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Some bridges are engineered with nothing but utility in mind -- for these, aesthetic design is secondary to safety and longevity. And given that San Francisco's Bay Bridge was just closed for six days, this makes sense. But advances in design software and construction materials have given bridge architects opportunities to focus on original, striking and sometimes whimsical designs that impress, while keeping function in mind. Here are some of our favorite unusual bridges and why they're architecturally striking.
Rolling Bridge
Background: UK-based design firm Heatherwick Studio created this 39-foot timber and steel bridge in 2004 to act as a walkway over a small section of London's Grand Union Canal.
Why It's Innovative: A hydraulic system built into the bridge's handrail allows it to retract and curl into an octagon. The retractable design allows for boats to cruise through the canal unobstructed, and the bridge rolls up every Friday at noon.
Donald MacDonald, an architect with more than 40 years of experience who has worked on over a dozen bridges, tells PM that, "this bridge represents an experimental gesture, and it's really complicated for what it does."
Henderson Waves
Background: The 118-foot tall Henderson Waves is Singapore's tallest pedestrian bridge, linking Mount Faber Park with Telok Blangah Hill Park.
Why It's Innovative: Pedestrian bridges allow for a certain amount of creativity that's not possible with structures that need to support heavy- duty use. The undulating outer shell of the Henderson Waves is striking, and the inside is shaped into benches where tourists can sit and gaze at nature or the nearby skyline of Singapore City. The bridge, which is about 900 feet long, is illuminated by an array of LED lights each night to bolster its snake-like presence in the midst of two national parks.
Sundial Bridge
Background: Renowned Spanish architect Santiago Calatrava designed this cantilever spar cable-stayed bridge to be made from a combination of steel, glass and granite. Completed in 2004, the pedestrian bridge crosses the Sacramento River in the Turtle Bay Exploration Park.
Why It's Innovative: As its name implies, the bridge's 217-foot-tall support tower acts as a giant sundial. The deck of this pedestrian bridge is made from nonskid glass panels to give walkers a greater sense of the river. MacDonald notes that the deck's truss bears the bulk of the bridge's load, while the cables appear slack compared with those of other bridges.
Juscelino Kubitschek Bridge
Background: Completed in 2002, this asymmetrical bridge crosses Lake Paranoa. Its name pays homage to former Brazilian president Juscelino Kubitschek, who established Brasilia as the nation's capital in the 1950s.
Why It's Innovative: Three steel arches that jump from side to side support the deck of the 3900-foot-long bridge. Architect Alexandre Chan, who designed the bridge, has said that he wanted to avoid straight lines on the deck in order to accentuate Brasilia's striking sunsets.
Millau Viaduct
Why It's Innovative: With its apex at 1125 feet, the Millau Viaduct is one of the world's tallest bridges. Despite its huge size, the construction period lasted only three years thanks to the use of GPS guidance systems, self-climbing formwork and prefabricated materials. "What is unusual is to have the towers taper in both directions as they rise," Macdonald says.
More than 1500 tons of cables support the 1.5-mile structure. Because its deck is 885 feet above sea level, drivers often find themselves above the clouds on foggy days.
Langkawi Sky Bridge
Background: This curved pedestrian bridge is 2000 feet above sea level at the top of Mount Mat Cincang in Malaysia. Tourists who want to walk across the 400-foot long structure must ride up to it in a cable car.
Da Vinci Bridge
Background: Leonardo Da Vinci sketched plans for this bridge in 1502 for Sultan Bajazet II of Constantinople, but it wasn't until 500 years later that the design was brought to life by Norweigan artist Vebjørn Sand. The pedestrian bridge opened in 2001 and crosses the E18 highway.
Why It's Innovative: For centuries it was thought that Da Vinci's sketch of the bridge was implausible. The bowed arches of the single-span design are dramatically thin where they connect with the walkway, but they flare out as they approach the ground. Da Vinci's original plan called for the span to be 720 feet long, but Sand and colleagues scaled it down to 300 feet and built the walkway from Norwegian pine.
Gateshead Millennium Bridge
Background: This award-winning tilt-bridge is the brainchild of Wilkinson Eyre, a London-based architecture firm. It crosses the River Tyne, connecting Gateshead and New Castle.
Why It's Innovative: A system of six hydraulic rams can pivot the bridge's walkway at a 40-degree angle in order to let boats pass. Wilkinson Eyre describes the bridge's movement as looking like a "slowly opening eyelid" when it raises, a process that takes fewer than 5 minutes. Perhaps even more impressive is the fact that the bridge was fully constructed before being installed as a single piece by Europe's largest floating crane, Asian Hercules II.
Oresund Bridge
Background: The Oresund Bridge crosses the Oresund strait and joins Sweden with Denmark. This marvel of ingenuity has a total length of 4.8 miles and was completed in 2000.
Why It's Innovative: The Oresund Bridge is an incredibly complex structure that begins as a cable-stayed bridge in Sweden and ends as a tunnel in Denmark. A small artificial island was built around the tunnel's entrance to keep water from creeping in. Not only is the bridge equipped to handle four lanes of traffic; it also has a double-track railway. Creating the tunnel was likely the best and cheapest solution to completing the crossing without impeding boat traffic, MacDonald says.
Octavio Frias de Oliveira Bridge
Background: This mammoth, x-shaped, cable-stayed bridge was completed in 2008 over the Pinheiros River in São Paulo. The peak of its pylon is 452 feet high, while the bridge's total length is just shy of a mile.
Why It's Innovative: The most noticeable attribute of the bridge is how its two levels of traffic cross one another as they pass through the pylon. A series of 144 steel cables were used to support the structure. Brazil spared no expense on this masterpiece; Philips was contracted to develop an energy-efficient system of LED lights that showers the bridge in changing colors and patterns, which the company says consumes 53 percent less energy compared to lighting on other bridges.
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