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High strength structural steel, more bang for buck?

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Australia’s top stadiums - An engineer's perspective

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Timber Design Standards - 2018 NDS

Timber Design Standards - Eurocode 5

Timber Design Standards - AS1720 and AS1684

Comparison of Current Timber Design Standards

High Strength Steels (HSS) and Ultra-High Strength Steels (UHSS) have recently made quite a splash. However, growing site constraints, construction demands, taller buildings, and longer-spanning bridges have necessitated enhancements in the mechanical and behavioral properties of materials.  

To date, HSS and UHSS have primarily been used in the automotive and crane manufacturing industries. But recently, they’ve become increasingly common in Civil Engineering applications, primarily due to their exceptional mechanical strengths and material efficiency. The possibility to design lighter and simplifying the transport of steel components through HSS integration into designs is the kind of good news we’ve been looking for! With HSS, the strength-to-weight ratio increases, resulting in lighter, leaner, and simpler fabrication and construction. The prospect of longer spans, net-weight reductions, greater load-carrying capacities, and reduced material usage are benefits engineers are keen to exploit.

Akashi kaikyo bridge, the second-longest suspension bridge in the world, incorporating 800 MPa steel in the truss system

Bridge spans are increasing, tunnels are getting longer, and building designs are becoming leaner. These modern design requirements are increasing the performance demand on structural materials. HSS's enhanced physical and sustainable credentials have accelerated its use in contemporary design and have increased demand for HSS products. Combined with that, an attractive 30% steel saving for most types of HSS applications compared to conventional steel will leave you asking why is HSS not being used everywhere.

Well, not so fast; there’s still a lack of research on HSS’s structural performance under certain conditions, particularly at elevated temperatures [1].  Most international design standards don’t even cover HSS design, which means the adoption of HSS products is pretty limited. Further analysis and research are needed before HSS can become ubiquitous in the industry. 

HSS will eventually become a key tool in any civil engineer’s toolbox despite the hold-ups, so it’s important to know when to consider using it. Below we break down everything you need to know about HSS, where to use them, and what to look for when designing them. 

 when they possess yield stress above 460 MPa up to 690 MPa. Steels with yield stress above 690 MPa are considered 

HSS and UHSS are used at an elementary level to make a wide array of structural members, from columns, beams, plates, and bolts to increasing hybrid applications. Common fabricated HSS sections include I/H sections, box sections, and tubular sections. To capitalize on substantial weight savings over conventional steel grades, hybrid elements containing HSS are becoming increasingly popular. Some examples of these include:

Behavior & properties of high-strength steel

High Strength Steels exhibit many different mechanical proprieties compared to ordinary steels; higher yield strength, higher yield-to-tensile strength ratio, better tensile stress, bending, and weld performance. Before getting to these differences in more detail, let’s lay out the key general mechanical characteristics of structural steels first:

Below, we’ll briefly go through each of these factors for HSS to assess their practical application over commonly used mild steels.

HSS has a higher yield strength than mild steel, yet the Modulus of Elasticity (E) is constant for all steel grades. This limits the deformation capacity of HSS, leading to potential issues concerning serviceability and stability. These issues primarily have to do with local strength and stiffness degradation that can impact global structural integrity, building sway, and member deflections. 

When HSS is specified in structural components, considerations must be made concerning the increased likelihood of stability problems, mainly when compressive stresses are involved. Serviceability considerations, like deflection criteria, usually govern design criteria in buildings. Further, structural integrity in buildings is achieved chiefly through appropriate element stiffness, which is not dictated by material strength but likely by factors like modulus of elasticity, support configurations, or the type of structural system or loading conditions. Even if HSS can carry more load due to its advances in material strength, its performance is often still limited by member deflection, so simple increases in strength may not be sufficient or economical[2]. For bridges, this is not always the case; fatigue and fracture criteria usually govern design requirements, so HSS could likely be more suited to bridge applications. 

