Design of the new Castle Mills bridge over the River Foss

One of the most exciting and talked about aspects of the Castle Gateway masterplan is the way it will open up new routes and spaces, and change the way we move around the city.

The new bridge over the Foss is central to this transformation. It will form part of a cycle and pedestrian route between the new river walkway, the public greenspace at the back of the museum and the business and residential community on a reinvigorated Piccadilly.

Like the Castle Mills development it connects to, the bridge design has evolved in response to a series of design challenges. Here Andy Kerr walks us through those challenges and the key features of such an important piece of new infrastructure.

Challenges

  • Height and changes in level

There are two main technical challenges linked to the height of the bridge. The first is that the Castle side of the bridge is 2.2m higher than the Piccadilly side.

This means that the bridge will have to slope from one side to the other. A longer bridge allows us to reduce the gradient of the slope to be safe for pedestrians and cyclists, with the aim to have a gradient no steeper than 1 in 20

The second issue makes this even more complicated. Because the river Foss is designated as a ‘navigation’ by an act of Parliament it has to be able to allow boats to pass up and down the river. This means that the bridge can’t simply slope from the high point on the Castle side to the lower Piccadilly side. Any new bridge built over the river has to allow enough space for boats to pass under it, meaning we need to have a 6.2m wide section where the minimum height of the underside of the bridge is 10.7m above ‘ordnance datum’ – the base level from which all altitude is measured in the UK.

Achieving this has restricted the options for the actual shape and look of the bridge. Because the underside of the bridge has to be at a certain height, and then slope down to Piccadilly without being too steep it is really important that the base of the bridge is as thin as possible. So the bulky structural steel that gives the bridge its strength and support has been moved to the sides of the bridge. This allows the bottom of the bridge, or the deck, to be very lightweight and much thinner, but means the sides of the bridge can’t be formed of glass or railings.

We did consider other options to support the weight of the bridge, but in consultation with heritage and conservation groups it was felt to be inappropriate. A suspension bridge would require high supporting columns which would detract from the setting of the Castle walls, and a bridge supported from below by columns would have impacted on the flow of the Foss, increasing flood risk upstream.

  • Yorkshire Water sewer

In the masterplan the new bridge was a kissing bridge located at the boundary between Ryedale House and the Castle Mills site. The now infamous major Yorkshire Water sewer that crosses under the Castle Mills site and continues under the river put paid to this idea. Due to the size and complexity of the sewer, it can’t be diverted. So we have had to redesign the residential building to create two separate buildings either side of the sewer, with the bridge in-between the buildings, and landing in a new public square created above the sewer.

In addition to not building directly over the sewer we also need to leave a protected zone on either side of the sewer called an ‘easement’. This is to allow future maintenance of the sewer and to make sure that the sewer is not affected by the foundations of the new buildings and the bridge. We have agreed with Yorkshire Water that we can build the bridge foundations slightly within the easement but this has to be kept to a minimum.

This is one of the factors that has influenced the design of the bridge, to ensure it requires a very simple foundation design that does not impact on the sewer or easement.

  • Costs and viability

The Castle Gateway’s ambitious masterplan will transform this area of the city. It will deliver world class public realm and sustainable transport improvements with huge public benefits. These are expensive to deliver. Whilst there is some commercial development opportunities within the masterplan to help pay for this, including Castle Mills apartments, making sure the scheme remains affordable remains challenging. We are applying for external funding to help pay for the masterplan wherever possible, but we also need to make sure that we make sensible decisions that are cost effective and value for money.

  • Maintenance

Just as we need to make sure we’re getting value for money on all aspects of the Castle Gateway project, it is essential that we consider the maintenance needs and costs of whatever we design. So the bridge has been designed by expert bridge engineers Witteveen +BOS in partnership with our architects, with input from the council officers who will be responsible for the long-term maintenance of the bridge. This has helped to design out long term costs and potential future issues such as avoiding opportunities for padlocks attached as ‘love locks’ and using robust materials which will need lower levels of repair.

