Forth Rail Bridge
The Forth Bridge is a cantilever railway bridge over the Firth of Forth in the east of Scotland, 9 miles (14 kilometres) west of Edinburgh City Centre. It is considered an iconic structure and a symbol of Scotland, and is a UNESCO World Heritage Site. It was designed by the English engineers Sir John Fowler and Sir Benjamin Baker.
Construction of the bridge began in 1882 and it was opened on 4 March 1890 by the Prince of Wales.[2] The bridge spans the Forth between the villages of South Queensferry and North Queensferry and has a total length of 8,296 feet (2,528.7 m). It was the longest single cantilever bridge span in the world until 1917 when the Quebec Bridge in Canada was completed. It continues to be the world's second-longest single cantilever span.
The bridge and its associated railway infrastructure is owned by Network Rail Infrastructure Limited.
It is sometimes referred to as the Forth Rail Bridge to distinguish it from the Forth Road Bridge, though this has never been its official name.
Prior to the construction of the bridge, ferry boats were used to cross the Firth.[3] In 1806, a pair of tunnels, one for each direction, was proposed, and in 1818 James Anderson produced a design for a three-span suspension bridge close to the site of the present one.[4] Calling for approximately 2,500 tonnes (2,500 long tons; 2,800 short tons) of iron, Wilhelm Westhofen said of it "and 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."[5]
Thomas Bouch designed for the Edinburgh and Northern Railway a roll-on/roll-off railway ferry between Granton and Burntisland that opened in 1850, which proved so successful that another was ordered for the Tay.[6] In autumn 1863, a joint project between the North British Railway and Edinburgh and Glasgow Railway, which would merge in 1865, appointed Stephenson and Toner to design a bridge for the Forth, but the commission was given to Bouch around six months later.[7]
It had proven difficult to engineer a suspension bridge that was able to carry railway traffic, and Thomas Bouch, engineer to the North British Railway (NBR) and Edinburgh and Glasgow Railway, was in 1863-1864 working on a single-track girder bridge crossing the Forth near Charlestown, where the river is around 2 miles wide, but mostly relatively shallow.[7][8] The promoters, however, were concerned about the ability to set foundations in the silty river bottom, as borings had gone as deep as 231 feet (70 m) into the mud without finding any rock, but Bouch conducted experiments to demonstrate that it was possible for the silt to support considerable weight.[9] Experiments in late 1864 with weighted caissons achieved a pressure of 5 tons/ft2 on the silt, encouraging Bouch to continue with the design.[9] In August 1865, Richard Hodgson, chairman of the NBR, proposed that the Company invest £18,000 to try a different kind of foundation, as the weighted caissons had not been successful.[10] Bouch proposed using a large pine platform underneath the piers, 80 by 60 by 7 feet (24.4 m × 18.3 m × 2.1 m) (the original design called for a 114 by 80 by 9 feet (34.7 m × 24.4 m × 2.7 m) platform of green beech) weighed down with 10,000 tonnes (9,800 long tons; 11,000 short tons) of pig iron which would sink the wooden platform to the level of the silt.[9] The platform was launched on 14 June 1866 after some difficulty in getting it to move down the greased planks it rested on, and then moored in the harbour for six weeks pending completion.[9][11] The bridge project was aborted just before the platform was sunk as the NBR expected to lose "through traffic" following the amalgamation of the Caledonian Railway and the Scottish North Eastern Railway.[9] In September 1866, a Committee of Shareholders investigating rumours of financial difficulties found that accounts had been falsified, and the chairman and the entire board had resigned by November.[12] By mid-1867 the NBR was nearly bankrupt, and all work on the Forth and Tay bridges was stopped.
The North British Railway took over the ferry at Queensferry in 1867, and completed a rail link from Ratho in 1868, establishing a contiguous link with Fife.[14] Interest in bridging the Forth increased again, and Bouch proposed a stiffened steel suspension bridge on roughly the line of the present rail bridge in 1871, and after careful verification, work started in 1878 on a pier at Inchgarvie.[14]
After Tay Bridge collapsed in 1879, confidence in Bouch dried up and the work stopped.[14] The public inquiry into the disaster, chaired by Henry Cadogan Rothery, found the Tay Bridge to be "badly designed, badly constructed and badly maintained," with Bouch being "mainly to blame" for the defects in construction and maintenance and "entirely responsible" for the defects in design.[15]
After the disaster, which occurred in high winds for which Bouch had not properly accounted, the Board of Trade imposed a lateral wind allowance of 56 lbs/ft2.[16] Bouch's 1871 design had taken a much lower figure of 10 lbs/ft2 on the advice of the Astronomer Royal, although contemporary analysis showed it would likely have stood, but the engineers making the analysis stated that "we do not commit ourselves to an opinion that it is the best possible" [design].[17] Bouch's design was formally abandoned on 13 January 1881, and Sir John Fowler, W. H. Barlow and T. E. Harrison, consulting engineers to the project, were invited to give proposals for a bridge.