High-strength steels exhibit superior ductility properties over mild steel by the measure of elongation.  Ductility in steel measures its ability to plastically deform under tensile stresses. Imagine a game of tug-of-war on a steel element, its ability to deform and stretch without becoming weak, brittle, or fracturing all refers to its ductility. Similarly, steel ductility enables structures to resist deformation from tremors or earthquakes, by dissipating the energy produced through plastic yielding. Ductility is also crucial during fabrication, allowing it to redistribute stresses and reduce cracks induced through welding, bending, and straightening of the steel.

Tests on HSS are typically measured through elongation at the yield of a steel specimen under high-loading events. Rotation capacities can reveal whether steels satisfy seismic and deformation demands. Recent research has shown that HSS steels achieve uniaxial elongations between 18-30%, deemed excellent [2].

However, experimental and numerical studies have revealed that HSS members could demonstrate poorer ductility under seismic loading due to higher yield-tensile strength and lower deformation capacity than mild steels. Despite some promises in ductility properties, before HSS steels can be applied broadly, current seismic design specifications within national standards need to be revised to accommodate HSS. 

When a whole steel section is subjected to compression, the strength of the steel member can be weakened in bending; this is overall buckling. Studies have shown that the overall buckling behavior of HSS elements and conventional mild steel can vary widely given differing geometric imperfections, residual stresses, and material properties within cross-sections. Despite this, the overall buckling strength of HSS is significantly higher than ordinary steel. However, using more slender sections means that stability issues like overall or local buckling remain a primary failure mode for HSS elements. Furthermore, since HSS elements tend to be thinner and if subject to the same load as an ordinary steel element, they are more likely to experience buckling [3].

Local buckling is due to compression in plate elements such as flanges or webs of an I beam. Conventionally, local buckling is controlled by making sections more compact.

Applying HSS in structures can lead to thinner, lighter, and longer sections aiming to achieve more economical solutions. As such, plate elements in HSS members tend to be thinner and wider than ordinary steel members.

The higher strength of HSS and UHSS steels allows for more slender sections with different slenderness limits to be utilized as structural components. However, this often means that local buckling would become the governing design criterion rather than the ultimate bearing capacity. Research has been done on improving local buckling and a post-local-buckling load of HSS sections by infilling sections with concrete, a potential (but expensive) solution to stability concerns [4]. 

Weldability refers to steel's ability to weld/join with similar materials. Steels require an appropriate chemical composition to improve weldability without propagating cracks and other imperfections. Filler metal selection can be an important consideration, but how the welding process affects the steel should be of particular focus. As metals change from liquid to solid during the welding process, stress is introduced in the weld as it shrinks and in the steel. It is the nature of these stresses on steel elements made of HSS (which tend to be longer and thinner), that needs to be looked into further. This can be done through careful heat and cooling, appropriate weld joint design, and low hydrogen or low alloy-metal cored or flux-cored filler metals[5], however, it introduces additional cost into the process.

HSS has superior weldability when subjected to proper heat treatment and filler, resulting in finished welds that are very strong. However, HSS can often be more susceptible to cracking over mild steels, so the welding process can be challenging, especially during the rapid heating and cooling process, requiring good heat treatment before and during welding; in other words, it's got to beat the heat! [6] 

Fracture toughness refers to a steel’s capacity to resist failure after the propagation of a crack. This mechanical property is significant for bridges, where fatigue cracking is prevalent due to repetitive and cyclical loading and service conditions.  

As a result of limits in steel processing technology, HSS can exhibit lower fracture toughness than mild steel. This indicates that the structural performance of steel structures fabricated with HSS can be significantly different from conventional steel structures when subject to dynamic, cyclic, and sudden loads or seismic conditions and should be reflected appropriately in design codes [2]. 

Corrosion can significantly degrade the properties of steel. Corrosion resistance refers to the ability of steel to resist weathering. Corrosion is a significant issue in many geographical locations, particularly for structures where elements are exposed to harsh environmental conditions.

Structural steels, particularly those in marine environments, are susceptible to corrosion. Generally, HSS steels demonstrate slightly better corrosion protection when compared to other steels. However, further research is needed to investigate HSS corrosion behavior, particularly in cyclic environments.

