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Is steel a green material?

TECHNICAL

Greenness does not immediately spring to mind when structural steel is mentioned. The recent international Steel in Green Building Conference1 aimed to address this image by making an informed environmental case for steel at each phase of its life cycle.

Steel begins well. Rather than just recycling steel as raw material, it has less resource-consuming potential for re-use as a structural material. A mass market for used steel has not yet developed, but in one case study - the Maine Audubon Butterfly House - a 500m2 building has been framed with 45 tonne of recycled tube. Re-use is not without its costs, of course, both cash and environmental. Steel must be removed. cleaned, maybe cut and refinished, stored and transported. (Much the same is true of timber - structural timbers would also need regrading).

Designers could play a role in promoting re-use, by specifying used material if available and in design for re-use through, for example, standardisation of components or design for dismantling. However, the long time-scale of building renewal makes any benefit of re-use a minor part of the overall environmental picture.

Much more work has been done on the impacts of recycling steel. Used steel is normally fed into the melt - usually 20-25 per cent currently in the uk. Steel is thus less energy-intensive per tonne than when making primary steel from the raw materials. Structural properties are not degraded by this mixing. Steel can cycle round many times. The steel of your next can of beans may have previously been a bridge, or the other way round.

Alex Amato of the uk's Steel Construction Institute (sci) estimates that primary steel manufacture requires 25.5GJ/T; with recycled steel the figure drops to an average of 18.9GJ/T. So not a massive saving. Figures from Mathias Borg from Sweden suggested a wider gap, from 28 down to say 10. Savings will vary from country to country with the number of cycles and the percentage of recycled material.

Keith Eaton, also of the sci, stressed the need to develop an understanding of materials in terms of functional units rather than mass. For example, with steel manufacture at 25-30GJ/T and concrete at 1-2GJ/T, the materials look very different in embodied energy terms. But, he said, in terms of function delivered to a building, say in GJ/m2, the materials are similar.

This is backed up by a broader sci study2, of life-cycle energy use, comparing office buildings framed in steel, concrete and composite construction. The study took 7m and 15m span buildings with differing servicing and looked at the life-cycle energy use and co2 emissions over a 60-year life. In both embodied and operational energy they found little difference in energy performance, whatever the framing materials.

An expected result of the study is that embodied energy is of the order of 10 per cent of total energy, though varying particularly with the choice of servicing system. And the frame is a modest part of total embodied energy. Building operation remains the design priority. However, reducing operational energy can increase the importance of embodied energy.

Life-cycle analysis

Greenness is more than just energy, of course. One of the spurs for the conference was a current major international study of the energy and environmental impacts of steel, 'from cradle to gate'. This means the life cycle up to the time when steel leaves the factory, thus embracing all inputs, including recycled cans and cars. The study includes raw materials, production consumables and production energy, transport and manufacture plus releases to the environment and co-products (such as by-products used as feedstocks in other industries).

It is being carried out by the International Iron and Steel Institute, whose members represent about 40 per cent of world steel capacity. The intention is to make this the definitive study worldwide on steel sustainability. Its potential uses include industry and producer-company benchmarking, contributing to comparative life-cycle analysis, prioritising process improvements and analysing government policy. Its inventory of impacts could later be extended to full life-cycle analysis in particular sectors, such as construction.

For construction, Agnes Schuurmans-Stehmann from the Netherlands described a national working party with 18 different construction product area organisations. They are trying to standardise environmental criteria so as to make differing products comparable. Then designers can make an informed environmental choice. Within the framework of the developing iso standard covering life- cycle analysis, they are seeking to reach agreements on functional units, system boundaries (what's included), service life, maintenance, recycling and allocation of impacts to particular causes.

Functional units are being defined as parts of reference buildings, such as a 5m2 dwelling window - what are the environmental imapcts of creating this in different materials? Building frames are currently seen as too complex and varied to define a typical functional unit, so designers are left to define their own; data is just to be offered per kg of material. Service life includes the functional service life of the building, durability of materials, maintenance and periodic replacement of materials and components.

To date the Dutch system covers only impacts on the external environment, not indoors. There will be a system for verification of manufacturers' data. The ambition, Schuurmans says, is to move on to providing data on whole buildings.

Though there was no presentation from bre, it is doing somewhat similar work on environmental profiles of materials with 24 construction trade sectors. And there are expected to be materials selection criteria in breeam revisions next year. The us is also starting on an environmental certification system, through the us Green Building Council3.

Building in steel

A variety of buildings were presented at the conference, showing mostly that you can design low-energy buildings in steel (as with other materials). The main distinguishing environmental claim was that steel framing is more flexible, prolonging building life.

There is a push in North America as well as in the uk towards light gauge steel framing for domestic-scale construction. It appears to be early days still, particularly for fastenings. It is not that the buildings fail, just that lack of standardisation adds to the cost.

A comparative study of steel and other housing construction in the Netherlands by Jaap Kortman came out with a generally good environmental bill of health for light steel framing, though he put a question mark over galvanising because of the resource-exhaustion of zinc. (He did not have specific data on this.)

The point was repeated several times that only the first 50-100mm thickness of an exposed concrete floor soffit is effective when used for its thermal capacity. Part of the message, of course, is that this thickness can be achieved with steel-framed composite construction, not just concrete framing. One problem for designers, though, is that the troughed steel soffit is rarely acceptable visually. Chris Kendrick of Oxford Brookes University is testing a range of perforated suspended ceilings to check whether room air will circulate through them and exchange heat with the floor soffit effectively. His preliminary results are:

a suspended ceiling of perforated tiles with only 14 per cent openings, of 2.5mm-diameter holes, reduces cooling effect by about 17 per cent

microperforations - holes of 1.5mm diameter or less - should be avoided

with larger holes, say 10+mm, the cooling effect is greater than with no suspended ceiling. The reason for this unexpected result is unclear. Two possibilities are heat absorption and re-radiation by the ceiling and/or ceiling perforations channelling the air more effectively across the concrete soffit.

The big picture

At a conference on one material, steel, it is easy to lose perspective on the global view that is essential to understanding sustainability. In terms of material options, there were systematic comparisons with concrete, but not for example with masonry or timber. How are they to be compared by designers? Even timber has its green limitations. Indeed, one of the exhibition stands was the Wood Reduction Clearinghouse, an organisation seeking green alternatives to timber so as to reduce felling of the forests of the North American Pacific Northwest. As Meghan Clancy-Hepburn asked, should they not learn from the mistakes of the western European lowlands, largely denuded of trees over the last few hundred years?

At the larger scale, making environmental advances relies critically on clients' agendas. James Grose from Australia, involved in designing stadia for the 2000 Olympics, described how the Olympics had gradually become paler green as the developers' agendas took over. (He has still managed some interesting natural ventilation systems and rainwater collection off the sheeted roofs.) The major software company wrq Communications in Seattle is focused on retaining its high-salaried staff, including responding to their environmental agenda. Unusually for the us, staff cycle to work, so they get a heated bicycle store.

Location can be the biggest variable in environmental performance of buildings. Lindsay Johnston from Australia, describing his near-autonomous steel house, noted that the biggest annual energy impact will result from changing to a smaller second car.

Steel is important but needs to be seen in context. There must be a steel equivalent of not seeing the wood for the trees. Answers on a postcard.

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