It is 8,094 feet (2,467.05 m) in length, and the double track is elevated 150 feet (45.72 m) above the water level at high tide. It consists of two main spans of 1,700 feet (518.16 m), two side spans of 680 ft (207.3 m), and 15 approach spans of 168 ft (51.2 m).[1][19] Each main span consists of two 680 ft (207.3 m) cantilever arms supporting a central 350 feet (106.7 m) span truss. The weight of the bridge superstructure was 50,513 long tons (51,324 t), including the 6.5 million rivets used.[19] The bridge also used 640,000 cubic feet (18,122 m3) of granite.[20]
The three great four-tower cantilever structures are 361 feet (110.03 m) tall,[1] each tower resting on a separate granite pier. These were constructed using 70 ft (21 m) diameter caissons; those for the north cantilever and two on the small uninhabited island of Inchgarvie acted as cofferdams, while the remaining two on Inchgarvie and those for the south cantilever, where the river bed was 91 ft (28 m) below high-water level, used compressed air to keep water out of the working chamber at the base.
The bridge is built on the principle of the cantilever bridge, where a cantilever beam supports a light central girder, a principle that has been used for thousands of years in the construction of bridges.[22] In order to illustrate the use of tension and compression in the bridge, a demonstration in 1887 had the Japanese engineer Kaichi Watanabe supported between Fowler and Baker sitting in chairs.[23] Fowler and Baker represent the cantilevers, with their arms in tension and the sticks under compression, and the bricks the cantilever end piers which are weighted with cast iron.
The bridge was the first major structure in Britain to be constructed of steel;[25] its French contemporary, the Eiffel Tower, was built of wrought iron.
Large amounts of steel had become available after the invention of the Bessemer process in 1855. Until 1877 the British Board of Trade had limited the use of steel in structural engineering because the process produced steel of unpredictable strength. Only the Siemens-Martin open-hearth process developed by 1875 yielded steel of consistent quality.
The original design required 42,000 tonnes (41,000 long tons; 46,000 short tons) for the cantilevers only, of which 12,000 tonnes (12,000 long tons; 13,000 short tons) was to come from Messrs. Siemens' steel works in Landore and the remainder from the Steel Company of Scotland's works near Glasgow.[26] When modifications to the design necessitated a further 16,000 tonnes (16,000 long tons; 18,000 short tons), about half of this was supplied by the Steel Company of Scotland and half by Dalzell's Iron and Steel Works in Motherwell.[27] About 4,200 tonnes (4,100 long tons; 4,600 short tons) of rivets came from the Clyde Rivet Company of Glasgow.[27] Around three or four thousand tons of steel was scrapped, some of which was used for temporary purposes, resulting in the discrepancy between the quantity delivered and the quantity erected.
The Bill for the construction of the bridge was passed on 19 May 1882 after an eight day enquiry, the only objections being from rival railway companies.[29] On 21 December of that year, the contract was let to Sir Thomas Tancred, Mr. T. H. Falkiner and Mr. Joseph Philips, civil engineers and contractor, and Sir William Arrol & Co..[30] Arrol was a self-made man, who had been apprenticed to a blacksmith at the age of thirteen before going on to have a highly successful business.[31] Tancred was a professional engineer who had worked with Arrol before, but he would leave the partnership during the course of construction.[32] The steel was produced by Frederick and William Siemens (England) and Pierre and Emile Martin (France), following advances in the furnace designs by the Siemens brothers and improvements on this design by the Martin brothers ,the process of manufacture was thus that it enabled high quality steel to be produced very quickly.