Advantages of using higher strength steels

The structural advantages that come from implementing HSS depend not only on the type of project but on the desired function and form of the structural element used.

Here are some likely advantages of using HSS steels in structural contexts:

High-strength steels have primarily been used in mechanical engineering, with frequent use in the automotive industry. There are, however, a few examples of HSS being used in structural and civil designs around the world. Here are a few:

The Latitude Building in Sydney, Australia, was completed in 2005 and sits on the very bustling George Street at World Square. The iconic 45-floor building was designed by Hyder Consulting and constructed by Multiplex, reaching skyward in high-strength steel for a total height of 222m. The building boasts some innovative composite steel construction. The 7-meter-deep transfer structures used 650 & 690 MPa HSS steel box sections filled with concrete and high-strength tubular sections. Similarly, 650-690 MPa steel plates were implemented for the basement columns and roof trusses. By using HSS in this context, column sizes and excavation costs were reduced, providing additional floor area and car-parking spaces in the building. 

The Sony Centre complex in Berlin is regarded as an iconic landmark at the busy Potsdamer Platz. Designed by Helmut Jahn, the buildings tie together the cultural and business dynamics of the space in a fluid, edgy design. While the roof above the Sony Centre looks like it is floating, it comprises 600 tonnes of steel. Using S690 HSS allowed the designers to achieve the significant weight reduction required to reduce supports and achieve its weightless look. It also enables the roof to be suspended several stories above the building, diverting the additional loading away from the heritage masonry building below and avoiding any strengthening works. 

Not only is this multi-span cable-stayed bridge an aesthetic marvel, but it has also cemented its status as a world record holder. It’s currently the highest bridge in the world, with a total construction height of 343m, crossing the Tarn River at an altitude of 270m. Michel Virlogeux was the leading engineer and civil engineering construction company Effiage constructed the colossal project.

The 2460 m long deck comprises six main spans and two end spans, all over 200 m wide. The bridge’s cross-section is a central box linked to seven steel pylons erected in an inverted Y-shape. Of the 43,000 tonnes of steel plates used, over 18,000 tonnes are HSS elements. HSS S460 steels were used for the entire central box, various girder panels, and erection auxiliaries. The pylons were also constructed from the S460 steel grade. 

The production process of the HSS steel allowed for the attainment of excellent high-strength performance and fantastic weldability. Applying an optimized and efficient welding process meant that time and weight savings were made. Furthermore, the higher strength of steel allowed the structure to resist high loads and bending moments without needing to increase the amount of steel used. Further cost savings were made by reducing transport weights and costs as a result of material savings[7]. Safe to say, this entire project would not have been possible without HSS.

Millau-Viaduct bridge cross-section (Adapted from

Despite the advantages of implementing either fully fabricated or composite HSS elements in structural applications, the use of steels with nominal yield stress above 460 MPa in broader contexts remains surprisingly low (approximately 5%). While there is immense potential for wider use in building structures, there are some drawbacks engineers should be aware of. The main one is research. 

There are gaps in experimental and numerical research on high-strength steel members' behavior and mechanical properties under varying and dynamic load conditions. For example, the local buckling behavior of HSS members under axial compression or the behavior of HSS under combined bending and axial loads have not been explored thoroughly.

There is even less research relating to the effect of elevated temperature on HSS. We know that steel's strength and elastic modulus drop sharply under elevated temperatures. Quantitative research of HSS under fire conditions is scarce, potentially making these structures something that engineers NEVER want to hear: unpredictable. Disastrous events like the collapse of the World Trade Center (WTC) towers in 2001 were an impetus for increased research activity into the impacts of fire on steel materials and structures. This research is far from over, particularly when HSS is concerned. Further research should not be neglected, and the behavior of HSS elements under fire loading needs to be investigated. 

