ce n'est pas un pont (THIS IS NOT A BRIDGE )
——Pedro Pablo Arroyo Alba (白德龙)
After finishing the Wieden+Kennedy China Office, our own CA-Group office (both in Shanghai), and several temporary constructions, we are about to finish our first public building: a bridge (?)
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On December 2004, we were commissioned the design of a pedestrian bridge in the Plot-18 of Qingpu District, Shanghai. This was actually the first design I did after arriving in China. The bridge would connect the two sides of the DianPuHe canal, separated 50m, and thus linking Hebin road at the north bank, with a new housing development at the south bank. Next to the bridge, inside the new development area, the master plan determines the location of a catholic church, designed by the Spanish architecture office S-MAO (Sancho-Madridejos).
Our strategy of borrowing concepts from local historical references and other variables of the site, together with the objective of optimizing, revealing and expressing the structural performance of the bridge, will generate the final form of the project.
Learning from the wonderful bridges of the great water town of ZhuJiaJiao, very near Qingpu, or those from the exquisite gardens in Suzhou, we decided to link both sides of the canal with a bent path, indirectly, instead of using a common direct line. The winding axis responds to different access conditions and also visually adapts to the surroundings. To the north, one landing direction is aligned with Pucan road. To the south, the other end is pointing to the entrance of the public square in front of the church. Both axes extend 12.5m over the waters from each bank, and a middle sector of 25m connects them. Since the central part of the bridge is facing the church directly, users will have a privilege perception of the building. As a result, this perceptual relation creates a very strong spatial tension between the two structures, which activates the area of the public square.
The load-span ratio recommends the use of the metal truss as structural model. In order to resist the strong torsion stresses associated with the support-less winding shape of the bridge, we need to activate structurally all the sides of its distorted volume. Not only the vertical, but also the horizontal surfaces of the bridge behave like a three-dimensional beam, working as a whole, like a tube, and deformed following the direction of the zigzag.
Both shores of the river have a contrasting character. At the north bank, the bridge lands on a perpendicular narrow street with traffic. At the south, the public space is wide and tranquil, free of vehicles. This fact is emphasized via the design of asymmetric constraints for the structure. The bridge has a simple support on the transversal ramp that faces the rapid street at the northern access. On the contrary, there is a rigid connection with the longitudinal ramp that welcomes the bridge to the slow and large southern square.
Both ramps are built with reinforced concrete and therefore belong conceptually to the ground, yet again, differently. The northern ramp seems to be lying over the stone riverside. On the contrary, the southern ramp advances towards the river, from which it emerges energetically.
Because of these very different supports, the resulting diagram of bending momentum of the overall structure is asymmetric, and this condition is translated to the design of the bridge’s elevation. The distance between the upper and lower profiles varies according to the structural requirements. Therefore, users walk inside the bridge through a sequence of spaces that contract and expand, emphasizing even more the alternating movement of the structure.
One of the final objectives of the process of structural optimization that I’m explaining throughout the article is trying to minimize the amount of different steel sections needed for the construction of the bridge. In this way we can lower the cost and ease the manufacturing of the structure.
Following this general strategy, the design of the bars that form each surface expresses the results of another important structural analysis, in this case the diagram of shear stress. For the layout of the profiles, instead of using a regular pattern whose bars become thicker when the stress is higher, we prefer to make the pattern denser, but still using the same structural section.
This feature is very common in structures we can find in nature. And for example, it is the same method used in the structural design of the CCTV building in Beijing. However, this solution is rather traditional in structural design, and we can trace it back to the iron bridges of the late XIXth century, when the metallurgic industry was not developed and just a limited number of profiles were available in structural catalogues.
What is most important in our case is the rhythm of the pattern itself, made of rhomboids. The cells are shifted one half the largest module, off the folding line of the bridge’s elevation, so the geometrical rhythm can be doubled seemingly. This feature is most evident at the highest vertical point of the bridge (9m), which doesn’t need the structural contribution of a vertical bar at the turning point, so the transition nodes can be observed very clearly.
Besides some square tubes that we had to use in the critical locations of the structure, most of the steel components are “open” structural profiles (H type). The total number of them is limited to a handful of different sections, after a careful balance between structural optimization, which would have needed more variety, and construction economy, which asks for repetition and standardization. In the end, all the profiles are custom made and assembled in small sets at the factory. They were later transported to the site and connected to each other in order to create the whole structure. The complete bridge was built on the southern riverside and finally transported on place with two cranes.
