FEATURE / Danjiang Bridge: How an 'impossible' design becomes reality

By Sunny Lai and Huang Chiao-wen, CNA staff reporters

Editor's Note: This is part two of a three-part series on the design and construction challenges of New Taipei's Danjiang Bridge -- a new landmark spanning the mouth of the Tamsui River -- and mixed views on its potential to ease traffic and boost tourism.

• More than a landmark: Danjiang Bridge to reshape traffic on Taiwan's north coast

• Danjiang Bridge stirs optimism and concern along the Tamsui River

After its planned inauguration in May, the Danjiang Bridge will connect Tamsui and Bali districts in New Taipei at the mouth of the Tamsui River across a nearly 1-kilometer span.

The world's longest single-mast asymmetric cable-stayed bridge has been described by veteran engineers as "a mission impossible" because it demands a careful balance between aesthetics and engineering.

Elegant simplicity

As one of the final works of the late Pritzker Architecture Prize winner Zaha Hadid, the Danjiang Bridge embodies her wish for a design that was "as simple and as elegant as possible," Huang Shao-wei (黃劭暐), an associate director at Zaha Hadid Architects (ZHA), told CNA while recalling the design principles set for the bridge by the "Queen of the Curve."

Huang, a Taiwanese architect who played a key role in ZHA's winning entry in the 2015 international design competition for the bridge, said the design drew inspiration from the renowned Taiwanese dance troupe Cloud Gate Theater (雲門舞集), based a few hundred meters away in Tamsui, particularly in the form of the bridge's main mast -- a curved tower that evokes the raised arms of dancers.

The tower begins as two separate legs beneath the deck, gradually merging into a slimmer vertical form before opening out slightly at the top, mirroring dancers bringing their arms together before unfolding their palms toward the sky.

A conventional two-tower suspension bridge would have left one of the towers blocking the path of the setting sun, Huang said, noting that the team eventually decided to design a cable-stayed bridge and remove one of the towers.

The single mast, he said, is positioned at a carefully calculated spot closer to the Tamsui side to ensure it does not block the sunset when viewed from major sightseeing spots in Bali and Tamsui.

Ultimately, ZHA's proposal stood out among five other entries, all of which opted for twin-tower bridge designs.

In addition, the team adopted a single-level deck to carry all types of traffic -- cars, motorcycles, pedestrians, and a future light rail line -- whereas some rival proposals used two or even three stacked levels.

"We wanted to minimize the impact on the environment," Huang said of the design, noting that the steel box girders -- prefabricated steel segments that form the deck and carry all the traffic -- are "extremely thin," at only about 4 meters in depth.

"When you look at [the bridge] from a distance, it really is just a single line hovering above the water," he added.

Engineering hurdles

The bridge's unconventional design, involving numerous custom components, along with its exposed location at a river mouth, posed serious construction challenges and deterred many builders, leading to seven failed tenders between 2017 and 2018.

The project only moved forward when local contractor Kung Sing Engineering Corp. won the bid.

However, Liu Yung-ching (劉永慶), the firm's deputy general manager, said he was initially opposed to taking on what he described as "a mission impossible."

"I did not think any construction firm in Taiwan could build something like this piece of art," he told CNA in an interview, recalling slamming his hand on the table when learning his boss's decision to take on the project.

As Liu had foreseen, his team ran into several major difficulties, including leaks in the underwater cofferdam used for the main tower's foundation.

"Why are there fish inside the cofferdam?" Liu recalled thinking. "If fish can get in, it means the holes are big."

Liu's team later drove an additional ring of steel sheet piles around the cofferdam to stop the leaks.

Another challenge came before the bridge's final span was installed, when strong winds caused unfinished sections to sway by as much as 40 centimeters.

To stabilize the structure, engineers installed underwater damping devices beneath the bridge that act as shock absorbers to reduce vertical movement.

In an effort to reduce the overall weight of the bridge, all the steel box girders that form the deck were designed to be welded rather than bolted.

However, high humidity and frequent rain at the river's estuary often made welding impossible.

To allow work to continue during unstable weather, the welding contractor for the project had to build sealed steel sheds directly on the bridge to create a pre-heated and dehumidified environment.

Once finished, "the welded joints were like a solid steel plate, with no cracks and no air bubbles," the company said.

The proposal to build the bridge was first unveiled in 1998 but was later shelved over environmental concerns.

It later received approval from the Executive Yuan in 2014 to proceed following several environmental reviews and design modifications to the surrounding road networks and infrastructure in Tamsui and Bali.

Construction of the bridge began in February 2019, and it was initially scheduled for completion in 2024.

However, the timeline was pushed back by factors such as the COVID-19 pandemic and labor shortages.

Those setbacks also pushed up the total budget, which was revised from the original NT$15.43 billion (US$494.11 million) to NT$23.03 billion.

With final road paving and lighting work underway and the bridge set to open to traffic on May 12, the Danjiang Bridge marks the end of years of pressure and perseverance for those involved in its planning and construction.

Liu, who once harbored opposition and doubt, now speaks with pride, noting that more than a thousand delegations -- many from overseas -- have visited the site to learn about their experience.

"If I had the chance to do it again, I actually would," he said.

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