tensile times

Efficient use of materials is vital in an eco-friendly society. Stuart Holdsworth of Capita Symonds explains how tensile structures are a viable, attractive, and economic solution for many structures…

The tensile properties of materials have long been exploited by structural engineers. Back in the 18th century, engineers designed and constructed suspension bridges exploiting forged chains to carry the deck loads.

Since then, the advance of material properties and manufacturing capabilities has given modern engineers far more scope for innovation. This, coupled with advances in desktop computing, has allowed a step change in the types of structure that can be economically designed and constructed. In fact, tensile structures provide the only current means for efficiently covering large areas under a single unified structure.

For example, areas exceeding 100000m2 can be covered using a single unifying membrane that can keep both the heat in and out (maintaining an even environment through 40˚C temperature ranges), whilst allowing almost natural levels of daylight to enter. All this can be achieved with a roof structure weighing on average less than 50kg/m2.

This ability arises from the efficient manner in which tensioned fabrics and cable grids can exploit the natural characteristics of materials to the maximum by the use of carefully tailored shapes. These structures can be exciting to behold, gaining presence from their scale and an efficiency of form that seems natural and pleasing to the eye. Durability need not be a major concern either. Repairs are relatively easy and do not require advanced technology or techniques.

It is noticeable that tensile structures are now being enthusiastically embraced in the ‘emerging’ countries of the world. Examples include the recent Khan Shatyr in Kazakhstan, the Fuda University Stadium in China and the soon-to-be constructed Jawaharlal Nehru Stadium in India. Ribbon bridges that exploit the tensile capabilities of steel or fibre cables have also become the new genre for long, unsupported crossings. Recent examples include the Mkomaas Valley Bridge in KwaZulu-Natal and the Maldonado Bridge in Uruguay.

Tensile bridge structures are also popular in the more advanced economies. We are all familiar with the cable stayed bridges that dot the European highway network, from the Pont de Normandie to the Millau viaduct and the Nový Most in Bratislava.

Such structures combine efficiency of form with an aesthetic that makes them compelling icons and a statement of pride and engineering prowess. They have a classic, almost sculptural form that makes them instantly recognizable, opening the possibility of them being associated with a single location, just as the Opera House is associated with Sydney.

The arguments for the continued development of tensile structures are compelling. The endeavour to cover the widest possible spaces with a minimum of internal supports, to build adaptable structures with a minimum of expense and yet achieve an elegance of form that captivates the imagination will continue apace, but the designer needs to be aware of and respond to a range of criteria that are less onerous with more traditional solutions.

Very lightweight structures have a different, almost exaggerated, response to certain loads, especially dynamic forces. The tools for analysing these factors and designing solutions for them are now part of the designer’s armoury, but there is as much art as science in crafting an optimal structural form. Computer power cannot yet replace intuitive understanding of structural forms and materials.

Tensile structures are attractive ambassadors and “must-haves” for many nations. The race is on for the longest, the tallest or the most elegant as a statement of national sophistication and prowess. As efficient users of materials they are an increasingly attractive solution in a more environmentally conscious world.

Stuart Holdsworth (stuart.holdsworth@capita.co.uk) is a Director in Capita Symonds’ Structures team.