The Architecture of British Bridges

Ronald Yee

• Éditeur: Crowood
• Année: 2021
• ISBN:
  9781785007958
• Sujet: Architecture
• Sous-sujet: Architecture General

During the Industrial Revolution, Britain was at the forefront of bridge innovation. Pioneering designers such as George and Robert Stephenson, Thomas Telford and Isambard Kingdom Brunel created Britain's rich bridge heritage that features many world firsts and we can learn much from their ground-breaking designs. Written by an experienced bridge architect, this book includes an introduction to bridge aesthetics; it gives an outline of British bridge development and advice on parapet treatment and bridge lighting. This book offers a comprehensive overview of how the best of British bridges marry aesthetic considerations with engineering ingenuity.

Chapter One

Architecture of Bridges

Not Just Looking But Seeing

One of the first exercises of my architectural education, entitled ‘Looking and Seeing,’ explored ways of seeing, understanding and interpreting an object, place or event, with an aim of critically looking beyond the obvious and analysing for myself the unseen quality of an object, building or space.

To help me organize my research and evaluation I developed a variation of the five Ws:

• What is it?

• Where is it?

• What purpose does it serve?

• What are the underlying principles?

• What relationship is there between the principles and the form?

I chose to investigate the bridge over the A2 that had now become a way-marker on my journeys down to Chattenden. The exercise turned out to be an epiphany in my understanding of bridge design, and of design in general. I found out that the bridge was a pre-stressed concrete footbridge at a place called Swanscombe in Kent. It was constructed in 1965 at the same time as the new A2 and carries a footpath that was severed by the new road cutting. The footbridge was designed by Kent County Bridge Engineer J.A. Bergg and is 294ft long (89.5m) with a deck that is 7ft 6in wide (2.3m) between railings, which is supported by a three-pinned arch with a span of 160ft (48.77m) rising to a height of 25ft 6in (7.83m).

Analysis of the Swanscombe Footbridge – elements and form.

The bridge comprises two parts: the deck (continuation of the footpath) and the supporting arch.

The deck is straight and level and connects to the top of the cutting on either side. To support the deck across the cutting a support structure is required, which is the arch. Structurally, these two separate elements need to be considered separately and then together. The deck in its natural state only carries its own weight (dead load) which is considered to be evenly loaded. The optimum shape for any arch that carries its own weight i.e. dead load evenly distributed over the entire span, is a parabola. A parabolic curve for any given span and height can be simply determined as an upside-down catenary i.e. the shape assumed by a chain suspended between two fixed points. Just as the links in the chain hang under gravity to form a sag curve where all the components are under tension, the elements of an efficient arch will follow a parabolic line of pressure, along which all the forces are compressive. Any deviation from the natural line of pressure will introduce stresses into the arch that makes it necessary to strengthen the cross section. Therefore, the most efficient structure is one that follows the natural parabolic line of pressure. However, the parabolic line of pressure of the arch is only applicable in a static situation i.e. deadweight of bridge alone, but to accommodate a load being carried at different points along the bridge the curve must be modified. When a load is asymmetrically applied to one side only, the arch naturally yields on the loaded side and thrusts up on the opposite side. The resultant deviation from the natural line of pressure introduces stresses into the arch, making it necessary to strengthen the cross section to prevent collapse. The opposite is true when the loading is reversed. By superimposing the lines of positive and negative deformation a lens-shaped zone is revealed on either side, with the largest deformation occurring halfway between the spring and the crown i.e. approximately quarter the total span. The most efficient shape would therefore have hinges at both ends and at the crown. A similar outcome is true when the same conditions are applied to the deck. When the zones of deformation for the deck and arch are combined, an obvious relationship can be seen between the lines of deformation and the outline of the footbridge’s arch and deck structure, resulting in an elegant solution that is both visually simple and structurally efficient.

Analysis of the Swanscombe Footbridge – load deflections.

Conclusion: the form of the Swanscombe Footbridge is a product of its functional requirements.

Bridge Architecture

More than just an elegant solution

Bridges, by their nature and scale, are usually prominent structures that are highly visible and should be beautiful. A beautiful bridge need not be more costly than an ugly one, but may need more time, care and skill in preparing its design and construction. Since bridges are built to last a long time, a high degree of care and design consideration should be deemed an investment. When a bridge design has been done well, its superstructure will soar, arches leap, balustrades embrace and columns march!

Beauty in relation to a bridge should be considered differently from beauty in art. An artist is free to paint a provocative picture or make a challenging sculpture in order to express his feelings or evoke emotions in the viewer who has the freedom to appreciate it or not. The enjoyment of art is truly a personal matter – if someone doesn’t like it then they can easily move on – however, a bridge’s appearance affects all who see or use it. Therefore, a bridge designer has a wider duty to society! An architecturally good bridge is more than just having elegant, clean and clear lines with unity of form and finish, proportion and composition. Experience of a structure involves much more than visual impression; our other senses have a strong influence too. Smell, touch and sound can affect our emotional response of awe, fear, security, claustrophobia, insignificance, etc.

When I first studied structures, my fellow students and I painstakingly produced technical design drawings of theoretical steel trusses with minimal struts and cleverly detailed connections that corresponded to bending moment diagrams, all neatly hatched in with pencil, and we were suitably pleased with our ingenious solutions and high grades. However, when I went on to study architecture, I was taught that even before we started designing, we first had to understand the genus loci – the distinctive atmosphere of our project location. For success any building (including a bridge) should be in harmony with its context, neighbours and environment. To test this, we were encouraged to produce ‘before and after’ renderings and scale models to ensure that our design would make a positive contribution to the setting or help us decide whether other solutions might be more suitable.

The context within which the bridge sits affects our perception of it. An industrial looking truss might suit an urban townscape but could look out of place in a flat pastoral landscape; the curvature of an arch member might blend harmoniously into the sides of a mountainous valley; and the loft towers of a cable-supported bridge might not be as dominant in a vast seascape.

Bridge architecture is not an exact science

Bridge architecture is not an exact science for which there is a definitive correct answer. For most bridging situations alternative solutions might be technically possible and a choice can be made. In an emergency, speed of construction may be the deciding factor and in temporary situations cost might be justifiable as a deciding factor. But with a permanent bridge, the time it took to build will not be remembered long, nor will the cost, whereas its beauty or ugliness will remain for its lifetime! Priority for choosing between alternative permanent bridge solutions should therefore be aesthetic.

Many believe that the appreciation of beauty is down to personal taste – ‘I like this; you don’t, you like that’ – however, aesthetic appreciation, like musical preference, is influenced by many things, including one’s culture, experience and education. Whilst there is room for personal preference in the appreciation of bridges, one must also account for societal consensus regarding the beauty of some and the ugliness of others. Later in the book, when we look at the examples that have been included, we will begin to see what good bridges have in common, and what the others lack. By critically looking at all bridges we should develop a better understanding of bridge architecture and our appreciation for what is successful will deepen.

What is good bridge architecture?

Based on the themes expounded by Roman...