Finally, the research done on HSS structures provides evidence that the mechanical behavior of HSS structures is different from that of ordinary steel structures, which means that current design methods, codes, and specifications from regulatory bodies are not necessarily applicable to HSS design and most definitely aren’t optimized. Research has shown that current codified practices can sometimes be both too conservative and not safe at the same time, so again, they’re unpredictable. To promote the application of HSS and take advantage of its potentially superior mechanical performance, new design specifications need to be regulated.  

The construction industry is material, labor, and energy-intensive. From having to source raw materials, transport them to the site, and everything in between. This is energy-intensive and contributes heavily to embodied carbon and operational emissions. On top of all of this, what do we do with all the materials in a building when it reaches its end of life? 

The application of high-strength steel in design means that we could use less steel in that design, significantly boosting a project’s economic, sustainability, and efficiency credentials. 

Less resource-intensive designs, quicker fabrication, time and cost savings in loading, transport, and unloading, and shorter supply chains. 

 The application of HSS assumes that strength is the limit state for the design, but as material and element configurations change, so do their mechanical properties and behavior under certain conditions. To fully realize the benefits of using high-strength steels in our buildings necessitates more research, more inputs, and changes implemented by regulatory bodies and more collaboration throughout supply chains. 

 What is CalcTree, we hear you say? And how can CalcTree integrate HSS into engineering designs? We aim to be at the forefront of materials innovation, supporting new and sustainable products within our engineering design services.‍

Structural engineers are integral in designing and executing our buildings and infrastructure. From residential homes to skyscrapers and from roads to rail, a structural engineer delivers projects that are safe, economical, and made to last. So, what should all structural engineers expect to know?

Structural engineers use their physics and engineering knowledge to design and assess and improve existing structures. When it comes to design, a structural engineer typically has to meet three main requirements: 

Strength, Serviceability, and Durability. 

In other words, a structure must not fail or collapse 

, it must not bend, sway, deflect, or vibrate excessively 

, and it must not deteriorate, corrode, or suffer fatigue 

. A structural engineer should make a structure as economical and efficient as possible if those three requirements are satisfied. 

Everything from analyzing structural systems under various loads to satisfying assumptions and allowable limits to determining construction methods and materials, interpreting and drafting plans, specifications, and drawings becomes part of your repertoire.

Structural engineering is a specialization of Civil Engineering; in other words: All structural engineers are civil engineers, but not all civil engineers are structural engineers.

As a structural engineer, you can expect to focus on more complex structures across various job types and industries. You might find yourself working on structures as varied as skyscrapers, train stations, hospitals, mining and industrial buildings, and large infrastructure projects.

Your design must consider all static and dynamic loads while balancing the requirements defined by developers, operators, architects, builders, services engineers, and more! You'll need to perform advanced calculations and simulations to analyze structural performance and explore options. You must also ensure the design meets all stakeholder requirements and physical and regulatory performance metrics.

On the other hand, civil engineers tend to design and supervise the broader aspects of public infrastructure. A civil engineer working on 

 can expect to deal with anything from road and pavement design to rail systems, landscaping, and drainage. They collaborate with structural engineers and other specialist engineers (like bridge and tunnel engineers) to design broader systems and communicate with stakeholders. They make recommendations and plans, ensure project sites meet performance requirements and respect regulatory standards.

To simplify it even more: civil engineers tend to look at the bigger picture, while structural engineers focus on the complex structural elements of a project.

There are several key groups of knowledge and skillsets that a structural design team needs to do their jobs well. 

Structural engineers must have a firm grasp of physics and the fundamentals of construction to understand how structures are built and behave and how they respond to environmental conditions.

An aptitude for physics, mathematics, and science is a must. These subjects are deeply intertwined into all the concepts you learn as you head into university and the workforce. 

From studying materials and mechanics to hydraulics and geotechnics to project management and, more recently, computer science, a structural engineer needs to be a jack of all trades to do their job well.

Being able to look at a real-life concept of a structure and turn it into an analytical model or free-body diagram is key to the job. Additionally, concepts like moment or torque, shear, and deflection should become second nature. Understanding and considering material properties like elastic modulus, the 

, and their impact on a system's performance are key skills you'll develop as a structural engineer. 

