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From AISC to AS/NZS: The Hidden Complexity of Steel Substitution on International Projects

Scott ChenDirector & Principal Engineer · North Star Design & Build

From AISC to AS/NZS: The Hidden Complexity of Steel Substitution on International Projects — image from the original publication

Over the past few months, I’ve been involved in a project in Timor-Leste that once again proved a simple truth in structural engineering:

“Swapping steel sections between different national standards is never just a like-for-like replacement — it affects everything from engineering calculations to constructability and services integration.”
Figure 1. Comparison of US/AU/CN Steel Sections (300 x 300 Class)
Figure 1. Comparison of US/AU/CN Steel Sections (300 x 300 Class)

🏗️ Project Overview

  • A U.S. client commissioned a portal frame warehouse in Timor-Leste.
  • The full construction drawings were developed in California using AISC standards and U.S. steel sections.
  • An Australian contractor won the bid — but in Timor-Leste, there is no stock of U.S. standard steel, and shipping from overseas would take at least two months.

So they proposed replacing all AISC sections with Australian Standard (AS/NZS) steel sections.

Figure 2. A Multi-span Steel Portal Frame Warehouse (Illustration Only)
Figure 2. A Multi-span Steel Portal Frame Warehouse (Illustration Only)

⚙️ Technical Challenges Behind a “Simple Substitution”

The contractor’s engineers and QS team first matched the closest AS/NZS UC/UB sections by geometry. Then they engaged my team to verify the substitution from multiple angles:

✅ 1. Section Geometry Compatibility Even small differences in flange width / web thickness led to clashes in Tekla with architectural and building services models — all connection details and many service penetrations had to be redesigned.

✅ 2. Structural Performance Check – Strength & Stiffness

  • We recalculated member capacities using Australian steel grades (typically 300MPa / 350MPa) vs AISC Fy = 345MPa or 50ksi.
  • We verified the modulus of elasticity (E ≈ 200 GPa) is comparable across both standards, to ensure similar stiffness and deflection behaviour for portal frames and rafters.
  • Where the section properties or slenderness differed, additional stiffeners or plate reinforcements were introduced.

✅ 3. Steel Material Certification & Weldability We reviewed the steel mill certificates and chemical composition reports for the replacement sections to ensure:

  • Carbon equivalent (CE) values were within allowable welding range;
  • Sulphur / Phosphorus limits met weldability criteria;
  • No deviation that could affect fabrication tolerances, bolt-hole punching, galvanising, or paint adhesion.

✅ 4. Multi-party Structural Review Process The U.S. structural engineer required full structural re-modelling using AS sections, performed by our team in Tekla + engineering calculations, and then reviewed and signed off by their office.

Figure 3. Steel Structure Connection Detailing Under AISC
Figure 3. Steel Structure Connection Detailing Under AISC

🛠️ Coordination Effort

  • More than 10 Zoom coordination meetings,
  • Revisions across architecture, structure, MEP, fabrication and erection sequencing,
  • And finally, a mutually accepted IFC (Issued for Construction) design package.

Due to strict confidentiality agreements, I cannot disclose project images or client details — but the experience is worth sharing.

Figure 4. Tekla - One of the Leading Structural Steel Design Software
Figure 4. Tekla - One of the Leading Structural Steel Design Software

💡 What This Case Taught Us

✔ Steel section substitution is never only about matching size on paper. ✔ It involves capacity, stiffness, weldability, chemical properties, and connection redesign. ✔ Success requires experience across multiple national standards (AISC / AS/NZS / Eurocode / GB) and the ability to coordinate engineers, fabricators, clients and certifiers across different countries.

If anyone is working on international steel design substitution, Tekla–based coordination, or structural review/sign-off across standards, feel free to connect — I’m happy to share more.

Figure 5. AISC - American Institute of Steel Construction
Figure 5. AISC - American Institute of Steel Construction

#StructuralEngineering#SteelStructures#TeklaStructures#AISC #ASNZS#SteelSubstitution#InternationalProjects#EngineeringCoordination#DesignReview#BIMCollaboration#PortalFrame#EngineeringLessons#GlobalConstruction