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Trail blazer - Bridge Design & Engineering (Bd & e)
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Construction 23 Nov 20
Trail blazer A 23km-long stretch of footpaths, elevated walkways and bridges weave their way across Xiamen’s urban and natural landscape to encourage green mobility and improve wellbeing, reports Khalifa Bokhammas
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Trail blazer - Bridge Design & Engineering (Bd & e)
The Xiamen Mountains-to-Sea Trail is an ambitious project by the City of Xiamen, which launched an international design competition for the development in 2016. Since the city centre is located on an island comprising densely populated urban districts, lush vegetation, rocky hills and waterfront areas, the local authority envisaged a project to boost access to these places and consequently improve the wellbeing of the local community. Dissing & Weitling won the competition with its ‘people oriented’ proposal of 11km of elevated footpaths and seven linking bridges. “They liked our approach to building bridges with a certain elegance and charm, so what we tried to do is to nd a balance between structural re nement and functionality,” says Steen Savery Trojaborg, partner and architect at Dissing & Weitling. “One of the client’s requests was that we made something that was fairly simple and easy to maintain but also something that had a certain visibility in the environment, so that the locals could discover it and start using it. This was one of the challenges,” adds Jesper B Henriksen, partner and architect at the same practice. To aid with the construction of the elevated walkways, particularly in the more topographically challenging areas of the island, the majority of the steel structural elements were prefabricated locally, meaning the relatively lightweight components could be transported with minimal heavy machinery and damage to the local environment. Since welding would also be a challenge in the more rugged terrain, most of the elements were designed to be bolted together. https://www.bridgeweb.com/Trail-blazer/7415
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“One of the nicest things about working in China was having a strong dialogue with the decision makers, including the mayor of Xiamen. We were able to put forward our ideas and test them with the city, and, therefore, de ne a shared solution,” highlights Trojaborg. The resulting collaboration is a series of raised pathways that are relatively minimalist in their appearance, with two-level handrails, steel fencing between the upper handrail and deck, and a steel deck, which has a depth of 66cm and an average width of 3m. The slender steel piers enhance the elegant pro le of the footpaths, helping them blend seamlessly with the existing trails they connect to. The deck is coated in a two-component resin, which was approved as the most suitable covering by the contractor. “The local authority has been very keen on safety, so they tested all the different coating options for the deck. It is a major attraction, but it is also a major piece of infrastructure for soft mobility, and has been used by a lot of families so far,” Trojaborg says.
The user journey from west to east is punctuated by seven architecturally ambitious footbridges, six of which act as transitionary points between the ve alignments that make up the entire project. For these structures, referred to within the project as ‘node bridges’, Dissing & Weitling partnered with Tonji University, Xiamen Municipal Engineering Design Institute, and Schlaich Bergermann Partner to execute detailed designs and engineering on the rst and sixth, fourth and fth, and second and third bridges, respectively.
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Trail blazer - Bridge Design & Engineering (Bd & e)
The rst node bridge, the Hexi Building, is the country’s rst multi-layered spiral steel structure suspended around an elevator tube. Ramps spiral around the 40mhigh, 6m-diameter reinforced concrete cylinder six-and-a-half times and are anchored to it via horizontal bracings and pull rods. The second, Hemei Bridge, is the longest curved mono-cable bridge in the world with a span of 217m. Completed in September, this was the most ambitious design due to its pioneering size and the small footprint available for the pylon. Due to a need to align with the principals of Feng Shui, the initial single-pylon design had to be rethought (see second article below). Bridge three, Yuanshan Bridge, is 130m long, 4.4m wide, and has a main span of 93m supported by a single-side suspension arch system connected to the deck via 13, D50 stainless steel cables. The arch crown is about 30m from the ground and the structure spans a metro station between the hills of Xianyue and Yuanshan. The fourth node is Wuling Bridge, which is roughly 200m in length, 4.2m in width and comprises a 2m by 42m prestressed concrete T-frame. Jinshang Bridge (pictured below), the fth node, connects two public parks. Its total length of 163.3m is made up of a suspended section of 70.5m and a simply supported continuous-beam section. The pylon for the suspension bridge is 20m tall.