  • Creating a safe environment for pedestrians and cyclists

Throughout the Castle Gateway process there were a number of workshops under the theme of movement and access, which were particularly well attended by cycling groups. The outcome of those workshops was a consensus that where possible the council should look to create shared spaces between pedestrians and cyclists, as this encouraged all users to be more aware of each other and be more considerate. Where there are segregated routes it was felt that cyclists were more likely to ride faster and pedestrians can often stray in to cycle lanes. This is also consistent with the wider council strategy where the new Scarborough Bridge is based on a shared space, and where existing cycle routes in the Castle Gateway are being changed to shared routes such as New Walk.

Decisions

Having considered all of the above challenges the decision has been taken to proceed with a simple single span, straight bridge, to be used as a shared space by pedestrians and cyclists.

Why is the bridge straight?

The engagement revealed a fairly equal split between people who wished to see a straight or a curved bridge with built in seating/ viewing points.

The proposed bridge is a straight “simple” design for several reasons:

  • Because the structural elements of the bridge need to be on the side rather than the base of the bridge, so the simple, straight design is easier to create and avoids more invasive foundations.
  • Whilst subjective, it was the view of the design team in conversation with planning and conservation officers that the straight bridge not only looked the best but also did the least to detract from the heritage environment due to its simplicity.
  • The addition of seating on the bridge would require a wider bridge. As well as significantly adding to the cost of the bridge, it would be very difficult to fit the wider bridge into the available space.
  • The use of the bridge for cyclists will require the sides to be 1.4m high, and the structural nature of those sides would mean any seating would have a very restricted view.
  • A straight bridge requires less steel than a curved bridge and is therefore more cost effective in terms of viability of the broader Castle Gateway masterplan.

Will the bridge be a place or just a function for movement? 

Across the My Castle Gateway engagement, movement was seen as key. There there was also a strong desire for the bridge to be a destination (like Millennium Bridge).

We have to make sure that pedestrian and cyclist flow on the bridge is not reduced, so the design creates a new sustainable transport route rather than a place to hang out. It will, however, connect brand new public spaces on either side of the river which will be destinations themselves.  You will be able to sit and stop in the new public green space to the west, and in the new public ‘square’ to the east between the new Castle Mills apartment blocks.

Benefits

  • New walking and cycling routes up the Foss and into the centre of York, open 24 hours a day.
  • Opens up two new public spaces at either side of the bridge where people can sit and see the Foss and appreciate its wildlife.
  • Moves cyclists and pedestrians away from the Gyratory traffic on the existing bridge.
  • The shared use bridge will avoid separate delineation on the ground and complex pedestrian/ cycle crossing points.

What happens next?

You can follow the Castle Mills planning application via the Planning Portal quoting reference number 19/02415/FUL. Further details about the bridge design can be seen in section 6 of the submitted Design and Access Statement.

What will be fixed through the Castle Mills planning application? 

The size, shape, and structural design of the bridge, with the use of materials being either fixed or needing to be agreed with planners in the future.

What can still be influenced? 

Some elements are still flexible. These include what the external visual details look like, including lighting, and any perforations/ transparency to create views and glimpses from the bridge.

Reference

Previous MCG blogs relating to the new bridge:

https://mycastlegateway.org/2019/07/03/pre-application-discussions-st-georges-field-multi-storey-car-park-castle-mills-apartment-and-castle-mills-bridge/

https://mycastlegateway.org/2019/05/04/trees-green-space-and-bridges-barry-otley/

https://mycastlegateway.org/2019/04/04/st-georges-field-car-park-castle-mills-apartments-and-the-new-bridge-exploring-the-proposals/

https://mycastlegateway.org/2019/04/01/a-new-bridge-over-the-foss-notes-from-the-workshop-20th-march/

Leave a comment