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.

Details of Individual Research

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.

Cantilevers and Viaducts

![](/content/images/2015/09/cantilever.jpg) Contruction of the main towers.

Each tubular member of the main tower was composed of separate plates of steel riveted together. The plates were manufactured in Queensferry, they were then heated in a gas furnace and pressed into the required curve with holes drilled for rivets. As the three main towers could balance on their own, they were constructed first upon the granite piers.

The viaducts were assembled from 15 spans each at a lower altitude, they were then slowly lifted up to the required 39.68m above high tide with hydraulic rams. Due to the precision required in connecting the spans and the main towers, the lifting was done at 1.07m every four days.

Discussion

At the time of its construction, the Forth Rail Bridge was the longest single cantilever bridge in the world. It was, and still is regarded as one of the most recognisable landmarks in Scotland. Being one of the first structures to be made completely from steel, it set a precedent for stronger, more durable materials to be used in large construction projects. Due to the national unease caused by the collapse of Tay Bridge not too long before, the Forth Rail Bridge was engineered so as to ensure that there are no doubts about its safety. Decades after its construction, countless computerised analysis and investigations reassured the engineering success. With new technologies and improvements reinforcing the bridge everyday, it continues its role as a major artery in the railway network.