A bridge's purpose is not usually up for discussion. Linking one parcel of terra firma with another has traditionally been a question of "how" rather than "why." But an exhibition in London shows a bridge's role can embrace more than the obvious.
Art gallery, arboretum, multistory car garage, convention center, even skyscraper apartments - all of these have been wrapped into new bridge designs in recent decades. As radical as it may sound, however, the idea is actually centuries old.
Hundreds of inhabited bridges stacked high with shops and houses thrived in European towns for 600 years. Ponte Vecchio in Florence and Ponte di Rialto in Venice, two of the few remaining and perhaps best known, still flourish commercially. In fact, says Jean Dethier of the Centre Georges Pompidou in Paris, who has pioneered research on the habitable bridge, "A bridge lined with shops can be regarded as the ancestor of our ... modern shopping mall."
But inhabited bridges offer more than an expanded marketplace. By extending town life onto the bridge itself, Mr. Dethier says, an organic relationship between the two banks is created that helps knit a naturally divided city together. The elegant Pulteney Bridge across Britain's River Avon, one of the last habitable bridges to be built in 1773, served just that purpose by drawing Bath's fashionable set to the less-developed south side.
Since the decline of these multifunctional thoroughfares in the 18th century, however, not a single new habitable bridge has been built.
The exhibition "Living Bridges: The Inhabited Bridge, Past, Present and Future" at the Royal Academy of Arts in London may end that hiatus. It aims to raise the profile of the inhabited bridge and help put it back into the lexicon of city planners, after decades of being viewed as historically interesting but functionally irrelevant.
For engineers and architects, on the other hand, the habitable bridge has long been a favorite form open to tremendous expressiveness and even flights of fancy.
In fact, most of the two dozen exquisite models in the exhibition are bridge proposals that never made it off the drawing board. One ambitious project by the American architect Raymond Hood in 1925 envisioned huge skyscraper bridges connecting Manhattan to the mainland, each housing 50,000 people. The plan fell by the wayside after the 1929 Wall Street crash.
The goal of reviewing the habitable bridge's development is not simply to copy a historical model, however, but rather to reinvent it for the 21st century.
To that end, the exhibition's finale includes scale-model designs from seven internationally renowned architects for a new "Old London Bridge" across the River Thames. As part of the Thames Water Habitable Bridge Competition, a diverse group of architects was given a brief to design a bridge connecting Temple Gardens to the South Bank (between Waterloo Bridge and Blackfriars Bridge). The bridge had to be a destination in its own right, help reduce the divisive nature of the river, and be self-financing.
Entries ranged from an organic-looking structure with an iridescent skin that shimmers at night to a lizardlike form of yellow and red to - one of the winning entries - a translucent collection of fingers fanning out across the river, by Zaha Hadid Architects in London. Graham Modlen, one of the team of architects who worked on the Hadid entry, describes it as a "confluence of bridge ideas into one" - a bundle of elements grabbed tightly on the north side that loosen out on the south side.
Although planning permission for such a bridge is still down the road, exhibition curator and competition organizer Peter Murray says the project has solid interest among developers and the strong backing of John Gummer, the secretary of state for the environment, who is keen to see it built by 2000.
London isn't the only venue for a modern habitable bridge, however. Rome, Dubai in the United Arab Emirates, and Boston all have proposals well on the way. The Rome Millennium Bridge by architect Cezary Bednarski, for example, would cross the River Tiber at the junction of two roads, one leading to Vatican City and the other to a new mosque. Also known as the bridge "between Islam and Christianity," it looks like a layered cake with swathes of translucent ribbons flanking a tree-studded public garden. Designed to provide temporary accommodation for some of the 4 million tourists and pilgrims expected to descend on Rome in 2000, the rooms could later be transformed into offices and restaurants.
Mario Bellini's Dubai Pearl Bridge would link the north and south of the city across the Al-Khor Creek. A huge arched conference center would span the divide with a 2,500-seat auditorium in the shape of a pearl suspended high over the water, an allusion to the country's pearl-diving heritage.
Boston's less-spectacular proposal is to revamp a turn-of-the-century swing bridge. The Old Northern Avenue Bridge spans Fort Point Channel, linking the downtown financial district to the soon-to-be-developed waterfront area. Right now, the bridge is swung permanently open because the city of Boston is unwilling to pay for maintenance and the navigation channel needs to be clear.
But a local architect, Wellington Reiter, at the invitation of the Boston Redevelopment Authority, has proposed a way to draw the bridge back into active use. Interestingly, he would leave the swing section open - like a central island - and would then connect it to each side with a series of smaller bridges.
With the renewed interest in the habitable bridge, one might wonder why such a versatile building type was abandoned in the first place. Its demise owes much to the Renaissance, which disdained most things medieval, including bridges piled high with buildings. Many were demolished and more fell during the 18th century, when the Romantics found they blocked the natural view.
The living bridge's newfound exposure - and receptivity - has been eased along by the advent of modern lightweight building materials that allow complex structures to be both delicate-looking and robust.
The Pompidou's Dethier believes the logic of the habitable bridge can no longer be ignored. With so much dysfunction in the modern city, he observes, "the time has come to ... appreciate their merits and devise for them new applications."
The exhibition 'Living Bridges: The Inhabited Bridge, Past, Present and Future' will be at the Royal Academy of Arts in London until Jan. 5.