Crossing Kruunuvuorensilta

When Kruunuvuorensilta, Finland’s longest and tallest bridge, opened to pedestrians and cyclists on 18 April this year, it came to life immediately: around 50,000 people crossed it on the first day. Despite having spent 13 years developing, protecting and realising the design, we weren’t prepared for how the opening would change the bridge in an instant. Alongside the ceremonial moments, as choirs sang and the mayors cut ribbons, it was the energy, noise and diversity of the users that felt most celebratory, that unlocked its true civic quality.

Design with nothing left to remove

Kruunuvuorensilta is the defining feature of the Kruunusillat Scheme - a 1.2 kilometre-long crossing that connects the new suburb of Laajasalo – unbuilt at the time of the design competition – to the centre of Helsinki. The city is known for its low-lying archipelago landscape, rocky shoreline and deep waters. In this context, the 135-metre-tall diamond in the centre of the bridge is a distinctive object, visible on the skyline throughout the city, from the sea and from the air. Its simple form is the product of a complex raft of technical challenges but also a desire to create a symbol that would be recognisable and speak very strongly of its place.

In Finland, the home of architecture and design pioneer Alvar Aalto, the prevailing design ethos is one of simplicity. A design is not done when there’s nothing left to add, it’s done when there’s nothing left to remove. That ethos, of utility, robustness and durability, of quality without unnecessary adornment, became one of our guiding principles across more than a decade of design work. 

Thirteen years is a long time to protect an idea, but that formed a large part of our work at Kruunuvuorensilta. Like many of our projects, there were two phases. First, there was an exploration phase, where we thought in depth about what the bridge was going to be. Once the initial design was secured, we moved into the protection phase, nurturing the essence of the original design through each stage of procurement and into construction.

Designing a two-hundred-year crossing

Throughout, our challenge was to achieve clarity from a very complex set of constraints. Many of these were physical: navigating extremely deep waters that made underwater piling expensive, enduring a harsh winter climate of ice and strong winds, and ensuring sufficient clearance for maritime traffic—all whilst supporting the heavy combined capacity of trams, cyclists, and pedestrians. This all needed to be delivered to a very ambitious 200-year life span – as opposed to a more typical 120 years. 

As always, our first consideration was alignment: where do people want to go, and where will the bridge be most useful? The gentle curve breaks down any foreshortening of the span to users, enabling them to see clearly where they’re going. Then, with that in our minds, we began to ask what typology would be most appropriate. We wanted to limit the number of times the bridge put its ‘feet’ down into the deep water, so we opted to use two tiny natural islands in the middle of the inlet as the base for a central pier. A cable stayed arrangement with two 250-metre long spans on either side of a central diamond tower emerged from the sketchbooks. An efficient form that we hoped would one day become a unique and identifiable symbol for the city.

For us as designers, this kind of symbolism is a careful balance. We’ll always look for ways to elevate our bridges from pure utility so they speak to more than function alone, but that must be done without whimsy – especially at this scale. The best symbols are those that people can project their own meaning onto. We were delighted to see the diamond form used in logos, as well as content creators and bridge fans photographing and filming the structure even before it was completed, because all of this suggested it was beginning to take on a meaning beyond its basic function. 

Enabling modal shift through infrastructure design

Bridges symbolise their place, but they also become symbols of their utility – how they serve people and planet. Kruunuvuorensilta is one of the world’s longest car-free bridges: it has been designed for trams, pedestrians and cyclists as part of a wider initiative to make the city carbon-neutral, and largely private-car free by 2030. It serves a new community that has been developed over the last decade and shaped by the same top-down ambition. The density of the development, as a consequence of the bridge, is far greater than it would have been if it was a vehicular bridge, because the apartments don’t need parking spaces. 

Instead, tram tracks run down the middle of the high street, and all of the apartments prioritise bicycle over car parking. A journey by car from here into the centre of the city would take five times as long as using the bridge, so it is firmly within the interests of new residents to travel by foot, bicycle or public transport.

Even a preliminary estimate of whole-life carbon shows that the new bridge – with its relatively high embodied-carbon spend – could be knocked down and rebuilt many, many times over and still use less total carbon than if it had been built as a road bridge with all the accompanying car journeys. Given that the lifespan of the bridge is projected to be 200 years, that’s of considerable net benefit to the environment.

The influence of infrastructure

Bridges like Kruunuvuorensilta are absolutely vital to the evolution of cities – you only have to look at a map of London to see how the density of development south of the River Thames directly relates to the connectivity across it. Our most civic-minded clients recognise how bridges can contribute to their vision of future growth, whether that’s by promoting a modal shift to help decarbonise the city, or by unlocking the technical or geographical challenges that curtail its development. 

In Helsinki, our client was understandably cautious because they needed to pick a design that they could deliver with certainty. A lot of the more flamboyant entries in the design competition were seen as potential reputational risks, a wasteful use of money from the public purse. People want their taxes to be spent on infrastructure that works for them. Generally they are not interested in flamboyance, but they still want a bridge that can tell the story of a place as well as a story of connectivity. For me, one of our strengths as a practice is in achieving this subtle balance – the emotional as well as the technical. 

When we consult on projects, we often sit at the threshold between the internal team and external stakeholders, leading or contributing to community engagement exercises, even when they’re not in our first language. Internally, we’ll be having a lot of detailed technical conversations about the ice build-up on the cables, how the parapet posts can protect pedestrians from the prevailing southerly winds without blocking views, or how long a granite surface is going to last relative to snowplough impacts. 

But when we engage externally, no-one is talking about those kinds of things. Instead, the community will be talking about change, they’ll be sharing their emotional responses to place. For new bridges there’ll be the excitement of a new connection: “I’m going to be able to get from A to B faster”. And there’ll also be the fear of the unknown, the unpredictable side effects of something which is this large in scale.