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 . 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.
What are high-strength steels?
Steels are considered “High Strength” when they possess yield stress above 460 MPa up to 690 MPa. Steels with yield stress above 690 MPa are considered “Ultra-High Strength”.
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:
- Concrete-filled HSS tubes
- Concrete-filled HSS tubes (HCFHST) stub columns
- HSS tubes at the corners of mild steel plates
- Cold-formed structural hollow sections with HSS (HCFHST)
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:
- Overall Buckling
- Local Buckling
- Fracture Toughness
- Corrosion Resistance
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. 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 .
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 .
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 .
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, 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! 
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 .
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:
- Reduction in Element Dimensions/Weight: HSS has high yield and tensile stresses, which allows for greater slenderness, longer elements, and reduced weight and dimensions, leading to savings in terms of welding, transport, and erection costs.
- Dead Load Reduction: In structures like long-span bridges, where steel dead weights are the predominant loads, HSS is an economical and efficient alternative. Elements are smaller and longer, and demands on foundations can be reduced.
- Connections Simplified: HSS plates and bolts can simplify their configuration where connections are highly-loaded, leading to time and cost savings in construction.
- Greater Load-carrying Capacity: Where buckling is not the governing mode of failure or does not significantly influence design outcomes, HSS is effective as a compression member, particularly in stub or stocky columns that are heavily loaded.
- Crack Propagation Resistance: HSS generally exhibits high strength, toughness, and hardness, excellent formability, and weldability, allowing elements to maintain crack propagation resistance; however, the caveat might be HSS's behavior under seismic or sudden loads, which still warrants further investigation.
- Environmental: smaller sections result in substantial materials savings and a reduction in the number of welding consumables, coating materials, and other non-renewable resources.
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: Sydney
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.
Millau-Viaduct Bridge: France
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. Safe to say, this entire project would not have been possible without HSS.
Millau-Viaduct bridge cross-section (Adapted from Dillinger)
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.
The benefit? Less resource-intensive designs, quicker fabrication, time and cost savings in loading, transport, and unloading, and shorter supply chains.
The drawback? 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.
The solution? 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.
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