CUTTING EDGE DESIGN FOR NEW FORTH CROSSING

The innovative plans and unique design for the New Forth Crossing – the key link in the most ambitious Scottish transport plan ever published – have early this year been displayed at a series of public information exhibitions arranged by Transport Scotland in the City of Edinburgh and the Fife and West Lothian regions. This significant infrastructure strategy has been developed by world leading consultants in a joint venture between Jacobs and Arup in collaboration with Danish architects Dissing+Weitling.

Most significant construction project in a generation
The project carries high international prestige first of all because of its size – it is the largest construction project in a generation in Scotland. The Firth of Forth is crossed daily by more than 66,000 vehicles and is a vital link in the transportation infrastructure serving Edinburgh and West Lothian to the south of the Firth, and Fife and the east coast of Scotland to the north.

Contemporary addition within a unique historical context
The replacement bridge will enhance the setting of two famous existing bridges – the world-famous railway bridge built in the 1880s and the suspension road bridge completed in 1964 – both listed and world-renowned structures that were ahead of their time in terms of engineering, architecture and fabrication.
To the designers from Dissing+Weitling’s project team, this context makes the new Forth bridge a particularly fascinating project: completing the architectural trilogy of bridges by adding a 21st century landmark bridge – promising yet again to move the boundaries of design and engineering
Aesthetically, the task is equally challenging, as engineering and design must merge in a modern structure that is unique – yet does not eclipse the existing bridges. Through long-standing co-operation with the world’s finest engineers, we have developed a unique, synergetic working method based on mutual, professional understanding and respect, allowing us as architects to make a significant contribution to large-scale infrastructure assignments.
Through a concurrent process of analysing the best engineering solutions, the project architects strive to express the aesthetic essence of the structure. The finalised design will communicate its primary function: a state-of-the-art infrastructure link for Scotland – and the UK.

First cross-stay bridge in the world
The new cable-stayed bridge, with its three impressive 200-metre mono towers and a span between them of 650 m, has been a magnificent engineering and design challenge for the team. The structural challenge in the design of a multi span cable stayed bridge lies in the need for longitudinal stiffness. To illustrate this, engineers model an asymmetric loading scenario: a large weight on one span opposed to a low weight on the second span. The result of this is twofold: the deck is deformed in the shape of an S-curve whilst introducing heavy bending forces into the centre tower, which effectively starts to “nod” towards the centre of the loaded span.
The potential for deformation introduced into the centre tower has historically been addressed by designing either a stiff, meaning deep, bridge deck or a stiff, meaning a broad based tower.
With regard to these restrictions, the bridge is revolutionary as it is a slender structure both in terms of it’s deck and tower geometry. This is achieved by adopting a system of crossing stays: instead of reaching half the span, the longest stays form a weaving pattern in the shape of a triangle at the centre of the span, thus improving the longitudinal stiffness of the entire structure tremendously.
This solution allows for the state-of-the-art design solution where hitherto unseen slender towers and cross sections afford it a spectacularly elegant lightweight appearance. Furthermore the design is cost efficient, reducing the total original budget by £1.7 billion and being the first cross cable stayed bridge in the world, it is one of it’s kind.

Aesthetic advisors for the entire project
As well as being part of the Main Crossing design team, Dissing+Weitling are as aesthetic advisors to the client also part of the Landscape/Infrastructure Design team. This is an equally challenging task as it is the client’s ambition to secure a continuity of design for the entire scheme, i.e. throughout all works necessary to lead to the new crossing on either side of the Forth.
To Dissing+Weitling this means taking part in the decision making process on highway alignment as well as designing secondary structures, such as flyovers and bridges leading over these highways. To designers, this means coordinating between factual, technical information, client needs as well as restriction imposed by safety issues and translating these into a consistent visual experience. Dissing+Weitling’s experience with comparable projects, such as the Great Belt Link or Stonecutters Bridge, makes all the difference – it allows to draw from a pool of know how, which is appreciated by partnering engineers and client alike.