The UNESCO World Heritage List is a collection of historic sites and landscapes of “outstanding universal value”. Civil engineers worldwide were understandably pleased when this gushing label was granted to the Forth Rail Bridge in Scotland. While UNESCO’s decision is certainly commendable, it should be noted that the new world heritage site is only the fourth to be centered around a bridge- the others being the Ironbridge in England, the Vizcaya Bridge in Biscay, and the Mehmed Paša Sokolović Bridge in Višegrad, Bosnia-Herzegovina. Other sites may include bridges- for instance, central Amsterdam and the Pontcysyllte Canal -, but only these four have been listed by UNESCO specifically to celebrate and preserve these bridges. That’s only four out of over a thousand heritage sites; this fact is astonishing to me. I’m not saying the other sites are not worth recognition- all historic sites are invaluable- but I can’t help but feel that bridges are underrepresented. To be a world heritage site, a landmark must meet one out of ten criteria. Here are three of the criteria:
- to exhibit an important interchange of human values, over a span of time or within a cultural area of the world, on developments in architecture or technology…
- to be an outstanding example of a type of building, architectural or technological ensemble or landscape which illustrates (a) significant stage(s) in human history
- to be an outstanding example of a… human interaction with the environment…
Bridges are stellar symbols of the technological achievements of mankind, for few other objects so perfectly represent our ability as humans to surmount obstacles, to tame our environments, to shape the world around us. Bridges can change history, linking cities, regions, and peoples to initiate cultural and economic changes. Finally, the greatest bridges pay testament to the inventiveness and ingenuity of their engineers, fulfilling UNESCO’s first and perhaps foremost criteria:
- to represent a masterpiece of human creative genius.
Here are four more bridges that UNESCO and the States Parties that submit tentative heritage sites should perhaps consider next year.
For a shallow stone arch bridge with a large span- in this case, forty meters- to survive a 7.2 magnitude earthquake is no mean engineering feat. To withstand such an event unscathed fourteen centuries after its construction is unbelievable. China’s sixth-century Zhaozhou bridge has entered into legend thanks to its longevity; the creator of the bridge has become mythologized, said to have waded into the river to hold up the bridge when two immortals crossed it. Whoever the actual engineer was- legends call him Lu Ban- they were incredibly skilled and resourceful. The graceful shallow arch- a daring structure which contemporary Europeans refused to attempt- was made possible through graceful spandrels (smaller archways that lighten the load) and the joining of stones with iron dovetails, adding strength to the bridge. The lack of mortar, which cracks under pressure, helps the bridge survive earthquakes. The Zhaozhou Bridge is one of the greatest feats of ancient engineering.
In 1826, Thomas Telford finally provided a safe crossing of the treacherous strait between Wales and Anglesey: the Menai Suspension Bridge. Nowadays, suspension bridges are the go-to designs when a large span is needed; every such bridge is a part of Telford’s legacy, for the Menai Bridge was the first large-scale bridge supported by suspension chains. It was a construction of biblical proportions, requiring stone anchorages 59 feet deep at either end to secure the four 133-ton chains, which were themselves each made of 935 iron sections. With elegant curves and monumental archways, the bridge is graceful as well as inventive. Its construction had strong cultural effects as well as a legacy of engineering: it cut the journey time from London to Dublin by 36 hours (ferries across the Irish Sea used Anglesey’s ports), providing a strong link between the British Isles.
One bridge that bears Telford’s legacy is New York’s instantly recognizable Brooklyn Bridge, which does not by any means lack inventiveness of its own. As I have previously written, the bridge was built on a massive scale, with a main span of nearly five hundred meters- when complete it was the longest span in the world. To support such a long roadway, the engineers John and Washington Roebling developed new and resourceful engineering strategies. Rather than iron chains, they used stronger steel cable- a feature still found in modern suspension bridges. For added strength, they combined suspension and cable-stayed supports, resulting in the cobweb of wires that users of the bridge can see. By literally connecting Manhattan and Brooklyn, the Brooklyn Bridge facilitated the rapid growth of both regions, and without it New York City might not have the immense size and cultural importance that it does today.
Finally, Switzerland’s Salginatobel Bridge serves a relatively low-key purpose- carrying a one-lane road across a gorge in a sparsely populated region- but pioneering engineer Robert Maillart saw the bridge as a perfect opportunity to demonstrate his engineering theories, including his prophetic belief that reinforced concrete was the material of the future. The beautiful monolithic structure is largely a three-pinned arch, with hinges at the center and at either abutment to combat compression and expansion of the concrete. This method of construction, developed by Maillart, is now commonplace in almost every bridge. Maillart’s reinforced concrete also allowed the bridge to be hollow, lightening the dead load and decreasing cost. For its futuristic appearance, the Salginatobel Bridge was at the time the most economical design that could span the gorge. Maillart’s perfect synthesis of technology and aesthetics makes this bridge deserving of landmark status.
There are of course many other bridges that deserve to be celebrated, as well as other forms of civil engineering, like dams and roads. Over the coming years, I hope- and I’m sure other engineers, architects, and others hope- that UNESCO will continue to recognize monuments like the Forth Bridge.
First image from asiavtour.com
Second image by Darren Glanville, Wikimedia Commons
Third image by Laura Choate, Wikimedia Commons
Fourth image by Rama, Wikimedia Commons
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