TA002

High(er) Strengths in Steel Structures - Engineering & Welding Considerations

Doug Hawkes (Principal Structural Engineer)

Abstract: This article examines the growing use of higher‑strength structural steels, commonly referred to as “Grade 350”, and the engineering, fabrication, and welding considerations that come with them. While these steels offer benefits such as increased strength, reduced weight, and wider global availability, their use also introduces challenges, particularly when materials are sourced from overseas standards like EN, GB/T, or ASTM. Differences in yield strength, thickness effects, and material properties can lead to significant design implications, making early alignment between design intent and fabrication materials essential. The article highlights risks associated with material substitution, weldability, and welding procedure qualification, noting that overseas steels may fall outside Australian design and welding standards. It also stresses long‑term maintenance concerns, urging engineers to understand international steel standards and ensure that material selection, quality control, and welding practices support reliable structural performance over a structure’s lifecycle.

KEYWORDS: High strength steel, welding, structural steel fabrication

1. OVERVIEW

Higher strength structural steels are becoming more highly utilised in structural steel construction, where ‘Grade 350’ type steels are often used due to their advantages of:

1. Additional strength for the same steel weight;
2. Less weight for a reduced transport cost, which is important for overseas fabrication; and
3. The ever-increasing availability (in various standard designation).

The days of 'Grade 250 mild steel’ are most likely numbered. Of course, there are many high strength steel options for both structural and other applications.

This article is generally reviewing the relatively common ‘higher’ strength structural steel around the yield strength of 350 MPa, and addresses a number of issues surrounding the use of these steels, particularly when being sourced from overseas steel mills.

In combination with these gains in material strength, additional consideration must be given to issues such as material substitution, weldability and long-term structural maintenance issues.

The consideration of the likely materials of construction is required as early as possible in a project.

Alignment of the original design requirements and design intentions with the fabrication practices, including the materials of construction, is of vital importance both technically and economically.

This is most aptly described in the following phrase from the foreword of ISO 2394 General principles on reliability for structures:

“It is important to recognise that structural reliability is an overall concept comprising models for describing actions, design rules, reliability elements, structural response and resistance, workmanship, quality control procedures and national requirements, all of which are mutually dependent.

The modification of one factor in isolation could therefore disturb the balance of reliability inherent in the overall concept.”

Many Design Engineers are not aware of the ‘common’ worldwide steel standards. Australian standard materials may not be used for a particular design, even if it is being fabricated in Australia! These could include Euronorm Standards such as EN 10025 and EN 10210, or Chinese Standards such as GB/T 1591, GB 700 and GB/T 8162, or American Standards ASTM A 36 or A 572, not to mention BS, JIS or DIN standards as well.

A key difference between most other international standards and the structural steel Australian/New Zealand Standards AS/NZS 3678¹, AS/NZS 3679² and AS/NZS 1163³ is that the international standards yield strengths are generally lower for the nominal and industry common, but poorly termed, ‘Grade 350’ type steel. The grade designation of steels (i.e. Grade 350, S355, Q345 etc) is generally defined as the yield strength for up to 16 mm thickness. As thickness increases, yield strength generally decreases for the same grade.

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