Bridge design: reaching higher, longer and taller
Image credit: Rex features, Dreamstime
Bridge design shifts as the structures grow, with the record for highest bridge now more than half a kilometre.
It’s a stunning view from the Duge Bridge connecting China’s Guizhou and Yunnan provinces, but you probably don’t want to look down. The Beipan River is half a kilometre straight down from a deck that often sits above the clouds and fog of the steep-sided valley, suspended by a network of steel cables. The first cable-stayed bridge to top 500 metres, the Duge Bridge stretches 1.3km across the valley, with its road deck up to 565m above the river. Five times loftier than Brunel’s iconic Clifton Suspension Bridge in Bristol, it is today the world’s highest bridge.
The Duge (or Beipanjiang) Bridge is the crowning point of China’s Hangzhou-Rulli expressway. This four-lane highway runs for almost 3000km from Hanghzou just outside Shanghai to the border of Myanmar (formerly Burma) near Tibet. Along the way drivers will be able to marvel as they travel over the Puli Bridge; at 485m high it’s the world’s highest suspension bridge, in contrast to the Duge’s cable-stayed design.
The Duge Bridge is one of a whole host of massive civil engineering projects that are transforming China. It is more than a structure; it brings together communities, will bring visitors and trade to previously ignored regions and improve integration. It is also a potent symbol of China’s growing financial strength and position in the world.
Bridges represent one of humanity’s greatest achievements, mixing art, science, design and engineering. To truly understand a bridge, you need to read it like a book.
“Every bridge tells a story,” claims David Blockley, emeritus professor of civil engineering at the University of Bristol and author of a number of books on structural engineering and bridge building. “We just need to be able to understand the language they are written in to read them.”
To extend his book analogy, while knowing the names of certain types of bridge – arch, suspension, cable-stayed or truss – is important, without understanding how these bridges are constructed you’re merely appreciating the cover.
Broken down simply, bridges are constructed from a combination of beams, arches, trusses and suspensions. By understanding the principles and function of these, the reader can begin to understand how bridges are designed, how they function and why they fail.
When designing a bridge, engineers must keep in mind three things: purpose, materials and form. A bridge’s purpose is simply its function in the world – the gap it needs to cover. The materials are the building blocks of its construction, and its form is the specific design chosen by the engineer.
A successful bridge is one whose design, form, construction and materials enable it to perform its primary function.
It’s easy to become confused, but the principles are simple – and it’s likely you’ve built your own bridges in the past. “A small stone across a ford can be classed as a bridge,” Blockley adds.
In selecting the materials you would have instinctively made a calculation on their compressional and tensional strength, ensuring that your chosen material was solid enough, and its size (function) big enough to cross the gap. If it collapsed in a heap under your weight, it failed. If you were conveyed safely to your destination it was a success.
Designing a structure like the Duge Bridge is of course more complicated, with many millions of calculations needed to ensure the structure is safe and secure. The process, however, is the same. Engineers have to identify the perfect balance of performance, materials and function, settling in this case on a cable-stayed bridge.
We can safely assume that humans have always found ways to erect structures that enable us to move across obstacles like streams, rivers, valleys and gullies. But it wasn’t until we discovered the powerful structure of the arch, with its ability to effectively transfer compressional force, that bridge building as we know it began.
The title of ‘world’s oldest stone arch bridge’ is attributed by some to the Arkadiko Bridge in Greece, which dates back to around 1300BC and was originally designed for chariot traffic. It’s a testament to those ancient engineers – and the solidity of the form – that this 22m-long, 5.6m-high bridge is still in use today.
Since then, the history of bridge design has roughly tracked the progress of human development. The Romans mastered the science of building arches, creating over 900 bridges during the reign of the empire, including masterpieces that remain today like the Pont Du Gard aqueduct. Built in the first century AD, this World Heritage Site remains a marvel of engineering, rising 48m into the air. The Romans used semicircular and segmental arches to construct a beautiful and functional structure that remains intact, with its associated road bridge continuing in use for pedestrians until relatively recently.