After painted white, the open profiles of the structure will cast self shadows that would make visitors to perceive the width of the bars thinner. Slimmer proportions will virtually alleviate the weight of the bridge, even if only perceptually, and will contribute to the final effect of “lightness” and “magic equilibrium” of our twisting jumping structure.
It is very common in Chinese tradition of bridge construction, especially in Southern China, to find cases, normally made in wood, where the structures are a sort of complete buildings that are hovering over the rivers. Indeed, we also understand that our bridge should provide for a dedicated space over the canal, becoming a room over the water, more than merely acting as an engineering device that solves a problem of physical connectivity.
Originally, we planned to envelope both the roof and deck with wooden skins. But after seeing the metal primary structure finished, its dedicated craftsmanship and beauty, we opted for leaving the steel exposed and then lift the wood covers above the beams. For the wood material we chose heat-treated pine. Compared with other methods of protecting the wood, heat treatment is more environmentally friendly since it doesn’t apply metal components. Moreover, the exposure of the wood only to temperature preserves the original brown color of the material, controlled by the length of exposure, and thus avoids undesired blue-green hues.
The orientation of the wood sticks follows the direction of maximum slope within each surface, in order to warrantee the rapid drainage of rainwater, with is evacuated directly on the river. The concave area of the wood cover next to the southern bank needed a special detail. Taking the artworks of the Italian artist Lucio Fontana as inspiration, the wooden surface is cut, sharply, and water is drained out through an “unhealed wound”.
The relation of the bridge with the different lighting conditions is one of the most important design features. During the day, the inner surfaces of wood will screen the glittering rays of the sun reflected from the water of the canal. At night, the sloped ceiling serves as a large reflector for the artificial lighting that is embedded onto the structure. A double, sometimes triple, line of fluorescent lamps is built over the diagonal beam that crosses the entire bridge. We use the space between the primary structure and the wood ceiling to disperse a cloud of indirect light.
The design of the lighting in this way will obviously render a very different image of the bridge in the day or in the night. This effect is perfectly intentioned. In general terms, we believe that trying to reproduce with artificial light the same visual conditions as with sunshine is not as interesting as giving the construction a new appearance after the sunset, where we can underline other formal and spatial aspects of the design. Under the sun, the most important feature of our bridge is the vertical surfaces, the elevation of the strong metal tracery, the play of self shadows within the structure, and shadows of the structure cast inside the bridge over the deck’s wooden floor. Under the moon, the attention is directed towards the wood ceiling, which is in the shade during the day. The ambient light created by the different reflections will transform the bridge into a large lamp over the water.
The design of the handrail is done with extreme simplicity, using steel rectangular tubes and panels of clear glass, so there is no formal competition with the primary aspects of the project. Hopefully, it would visually disappear. The section of the bars is minimized, and the rhythm of supports follows the same pattern of the main structure. The lower profile of the handrail integrates a line of security led light that will be permanently operating for the safety of users.
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The aforementioned explanation of the project is very rigorous and analytical; very rational. Of course, the process of the construction is mixed with infinity of difficulties and emotional moments, which are far more complex to explain, and whose relation would need a lot more extension that the one of this article. At this moment, the reader would have perfectly understood the title of the article, why I believe that this bridge is not a bridge (in reference to Magritte’s famous painting). Because other dimensions of the project are more important than strictly the engineering ones. It is more a playground for children, a house for the sun, a landscape of shadows, a fish jumping the river, a balcony for lovers, a lamp that illuminates dreams, …, a personal adventure for a very young design team.
One very famous Spanish architect said that in order to have interesting architecture it is necessary to have a good architect and an intelligent client. From my experience in this project I can only but totally confirm this definition, if I’m forbidden the lack of modesty. Moreover, I should add that it is equally necessary a supporting engineering team and capable builders, all understanding the task and developing the design in the same direction.
Many people said that the project was impossible to be realized, but by the time this article is published, you may be able to visit it. The project will be finished before the Chinese National Day, October 1st of 2008, almost four years after the first sketch.
建築設計：上海文築國際結構設計：同濟大學建築設計研究院、同濟大學橋樑系
建設單位：上海青浦新城建設與發展（集團）有限公司
施工單位：上海綠地建設集團
建築面積：1280平米
設計週期：2004-2008
施工週期：2007.10-2008.10橋寬：8米橋跨：50米（河寬）材料：引橋：素混凝土，C30結構：Q345qD； 用鋼量：170噸，鋼構件鋅加塗層防腐處理及聚氨酯白色面漆鋪裝：碳化南方松，表面塗透明水封漆
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