Historically, materials and their implications Flashcards
Timber
Was commonly used in the earliest bridges, perhaps in as simple a form as a log between two banks. Timber offers ease of manufacture and has historically been a readily available resource, mainly limited by its strength and its susceptibility to rotting.
Timber’s use as fuel as well as its use in construction often resulted in deforestation of surrounding areas. Trees take centuries to regrow and many areas are yet to recover from the destruction.
Rope
Another material used in earlier bridges, rope suffered the same limitations as timber; lack of strength and rotting.
Stone
A far more durable and permanent material than many of the alternatives used in early bridge construction, stone offered great strength in compression, albeit sub-par strength in tension, which made it suitable for arch bridges.
Stone is not commonly found in desirable quantities and shapes above ground, necessitating the use of quarries which inevitably resulted in the destruction of landscape, with similarly devastating impacts on flora and fauna.
Brick
Though similar in properties to stone, bricks are manufactured from clay and can be formed to shape as opposed to chiselled to shape in the way stone is.
A similar issue to stone quarries, bricks require vast amounts of clay and shale to manufacture, so large pits were dug to find these resources. These pits, once completely hollowed, were filled back in and reclaimed. These sites often find issues with subsidence (the sinking of the ground’s surface).
Cast iron
Cast iron found widespread use throughout the Industrial revolution, and by the late 18th century was available for use in bridge construction. Similar to stone, cast iron had great strength in compression but was weak in tension. However unlike stone, cast iron could be formed into shape, saving time and introducing the concept of pre-fabrication. It could also be formed into frames, maintaining similar strength to its stone counterparts, but with a far greater strength-to-weight ratio.
Cast iron is just one of the many materials that required use of fossil fuels to manufacture. With both mining and smelting involving processing plants and transport facilities, all associated industries have had a huge impact on the landscape, and we are starting to feel the effects of such a legacy today.
Wrought iron
Wrought iron is loosely defined as a mixture of elemental iron with an added 1-2% of slag, the by-product of iron smelting, generally consisting of silicon, sulphur, phosphorous and aluminium oxides. It was originally used in the chains from which the bridge decks were suspended, however due to the unreliability and inconsistency of the material as a result of the fibrous structure present in the ferrite, these suspension bridges were often limited in length.
Implications - refer to Cast iron.
Steel
Steel was mass produced in 1856 by Henry Bessemer and it changed bridge design. Steel is equally great in both tension and compression compared to stone and cast iron, and so suspension bridges, as opposed to the more traditional arch-bridge, could be achieved, which allowed for greater spans.
The impact of steel is similar to cast and wrought iron, however on a greater scale as a result of its proliferation throughout different industries.
Concrete
Concrete was not used in bridge construction until the issues of shrinkage and weakness in tension were addressed. Reinforced concrete has been developed into pre-stressed and post-stressed concrete to improve strength and performance. Most 20th century bridges now incorporate reinforced concrete into their designs.
As with other resources, concrete requires vast amounts of mineral to manufacture it, so with it comes adverse costs to the environment. Some (i.e Mr Copeland) may also be inclined to argue that it is
also visually unappealing.
Stainless steel
Stainless steel is now being used in certain bridges, often pedestrian bridges. Its main advantages of course are the hallmark qualities of steel, in addition to the bonus corrosion resistance.