The sixth node, Guanhutai Bridge, is a beam string structure that straddles Xianyue Road and acts as an entrance to a lakeside reservoir. At the waterfront end of the bridge is a 28m-tall tower, which offers pedestrians the highest point of the whole project. https://www.bridgeweb.com/Trail-blazer/7415
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Trail blazer - Bridge Design & Engineering (Bd & e)
Using an elevator integrated into the tower’s structure, pedestrians can travel to the top to enjoy the surrounding views and cross the bridge. The upper chord of the beam string is a stiffened steel box girder, and two symmetrical steel cables are anchored to the girder at one end and the tower at the other, with struts connecting the upper chord and cables via hinged connections. The self-balanced system, which greatly reduces demand for horizontal restraints at the support, was partly chosen to reduce material usage. Much of the footpath network was opened to the public in January 2020, with the inauguration of Hemei Bridge on 1 September signifying the completion of the entire project. While Covid-19 has stopped the design team from visiting the site since early 2020, they highlight that their intimate knowledge of the location gleaned from numerous trips to it from Denmark helped hone their initial design and create an end product that all parties are happy with. “It’s been a combination of knowledge and calculation. We’ve been there many times and walked many kilometres with the clients and engineers, so we got the feeling of the city and the mountains. It’s fundamental for us to start with the uses and site constraints, and not just design a bridge that looks good,” Trojaborg says.
Learning curve The most ambitious crossing in Xiamen’s Mountains-to-Sea Trail is a remarkable curved mono-cable suspension bridge that required complex detailed design
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The V-shaped mast’s design was in uenced by Feng Shui (Gustav Krieg)
When Dissing & Weitling created the concept for the Xiamen Mountains-to-Sea Trail, they initially asked Schlaich Bergermann Partner (SBP) for support in realising three of the node bridges. However, later in the planning phase the work was again split between the engineering consultants, and SBP conducted design and engineering from concept stage to construction drawings for the Hemei and Yuanshan bridges. As a mono-cable suspension bridge with a main span of 217m, a single pylon and curved deck, Hemei Bridge was considered the most ambitious of the node bridges. The context of the site helped guide the typology selection: the bridge was required to span Xianyue Road and connect two existing pathways whose trajectories curved towards each other from either side of the highway. This made a curved deck the most logical choice, and, as Gustav Krieg, project manager at SBP, explains, “There are signi cant structural bene ts to using a mono-cable suspension bridge design when there is a curved deck and a large span. If you have a curved bridge deck and only suspend it on one side, you don’t get a lot of torsion. With the rotating moment created by supporting it only on one side, when you add loads to the beam, you split the forces into two horizontal forces, one on top on the deck and one on the bottom.” With the mast on the outside of the bridge, as found in Xiamen, the forces are handled by a compression ring around the top of the deck, and tension rods around the bottom. The original plan was for a single pylon founded in the island between the two sides of Xianyue Road, with abutments located at the end points of existing paths on the hills on either side of the highway. However, SBP found that there were some problems with this arrangement. “We had three problems: our back-stay cables were https://www.bridgeweb.com/Trail-blazer/7415
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hitting the streets, our abutments were running into the slopes, and we had con icts at the mast base with the power lines. So, what we proposed was to rotate the bridge slightly, which reduced the span a little and overcame these problems,” Krieg says. The main bene t of the optimised design is that it allowed for all the backstay cables to be founded within the green traf c island separating the north and southbound sides of Xianyue Road. While the new arrangement meant the bridge would no longer link seamlessly with the path on the northern side, it created the opportunity for the architect to create a covered plaza between the existing path and the bridge, helping enhance its credentials as an attraction in itself. Another aspect of the site that in uenced the bridge’s design was the 6% downward gradient of the existing paths. “Normally, you don’t want the bridge deck sagging for aesthetics and drainage reasons, but it was an architectural decision from Dissing and Weitling to have the bridge deck sagging on this structure. They did some studies, and we agreed, that to transition from a 6% downward slope to a 2% upward slope, for example, would have looked pretty strange,” notes Krieg.
