The Forth Rail Bridge: Materials and Construction Process

The Forth Rail Bridge was the first major structure in Britain to be constructed entirely of steel. This was mainly due to advancements in steel production making it a competent structural material. The design called for 55,000 tonnes of steel and 110,000m$^3$ of masonry, 6.5 million steel rivets and 21m diameter steel caissons. The construction process was a massive project spanning eight years and eventually reaching completion in 1890 where it was tested with 50 wagons loaded with coal weighing 1880 tons - twice the design load of the bridge. The wagons were stopped frequently to measure the deflections in the bridge; results were as expected.

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Materials

In 1865, Pierre-Émile Martin's regenerative open-hearth furnace was a breakthrough in steel production that allowed for a strong and consistent yield. Although slower, the resulting Siemens-Martin process can be easily controlled and the structure of the steel can be frequently analysed during the process.

This steel was chosen for the entire bridge due to advantages including its strength and predictable fatigue life. Corrosion in steel rarely compromises structural integrity and is readily apparent and easily addressed without the need for further investigation.

Construction Process

Caissons and Piers

![](/content/images/2015/09/caisson.jpg) One of the caissons used in the construction of the piers.

After the landing areas were surveyed and levelled, six steel caissons were floated to their designated position by barge. As the tide came in, the caissons were supported by buoyancy and the space under them were cleared with drilling or blasting. A problem that arose was that, due to the exceptionally low tide at the northwest pier, the caisson sunk with a tilt. As the tide rose, the lower edge was flooded and the imbalance caused the caisson to tilt further. To reinforce the caisson and pump out the water took 10 month to accomplish.

The positive air pressure inside the caissons keeps the water out and ensures dry working conditions up to 27m. To form the piers, dirt and rubble were piled up inside the caissons and then topped with granite at 11m. A smaller, temporary caisson was constructed on top of the permanent caisson to allow work to be done on the granite top. Some piers were at shallower areas and were therefore dried using cofferdams to hold back the water instead, this method was both cheaper and safer than using caissons.