Over the last few days UNESCO has announced the latest additions to its list of World Heritage Sites, historic or natural landmarks of “outstanding universal value”. Recognized were sites such as Mongolia’s Khaldun Mountain, Hamburg’s Speicherstadt, and Champagne’s vineyards. Amidst these historic landscapes and buildings was an engineering landmark: the Forth Rail Bridge in Scotland. The iconic crossing is heavily praised by UNESCO as “a world wonder of its age” with “the perfect balance of structural elegance and quality of design”, and the organization’s selection of the bridge has been celebrated by locals, preservationists, and engineers. Hopefully, UNESCO’s choice will set a precedent and pave the way to further recognition of landmarks of civil and structural engineering.
Ever since its construction, the Forth Bridge has been viewed as one of the world’s most beautiful and distinctive bridges. The claret behemoth carries a railroad across the mouth of the Forth River, near Edinburgh, between the villages of North and South Queensferry. Many qualities of the bridge are stunning: one is its color, a striking, high-visibility red that stands out in the oft-bleak Scottish weather. Another is the sheer scale of the bridge. The whole bridge is two and a half kilometers long; the superstructures are a hundred meters high and twice as long as the Eiffel Tower- a contemporary construction- is tall. “Painting the Forth Bridge” has become an idiom in Britain meaning an endless, Sisyphean task for, prior to the recent invention of a long-lasting, multilayered paint, workers were perpetually repainting the massive bridge. A final striking quality is the design: the structure is a cantilever bridge, a type that is rare and has now largely been replaced by suspension and cable-stayed bridges,where the diamond-shaped, truss-filled superstructures support lighter sections at the center of each span. The cantilever structure was one of the many ingenious features of the design by its engineers John Fowler and Benjamin Baker.
Sir John Fowler was one of the Victorian era’s most prolific engineers, and before the Forth Bridge had been largely responsible for the development of England’s railways. Baker was younger and less experienced, but would later go on to build Egypt’s Aswan Dam. They were hired together to span the Forth at a time when the British public were wary of engineers following the deadly collapse of the Tay Bridge in Scotland in 1871. Their employers, the Board of Trade, demanded a rigid and stiff structure that could withstand maximum loads. Their demands led to two groundbreaking decisions by Fowler and Baker: to build a heavy and sturdy cantilever bridge and to build almost entirely with steel, making the Forth Bridge the first major structure in Britain to be built with the material, instead of wrought or cast iron. 53,000 tons of steel were used, making the design expensive (3.2 million pounds at the time) but extremely strong, capable of carrying even large modern locomotives.
Though the Forth Bridge was not the first cantilever bridge, it was built on a much larger scale than any previous similar structure. It remains the world’s largest cantilever bridge, even though the Quebec Bridge’s single span is longer than either of the Forth Bridge’s. Cantilever bridges in their most simplified form are two T-shaped structures- TT- where the horizontal lines act as lever arms and the vertical lines as fulcrums. The weights of the lever arms are balanced on either side so that the weight of one half supports the other. The outer ends are anchored to counterweights for maximum stability. The Forth Bridge has diamond-shaped cantilevers instead of T’s, made rigid by the steel trusses. The upper sides of each diamond are in tension and the lower sides in compression, and stone counterweights allow the central spans to be supported. A famous image of Fowler, Baker, and an assistant demonstrated the safety and logic of this design to the public:
The result of Fowler and Baker’s engineering is a masterpiece of design that is undeniably graceful. It is without a doubt one of the world’s most important bridges, a key structure from what could be described as a golden age of engineering in the late nineteenth century, when leading figures praised the “art of building”- the elegance of modern engineering- as superior to the useless ornamentation of earlier architecture. As well as its aesthetic merits, the Forth Bridge has developed a magnificent legacy in the engineering world, repopularizing the profession in Britain and initiating the widespread use of steel in bridge construction. Its new status as a World Heritage Site should protect it for future generations. It will also limit future development around the bridge, ensuring that the structure will continue to dominate the Firth of Forth.
First image by Accent Garden Designs, Pinterest
Second image by soozq, Pinterest
Third image by nas.gov.uk
Fifth image by Kim Traynor, Wikimedia Commons
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