1. Introduction to Brunel
Isambard Kingdom Brunel can be argued as the most ambitious, innovative and successful engineer of the 19th century. Most of his work was centred on his role as engineer for the Great Western Railway. The job demanded the construction of hundreds of bridges across the south-west of England. As the Great Western Railway grew, the demand for bridges grew, posing many challenges for Brunel. This report will address the different bridges he constructed using mainly the specified materials. As a result it will give an insight into key designs and his contributions to bridge engineering.
2. Iron Work
Up until Brunel’s time, cast iron was widely used in bridge engineering. It was well understood and met the requirements needed at that time. But by the 1840’s, a different approach to the design of suspension bridges was needed as they were too flexible to carry the heavy load of a railway train (Brindle, 2005: 170). The Great Western Railways had grown and more railway lines were being constructed; this demanded the construction of stronger suspension bridges. In 1847, Brunel began work on designing bridges with wrought iron. The material had many benefits over the traditional cast iron but was not as well understood. This meant that Brunel had to engineer a way of incorporating wrought iron into his designs.
2.1 The Chepstow Bridge
The Chepstow Bridge had to be designed to carry railway 50-feet above a 600-foot wide River Wye. The challenge was constructing this on uneven river banks. One side of the river was made of 120-foot limestone-cliffs and the opposite side rose just above the high water level. Furthermore, a 300-foot clear span was required for ships to pass under. Brunel created a tubular design using wrought iron on the Chepstow Bridge which was not only “a marvel of economical design” (Brindle, 2005 p.170) but was a design unlike any that had been seen before. He devised a truss-like suspension bridge. The first 300 feet blended tubular girder design with the design of a suspension bridge. Instead of transferring stresses to “anchorage points in the ground at either end, the stresses were contained within the structure” (Brindle, 2005 p.175). This is known as a closed system. The design allowed for the clearance below the deck for ships to pass under and meant the structure could deal with the load and vibrations of railway trains (Brindle, 2005: p.170-175). The rest of the deck was supported by cast iron pillars and met a raised embankment at the other side (see figure 1 below).
Truss design creating a closed system
Raised embankment
(Figure 1: NOT TO SCALE) River bank just above high water mark
2.2 The Royal Albert Bridge
The Royal Albert Bridge still remains unlike any bridge constructed. It is designed (much like the Chepstow Bridge) on the principles of a suspension bridge. It uses tubular arches, chains and cross-bracing using wrought iron to strengthen the bridge. The key challenges for this project were: spanning 1,100 feet, to allow for sailing ships to pass underneath and to create a seaming-less journey for railway passengers. Many designed were considered including just one span made from timber. Eventually Brunel settled on a design, two 465 feet spans with a central pier between them. Two main difficulties arose from this design: constructing the central pier and raising the main spans. To construct the central pier he had a wrought-iron tube constructed which was floated out to the site and acted like a cofferdam. The main spans were built on the foreshore and floated into position. The pier on the Cornwall side was erected first. This pier was raised by a jack 3-feet at a time. This method was used to allow for the central pier to have ironwork constructed, and the landward side to have brickwork constructed below the pier (Building the Bridge (2015)).
2.3 Summary
Brunel contributed the use of closed systems combined with a standard deck in the Chepstow Bridge. This design had never been seen in bridge engineering. He furthered this design with the Royal Albert Bridge. It can be argued that Brunel invented a new design into the theory of suspension bridges through his tubular arch and cross bracing design.
3. Brick and stone
Brunel can be famed not only for his work with wrought iron, but also his work with standard materials such as brick and stone. He pushed the limits of these materials to create bridges of seeming-less design. The most notable of which is the Maidenhead Bridge.
3.1 Maidenhead Bridge
The Maidenhead Bridge was constructed to allow trains to cross a 300-foot-wide part of the river Thames. It boasts two of the shallowest and widest brick arches in history. The bridge had two spans separated by a central pier. The spans have a width of 128 feet with a rise of only 24 feet. This allowed for two clear paths under the bridge for sailboats. A scaffolding of timber was used to aid construction and many doubted the bridge would remain standing once this was removed. Brunel proved to the critics what could be accomplished with brick built bridges, as the bridge continues to carry railway to this day.
3.2 Summary
Brunel pushed the limits of basic materials and created a bridge that defied what many engineers thought possible. The contribution made to bridge engineering was a further understanding of what can be done with basic materials.
4. The Clifton suspension bridge
Proposals had already been made to cross the Avon Gorge well before Brunel was born. As Clifton village grew as a home for wealthy merchants, the demand for a bridge to cross the gorge grew. To add, constructing the bridge could bring more trade to Bristol from the west and south. Many of the merchants realised that the cost would be close to 90,000 as oppose to the proposed 10,000 if the bridge were to be stone-built. This lead to a competition for engineers to submit their designs for a suspension bridge, in which Brunel entered four out of twenty-two submissions. Brunel’s entries had designs that spanned from 760 feet up to 1,180 feet (Christopher, 2014: 9).
Engineer/Name of bridge
Span in feet
Thomas Telford/Menai Bridge
600
Marc Brunel/ Île de Bourbon suspension bridge
132
Isambard Kingdom Brunel/Clifton suspension bridge proposals
760 – 1,180
(Figure 2)
‘Figure 2’ above shows a comparison of suspension bridges built at that time, to Brunel’s proposals. Brunel had entered a design almost double the span of the longest suspension bridge built. Brunel’s father, Marc, wrote to him saying that there must be a support at the centre of the bridge otherwise it will fail. Brunel went forward with his design spanning just over 700 feet. The Clifton Suspension Bridge began construction in 1831 but was not completed before Brunel’s death. The bridge was completed in 1864 and spans 702 feet.
4.1 Summary
Although the Clifton suspension bridge was not completed by Brunel, his design (a bridge to carry loads from horse and carts) carries modern day traffic across the Avon Gorge. He contributed unusual designs that pushed the understanding of suspension bridges at that time. Moreover designs surpassed the requirement at that time and continue to carry much heavier loads on a daily basis.
5. Conclusion
Brunel’s contributions to bridge engineering include:
1) His work with wrought iron (Chepstow and Royal Albert Bridge)
2) His work with basic materials (Maidenhead Bridge)
3) The way he pushed the boundaries of understood concepts (through work on all bridges discussed)
His work with wrought iron introduced an unseen design for br
idges using known concepts. He combined the concepts of suspension and truss bridges to overcome problems. His work with basic materials demonstrated the potential that brick and stone had in bridge engineering. As a result of overcoming many engineering critics (with the Maidenhead Bridge), he contributed a further understanding of brick and stone. Finally, throughout his career he pushed the boundaries of understood concepts. This is a more abstract contribution but proved (to many other engineers) that more minimal and cost-effective designs can be accomplished with basic materials.
Bibliography
Brindle, S. The man who built the world (2005)
Christopher, J. Brunel’s bridges: Clifton suspension bridge 150th anniversary (2014)
Building the bridge (2015). Available at: http://www.royalalbertbridge.co.uk/building-the-bridge.html (Accessed: 1 December 2015).