The Mechanics Behind Floating Bridges

Imagine a future where cars transform like Transformers, shifting from road vehicles to helicopters and boats. While we await such futuristic marvels, we’re limited to traveling on asphalt and concrete. However, some highways achieve remarkable feats of engineering, such as floating bridges.

Floating bridges, often referred to as pontoon bridges, lack sails or motors, making them unsuitable for ocean adventures. However, they provide a unique driving experience, allowing vehicles to traverse water bodies with ease.

While floating bridges may seem like a novelty, their primary purpose is highly practical. These innovative structures are essential for many metropolitan areas, ensuring smooth traffic flow and supporting local populations and economies. Without them, some cities would struggle to maintain efficient transportation networks.

Permanent floating bridges remain a rarity, with only about 20 such structures worldwide. Notably, Washington state is home to four of them, driven by its dense population, thriving economy, and water-rich urban areas, making it a hotspot for these unique bridges [source: Washington DOT].

Floating bridges come in diverse designs tailored to their location and function. Some are massive constructions of concrete and steel, engineered to bear the immense weight of relentless commuter and commercial traffic.

On the other hand, simpler wooden designs cater to lighter loads. A prime example is the historic 330-foot (98-meter) wooden bridge in Brookfield, Vt., now reserved for pedestrians. This bridge gained fame partly because vehicles crossing it often ended up with wet wheels.

Temporary pontoon bridges are far more common than their permanent counterparts. Often deployed in emergencies, they prove invaluable for military operations, enabling the movement of troops and equipment across rivers or lakes. These short-term structures can be crafted from wood, metal, or even inflatable materials.

Despite their varied materials, all floating bridges share one key feature: they genuinely float. Delve deeper into their history on the next page, and you'll discover how these remarkable structures have bridged diverse engineering challenges over time.

A Boat-like Bridge to History

In ancient times, people undoubtedly gazed across rivers at distant shores, yearning for bridges to connect them. While wishes alone couldn’t create such structures, the demands of war often made it possible.

Many of the earliest floating bridges were constructed for military purposes. Civilizations like the Chinese, Romans, Greeks, and Mongols employed pontoon bridges to transport troops and supplies across rivers that were too deep to cross on foot.

The simplest floating bridges consisted of wooden boats arranged in rows, with planks laid across them to accommodate pedestrians, horses, and carts. These bridges were anchored to the shores, often with ropes, to prevent them from drifting due to currents or wind.

One of the most legendary floating bridges from ancient times was constructed under the orders of Xerxes, the Persian king. In 481 B.C., Xerxes aimed to conquer Greece, but his massive army faced the challenge of crossing the Hellespont, a strait spanning approximately 4,409 feet (1.3 km).

Xerxes ordered his men to build a floating bridge, but a fierce storm destroyed their efforts. In a display of rage toward the sea, Xerxes instructed his men to symbolically whip the water 300 times and threw leg shackles into it. He also executed the engineers responsible for the failed bridge.

A second attempt succeeded, utilizing nearly 400 ships to keep the bridge afloat. These ships were bound together with strong flaxen and papyrus ropes, secured with heavy anchors. An opening was left to allow smaller vessels to pass through the strait.

The bridge's surface was made of logs, covered with brush and soil. Barriers were added on both sides to prevent horses (and possibly soldiers) from being frightened by the proximity to the water. This bridge held firm, enabling hundreds of thousands of soldiers to cross and ultimately attack and seize Athens.

Floating bridges have played a crucial role in numerous wars since then, including the American Civil War and both World Wars. They remain essential for military operations requiring the crossing of water bodies to engage or retreat from enemies.

Modern pontoon bridges are highly portable, often transported on semi-truck trailers. They typically feature an inflatable base paired with a metal framework capable of supporting heavy vehicles and personnel.

While many pontoon bridges are designed for military use, others serve peaceful purposes. Continue reading to discover how more permanent versions are built for civilian applications.

