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The Mobile Walkway of Geneva

The Mobile Walkway of Geneva

Last week I had the opportunity to travel to Switzerland. My days were filled with beautiful scenery, decadent chocolate, and too much cheese for my own good. While walking around the city of Geneva, a bridge caught my eye. Located right in the city center, this pedestrian bridge connected the esplanade to the main area, directly in front of the Jet d’Eau. When I got home that night, I knew I had to look it up and see what this bridge was all about.

Structure Information

Figure 1. Mobile Walkway located in Geneva in its upright position for boats to pass through [1]

The pedestrian bridge at the esplanade is located south of the Pier Eaux-Vives and officially called The Mobile Walkway of Geneva. The project was completed on June 25th, 2016 and serves as a symbol of inclusiveness to the Swiss. The project cost 4.5 million francs ($4,536,225) and was funded by the State of Geneva, the GIS, and a private foundation that wishes to remain anonymous.  Previously, the pier could only be reached by walking up a set of narrow stairs, making it not accessible to everyone. Designed by the structural firm Ingeni, this new walkway is a flat path, making it accessible to wheelchairs and strollers. Also, the unique design folds up into a set of stairs when a boat needs to pass underneath. Barbara Tirone, the vice president of the associated Handicap Architecture Urbanism (HAU) stated “It is symbolically very strong to allow everyone to finally reach this emblematic site of Geneva” [2].

Historical Significance

This new pedestrian bridge is the definition of innovative.

Figure 2. Jet d’Eau in Geneva [3]

The engineers, Etienne Bouleau and Gabriele Guscetti at Ingeni, even patented their invention of a mobile walkway. This pathway is typically a flat pathway allowing access to all pedestrians, but when a boat needs to pass below, it compresses together to form an arch bridge and allows the boat to pass. This project proposed the “widening and extension of the existing pontoon by a generous wooden esplanade” [4]. The bridge uses a scissor mechanism in order to change from an arch bridge to a cantilever bridge.

From my research, this seems to be the first bridge of it’s kind, hence the engineer’s patent on their design. The only previous example I could find is a trial run for scissor-like bridge to be used during emergencies. But that model was for transportable bridge, not ones that are meant to be permanent [5]. Because of this innovative design, I am sure that we will be seeing this style of pedestrian bridge popping up around the world within the next 5-10 years. The concept of allowing boats underneath the bridge while allowing accessibility for everyone to cross is a tremendous deal.

Cultural Significance

Figure 3. The mobile walkway lays flat to provide accessibility to the jetty for everyone [1]

The area around the Jet d’Eau is known as the Mecca of Geneva. The Jet d’Eau was built in 1886 to “control and release the excess pressure of a hydraulic plant at La Coulouvrenière” [6]. Soon afterwards, the fountain became a symbol of the city, so the pressure was amplified, and it was relocated to the center of the lake. To the people of Switzerland, the fountain became “the symbol of strength, ambition, and vitality” [7].

Before this new foot bridge, the only way to get to the jetty was by crossing a narrow wooden bridge with steep steps. This made it nearly impossible for people with walking difficulties, or even strollers, to get to the other side. By constructing this new walkway, it enabled all people to cross over to the pier and see in Jet d’Eau in its full glory. The people of Geneva loved this new installation. They felt like their pride of the city was finally able to represent their feelings towards being welcoming to all. The bridge normally lays flat and raises into an arch when a boat needs to pass. Unfortunately, I found no details on the mechanism used to control the bridge’s movement; I assume it is a hydraulic system given its location.

Structural Art

Figure 4. Front view of the Geneva Walkway in arch form [1]

In my opinion, this structure is a great example of structural art. In regards to Billington’s first structural art criteria, the bridge checks the box for being scientific. This was the first of its kind in terms of engineering. The engineers worked hard to be able to please everyone, and certainly succeeded, justifying their decision to get a warrant for their design. Socially, this new bridge brought the people of Geneva together at their city center. It made everyone who visited the city feel welcome because they are now able to see such a big attraction. Symbolically, this bridge represents the kind and inclusive people of Geneva and how important the Jet d’Eau is to them. They spent 4.5 million Francs on this bridge in order to make it accessible to everyone. This certainly was not a requirement for the city,as you can see the Jet d’Eau from farther away (I saw it from the airplane when we were landing!), but the city decided to spend their money to truly showcase this symbol of the city.

Structural Analysis

The first thing you notice about the structure is the groundbreaking scissor mechanism that allows it to move up and down. The scissor mechanisms were cut using a water jet from a 20-60mm steel plate and are about 1.2 meters high and weigh 400kg. The pairs of the scissors are linked by a 12cm diameter metal aces that includes porous bronze rings in order to reduce friction and to be able to adjust the pairs of scissors easily. Duplex 1.4462 grade of steel was chosen because of it’s great resistance to wear, good corrosion resistance, and it has an elastic limit of about 500MPa [1].

Because of the design of the deck and how it extends, a standard bracing system couldn’t be used. Instead, horizontal stability is provided by the cross members in the load-bearing scissor mechanisms at both ends of the walkway [1].

I chose to analyze this structure when it is in it’s cantilever form because the entire time I was in the area, I only saw it lay flat.