Offices and stores erected by Arrol in connection with Bouch's bridge were taken possession of for the new works, and would be expanded considerably over time.[35] An accurate survey was taken by Mr. Reginald Middleton, to establish the exact position of the bridge and allow the permanent construction work to commence.[36][37]
The old coastguard station at the Fife end had to be removed to make way for the north-east pier.[38] The rocky shore was levelled to a height of 7 feet (2.1 m) above high water to make way for plant and materials, and huts and other facilities for workmen were set up further inland.[38]
The preparations at South Queensferry were of a much more substantial character, and required the steep hillside to be terraced.[38] Wooden huts and shops for the workmen were put up, as well as more substantial brick houses for the foremen and tenements for leading hands and gangers.[38] Drill roads and workshops were built, as well as a drawing loft 200 by 60 feet (61 by 18 m) to allow full size drawings and templates to be laid out.[38] A cable was also laid across the Forth to allow telephone communication between the centres at Queensferry, Inchgarvie, and Fife, and girders from the collapsed Tay Bridge were laid across the railway to the west in order to allow access to the ground there.[38] Near the shore a sawmill and cement store were erected, and a substantial jetty around 2,100 feet (640 m) long was started early in 1883, and extended as necessary, and sidings were built to bring railway vehicles among the shops, and cranes set up to allow the loading and movement of material delivered by rail.[38]
In April 1883, construction of a landing stage at Inchgarvie commenced.[38] Extant buildings, including fortifications built in the 15th century, were roofed over to increase the available space, and the rock at the west of the island was cut down to a level 7 feet (2.1 m) above high water, and a seawall was built to protect against large waves.[38] In 1884 a compulsory purchase order was obtained for the island, as it was found that previously available area enclosed by the four piers of the bridge was insufficient for the storage of materials.[38] Iron staging reinforced wood in heavily used areas was put up over the island, eventually covering around 10,000 square yards (8,400 m2) and using over 1,000 tonnes (980 long tons; 1,100 short tons) of iron.
The bridge uses 55,000 tonnes (54,000 long tons; 61,000 short tons) of steel and 140,000 cubic yards (110,000 m3) of masonry.[39] Many materials, including granite from Aberdeen, Arbroath rubble, sand, timber, and sometimes coke and coal, could be taken straight to the centre where they were required.[39] Steel was delivered by train and prepared at the yard at South Queensferry before being painted with boiled linseed oil before being taken to where it was needed by barge.[39] The cement used was Portland cement manufactured on the Medway.[40] It required to be stored before it was able to be used, and up to 1,200 tonnes (1,200 long tons; 1,300 short tons) of cement could be kept in a barge, formerly called the Hougomont that was moored off Queensferry.[40]
For a time a paddle steamer was hired for the movement of workers, but after a time it was replaced with one capable of carrying 450 men, and the barges were also used for people carrying.[39] Special trains were run from Edinburgh and Dunfermline, and a steamer ran to Leith in the summer.
The three towers of the cantilever are each seated on four circular piers. Since the foundations were required to be constructed at or below sea level, they were excavated with the assistance of caissons and cofferdams.[40]
Six caissons were excavated by the pneumatic process, by the French contractor L. Coisea.[41][42] This process used a positive air pressure inside a sealed caisson to allow dry working conditions at depths of up to 89 feet (27 m).[42][43]
These caissons were constructed and assembled in Glasgow by the Arrol Brothers, namesakes of but unconnected to W. Arrol, before being dismantled and transported to Queensferry.[41][44] The caissons were then built up to a large extent before being floated to their final resting-places.[45] The first caisson, for the south-west pier at Queensferry was launched on 26 May 1884, and the last caisson was launched on 29 May 1885 for the south-west pier at Inchgarvie.[45] When the caissons had been launched and moored, they were extended upwards with a temporary portion in order to keep water out and allow the granite pier to be built when in place.[45]
Above the foundations each of which is different to suit the different sites, is a tapered circular granite pier with a diameter of 55 feet (17 m) at the bottom and a height of 36 feet (11 m).
The rock on which the two northern piers at Inchgarvie are located is submerged at high water, and of the other two piers, the site of eastern one is about half submerged and the western one three-quarters submerged.[47] This meant work initially had to be done at low tide.[47]
The southern piers on Inchgarvie are sited on solid rock with a slope of around 1 in 5, so the rock was prepared with concrete and sandbags to make a landing-spot for the caissons.[48][49] Excavation was carried out by drilling and blasting.
Once the positions of the piers had been established, the first task at Fife was to level the site of the northernmost piers, a bedrock of whinstone rising to a level of 10 to 20 feet (3.0 to 6.1 m) above high water, to a height of 7 feet (2.1 m) above high water.[40] The south piers at Fife are sited on rock sloping into the sea, and the site was prepared by diamond drilling holes for explosive charges and blasting the rock.
