Calvin Wilson of Metal Technology looks at the relative pros and cons of steel and aluminium from a structural perspective
Architects need no introduction to the benefits of metal as a structural material with design flexibility, robust performance and aesthetic benefit. Metals have been used in buildings for centuries, and every building constructed today is likely to contain metal, regardless of the construction method chosen. Copper, brass, bronze and steel have been prized throughout human history. Aluminium is the ‘new kid on the block’, dating back a mere two centuries. We now know that aluminium is our most abundant metal, forming about 8 per cent of the earth’s crust and our third most plentiful element, surpassed only by oxygen and silicon. When it comes to the structure of modern buildings, among the metals the choice tends to come down to steel and aluminium competing for the architect’s favour. So, how do they compare?
Steel is an alloy, usually produced by a reduction of iron ore with carbon as the alloying material. Its great strength as a building material is simply that – its strength! The yield and tensile strengths of steel are higher than aluminium and are in the range of 250-1000 N/sq mm and 400-1250 N/sq mm respectively. Steel is also relatively dense, usually ranging between 7.75 and 8.05 tons/cum, which makes steel stronger but significantly heavier than aluminium. Aluminium is not an alloy, but a pure element. It is soft, durable, lightweight, non-magnetic and ductile in nature. The yield and tensile strengths of aluminium and its alloys vary from 30-500 N/sq mm and 79-570 N/sq mm respectively. There is no question that steel is the stronger of the two but with that comes bulk. Lightness is the outstanding – and one of the best-known – characteristics of aluminium. It has a density around one third that of steel. This makes it lightweight, but does not affect its strength. Indeed, because aluminium is one of the lightest engineering metals, its strength to weight ratio is superior to steel. Aluminium is highly ductile, significantly more so than steel, and this is another of its great strengths, particularly from an architect’s design perspective. Its flexibility makes aluminium easier to cut and form and thus customise to bring the architect’s vision to life. When it comes to corrosion resistance, aluminium outperforms steel. Aluminium doesn’t need a protective coating – it creates its own, through self-passivation, generating a thin but tenacious layer of oxide. With iron as a base component, steel will oxidise to produce rust, however it can be coated or otherwise treated in order to mitigate against this.
Aside from its lightness and malleability, aluminium’s other great asset is its environmental performance. Because of its inherent thermal properties, aluminium can provide insulation, which can be enhanced with thermal break technology. Since it is most often used in combination with glazing, it also facilitates the cost and health benefits of natural light. Famously, it is 100 per cent recyclable and there is no difference in quality between virgin and recycled product. Steel manufacturers and fabricators will extol the virtues of steel, aluminium manufacturers and fabricators (like Metal Technology) will step up on behalf of aluminium. For an architect, the logical answer is to employ the benefits of each, and both together, as appropriate. One quite magnificent example of this is the Cocoon Tower in Toyoko, designed by Tange Associates and completed in 2008. It is the second tallest educational building in the world, comprising 50 floors and standing 204 metres high. Tange Associates created the elliptical shape of the structure to allow for more varied spaces, with the cocoon theme reflecting the incubatory nature of education to transform its students’ futures.
In its construction, both steel and aluminium are harnessed for their most prized attributes. Steel comes to the fore as the main core structure of the tower, providing strength and stability in a modular construction allowing this, like many high-rise buildings, to achieve greater heights. The magnificent, curved expanses of curtain walling that give Cocoon Tower its name pay homage to aluminium. Here the design flexibility of aluminium came to the fore and, indeed, Tange Associates’ blue glass and white aluminium curtain wall grid composition won the Emporis.com 2008 Skyscraper of the Year award.
As a systems company, we continue to be impressed by the virtues of aluminium, which enables us to design complex and intricate, project-specific extrusions. Its robustness withstands high levels of usage in the form of doors and windows, and its strength in forming slim curtain walling supporting structures to large expanses of glass allows light to be the prominent feature. All this from a corrosion resistant material that is 100 per cent recyclable!
Calvin Wilson is managing director of Metal Technology