Forces are handled by a compression ring around the top of the deck and tension rods around the bottom (David Sommer)
Optimisation was also required to adjust the forces in the main cable. “Following our concept report we showed forces of 13MN in the main cable, and then people got a little worried, as the main cable would have needed to be 180mm in diameter, which is not standard.” Accordingly, the team moved to reduce this to 140mm — in line with what most locked-coil cable manufacturers produce. To achieve this, a multi-stage optimisation process was undertaken to reduce the cable forces without sacri cing dynamic performance by adjusting six parameters: the distance between the main cable anchorage and the deck; the minimum distance between the main cable and the deck; the horizontal distance between the bridge deck and the mast tip; the mast https://www.bridgeweb.com/Trail-blazer/7415
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height; the spread between the two guy cables; and the mast inclination at the base. The physical form- nding procedure to obtain these values involved modelling the perfect parabolas for the horizontal and vertical forces. “Because the hangers are inclined, the hanger force has a vertical and a horizontal force component. For the main cable, this means that you need a vertical and horizontal parabola. And by moving the mast tip in the horizontal and vertical direction you can nd the optimal spot where both parabolas have their optimal curvature, resulting in the lowest cable forces possible,� explains Krieg. Through the optimisation, the team achieved a 45% force reduction in the main cable compared to the initial geometry. The main cable diameter is 140mm and was produced locally by Juli Sling. After this process, SBP submitted its schematic designs. Since the bridge type was not fully governed by Chinese codes, the client hired an expert panel to analyse the submission. One of their main concerns was that the eigenfrequency of the bridge was 0.36Hz, compared to 0.50Hz from the two reference projects SBP had presented to the panel from its portfolio (Sassnitz and Grimberg Harbour footbridges in Germany). To bring the eigenfrequency closer to the level of the reference projects, the team increased the guy cable spread from 30m to 46.5m, the diameter of the guy cables from 100mm to 125mm, and the thickness of the bridge deck from 800m to 1,000mm. The former two adjustments helped stabilise the mast tip, while the latter increased the stiffness of the deck.
The two mast tips are at a height of 49m
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The biggest change to the design, however, was driven by the mayor of Xiamen, who voiced his concerns about the Feng Shui of the bridge, namely that the single mast resembled a knife too much, and that it represented the number one — a symbol of loneliness in China. This forced everyone involved to take a big step back in the middle of the project in October 2018 and reanalyse the boundary conditions to generate new design options. These included a double mast option, a Y-shaped mast and a V-shaped mast. The latter was preferred by the mayor and it is the design that stands today. “I’m really happy about the change, because it makes our bridge unique and even more impressive. All other bridges of this type have a single mast tip,” Krieg says. The six optimisation values obtained for the single mast were still valid for the V-shaped mast as both had the same theoretical tip, with the 62m single mast cut at a height of 49m to accommodate the new V-shape. However, one problem that arose from the new V-shaped pylon was that the eigenfrequency was back down to around 0.4Hz. To increase it, the team added another two guy cables and reinforced the bridge deck, taking it from 1m to 1.2m thick. Due to the site being vulnerable to typhoons, the nal design then had to undergo wind tunnel testing, resulting in further adaptations. The recommendation from the studies was that a baf e plate be installed on the edge of the deck to reduce turbulence. “Another thing they wanted us to do was make the sections of deck more symmetrical. The rst iteration was closer to a triangle in shape, which is optimal for this kind of bridge because you want to have the dead load close to your cables and not too far away. But the potential wind loads forced us to go towards a more balanced solution, which increased the cable forces a little.” While the maximum cable force decreased from 13MN to 7MN in the earlier optimisation phase, these changes saw it rise to 8MN. A mast tempering study was also conducted, which found that with tempering starting 25% away from either end of the mast, a near constant stress level over its entire length could be achieved. The mast is connected to the base via a 600mmdiameter cast steel spherical bearing. “To our knowledge this is the biggest such bearing in the world.” As construction progressed, the soil quality was found to be much worse than the initial analysis had shown, so the number of piles at the north abutment was increased from nine to 12. Due to the complex ow of forces at the abutments, SBP arranged large steel sheets vertically in a grid shape at the head of the piles help to control the load path. “A reinforced concrete abutment would have had very sophisticated reinforcement geometries. Building the rst large-scale bridge of this type in China, we did not want to take an unnecessary risk and decided to go for the embedded steel sheet option at the abutments.” Krieg says. https://www.bridgeweb.com/Trail-blazer/7415
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During deck erection, since the mast is inclined away from the bridge deck in its end state, the mast tip was tilted closer to the deck to be able to install the hangers and main cable without any prestress force. The operation was more-or-less reversed to prestress the cables, whereby the mast tip was pulled 1m backwards. Although construction generally progressed smoothly, a ve-month delay was caused by problems fabricating the sockets for the main cable. Although the contractor initially tried to follow SBP’s request to construct the socket using high-strength cast steel, these failed during testing and machined sockets had to be made instead. Cable installation and prestressing were completed in August and on 1 September 2020 the bridge was inaugurated. Hemei Bridge Architect: Dissing & Weitling Structural Engineer: Schlaich Bergermann Partner Local Design Institute/General Planner: JSTI Xiamen Municipal Engineering Design Division Contractor: Xiamen Municipal Construction Group #Construction (../../Construction) #Footbridge (../../Footbridge) #China (../../China)
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