electrical lines do not fully leverage the plastic behavior of stocky sections, thereby diminishing the benefits of using compact sections.

Modern standards, such as Eurocode 3 (EC3), recognize the activation of plastic reserves as a critical aspect of structural design. EC3-1-1 dictates that a cross-section's slenderness determines its classification (Class 1, 2, 3, or 4), which influences whether plastic design is permissible. A significant issue arises when transitioning from Class 2 to Class 3 sections, where EC3-1-1 prescribes a sudden, often unjustified, drop in resistance. This paper addresses this mismatch, presenting research aimed at optimizing the ultimate load of polygonal sections.

Why Understanding Plastic Behavior Matters for Steel Pole Design

Steel pole bending under pressure, showing plastic deformation.

The classification of cross-sections in EC3 depends significantly on the c/t ratio (width-to-thickness ratio) of the individual plate strips that form polygonal hollow sections. This ratio dictates whether a section can fully utilize its plastic capacity, which directly impacts its load-bearing potential. According to EC3 Part 1-1, Table 5.2, the limits for cross-section classes are defined by the following equation:

Where: c/t is the width-to-thickness ratio, ε (epsilon) is a strength parameter calculated as √(235/fy), where fy is the yield strength.

Class 1 and 2 sections are capable of developing a plastic moment (MRk,pl/Mel ≈ 1.27), allowing for plastic design.
Class 3 sections can only reach the yield limit at the extreme fiber, leading to elastic design.
Class 4 sections experience buckling before reaching the yield limit, requiring resistance to be determined using effective cross-section properties (Weff/Wel < 1.0).

This classification system, while comprehensive, introduces a discontinuity. Specifically, the bending moment capacity for polygonal sections decreases sharply as sections transition from Class 2 to Class 3, which many researchers and engineers consider an oversimplification. This paper investigates methods to refine these classifications to better reflect the actual behavior of steel poles under bending.

The Future of Steel Pole Design

The research presented here underscores the critical need to refine current design standards to better account for the plastic behavior of steel sections. By understanding and leveraging plastic reserves, engineers can design more efficient, cost-effective, and resilient structures. Ongoing research and revisions to standards like Eurocode 3 promise to bridge the gap between theoretical design and real-world performance, ensuring the safety and reliability of future steel pole structures.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.1002/stco.201710027, Alternate LINK

Title: Plastic Behaviour Of Polygonal Hollow Sections In Bending

Subject: Metals and Alloys

Journal: Steel Construction

Publisher: Wiley

Authors: Katharina Bräutigam, Peter Knoedel, Thomas Ummenhofer

Published: 2017-08-01

Everything You Need To Know

Why are polygonal hollow sections preferred over circular hollow sections in steel pole design?

Polygonal hollow sections are favored over circular hollow sections primarily due to their economic advantages and aesthetic appeal. In applications like lighting towers and transmission line pylons, they offer a more cost-effective solution. Additionally, their design allows for minimizing land usage and achieving a visually subtle profile, crucial for modern infrastructure projects.

How does the c/t ratio influence the design of steel poles according to Eurocode 3?

The c/t ratio (width-to-thickness ratio) of the individual plate strips in polygonal hollow sections is critical in Eurocode 3 (EC3). This ratio determines the cross-section's classification (Class 1, 2, 3, or 4), which then dictates whether plastic design is permissible. The classification, as per EC3-1-1, depends on the c/t ratio and the yield strength, influencing the section's ability to utilize its plastic capacity and, consequently, its load-bearing potential. Understanding and correctly classifying sections based on their c/t ratio is fundamental to ensuring accurate and efficient steel pole design.

What are the key differences between Class 1, 2, 3, and 4 sections in the context of EC3, and how do they affect steel pole design?

In EC3, the classification of sections significantly impacts design considerations. Class 1 and 2 sections can develop a plastic moment, allowing for plastic design, which fully utilizes the material's capacity. Class 3 sections can only reach the yield limit at the extreme fiber, necessitating elastic design methods, thus limiting the full plastic capacity. Class 4 sections experience buckling before reaching the yield limit, requiring the use of effective cross-section properties. The transition between these classes, particularly from Class 2 to Class 3, can result in a significant, and sometimes unjustified, drop in resistance, which research aims to address.

Why is understanding plastic behavior crucial in the design of steel poles?

Understanding plastic behavior is crucial because it allows engineers to design more efficient and cost-effective structures. By leveraging plastic reserves, engineers can design steel poles that can withstand higher loads before failure. This is particularly relevant for polygonal sections where current design standards may underestimate the actual load-bearing capacity. Refining design standards to better reflect plastic behavior ensures more resilient and reliable steel pole structures, optimizing both material usage and safety margins.

How does the research presented aim to improve current standards like Eurocode 3 for steel pole design?

The research focuses on refining current design standards, particularly Eurocode 3 (EC3), to better account for the plastic behavior of steel sections in polygonal hollow sections. It addresses the discontinuity in bending moment capacity that occurs when sections transition from Class 2 to Class 3. By investigating methods to refine these classifications, the research aims to bridge the gap between theoretical design and real-world performance. This involves a deeper understanding of how steel poles behave under stress, leading to more accurate designs and potentially more efficient use of materials, ultimately improving the safety and reliability of steel pole structures.