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A bridge is more than just a path from A to B

Structures By Dong Chen, Director, Structural Engineering – 10 November 2021

Rendered design of a curved large white bridge across a courtyard with pedestrians

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Dong Chen in black jumper with office wall background

Dong Chen

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For most of my career I have been designing unconventional structures. Anything curly, spiralling or long-span fascinates me. The engineering challenge is not just to make a structure strong and stable from an integrity perspective, but also to design for the intangibles that shape the experience of people.

Bridges are not just pieces of infrastructure; they have an emotional dimension. Bridges anchor memories and sense of place. We locate ourselves in relation to them, they appear in songs, stories and art works. They have deep symbolism in many cultures.

They are also, from a material perspective, designed to last over 100 years – no-one decides to build a bridge on a whim! Every bridge must have a reason for being; whether that is to connect two sites of social importance, carry people over a river, or connect two communities. It must also have qualities that make people want to use it, particularly when it is a pedestrian bridge. As a designer, I consider how the bridge will add to user experience of being in a place.

Thinking about the user experience means considering how dynamic effects, such as wind and human activities will interact with the structure and the people on it. Vibration tends to be the biggest concern for a pedestrian bridge, as it can make people feel uneasy and unsafe. We often consider a more advanced design approach, in comparison with building design, by simulating the pedestrian footfalls under different activities, i.e. walking, jogging, running, jumping etc., on the structure and see how the design responds.

The vibration must be within the tolerance of human perception, and that varies from person to person. But if we err too much on the side of very low tolerance for vibration, we design with too much structural material waste. A bridge needs to be as sustainable as possible from engineering performance, material usage, fabrication and construction perspectives, so being too conservative is best avoided.

In some cases, we also consider glare analytics, looking at how sunlight will affect people on and near the bridge, and how the design, materiality and finishes of the bridge will potentially cause inconvenience to people in the vicinity. We also test how the design will affect pedestrian wind comfort, i.e. wind effects on the comfort and safety of pedestrians and cyclists, and how that impacts the environment and community nearby.

We also consider and design for potential accidental loads, such as vehicle, vessel impacts, natural disasters, terrorist incidents and climate change. Climate adaptation for such infrastructure certainly adds another level of design consideration. Many structural materials and finishes have been tested for criteria according to existing codes and regulations that may not necessarily have incorporated future climate change, especially those related to temperature, wind, flood risk etc. Our detailed analysis further extends to social and environment impact and traffic levels.

There are a lot of moving parts and intangibles involved in bridge structural design. Then we add the layer of landscaping, artworks, seating, social spaces, as well as lighting, water and electricity – the equivalent of fit-out and building services in a building.

When people walk across a bridge, they are also having a sensory experience: lighting becomes very important. We take a dark sky lighting approach that minimises light spill into the surrounding environment to protect biodiversity and amenity for the surrounding community. Lighting for a bridge at night needs to feel safe, welcoming, and comfortable. And it’s important not to forget under the bridge, spaces which can often be quite intimidating and dark. Our lighting designs illuminate those areas, so they become a feature and are safe and inviting spaces.

Of course, no-one wants to see the services conduits and water pipes, so detail design incorporates ways to hide them from the public while also making them safe to access for regular inspection and maintenance. As a structure that will be around for over a century, it WILL need to be looked after very carefully; we also consider the safety of the people who will need to perform such work in future and establish a comprehensive maintenance strategy with the owner during the design phase.

This is also why careful thought is given to finishes – they need to be durable and not require frequent maintenance. Bridge cladding for example, is a high-risk logistical feat, so best to design out the need for it. Painting finishes need to be resilient to climate and people, because if a bridge starts to look ugly, people will not want to use it. Research shows that people even experience vibration differently depending on the colour of a bridge!

When we bring all these pieces together, the result can positively change people’s urban experience. The award-winning Manhattan High Line in New York (a disused elevated train line) is a leading example of how bridges and elevated walkways can inject vibrancy and improved amenity in our cities and communities.

In Manchester, we are working on the Salford Rise landscape pedestrian bridge which demonstrates another example. This will be a destination in the sky, with leafy promenade, dining and social connection spaces as well as fulfilling a pragmatic function to connect a university campus to other parts of the city.

A bridge is the ultimate multi-tasker, doing many things at once.

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