Fran Williams summarises the key points about making steel more sustainable
The role of steel has long been central to the construction industry and will be for many years to come. It is unique: unlike any other major construction material, once it is made it is indefinitely recyclable without the loss of quality or downgrading over time. Current recycling rates in the EU are 99 per cent for structural steelwork and 96 per cent for all steel construction products and, on average about 85 per cent of steel is recycled at the end of its ‘first’ useful life. These properties suggest it could therefore be the ultimate sustainable construction material in a circular economy.
However, the energy consumed in its production and re-making generates huge amounts of carbon emissions. It is made from iron ore, which is dug from the ground and heated to a high temperature with coke (a fuel produced from the kiln treatment of coal) to produce brittle, high-carbon pig iron. This is then smelted with other alloying elements, mixed with a proportion of scrap and treated with oxygen to reduce its carbon content and produce varying properties. Once the impurities have been separated off, the resulting steel is then cast into ingots, extruded or rolled and delivered to manufacturers for fabrication into components.
The material has been around for a long time. The earliest known steel objects date from 1800BC. The first steel-framed building was the Home Insurance Building in Chicago, designed by William Le Baron Jenney and built in 1885, four years before the Eiffel Tower.
Most steel in Europe is manufactured via two production routes – BOS (basis oxygen steelmaking – essentially a blast furnace) and EAF (electric arc furnace). Both routes make use of a significant recycled content – an average of 60 per cent for structural steel globally. Tata Steel and British Steel (both major producers in the UK) recycle or reuse over 90 per cent of their process residues, which include sludges, slags and dust. However, most UK constructional steel sections are made in BOS furnaces, with a maximum recycled content of about 35 per cent. Steel imported from Europe may have a greater recycled content, but at the cost of more emissions due to its transportation.
As steel production has a very high carbon footprint, how can we make the way we use the material more sustainable? Unlike concrete, it’s not necessarily a case of changing the material’s properties to make it more environmentally friendly. It is more about using it more efficiently and economically and ‘designing out’ demand for structural steel. Below we examine seven options showing how this can be done.
1. Embrace the circular economy
The Steel Construction Institute (SCI) published a 46-page guide in October last year aiming to help the construction sector to reduce greenhouse gas emissions from structural steel – in an economical way – while encouraging the reuse of the material. Structural Steel Reuse: assessment, testing and design principles provides guidance on the procedures and processes for reclaiming steel used in existing structures and for utilising surplus steel, such as that from cancelled projects.
2. Reuse, reuse, reuse
Steel is routinely recycled but much less often reused, which would be much cheaper and less damaging environmentally, given that the process of recycling steel produces nearly as many emissions as producing the material from scratch. Barriers obviously include traceability and certification issues but reusing surplus steel from cancelled projects can reduce carbon emissions by up to 96 per cent compared with newly milled steel and is much more sustainable than sending steel back to the mill for recycling.
Because steel frames are basically a kit of parts, they can be dismantled and reconstructed. Steel sections are highly robust and being usually bolted together (rather than welded) means that they are demountable. Therefore, entire steel structures can be fully deconstructed and reconstructed in a different location in a matter of months without creating dust and noise.
After saying that, shear studs, which are used in composite beams and are welded to the steel section, have a huge impact on their demountability. There are some bolted options being developed at present to help mitigate this.
The SCI’s document is recommending that data collection, inspection and testing can be used to ensure that reclaimed structural steelwork can be reused with confidence. They suggest that reusing steel is limited to only steel produced after 1970 and to applications where the steel hasn’t been subjected to fatigue (such as bridges) or damaged by corrosion, fire or high impact.
3. Explore alternative materials where steel is not required
Steel is used in building construction for its high tensile strength and low cost. However, there are lower-carbon alternatives, such as timber, low-carbon concrete, stone or other materials that are also useful in compression. It’s about designing cleverly and prioritising: if large spaces and a lightweight structure aren’t required, why use steel?
4. Reduce spans
Steel’s most useful property – good strength-to-weight ratio – means that a little goes a long way when it comes to the material – giving architects flexibility to innovate and create new and exciting buildings. But reducing the spans of steel beams means that the length-to-depth ratio is reduced exponentially and less of the material is required.
5. Design for adaptability
Steel-framed buildings are by their very nature often easily adaptable if the internal configuration needs to change. Steel can span large distances, meaning that its steel-framed buildings often contain large, open-plan spaces that are easily reconfigured with lightweight partition walls. The steel frame itself can be designed to be easily adapted as well with parts added or taken away and being relatively lightweight in comparison with many other construction methods means that floors can be added without overloading existing foundations.
Steel structures are also often used to renovate heritage buildings – especially behind retained facades – meaning that the character of the building’s elevation can be retained while the building structure is reconfigured.
6. Avoid over-engineering
Early last year New Civil Engineer reported on a study that suggests that over-designed buildings in the UK are a secret contributor to carbon emissions. The research, by engineering consultancy Ramboll, found that buildings are being constructed in the UK with over engineered systems that have more electrical capacity, heating and cooling and structure than is needed. This comes because of trying to achieve technical compliance (as well as project pressures) and meet building regulations – causing a gap between predicted performance and reality. Therefore, architects and engineers need to design to suit the structural loads on every member so that steel isn’t wasted. They also need to challenge design load assumptions, so as not to over-engineer structures which are deemed as perhaps overly ‘safe’, further stifling innovation. Factors of safety or code compliance issues can lead to over-design.
7. Use steel efficiently
Finally, think carefully about what is driving the design of a steel structure. If it’s deflection, beams can be pre-cambered to reduce the size required. If it’s vibration, alternative structures options can be used such as perpendicular tie beams to save throwing weight behind the main beams and thus reducing the amount required.
A composite design for increased beam depth can be incoporated where possible to reduce the weight of the steel beams. This can drive up embodied carbon within a structure (especially when combined with the use of concrete) but this needs to be balanced against any increase to the building height/volume.
|Embodied carbon (CO2e) impacts of common steel construction products|
|Sections and plate||Hot finished/ formed tubes||Steel deck|
|0.84 CO2e (tonnes)||1.17 CO2e (tonnes)||1.13 CO2e (tonnes)|
From feilden Clegg Bradley Studios’ ‘Carbon Counts’ exhibit
- Carbon impact per m³ of steel: 12,170 kgCO2e
- Global production: 1,809 Mt (2018)
- Associated emissions: 7-9 per cent of direct global CO2 emissions
- UK production: 7.7 Mt (2018)
- Net imports: 3.1 Mt (2017)
- Forecasted consumption: Set to double by 2060
With thanks to Heyne Tillett Steel and Feilden Clegg Bradley Studios