Finding a Solution (Design Process)
The shape, structure, and overall design of this bridge were a natural response to the constraints of the site, uses, and structural and material needs. Everything in this design has at least one function other than aesthetics. As I tried to solve the different problems, I was presented with opportunities and possibilities that would not have existed otherwise.
The first challenge was to make the bridge high enough to allow for container ships and other boats to pass by underneath. This meant that, at the highest part of the bridge, there had to be at least 25 meters of height clearance between the water and the bridge above.
- The main support structure for the top of the bridge was located where it would not interfere with ongoing traffic.
- The top would also need to have long spans, wide enough for container ships to fit.
- A higher bicycle bridge meant that ramps had to be longer. For cyclists, a slope higher than 5% is considered a bit difficult, so the ramps had to be less than 5%. Ramps also had to be located far away from buildings, as to not block their views. Finally, ramps and their support columns could not interfere with existing traffic (boats, ships, pedestrians, cyclists, automobiles, trains).
- Location of entries to the bridge was dictated by existing bike and pedestrian circulation trends and by the constraints mentioned above.
- Not to forget that cruise ships also have to pass by. The bridge could not be high enough to allow cruise ships to fit underneath, so…
- The top of the bridge swings open to let cruise ships pass by. It would take the bridge about 2 minutes to swing 90 degrees. Then, it would take the cruise ship about 3.5 minutes to pass by (at a speed of 4.34488 Knots or 5 miles per hour). So the approximate time the bridge would remain open is 7.5 minutes.
- With height come better views. As a response, the perimeter of the top of the bridge is modified to maximize views and to allow more people to enjoy the views at once.
Long and winding ramps can become tedious and tiring. However, if the top of the bridge is turned into a destination, with amazing views and plenty of places for rest and leisure along the way, the experience becomes more like being on a hill, and not just crossing a bridge.
- What’s a hill without vegetation? Provide vegetation (grass and trees) along the ramps and at the top of the bridge. Now, the top of the bridge feels more like a park than a bridge. Welcome to Island Hill Park!
Trees and grass turn an inert structure into a lively ecosystem by attracting animals and insects, and providing shade and places for respite, play, and socializing. Trees should be spaced far apart so that wind pressure does not become a problem.
As it rains often in Amsterdam (about 200 days a year), no irrigation is necessary for the bridge’s vegetation. All paving should guide rainwater into the vegetated areas. Within the truss system, there is space for some rainwater collection to use if necessary. However, it is not likely to be needed. In fact, too much rainwater may be an issue, so excess water should drain down into the river.
Structure and Materials
All these features need to be structurally supported and we want to use as little materials as possible. We also want to use recycled or recyclable, and reused or reusable materials where possible and a structure that can be disassembled if needed.
The top of the bridge is the swinging part of the structure. To allow for the structure to swing, it requires a single column that can support the whole swinging bridge. The swing bridge also needs to be able to span wide enough for 85m (279ft) wide cruise ships to easily fit when the bridge swings open. The best structure I found for meeting all these requirements was a mixture of a truss bridge (to support the structure longitudinally) with a suspension bridge (to support the decking transversally).
Because the main column is a spherical tower made of a steel truss system, it is hollow, with interior space being utilized for equipment, elevators, stairs, conduits, etc.
After locating the ramps, attention was given to allow as much of a span as possible without jeopardizing their structural identity. For this reason, as for requirements mentioned above, a steel truss system was also chosen for the ramps, allowing an average span of about 37m (121 ft).
The columns for the ramps are made of concrete, or a recyclable and/or reusable material with similar strength and durability.
To allow for reuse, rough (non-slippery) cobblestone (or a similar local material) paving was chosen for bicycle lanes and sidewalks, both on ramps and on the swing bridge.