1) The drawbridge was built to speed up travel

The Eshima Ohashi bridge is the largest rigid-frame bridge in Japan and has an extraordinary design. It was built to replace a drawbridge that used to cause traffic delays and weight restrictions for vehicles crossing the lake. The bridge had to be high enough to allow ships to pass underneath without interruption, giving it a roller coaster-like appearance when viewed from certain angles.

The previous bridge across the lake was a drawbridge that could be lowered for road vehicles to pass through on the road and then raised for the ships to pass through, but it could sometimes take ships eight minutes or more to pass underneath, slowing traffic significantly.

Additionally, the drawbridge restricted vehicle weight and size, reducing its usefulness and frustrating travellers. As a result, the Japanese government decided to replace it with a tall bridge that could allow the movement of ships and heavy road vehicles at the same time.

The new bridge can withstand earthquakes up to magnitude seven on the Richter scale thanks to its rigid-frame structure that distributes the load evenly. 

From this angle, the bridge looks normal. Image: mstk east – eshima oohashi 09 CC BY 2.0.

2) Construction began in 1997

The construction of the Eshima Ohashi bridge began in 1997. It was designed to connect the Matsue and Sakaiminato cities, located on opposite sides of the lake. The undertaking was entrusted to a consortium of construction companies, including Sumitomo Mitsui Construction Co, Ltd, IHI Infrastructure Systems Co, Ltd, and Japan Steel Works Ltd.

The bridge is constructed primarily from prestressed concrete. This material was chosen for its ability to withstand the immense weight and stress placed on the bridge due to its length and height.  

Engineers employed a sophisticated prestressing technique during construction to achieve the required strength and flexibility. By embedding steel wires within the concrete and applying tension, they created a structure capable of handling heavy traffic and resisting the powerful forces of nature.

Furthermore, the bridge's cross-section exhibits remarkable breadth, with a girder height of 15.5 metres and a lower floor slab thickness of 2.55 metres. This substantial size provides ample space for vehicular traffic while contributing to the bridge's overall stability and resilience.

After overcoming significant technical challenges, the bridge's construction took seven years, and it was finally opened to the public in 2004.

3) The largest rigid-frame bridge in Japan

The impressive size of the Eshima Ohashi bridge becomes even more apparent when considering the waterway it spans. Nakaumi Lake, a bustling shipping lane, stretches approximately 500 metres wide beneath the bridge's towering structure.

The bridge's central span, the distance between its two main supports, measures 250 metres, making it one of the most extended spans in the world for a concrete bridge.

Its width varies – the standard section is 11.3 metres wide, divided into a 1.5-metre path on each side and a 7.5-metre wide roadway in the centre. In emergencies, the bridge can expand to a width of 15 metres to accommodate wider vehicles, with an approximately 10-metre road and a three-metre path on each side. 

4) The cable-stayed architectural design is a critical element

The Eshima Ohashi bridge utilises a cable-stayed architectural design, widely used for constructing bridges with large spans because it offers a combination of structural robustness and visual appeal. In this architectural layout, the primary section of the bridge is upheld by cables fastened to tall pylons.

The cables, generally composed of high-tensile steel, stretch from the pinnacles of the pylons to different locations along the bridge's deck. The bridge features a cable arrangement that resembles a fan, with several cables extending from each pylon. This design improves the bridge's stability and enhances the structure's visual appeal.

The foundations of a bridge are constructed using steel pipe sheet piles, ensuring the bridge remains firmly anchored and can withstand significant loads.

5) Here's why Eshima Ohashi Bridge is called the 'rollercoaster bridge' 

The bridge's dramatic incline, reaching a 6.1% grade on the Shimane Prefecture side and a 5.1% grade on the Tottori side, creates an optical illusion that makes it appear as if cars are climbing a vertical cliff before disappearing over the edge.

This illusion is further amplified by the use of telephoto lenses in many photos, which compress the distance and exaggerate the steepness of the incline. The Eshima Ohashi bridge is nicknamed the Betabumizaka ('acceleration bridge'), meaning the slope where you must accelerate like mad.

Despite its nickname, the bridge is designed with safety and functionality in mind. The grade allows for smooth traffic flow and meets all safety standards. However, its unique appearance has captivated imaginations and earned it a place as one of the most iconic bridges in Japan.

So, the next time you see a picture of the Eshima Ohashi bridge, remember the story behind its nickname. It is a reminder that sometimes, even the most impressive engineering feats can leave us feeling like we're on a thrilling rollercoaster ride. 

6) How much did the Eshima Ohashi bridge cost?

While official figures vary, the estimated cost of building the Eshima Ohashi bridge falls between 23 and 30 billion yen or about $200m to $260m. This significant investment reflects the complex engineering challenges in constructing a bridge with such a steep incline and high clearance over a busy waterway.

7) Balancing earthquake resistance and ship clearance

Japan lies on the boundary between two tectonic plates: the Eurasian Plate on its west side and the North American Plate on its east side. This makes it highly prone to seismic activity. Being a seismically active region means that earthquake vibrational resistance is critical for any structural project. The bridge is designed to withstand seismic activity, incorporating sturdy materials and isolators.

Another critical challenge for the bridge engineers was to provide adequate clearance for the ships and vessels to pass beneath it. This required precise calculations to determine the optimal slope for the bridge's incline, allowing ships of various sizes to navigate the Nakaumi Lake without any obstruction.

8) The unique design captivates tourists and the media

Despite the significant investment, the Eshima Ohashi bridge has already proven to be a worthwhile undertaking. In addition to facilitating trade and travel between Matsue and Sakaiminato, the bridge has boosted tourism connections and made transporting goods more efficient.

Moreover, it has improved traffic in the eastern part of Shimane Prefecture, leading to positive outcomes like lower transportation costs and quicker delivery times. 

Attracting numerous tourists and photographers, the bridge's unique appearance and optical illusion have garnered widespread fascination. It has also been featured in various media, including commercials and video games.