Modern construction using long-forgotten techniques

In the urban environment, space is at a premium. Architects therefore look for ways to make buildings more compact ¨C and as cost-effective and environmentally friendly as possible. One approach to this is lightweight construction: the thinner the ceilings of a multi-storey building, for example, the more space is left for extra floors. As there is less weight for them to support, it is possible to save on construction materials in the foundations and walls, which in turn reduces costs.

However, there is almost no scope for weight reductions with the concrete construction techniques typically used today: to ensure that the floors of each storey can support the necessary load, they must be an average of 25 centimetres thick and must also be reinforced internally with steel bars or grids. This amount of material makes them very heavy.

Load-bearing without steel

A new approach has now been presented by researchers from ETH Zurich¡¯s Institute of Technology in Architecture, who have developed concrete floor elements in which the load-bearing slab is just two centimetres thick and yet extremely stable. ¡°This makes them 70 percent lighter than conventional concrete floors,¡± says Philippe Block, Associate Professor of Architecture and Structure and Deputy Director of the National Centre of Competence in Research (NCCR) in external page Digital Fabrication. At the same time, these elements help to protect the environment as they require less concrete, whose production gives rise to large quantities of CO₂.

The weight reduction is possible because, instead of being flat, the slabs are arched like the vaulted ceilings of Gothic cathedrals. Simply by virtue of their shape, they can support very heavy loads and so do not need to be strengthened with reinforcing steel.

Building like the old masters

¡°We based our design on historical construction principles and techniques that have since been forgotten,¡± says Block. To this end, the researchers analysed structures in various styles, including the Catalan vault. This traditional construction method was brought to the USA in the late 19th century by the Spanish architect Rafael Guastavino, who reinforced his brick vaults with narrow vertical ribs on the upper side. These ribs serve to create a flat surface for the floor, and also provide the vault¡¯s stability under asymmetric loading ¨C that is, when the weight of people or objects is not distributed evenly throughout the room.

The ETH researchers adopted this principle of strengthening ribs for their concrete elements. Using specially developed computer software, they calculated how the ribs would need to be arranged to produce the optimum distribution of compression forces under loading, resulting in an intricate pattern of thin lines that converge at each of the corners. The supports are connected by a set of steel ties that absorb the resulting horizontal forces ¨C performing the same role as the flying buttresses that support the vaults in cathedrals. ¡°The construction is extremely stable,¡± says Block. Stress tests have shown that it can withstand an asymmetric load of 4.2 tonnes, which is more than two and a half times what the applicable building codes in Switzerland stipulate.