Floating Bridge Design and Construction

Floating bridges are rare due to the specific conditions required for their construction. They are primarily used in densely populated areas with very wide and deep bodies of water, where soft lake or ocean floors make traditional bridge piers impractical.

Floating bridges are more economical than traditional bridges in situations where the water depth exceeds 100 feet (30 meters) and the width spans over half a mile (900 meters). However, regions with intense winds or waves often render floating bridges impractical.

Modern floating bridges are typically constructed using steel, concrete, wood, or a combination of these materials. Concrete is particularly advantageous in saltwater environments due to its resistance to corrosion and its ability to absorb vibrations caused by traffic, wind, and waves.

Engineers must account for both horizontal forces from water and wind, as well as the vertical load from traffic. This is where buoyancy plays a critical role. Pontoons, which are hollow and watertight, provide the necessary buoyancy to support significant weight, even when made from heavy materials like concrete, by displacing an equivalent volume of water.

Pontoon bridges fall into two main types: continuous and separate. Continuous pontoon bridges feature a single, uninterrupted pontoon structure that runs the full length of the bridge, often topped with a traffic deck. Separate pontoon bridges, on the other hand, use multiple pontoons spaced out along the bridge's length.

While making bridges float is relatively straightforward, the real challenge lies in their design and the complications posed by land rather than water. On the next page, you'll explore these design intricacies further.

Making the Critical Connections

Each location presents unique geographical challenges, prompting engineers to select the most suitable bridge design. For floating bridges, pontoons are typically constructed onshore near the site and then transported into position by barges. Once anchored, the bridge's support structures and deck are installed.

Engineers often need to accommodate water traffic, such as barges or naval vessels. To address this, bridges may feature elevated sections (known as high rises) for boat passage or drawbridges that can be raised, though this requires halting vehicle traffic.

To ensure floating bridges remain stable and stationary, engineers employ a mix of anchors and mooring lines. These anchors, weighing several tons, along with the mooring lines, allow the bridge to withstand stress and flex without breaking.

While the floating sections of bridges are relatively straightforward, the shorelines often present the greatest challenges. Shores are rarely level with the water surface, frequently uneven, and situated at higher elevations, complicating the design process.

Engineers must design approaches that allow vehicles to transition safely onto the bridge without steep inclines. This can involve gradually elevating the shoreline with soil or rock to create a smoother slope. Alternatively, supports are driven into the underwater soil to anchor a deck that slopes gently toward the floating sections. Some bridges even use tunnels to connect to the water-based portions.

As engineering expertise has advanced, the achievements in floating bridge construction have grown increasingly remarkable. Among these, Seattle is home to one of the most astonishing floating bridges in the world. On the next page, you'll discover the details of the largest pontoon bridge ever built.

The World's Biggest Floating Bridge

The Governor Albert D. Rossellini Bridge—Evergreen Point bridge, commonly known as the SR 520 bridge in Seattle, holds the title of the world's longest floating bridge. Stretching 7,497 feet (2,285 meters) or 1.4 miles, it spans Lake Washington, connecting Seattle and Bellevue, Washington.

Following three years of construction, the four-lane toll bridge opened in 1963. It wasn't the first floating bridge on Lake Washington; that distinction belongs to the Lake Washington Floating Bridge, built in 1940 and later incorporated into Interstate 90. However, the SR 520 bridge far surpasses its predecessor in size.

The bridge features four lanes and is 60 feet wide, supported by 33 massive pontoons. Each pontoon stands approximately two and a half stories tall and measures 360 feet (110 meters) in length—longer than four semi-trucks lined up end to end. These pontoons are anchored by 62 anchors, each weighing around 77 tons.

If those figures aren't impressive enough, consider this: the SR 520 bridge is undergoing a significant upgrade. The new version will include six lanes, a bicycle path, and a width of 115 feet. It will be supported by 77 pontoons, each 360 feet long, 75 feet wide, and 28 feet tall. Remarkably, each pontoon will weigh approximately 11,000 tons, equivalent to 23 Boeing 747 jets [source: Washington DOT].