Until the Age of Enlightenment in the 18th century, arch bridges like those used by the Romans remained the dominant form. As science and engineering developed, coupled with new materials like iron – which was used for the first time on Shropshire’s elaborate and beautiful Iron Bridge – and subsequently steel, bridge builders were able to develop now-common designs like the suspension bridge, cantilever and truss bridges.
In more recent times, cable-stayed bridges – familiar since the 19th century – have grown in popularity as developments in materials, construction and engineering make them more cost-effective
The industrial revolution placed Britain at the forefront of the development of bridges. Iconic designs like the cantilever Forth Bridge, Brunel’s spectacular Royal Albert Bridge whose two lenticular iron trusses span the Tamar and his iconic Clifton Suspension Bridge remain to this day as a testament to the ambition of these engineers, coupled with confidence in their calculations and the materials they were using.
The longest single-span suspension bridge in the UK is the Humber Estuary bridge at a length of 1,410 metres. It was completed in 1981 and was at that time the longest in the world. The longest British bridge in its entirety is the Second Severn Crossing, which has a total length of 5,128 metres.
Cutting a swath through the French countryside, the cable-stayed Millau Viaduct sets a different record. The breath-taking and iconic structure, designed by Sir Norman Foster and French structural engineer Michel Virlogeux, can claim by virtue of its 343-metre tower to be not the world’s highest but the world’s tallest bridge. In terms of the deck height, it’s a less impressive 17th in the world, at only 270 metres, but measurements fail to capture the impact of the structure. Viewing the bridge from the ground, or driving across it, it’s hard not to be moved.
Professor Blockley believes bridges themselves hold a great power to inspire. “The interesting feature of bridges is that function and form combine as one,” he claims, describing the inherent beauty when “human purpose and the natural processes of the flow of forces come together”.
Beyond aesthetics, bridges are highly symbolic structures that can imprint themselves in the human mind. A mere glimpse of the burnt ochre towers of San Francisco’s Golden Gate Bridge or the symmetrically stunning arch of the Sydney Harbour Bridge are enough for most people to immediately recognise the location.
Some bridges – like the Millau Viaduct for instance – are designed to be iconic. Others become that way over time. Tower Bridge, one of London’s defining landmarks, was at first described in unflattering terms by aesthetes who objected to its gothic styling, deeming it an unnecessary affectation. The bridge now is one of the defining features of the city’s skyline.
The reason that bridges inspire is that they perfectly marry form and function. “Architectural form comes naturally from structural engineering form.” Blockley adds. Using the perfect symmetry of the suspension bridge as an example, the beauty is in the engineering, not in the architecture.
In fact, the romance of bridges can even stir the hearts of dictators. As the German Army retreated from Italy in 1944, it’s said that Hitler ordered the destrution of bridges to halt the advance of the allies – except for one, Florence’s Ponte Vecchio. Apparently the Fuhrer, enchanted with the medieval structure, spared it – making this arguably the luckiest bridge in the world.
In a life spent educating and training engineers, including those in charge of building projects like the new Forth road bridge, Blockley is keen to champion the profession, and address the disproportionate attention focused on architects by the media.
“Bridges are constructed by engineers,” he states. “For most bridges architects do, say, 5 per cent of the work.” In structures like the Millau Viaduct, this proportion is greater, but even then the need for structural integrity defines what’s possible.
The key to successful bridge design is in marrying engineering knowledge with an understanding of how people live. Martin Hooton, associate bridge engineer at Arup, claims that, as a profession, engineers need to challenge how bridges integrate into the landscape and how people interact with these structures. “We encourage our engineers to appreciate and be curious about the world around them.”
Although the core designs for bridges have remained stable for more than a century, bridges are themselves learning about their environment. Engineers are beginning to fit the structures with smart sensors that listen for subtle changes in vibration and detect problems before they present a danger. And new materials are being brought into construction to push them higher and further than ever.
Case study: the Hanging Garden of London?