Understanding the individual components of a structure, like columns, beams, slabs, struts, etc., are the building blocks of design. From there, conceptualizing how these components interact to create sub-systems within a structure, like lateral stability systems and floor plates, is also key. 

Building a strong knowledge base about local and national building codes and standards is important too. Understanding and applying codes of practice and typical assumptions are just as important as crunching numbers. 

Knowing your trade also means knowing how it all comes together on-site. Construction processes and how to inspect and evaluate a building site will be a skill you learn on the job. As a structural engineer, you should be able to inspect a building or construction site and compare what you see with the building plans. You will inspect the building envelope, walls, insulation, etc. Discrepancies are then noted and remedied. Finally, identifying defining risk and, therefore, the margins of safety to apply to global and local parts of a structure is the final piece of the puzzle to designing a safe and robust structure.

From simple excel spreadsheets to niche tools and advanced simulations, structural engineers should have strong computer skills and knowledge of software solutions.

Hand calculations in the structural analysis will always be a critical skill. In the early stages of design, when you are still in the planning phase, you can't simply input unfinished concepts into a software tool; you need a solid understanding of what you intend to input. In short, 

Software tools can boost your understanding of structural concepts to help you see how structures work at scale and how to iterate a design quickly. The software can be used in various ways, from modelling existing or new systems to modelling the physical environment, evaluating alternatives, and integrating all essential design tools, features, and international and local building codes. More conventional software includes features to analyze a diversified profile of structural elements, outputting analysis and graphics, and checking for errors.

Structural engineers can add immense value to an organization with coding skills. While structural engineers are typically taught to use programs, a small proportion knows how to code. Engineers have a lot of transferable skills needed to learn how to code, and learning to do so will set you apart. Design automation, AI, and other digital transformations within the industry are gaining momentum; understanding code will future-proof your skills.

The ability to document, interpret and communicate engineering drawings, plans, 3D models, and specifications is essential. Structural engineers must know how to interpret and create drawings and models, whether by hand or with tools like Revit, AutoCAD, or Bluebeam. It allows structural engineers to understand project developments, functions, outcomes, and how they all come together. Structural engineers will never work in a vacuum; their work must be coordinated within the bigger picture of the project with designers, architects, contractors, engineers, and clients.

Being able to communicate design decisions and outcomes visually to all major participants in a project and to do so effectively is a skill and core part of the job.

This one is quite equivocal and only truly developed after years of study and professional experience. It's not just about having the answer to a problem or delivering the objective; it understands how to do so by applying creativity, ingenuity, intuition, innovation, and judgment to all tasks. Solutions may be found, but problems can persist; navigating around this separates great engineers from good ones.

Engineering judgment is the ability to formally and intuitively decide on a course of action and know how to support it. It is anything from communicating highly technical concepts to non-technical persons or understanding the nuances, intricacies, and best ways to make decisions.

The industry is changing. The need to remediate the building sector's enormous impact on natural resources, livelihoods, and the economy has gained momentum. As governments and organizations regulate a commitment towards sustainability and innovation, all industry participants must be aware of this. Structural engineers need to become more responsive to the tenets of sustainability and the environment. Understanding sustainable materials, working to produce and implement energy and cost-efficient designs, and having a grasp of supply chains, material availability, and new technologies are some new challenges engineers must take on. 

As the industry shifts to being more automated and data-driven, how engineers collaborate and how existing tools and workflows are organized will evolve. All existing tools in design and analysis are effective, but there are ways to enrich them and make them work better at scale. That's our 

with CalcTree. We provide engineers with a 

 to help wrangle data across projects. We want to help you organize and leverage calculations and designs like never before. Helping engineers build and share creations faster and smarter. To learn more, join our waitlist and 

Australians love sports and for construction engineers, the best projects to work on are stadiums. They’re big and complex and when the project emails finally go silent and the last bolt is fixed, you get to enjoy them yourself!

But which are the best stadiums in Australia from an engineering perspective? In this article, I’ll list what I believe are the three most interesting and iconic sporting structures in the country.