The bridge is designed for future upgrades. Currently, the two center lanes are reserved for HOV (high-occupancy vehicles), but they could later accommodate a light-rail system, though this would require additional pontoons to support the extra weight.

You might question why Washington State would construct such a colossal bridge. The Seattle metropolitan area hosts major corporations with vast workforces, all requiring efficient transportation. The most direct route across Lake Washington is the quickest, but its curvature makes a suspension bridge impractical, and the lake's depth rules out conventional bridge designs.

However, this bridge comes with a hefty price tag. The estimated budget for the new crossing is currently $4.65 billion [source: Washington DOT].

Given the enormous cost, we can only hope this bridge stands the test of time. Yet, as Murphy's Law often proves, even floating bridges are not immune to failure. Continue reading to learn about famous bridges and their notorious mishaps.

These Bridges Don't Burn (But They Do Sink)

Natural disasters can devastate human-made structures, and floating bridges are no exception. Wind, waves, and corrosion pose significant threats to these primarily concrete and steel constructions. Despite meticulous planning and overengineering to account for extreme conditions, floating bridges can and do fail.

When designing floating bridges, engineers carefully consider local environmental factors, such as wind and wave patterns. They also account for rare events like ship collisions or severe storms. Typically, bridges are built to endure a once-in-a-century storm, with designs that allow for flexibility in response to temperature changes, wind, and wave activity.

During intense storms with high winds and waves, traffic is often halted. If the bridge includes a navigational opening, such as a drawbridge, it is opened to reduce stress. This was the approach taken by the Hood Canal bridge operators during a fierce windstorm in February 1979, though it ultimately proved ineffective.

In that storm, the doors on the western pontoons were either blown open or accidentally left open. Rainwater and waves, reaching heights of 10 to 15 feet, gradually filled the pontoons. The relentless pounding of the waves caused the bridge to break apart and eventually collapse.

In November 1990, renovation crews working on the Lake Washington bridge left for Thanksgiving weekend as a major storm approached. With engineers' approval, they had stored wastewater in some of the bridge's large concrete pontoons, temporarily removing the watertight doors. As the storm hit, seawater and rain flooded the pontoons. The bridge slowly tore apart and sank, captured live on television.

Both bridges were eventually rebuilt using advanced engineering techniques and modern construction methods. However, concerns persist about whether massive floating bridges are durable enough to justify their high initial costs. Additionally, these bridges are notoriously expensive to maintain, as they are constantly exposed to natural forces that require ongoing monitoring for cracks and other issues, necessitating costly repairs.

Despite these challenges, engineers continue to propose and construct floating bridges. Read on to discover more about famous floating bridges around the world.

A Global Gathering of Pontoons

While Washington State is home to the largest and most numerous floating bridges, they are not exclusive to this region. Countries worldwide have constructed their own permanent pontoon-based bridges.

Similar to the Seattle area, Georgetown, Guyana, faced economic challenges due to the Demerara River. To address this, the Demerara Harbor Bridge was built in 1978, connecting communities and fostering commercial growth.

Stretching 1.2 miles (2 kilometers), this bridge ranks as the fourth-longest pontoon bridge globally. It features a two-lane roadway supported by 122 steel pontoons. Similar to its counterparts in Washington, this bridge has faced challenges, including deterioration from inadequate maintenance, collisions with ships, and sections sinking due to strong tides.

Norway, like Washington, has significant water bodies near its urban areas, necessitating structures like the Nordhordland Bridge. Its floating section spans 4,088 feet (1,246 meters) and is supported by 10 pontoons of varying sizes. This section connects to a cable-stayed bridge, allowing it to cross a fjord.

Not all floating bridges are massive or modern. The Dongjin Bridge in China, for instance, has been in use since the Song Dynasty (960–1279). This 400-meter bridge relies on around 100 wooden boats for buoyancy, with a wooden plank deck secured by chains.

Floating bridges have been integral to human transportation for centuries. Despite facing harsh winds and waves, they have endured over time. As long as there is a need to cross deep waters, these bridges will continue to play a vital role.