THE NEW MOBILE WALKWAY OF GENEVA'S JET D'EAU
Like a ribbon that gradually unfurls across the water, the new walkway is a fluid area that invites you to stroll and wander around the foot of the most emblematic monument in Geneva: the Jet d'Eau. The watchword for this project was accessibility for all: boats, yachtsmen, walkers, tourists and people with reduced mobility. Everybody is welcome to this new public space that imposes itself as a bold gesture, but seems only natural.
Prize for Pedestrian Bridges, Structural Awards 2017 / Golden Prize and Prize for Infrastructure, Building Award 2017 / Prizes for Public Landscape & Other Architecture, American Architecture Prize 2017
This special object was designed as a part of a larger project called the "Esplanade du Jet d'Eau".
The esplanade runs for a distance of about 200 m from the wharf to the rotunda around the Jet d’Eau. The decking is about 4 m wide and is made from solid oak beams sourced exclusively from the forests of Geneva. Solid oak is the most suitable wooden material for the external conditions and the humidity of the lake, and the fact that is was locally sourced is one of the project's major assets. Other solutions with treated wood were not permitted because of the risk of toxins.
The bearing structure for the decking that includes pre-cast concrete beams and metallic piles, is intentionally hidden to give the impression that the decking is floating on the water.
Thirty metres from the wharf, the esplanade crosses an access channel to the port for small vessels. To maintain the fluidity of the space, we designed a whole new type of walkway: The structure rises like a wave to let boats go through, without interrupting the flow of pedestrians who can keep on walking.
The bearing structure for the decking is a scissor mechanism with a single axis of movement. Eight pistons at the ends can be used to separate the supports by sliding them on a rail, automatically all the scissor mechanisms open and the walkway lifts in the middle to a height of about 2 m, and lengthens by about 1.50 m at either end.
The scissor mechanisms are about 1.20 m high and each weigh 400 kg. They were cut from 20-60 mm stainless steel plate using a water jet cutter. This mechanism has to be produced with enormous precision, the pairs of scissors are linked to each other by 12 cm diameter metal axes including porous bronze rings to reduce friction, and adjust to the pairs of scissors without play.
The walkway is supported at two points on either end, so it is mounted like a cantilever bridge. The stresses on the support are huge, especially when the bridge is raised.
The chosen grade of steel was Duplex 1.4462, which has very interesting mechanical properties: great resistance to wear, good corrosion resistance and an elastic limit of about 500 MPa. The external support is under tension and the internal support is under compression, both sliding on the top of the rail thanks to bronze wear plates, which can take the loads whilst limiting friction.
The deck is a self-contained sub-mechanism that does not induce any spurious force on the main bearing system. When the walkway is raised, the deck turns into a stairway thanks to a series of conrods and ball joints at the end of each step.
Because of the extension of the deck, a standard bracing could not be used. Horizontal stability is provided by the embedding of the cross members in the load-bearing scissor mechanisms at either end of the walkway. The cross members are 13 cm diameter steel cylinders on which the deck rests. The absence of any standard bracing made us take care with the vibrational behaviour induced by pedestrians walking. A series of in situ measurements were taken to confirm the calculated values; these indicated that the frequencies of the vibrations are outside the problematic values defined by the SIA standard with the exception of the 1st vertical mode when the walkway is raised. However, the in situ measurements revealed that the rate of absorption was very high due to the assembly of the mechanism, which considerably reduces the risk of sympathetic resonance.
The walkway was entirely pre-assembled in a workshop in order to carry out a series of tests prior to final implementation. The structure was mounted on a metallic frame that was used for constructing, transporting and hoisting the assembly. The walkway was put in place on the supports, whose level had been previously adjusted with precision, using a crane on the wharf.
The project was opened to the public on 25 June 2016. The operation of the walkway is directly related to the operation of the Jet d'Eau: the walkway is in the raised position when the fountain is off and respectively in a horizontal position with the water is turned on. In this way, the public space is consistent and perfectly meets the requirements of the different users