The two towers are in concrete until level +175m and in composite construction consisting of an inner concrete ring with a stainless steel skin for the top 120m. The original concept had a conventional steel structure above level +175m but Arup found that this configuration would be too lively and lead to unacceptable vibrations of the stay cables. Using a heavier composite section instead of a pure steel structure solved this problem. For reasons of durability and to enhance the appearance, further studies concluded that the tower skin should be fabricated from a duplex stainless steel (grade 1.4462 to BSEN10088) with a shot peened surface finish. The deck itself is made of steel in the main span and of concrete in the side spans.