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Metal Works opinion

Stephen Morley, design principal with Modus and Sinclair Knight Merz, explains the joys and frustrations of engineering dramatic canopies for sports structures

'We want a roof covering tens of thousands of people; lightweight, very cheap and which looks as if it floats above the arena with no visible means of support.'A familiar brief? It should be to any engineer of sports structures.

From this we set to work producing sketch ideas of cantilevers, arches, frames, ring systems, trusses, shells, tensegrity systems and a myriad of other ways of combining tensions and compressions, catenary and arch forces, the building blocks of long- span structures. It may seem strange with such a broad exciting brief, but the first thing we look for are any constraints; rocks from which solutions can start to grow in the ordered world of engineering. Approaching these constraints from our slightly different perspectives turns them instead into opportunities and, sometimes sooner, often later, a favoured solution emerges. (For instance, Chelsea FC's unique inclined-catenary and flat-arch roof was born from the need to accommodate development immediately behind the cantilevered stands. ) The principles of structures, whether short or long span, are very simple and one can get the measure of a structural system quite quickly. The sizes of the main arches at Stadium Australia, spanning 286m, were estimated in one line from the ubiquitous formula WL/8e; it just took eight months to prove it! Eight months of very necessary and well-executed work I would add for the benefit of my office. From such back of 'vegetarian meal for two' packet calculations (more PC nowadays! ) an initial estimate of element sizes, structural depths and unit weights is derived for outline costing. Then comes the first team panic: 'It costs how much?'

Several late nights and hopefully not more than one 'value engineering' session later we are collectively back on track with costs having also passed through the phase of 'can you get rid of those fussy diagonals?' relatively unscathed.

As engineers we have an easier time defending what we believe to be the right solution with the full weight of the planet (gravity) behind us instead of just our convictions and instincts. We can use a simple statistic to explain the degree to which we rely on high-strength materials, such as steel. For example, explaining that the pins holding the Wembley arches sustain a stress equal to the pressure under 25 miles of water, soon puts paid to any suggestion of a reduction in their size!

A passing reference to that last stadium you did with 'the brilliant engineer from Wagga Wagga' which was 'a fraction of the weight' is also quickly dismissed when we point out that this same roof, designed to the Australian codes for a maintenance load of 1,000 tonnes, would need to sustain a maintenance load of 2,400 tonnes when designed in the UK - interestingly it would only have to support 10 tonnes under the French codes.

Detailed structural design seems unavoidably complex when one is dealing with long-span systems. Effects that are of little consequence with normal-scale systems, such as geometric changes under load and residual effects from erection, become significant and require careful analysis. It is little wonder that we pale when you discover that the WCs don't fit and tell us that the only solut ion is to make a sma l l change to the arena geometry two weeks before the steel tenders are due out!

Still, we quickly reach the stage where we can relax and watch the cloud take shape while the architect wrestles with door schedules and the lack of co-ordination of services. Finally, the big day arrives, the opening ceremony. Walking among the crowds entering the stadium, I hear people say 'so light - it appears to float like a cloud over the stadium; which architect designed this?'

Do we mind that our role is so little recognised by the public? No, because for a while we were allowed to play with the biggest Meccano set in the world, and we know that it will be there to inspire the next generation of potential cloud trainers.

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