About the competition, we know that the city of Turin hosted the 2006 winter Olympic games and had embarked on an extensive programme of construction of various venues and infrastructure for the games. The city took advantage of the infrastructure programme to incite urban renewal in the southern part of the city that has been affected by the economic downturn in the car industry. The Olympic village, situated in the disused Mercati Generali in the Lingotto district adjacent to the main railway lines entering the city from the south, is part of this new infrastructure and will contain housing and a logistics centre.
In this context, the footbridge by Hugh Dutton Associes (HDA), was exploited to provide a symbolic focal point for the entire village as well as provide a more ambitious role of a sculptural symbol, both for the Olympic Games and to represent the dynamism of the changing city’s regeneration of the Lingotto area beyond and after the event of the games.
The footbridge provides a 365m link, 4m wide, between the main Mercati central ‘gull-wing’ hall and the existing parking access footbridge on the Lingotto development with a free span of 150m over the railway tracks. It is destined for pedestrian and bicycle traffic.
The safety considerations with regards to the railway have a major impact on the design and realization of the bridge. Strict time limitations were given to ensure that construction procedures did not interfere with or provide any danger to railway traffic. A 2.5m high protective barrier, of which the lower 1m must be solid and the remaining 1.5m above in safety netting, is to be provided above the railway tracks for pedestrian safety and to prevent objects from falling on the tracks. The railway tracks are electrified and therefore the acceleration of corrosion of the steel components, notably in the foundations, due to residual electricity in the damp earth is an important consideration. All activities over the tracks during construction and future maintenance are subject to specific safety constraints.
The parabolic arch, inspired by arch framing of the existing Mercati halls by the architect Cuzzi in 1932 is an optimal structural form – the supported loads travel to the ground in pure compression. Traditionally, arches are stabilized by the masonry they carry as these serve to provide geometric stability, keeping the compression rim in it’s plane. In the case of the Passerella arch the suspension cables carry out the same stabilizing role, preventing it from buckling. The concept is made clearer for the passerella arch and it’s cables if we consider their performance as analagous to a bicycle wheel. The rim supports the weight of the bicycle and it’s rider through the thin wire spokes that connect it to the axle. The spokes, by both their radial geometry and sectional triangular configuration, can resist considerable loads and remain stable. The rim is in pure compression and the spokes are in pure tension. The curved parabolic shape is adapted to optimize the path of the compression forces so as to reduce bending loads in the section.
The arch is founded on a thin strip of land between the railway lines and the via Zino Zini that borders the Mercati site to the east. This strip is one of the only available bearing points from which to support the bridge in the congested urban context. The planar arch structure exploits the potential of a laterally wide base whilst remaining in a singular sectional plane.
The arch consists of 370T of welded FeS355K steel plate, in a hollow 3m equilateral triangular section determined both by the necessary structural performance against buckling between the points at which the cables are attached, and the requirement for maintenance access for inspection. The triangular profile is constructed from pre-cut sections curved to conical surfaces and includes stiffeners and diaphragms at the cable attachment points.
The inclined arch is supported by eight pairs of 75mm diameter locked strand galvanized cables on the Mercati side gathered in double anchor points on either side of the deck corresponding to the four ‘piedritti’ column supports that transfer the tension loads directly to the foundations. The deck spanning the railway tracks is suspended from the arch with another eight pairs of 55mm diameter steel cables, matching the ones that support the arch. Additional cables at the base of the arch in a diamond configuration tie the arch and deck together.
The deck is divided into two distinct parts that are structurally independent. The larger portion, called ‘strallata’, spans the railway tracks suspended from the arch with extensions at both the Mercati and Lingotto ends, totaling 235m in length. The smaller portion, called ‘Lingotto’, provides the link between the Strallata section and the existing parking access footbridge of the Lingotto shopping centre building.
The 150m span strallata deck is suspended from the arch by the cables on 18m spacings. At the Mercati end, it extends 95m to the stair access point in front of the ‘gull wing’ Mercati building. The Lingotto portion extends a further 120m to the existing parking access footbridge. The total length of the two portions is therefore close to 365m in total. The strallata portion spanning the railway is gravity supported by the arch while the Mercati and Lingotto portions by the ‘Piedritti’ columns.
Three dimensional geometry models were prepared on Autocad at HDA, for the wire frame geometry and surface modeling for detail studies, notably of critical assemblies such as the cable anchors, arch steel plate geometry, foundations, staircases, cable end socket connection details, countercable struts and piedritti details. Architectural modeling using 3D renderings developed from the Autocad models were an important communication tool.
The footbridge is designed to accommodate loads of public crowds, taken as full or partial loading in the most unfavourable conditions. Other applied loads incluye prestress of the cables, snow, wind and thermal stresses. The most significant loadcases were wind and partial crowding. Even though Turin has no history of seismic activity, analysis was carried out at the request of the railway authority, but was shown to have a negligeable effect on the structure.