The Forth Bridge: An Incredible 19th Century Achievement - Megaprojects (2022)

The Forth Bridge crosses the Firth of Forth, an estuary nine miles west of Edinburgh, to connect the villages of South Queensferry and North Queensferry by rail via the Edinburgh-Aberdeen line.

The Forth Bridge, sometimes called the Forth Railway Bridge to distinguish it from the more recently built Forth Road Bridge that carries automobile traffic, was designed by the British engineers Sir John Fowler and Sir Benjamin Baker who began construction in 1882. Eight years later in 1890, King Edward VII—who was still the Duke of Rothesay at the time—opened the completed bridge.

Recognized at the time as a marvel of science, the Forth Bridge has only gained reputation over the last 130 years. It is a UNESCO World Heritage site and considered the greatest man-made wonder in Scotland, actually voted as such in 2016.

This is because the bridge has a total length of 2,467 meters, which is over 8,000 feet, roughly the depth of the Grand Canyon. Plus, it’s a cantilever bridge, meaning the bridge’s spans merely extend out from central supports rather than having any kind of suspension. In fact, at the time of completion, the Forth Bridge had the longest cantilever span of any bridge in the world at 521 meters (over 1,700 feet).

Although this was ultimately surpassed by the Quebec Bridge in 1919, the Forth Bridge has to this day the rank of second longest cantilever bridge span in the world and is one of the foremost engineering accomplishments not only of the 19th Century but all time.

DISASTROUS BEGINNINGS

Until the 19th Century, people used ferries to cross the Firth of Forth. As industrialism increased both population and the need to move people and goods around Scotland, these became decreasingly efficient and effective. Moreover, train lines were spreading all around the UK as the dominant form of transportation. Interest in a way to put railroad tracks across the Firth of Forth quickly grew.

TheRo-PaxferryBlue Star 1passing under the Forth Bridge in the Firth, en route from Rosyth to Zeebrugge. By Emoscopes, is licensed under CC-BY-SA

Initially, engineers viewed tunnels as a solution due to the success of mining tunnels under the firth and surrounding sea as early as 1618. However, this idea was soon discarded in favor of a bridge. Scottish civil engineer James Anderson proposed a suspension bridge made of 2,500 metric tons of iron, which equates to 2,800 short tons.

For reference, the Golden Gate Bridge, which is only about 12% longer than the Forth Bridge, weighs some 811,000 metric tons, or nearly 900,000 short tons. As you can imagine, then, this was an impossibly inadequate amount of materials, and the design was subsequently criticized. The famous German engineer Wilhelm Westhofen was quoted as quipping that “this quantity [of iron] distributed over the length would have given it a very light and slender appearance, so light indeed that on a dull day it would hardly have been visible, and after a heavy gale probably no longer to be seen on a clear day either.”

Then in 1863, the North British Railway and the Edinburgh and Glasgow Railway entered into a joint venture with the hopes of putting a bridge across the Forth River. They ultimately appointed Sir Thomas Bouch to design it. Apparently there was not as much specialization in engineering in the 1860s as Bouch was chosen thanks to his previous work designing ferries to carry railcars across the Firth of Forth.

Before construction even started, the Forth Bridge became mired in financial problems, including the ultimate revelation that the projects’ accounts had actually been falsified to mislead the shareholders. This led to the resignation of the chairman and board of directors. However, in 1870, the North British Railway did use Thomas Bouch’s design to begin work on the Tay Bridge to cross the Firth of Tay to the north.

The Tay Bridge opened in 1878. Queen Victoria traveled over it herself and knighted Thomas Bouch as a result of its success. Things looking up, the North British Railway decided to resume work on the Forth Bridge using Bouch’s design for a suspension bridge.

Tay Bridge at Dundee, Scotland from the Dundee Law

Then, shortly after the Tay Bridge’s opening, disaster struck. On December 28, 1879, the storm of a generation hit the east coast of Scotland, bearing down on the bridge with wind speeds of 129 kilometers per hour, over 80 miles per hour, enough to break the interstate speed limit in most places.

After dark, a passenger train from Burntisland crossed onto the single-track bridge on its way to Dundee at which time sparks began flying from the wheels, something not especially unusual for trains crossing bridges in that era. However, as the train entered the “high girders,” the recognizable part of a girder bridge where a triangular network of steel girders creates a canopy over the bridge bed, there was a bright flash of light, and the train disappeared from sight.

The train never appeared on the far side of the bridge, prompting the signalman to try to contact his counterpart on the near side, but he found all communications had been lost. On inspection, the train had fallen off the bridge along with a significant portion of the high girder section. 75 people died. There were no survivors.

Almost all the blame fell on Thomas Bouch, who it was claimed failed to take into account enough wind loading in his plans. His reputation was destroyed. His design for the Forth Bridge was abandoned, and he died just a year later.

OLD DESIGN MADE NEW

John Fowler and Benjamin Baker won the contract for a new design for the Forth Bridge with their idea for a cantilever bridge. Cantilevers are one of the oldest bridge designs known to humanity, used thousands of years before industrialization. Fowler and Baker, however, wanted to take the concept to the next level.

cantilever bridge

In a cantilever bridge, horizontal girders stretch out in either direction from centralized cantilever supports. On one side, the side reaching the shore, the girders are then anchored. This allows the other side, which is suspended in the air, to support a further suspended span of bridge which then meets girders supported by a cantilever on the other side. The distance between two cantilevers is called the “main span,” and the Forth Bridge had the longest one for 29 years.