Bridges themselves are powerful structures that bring people together, but they can also drive them apart. London’s plans for a Garden Bridge are ambitious but the structure has run into problems over the funding for its construction and how much access the public will have.
The proposed 366-metre structure will cross the Thames from the top of the Temple Underground station to the South Bank. Anthony Marley MIET, programme director at the Garden Bridge Trust, describes the bridge as an “enchanted space in the middle of the city,” and one that has actually been “designed to become iconic”.
His enthusiasm and that of famous benefactors like actress Joanna Lumley hasn’t been reciprocated by everyone. London Mayor Sadiq Khan has been lukewarm, refusing to underwrite the £3m per year maintenance costs for the bridge. Marley is undaunted, describing how the Trust behind the bridge has secured the necessary permits and licences and is well on their way to raising the £185m needed to kick off the project.
If construction does go ahead, the bridge will be built in Italy, with pieces delivered on barges up the Thames and lifted into place by floating cranes. The bridge’s concrete and steel interior will be wrapped in a copper-nickel alloy called cupro-nickel that Marley claims will be maintenance-free for over a century. The choice is both practical and aesthetic, with bridge designer Thomas Heatherwick’s vision for a structure that sympathetically ages, blending in and not standing out.
It’s a question of if, not when the project will begin, but the muted reception afforded Tower Bridge when it was unveiled should encourage Marley and colleagues that Londoners may in time consider the Garden Bridge a defining part of their landscape, the controversy of its creation long forgotten.
Bridges to nowhere
A whole host of ambitious bridge-building projects have never seen the light of day. Anyone heading from Britain to France via the Channel Tunnel might consider how very different their journey would have been had Margaret Thatcher, when prime minister, agreed to the LinktoEurope plan presented to her in 1981.
An influential consortium proposed a suspension bridge over the English Channel. Drivers would have paid £5.60 to make the 21-mile crossing. However, the Iron Lady dismissed these plans for a steel bridge with a stroke of a pen, settling instead on the tunnel, which began construction in 1988.
The idea of a bridge this size across the sea may seem fanciful, but the Jiaozhou Bay bridge in China does just that, carrying a four-lane highway for 26.4 miles. Linking Huangdao District and the port city of Qingdao on opposite sides of the bay, and with a spur to Hongdao Island, the incredible structure is supported by over 5,000 concrete piles sunk into the seabed below.
A slight question of synchronised stepping
On 10 June 2000 the Millennium Bridge in London opened. Famously dubbed the wobbly bridge, it was shut shortly after and remained closed for almost two years. The 100,000 pedestrians who trooped across the bridge at the time noticed that the iconic structure – designed and built by a consortium including Arup, Foster & Partners and the sculptor Sir Anthony Caro – moved in an unnatural way, which engineers subsequently considered dangerous.
The Millennium Bridge is an exceptionally shallow suspension bridge, with a central span of 144m. The design of the bridge embodied a bold aesthetic, but it also had an inherent problem that engineers couldn’t foresee: a phenomenon called lateral synchronous excitation.
When the bridge opened, as pedestrians began to make their way across it, their chance correlation of steps caused a slight sideways movement of the bridge. These pedestrians began to walk harmoniously with this movement, matching the resonant frequency of the bridge and amplifying the sideways motion. Within normal operation, suspension bridges move, but this 70mm sway was beyond what engineers considered normal or pedestrians regarded as comfortable.
Once the problem was identified, Arup instigated a large research programme to worked with researchers at UK universities to understand the issue. While it turned out that the phenomenon had been observed elsewhere in the world – most notably in Japan – it was unknown at the time to engineers. Solving the problem was relatively simple, with 37 viscous dampers and 54 tuned-mass dampers helping to reduce movement to acceptable levels.
The problems that affected the Millennium Bridge are a salutary reminder that engineers continue to face new challenges, which will necessitate new engineering, design and technical approaches. As Martin Hooton, associate bridge engineer at Arup, explains: “The research and findings from this project led to the re-writing of many codes around the world.”