The National Stadium is two structures in one - a massive steel lattice wrapped around a giant spectator bowl. The bowl had to be designed first to seat 91,000 people, says Michael Kwok, from engineering giants Ove Arup and Partners Hong Kong. 'First you have to work out the bowl to see how it can be most efficiently angled ... so people can see the whole event,' Kwok said. 'The bowl has to be as compact as possible. It is relatively conventional. Then a skin is wrapped around it.' And that is where the real cleverness begins. Kwok says several geometric patterns were overlaid on each other to create the random look of the 'Bird's Nest' exterior. 'The several layers create a chaotic effect,' he said. 'But the shell structure has a geometry and logic.' Two dozen sets of steel columns around the stadium form the main structure and trusses join each of those sets to two others. 'If you look at the main structure it is regular,' Kwok said. Then several other layers of interconnecting steel beams were added to the metallic web. 'You can't really pick out the main structure and the secondary structure,' he said. 'Visually they are the same.'