Cantilever bridges have the advantage of simple construction and fairly straightforward engineering principles. However, they’d never been used to bridge such a long distance.

In addition to solid math and science, Fowler and Baker were able to convince the North British Railway that the cantilever approach was the best idea thanks to a clever gimmick where they demonstrated the principles by forming a bridge with their own bodies. The two British engineers were the cantilevers, and using brick anchors and wooden supports, they were able to support the Japanese engineer Kaichi Watanabe on a span of wood between them.

CONSTRUCTION BEGINS

In 1882, the contracts for construction were awarded to Sir Thomas Tancred, T.H. Falkiner, Joseph Philips, and Sir William Arrol & Co. They would ultimately use 55,000 metric tons, or 61,000 short tons, of steel on the project, making it the first major structure in the UK to be made of the material.

Other required materials included cement, lumber and granite as well as coal for ovens. These were sourced from nearby locations and delivered by train, after which barges carried the materials to where they were needed along the bridge.

Considerable infrastructure had to be built simply to support the Forth Bridge’s construction. This included piers for the barges, additional rail for the trains, and numerous cranes. Most significantly, houses and workshops had to be built for the over 4,600 laborers who would help build the bridge over its eight years of construction.

The first structures that had to be built were the circular underwater piers that would support the cantilever towers. Each of the three cantilever towers is supported by four piers for a total of 12 altogether. Although some of this work could be done at low tide when the riverbed was exposed, most of it had to be done using caissons.

Caissons are watertight structures that can be sunk into the riverbed. Water can then be pumped out one way or another allowing the building of piers. In this case, the French contractor L. Coisea used pneumatic technology to remove the water. The builders then poured cement into the bottom of the caissons onto which they built the main granite body of the pier.

The Caissons illustrations

Because the caissons involved pressurizing the inside of the structure to keep water out, workers had to depressurize when exiting, much like scuba divers rising out of deep water. Improper depressurization can lead to a kind of decompression sickness called caisson disease, which was responsible for one of the 73 worker fatalities during the Forth Bridge’s construction.

Indeed, building the Forth Bridge was a dangerous undertaking for its workers. At 38 fatalities, falling was the most common cause of death. In fact, row boats had to be placed under the working areas to rescue men who fell into the river. Additionally, compulsory membership in The Sick and Accident Club paid for the treatment of injured workers or their funerals if they died.

Along with the piers, the builders had to create the “approach viaducts” on either shore. Interestingly, the engineers decided it would be best to build the viaducts at a lower height than the final plans called for. They then raised the viaducts as they built the granite piers. They did this using hydraulic rams, another impressive feat for 19th Century builders.

The last stage of construction was the cantilevers themselves. The steel beams used for the cantilever towers were shaped in workshops there at the bridge site. They were then installed in the towers using cranes and hydraulic riveters, bridge building technologies you would still see in use at any construction site today.

ONE FOR THE RECORD BOOKS

After the Tay Bridge disaster, British officials weren’t taking any chances and put the completed Forth Bridge through rigorous testing before opening it. Finished in December of 1889, formal testing took place in January of 1890 in which two trains overloaded and weighing nearly 2,000 tons each crossed the bridge without problems.

Cantilever Forth Bridge Queensferry Scotland Bridge

In February, chairmen of the railway companies involved in the project crossed the bridge, and on March 4th, it officially opened. The Duke of Rothesay, who at the time was future King Edward VII, completed the ceremony by driving the last rivet, gold-plated and inscribed, into the bridge.

Railway companies quickly built more tracks to connect with and take advantage of the Forth Bridge. Even now in the 21st Century, around 200 trains cross it each day. Plus, it appears in art and media and remains a major symbol of the Scottish nation.

That isn’t just due to the record-breaking cantilever span. It’s thanks to the incredible ingenuity that went into designing and building such an effective and durable structure, especially at a time when they did not even have widespread access to electricity. From pneumatically pressurizing caissons to hydraulically lifting viaducts, the Forth Bridge is truly a marvel and testament to the intellect and tenacity of the 19th Century.

SOURCES

The Forth Bridge. Wilhelm Westhofen. https://en.wikisource.org/wiki/The_Forth_Bridge

“History of the Forth Bridge.” TheForthBridges.org. https://www.theforthbridges.org/forth-bridge/history

“Kaichi Watanabe.” University of Glasgow. https://universitystory.gla.ac.uk/biography/?id=WH17361&type=P

“The Great Storm and the fall of the first Tay Rail Bridge.” Natural Resources Institute, University of Greenwich. https://rmets.onlinelibrary.wiley.com/doi/abs/10.1256/wea.199.04

“Thomas Bouch: Architect of the Tay Bridge disaster.” Independent. https://www.independent.co.uk/independentpremium/long-reads/thomas-bouch-tay-bridge-disaster-trains-dundee-a9229646.html

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