Due ponti sull'Arno a Firenze | Maurizio Mosca

The designs evolved directly from an analysis of the topography following an extended visit to the site, and each one belongs uniquely in its setting; the two could not be interchanged. The form of the suspension bridge and the arch in each case responds to the views along the river and the landscape setting of the background. In one case, the gentle river and wide landscape, open to the sky and the hills behind, invites the subtle serenity of the suspension bridge, while in the other case, the steep wooded river banks and ravine-like setting suggests the need for containment by the embracing form of the arch.
In both cases, the main supporting structures are inclined slightly outwards, opening up the views along the road and enhancing the experience of the route. The second bridge can be seen ahead while crossing the first, emphasising the relationship between the two. Both designs adopt a reflected symmetry to create an asymmetric form, and this is one of their common characteristics which reveal the family resemblance.

In the case of the suspension bridge, the heavily skewed alignment and wooded river banks mean that it will be impossible to see a clear true elevation. Instead the important views will be along the river in both directions. So we have placed the masts in line with the river bank on each side, and not perpendicular to the road, thus avoiding the visual clutter which would otherwise occur due to the confusing interplay of the two overlapping planes of suspension cables and hangers. The masts are also partly concealed among the trees so as to emphasise the lightness of the bridge as it sails apparently effortlessly across the river with no visible means of support. By inclining the masts backwards and outwards, and allowing the hangers to follow their lead in an open fan shape, the design achieves a delicacy and lightness which respects the serenity of the landscape. The asymmetry in this case is achieved by taking two symmetrical forms and moving them longitudinally relative to each other.

In the case of the arch bridge, the two arches are asymmetric and opposite handed, but placed symmetrically about the centreline of the bridge. This creates a unique and dramatic shape in a fluent sequence responding well to the shapes of the hills behind. The soft form expressed by the arches is drawn out by the quiet dynamism produced by their contrasting symmetry. The presence of the two bridges can be interpreted as the result of a single architectural plan, separated into two phases. The suspension bridge opens up into the landscape whereas the arch bridge closes the whole composition, providing a continuous visual relationship with the background hills, the river and the rich vegetation. Both individually and with their combined dialogue the bridges elevate the new infrastructure to reflect the awesomeness of the surrounding natural beauty.

The form and detail of the bridge deck is identical in both cases. This serves not only to underline the family resemblance but also to maximise the economy of the scheme. The supporting hangers are at 7m centres in both cases, permitting the use of repeated identical pre-fabricated deck sections and details throughout.

The real innovation of this proposal is the use of Glass Fibre Reinforced Plastic (GFRP) as a principal structural material in the suspended bridge deck. Having been used in major structural applications for over 50 years, this self-finished, lightweight, durable material is now becoming widely accepted as a primary structural material in bridge decks. It is acknowledged that this material uses less energy to produce than the equivalent steel structure and that it performs well against most sustainability criteria. Its use in this project brings this state of the art sustainable technology and low whole-of-life costing to contemporary highway bridge design in Italy.