Figure 5 illustrates the load path of the distributed load of pedestrians. I chose to use a distributive load because while the Jet d’Eau has light shows and during holidays, the entire lakeside it packed full of people. The load first goes to the deck where it then transfers to the pin supports. From the pin supports, the load travels to the trusses and works it way outwards to the column supports at the two ends.

 

Figure 5. The load path on the bridge due to pedestrians

Figure 6. Stress forces on the truss (plate) members

Figure 6 demonstrates where the stresses are the largest when the bridge is in cantilever form. Note, the stresses are almost the same throughout the whole bridge, except for the ends which have a slightly higher stress. This is due to the bridge being symmetrical and evenly distributing the weight of the live load.

I wanted to calculate the stress in the individual truss members. To do so, I first calculated the tributary area. Figure 7 shows how the forces are divided among the pins; there are 16 pins (represented by white circles) along each side. The red lines show how the tributary area is divided. I also added the green dashed line to show how the tributary area aligns with the elevation view. Figure 9 shows all the calculations.

Figure 7. Tributary area of the pin joints

Figure 8. Idealized truss system of the center 9 joints

I used a live load of 85 psf because that is an average pedestrian live load. I found the width of the bridge to be 12.5 ft. Figure 7 illustrates all the forces acting on it [8]. I calculated the dead load of the steel and assumed a width of .5 ft for the steel plates and found it to have a thickness of .2 ft. For the deck, I assumed it to be made of redwood because that is a popular wood for decks. I also assumed a thickness of .67 ft because that is the minimum standard according to codes for wood jetties. I started by calculating the tributary areas, then applying the area to the density of my materials to find all the loads on the connections. Next, I used method of joints to calculate the internal forces in the truss.

Tributary Area

Deck: (12.5 ft/2)(892 ft/ 16) = 348.44 ft2

Steel plates:

(sqrt(2.52+42))= 4.7 ft

(4.7 ft)(.5 ft)(.2 ft) = .47 ft3

Load on each pin

Live load: (85 lb/ft2 )( 348.44 ft2) = 29,617 lb

Dead load (steel): (490 lb/ft3)( .47 ft3) = 230.3 lb

Dead load (redwood deck): (32 lb/ ft3)( 348.44 ft2 )(.67 ft) = 7470.56 lb

Figure 9. Method of joints

Calculating internal forces

∑Fy = FAC(2/2.36) + FAB(2/2.36) – (29617+230.3+7470.56) = 0

∑Fx = FAC(1.25/2.36) – FAB(1.25/2.36) = 0

FAC  =  FAB

∑Fy = FAC(2/2.36) + FAC(2/2.36) – (29617+230.3+7470.56) = 0

FAC  =  FAB = 22,017.53 lbs or 22 K

Because the bridge is symmetrical, all the members of the truss have the same force of 22 K. Due to all the assumptions, my calculations were quite off from the stress values shown in Figure 6, however I verified the idea that the stress should be consistent throughout the structure.

Figure 10. Construction of the mobile walkway.

The construction of the walkway was unique because of the mechanisms. The entire walkway was pre-assembled in workshop shown in Figure 10. This way, the engineers were able to run a series of test on the structure before it was available to the public. The structure was mounted on a metallic frame that was used for constructing, transporting, and eventually hoisting the assembly.  Once on the wharf, a crane was used to place the walkway on the supports [1].

I think it was quite simple to show this design to the stakeholders considering it was built indoors first. Because it was the first design of it’s kind, many different tests were run on it and I am sure that the city of Geneva was involved throughout the process because they were funding it. The other known company that helped fund the project was GIS, who is actually a lifting solution company. I am sure that when a crane was needed GIS was the company that was selected. Sounds like a conflict of interest to me, but if it gets the job done I guess it’s alright?

Personal Response

From visiting this bridge, it was nice to see how many people were just sitting outside enjoying the view. From tourists to natives, there were many people outside just taking in the sunshine, laughing, and appreciating their time together. I understand how this place can be viewed as the “Mecca” because of the energy that is felt when you are there. I’m glad the city of Geneva wanted to make everyone feel welcome and made sure this bridge was a priority.

References

[1] https://ingeni.divisare.pro/projects/338960-the-new-mobile-walkway-of-geneva-s-jet-d-eau
[2] https://www.tdg.ch/geneve/actu-genevoise/jet-eau-presente-nouveaux-atours/story/22163401
[3] http://www.geneva.info/jet-d-eau-fountain/
[4] (http://www.ge.sia.ch/node/182
[5] http://www.engineersjournal.ie/2015/07/28/novel-scissor-like-bridge-structure-use-emergencies/
[6] (http://www.geneva.info/jet-d-eau-fountain/
[7] http://www.geneva.info/jet-d-eau-fountain/
[8] http://www.conteches.com/DesktopModules/Bring2mind/DMX/Download.aspx?Command=Core_Download&EntryId=9850&language=en-US&PortalId=0&TabId=144

Comments

  1. adiabre3 says

    I remember seeing your work on this bridge and I thought it was so cool. It kind of looks like something out of Game of Thrones. The design really is innovative. Instead of making a bridge that opens up, they decided to keep it as one piece and just modify its shape. Has the bridge had any mechanical failures since its inauguration? Have there been any major issues with the design that you know of?