This list is in no specific order, but if it was Optus would be at the top. Not only has it been voted the world's 

, but the building and the surrounding precinct are friggin’ cool!

Optus can seat 60,000 spectators and boasts some pretty awesome facilities. The entire stadium receives 4G coverage, and over 1000 individual TV screens and 340 m2 of super screens make it impossible to miss a second of the game. Safe to say, the electrical engineers had a few late nights on this one…

And what about that roof?! The ‘halo’ roof design is made to look like it’s floating above the seating bowl and covers 85% of the seats in the stadium. It’s built out of a double layer of fabrics supported by 50 cantilever triangular trusses spanning 35m and made from tubular steel.

The trusses can span so far yet remain robust due to the vierendeel truss design of their bottom webs. This means the roof can remain laterally stiff without the need for in-plane bracing. It also achieves robustness in the case of a local segment failure, avoiding any kind of progressive collapse. This approach also saves on steel tonnage as diagonal braces between trusses aren't needed!

All in all, the roof of Optus stadium contains some pretty awesome engineering and construction processes to get it all in place. As a sports lover and Melbournian, it took some convincing, but after watching the 2021 AFL grand final in Perth (Go Dees!) I was sold!

No list of Australian sporting arenas would be complete without mentioning the MCG. The oval-shaped Melbourne Cricket Ground is highly decorated and can seat over 100,000 spectators. From AFL Grand Finals, Boxing day cricket test matches, Commonwealth Games, the Olympics in 1956, and even becoming a make-shift barracks for thousands of U.S Marines in WWII, the MCG has seen it all! So while it’s not the most spectacular entry for engineering, its history and persistence earned it a spot on my list.

Nothing gives you more goosebumps than hearing 100,000 fans cheering their lungs out at the AFL Grand Final at the ‘G’.

What’s interesting about the MCG structure is that it’s not one continuous structure. From its humble beginnings as a single clubhouse watching over Police Paddock in 1853, the 'G' has been built up piece by piece into what it is today. Throughout its history, it’s actually been made up of a varying amount of individual structures linked together.

The Great Southern Stand is the largest of these structures and runs around ~ one-third of the ground. It’s flanked by two massive big screens providing visibility to all 100,000 spectators.

 The tragic passing of Australian legend Shane Warne sparked the MCG to rename this stand the 'Shane Warne Stand'. This is a fantastic memorial to a man who brought so much joy to the spectators sitting in this stand over the years, and another piece of history that'll stay with the 'G' for decades to come.

An aerial snap of the ground with the old MCC members standing front and centre from 1928.

The historic MCC member's stand sits north of the stadium and has been through a few iterations since the 1800s. Most recently in 2002-6, the MCC along with the Ponsford and Olympic Stands were demolished to make way for the Northern grandstand. The northern stand is now one continuous structure around the northern boundary of the stand.

The intricacies of the MCG's development are pretty incredible when you dig into them, and that's why I had to include it in my list. Check out this cool summary of the ground's evolution here for more images and information!

3) Sydney Football Stadium (In construction)

The new Sydney Football stadium is currently being built on the site of the old Sydney Sports Ground. It will be the new jewel in the crown of Sydney’s sporting scene. The rectangular stadium was planned to be a timber diagrid roof, however, cost limitations meant the design changed to be a more traditional steel diagrid roof shell. The stadium is set to seat 40,000 to 45,000 spectators, all of whom will be completely covered by the roof!

If this image is anything to go by, the lighting system could rival Optus Stadium!

Currently, in construction, the roof consists of 4 individual diagrid shells which are supported by four massive derricks at the stadium's corners, and 4 steel arches at their inner edges. The shells are then pulled together by a massive steel tension ring that runs around the outer perimeter of the roof, holding it in shape.

The project was highly politically contentious at the time of breaking ground, and for this reason, it has many naysayers. But putting that aside, the structure has some serious engineering and construction behind it that can't be ignored. The new Sydney Football Stadium is sure to be one of Australia's finest when complete!