The footbridge will straddle the Padua-Bologna highway at the Bologna tollgate end and is designed to be an imaginary gateway to the town. Lightweight and elegant, the structure will be a notable urban feature while ensuring low-environmental impact.
The main weight-bearing structure comprises two inclined “A”-frame pylons, reinforced by cross-members to allow a span of 90 metres from one support to the other and a height of 20 metres. At each support, the two posts of the “A”-frame column are some 10 metres apart and the thrust – generated by a weight-bearing mechanism very similar to that of a three-hinges arch – is balanced by a system of high-resistance woven steel cables. The two shafts of the “A”-frame columns are a series of variable-size, box-shaped steel sections.
The secondary weight-bearing system comprises a series of stays and a “ridge beam” made up of welded steel plates. The stays fan out from the centre point of the crown to support the deck together with the welded-plates “ridge beams”. The woven stay cables are made of high-resistance elements with nominal diameters of 40 and 28 mm. The stays anchor at the points of convergence of the stabilising cables and the traverse beams - set at regular 5 m intervals.
A transverse tensile structure at the outer perimeter of the deck provides dual effect stabilisation: four, 42 mm diameter, pre-tensioned cables on either side of the deck offset bending stresses on the horizontal and vertical planes induced by gravitational loads or the lift and drag caused by wind forces. On the vertical plane, the stabilising cables are connected to the stays by means of beam struts and contribute to offset the thrust generated by the arch.
The principal force generated at the supports is the vertical thrust induced by the structure itself, whereas the horizontal stress is mainly longitudinal and transversal drag caused by wind and seismic forces. Windbracing and stress-bearing systems have been dimensioned for minimum seismic intensity values of s=6. Two concrete abutments offset the structure’s elevational thrust. They will be hidden from view by an embankment of packed earth to minimise visual impact. The whole structure will rest on a foundation of sunken piles.
As well as the four supports located at the foot of the “A”-frame columns to minimise both vertical movement and rotational forces around the horizontal axis, two further supports on the longitudinal axis of the footbridge at the top of the foundation abutments will oppose horizontal movement. Four other struts will offset horizontal longitudinal movement: two fixed-type and two Domain-Jarret elasto-plastic type dampers. While these will allow horizontal movement at the foot of the “A”-frame columns induced by variations in length of the “ridge beam” caused by the “slow” action of the weight of people, snow and thermal excursion, they will block after just a few millimetres of movement if induced by “rapid” movements of wind forces or “seismic” type activity.
The technical literature recommends avoiding frequency ranges between 1.6 ´ 2.4 Hz and 3.5 ´ 4.5 Hz. The structure will be provided with a connection to a TMD (tunned mass dampers) system so that if, during the testing phase, alterations prove necessary, the mechanical features of the structure can be altered to avoid typical phenomena of vibration-induced resonance generated by people using the bridge or by other activities.