The four Queensferry caissons were all sunk by the pneumatic method, and are identical in design except for differences in height.[41] A T shaped jetty was built at the site of the Queensferry piers, to allow one caisson to be attached to each corner, and when launched the caissons were attached to the jetty and permitted to rise and fall with the tide.[45][50] Excavation beneath the caissons was generally only carried out at high tide when the caisson was supported by buoyancy, and then when the tide fell the air pressure was reduced in order to allow the caisson to sink down, and digging would begin anew.[43]
The north-west caisson was towed into place in December 1884, but an exceptionally low tide on New Year's Day 1885 caused the caisson to sink into the mud of the river bed and adopt a slight tilt.[48] When the tide rose, it flooded over the lower edge, filling the caisson with water, and when the tide fell but the water did not drain from the caisson, its top-heaviness caused to tilt further.[48] Plates were bolted on by divers to raise the edge of the caisson above water level, and the caisson was reinforced with wooden struts as water was pumped out, but pumping took place too quickly and the water pressure tore a hole between 25 and 30 feet (7.6 and 9.1 m) long.[48] It was decided to construct a "barrel" of large timbers inside the caisson to reinforce it, and it was ten months before the caisson could be pumped out and dug free.[48] The caisson was refloated on October 19, 1885, and then moved into position and sunk with suitable modifications.
The approach viaducts to the north and south had to be carried at 130 feet 6 inches (39.78 m) above the level of high water, and it was decided to build them at a lower level and then raise them in tandem with the construction of the masonry piers.[51] The two viaducts have fifteen spans between them, each one 168 feet (51 m) long and weighing slightly over 200 tonnes (200 long tons; 220 short tons).[51] Two spans are attached together to make a continuous girder, with an expansion joint between each pair of spans.[51] Due to the slope of the hill under the viaducts, the girders were assembled at different heights, and only joined when they had reached the same level.[52] Lifting was done using large hydraulic rams, and took place in increments of around 3 feet 6 inches (1.07 m) every four days.
The tubular members were constructed in the No. 2 workshop further up the hill at Queensferry.[53] To bend plates into the required shape, they were first heated in a gas furnace, and then pressed into the correct curve.[53] The curved plates were then assembled on a mandrel, and holes drilled for rivets, before they were marked individually and moved to the correct location to be added to the structure.[53] Lattice members and other parts were also assembled at South Queensferry, using cranes and highly efficient hydraulic rivetters.
The bridge was completed in December 1889, and load testing of the completed bridge was carried out on 21 January 1890. Two trains, each consisting of three heavy locomotives and 50 wagons loaded with coal, totalling 1,880 tons in weight, were driven slowly from South Queensferry to the middle of the north cantilever, stopping frequently to measure the deflection of the bridge. This represented more than twice the design load of the bridge: the deflection under load was as expected.[21] A few days previously there had been a violent storm, producing the highest wind pressure recorded to date at Inchgarvie, and the deflection of the cantilevers had been less than 25 mm (1 in). The first complete crossing took place on 24 February, when a train consisting of two carriages carrying the chairmen of the various railway companies involved made several crossings. The bridge was opened on 4 March 1890 by the Prince of Wales, later King Edward VII, who drove home the last rivet, which was gold plated and suitably inscribed.[20] The key for the official opening was made by Edinburgh silversmith John Finlayson Bain, commemorated in a plaque on the bridge.
At its peak, approximately 4,600 workers were employed in its construction. Wilhelm Westhofen recorded in 1890 that 57 lives were lost; but in 2005 the Forth Bridge Memorial Committee was set up to erect a monument to the briggers, and a team of local historians set out to name all those who died.[55] As of 2009, 73 deaths have been connected with the construction of the bridge and its immediate aftermath.[56] It is thought that the figure of 57 deaths excluded those who died working on the approaches to the bridge, as those parts were completed by a subcontractor, as well as those who died after the Sick and Accident Club stopped.[56] Of the 73 recorded deaths, 38 were as a result of falling, 9 of being crushed, 9 drowned, 8 struck by a falling object, 3 died in a fire in a bothy, 1 of caisson disease, and the cause of five deaths is unknown.[57]
The Sick and Accident Club was founded in the summer of 1883, and membership was compulsory for all contractors employees.[58] It would provide medical treatment to men and sometimes their families, and pay them if they were unable to work.[58] The Club also paid for funerals within certain limits, and would provide grants to the widows of men killed or the wives of those permanently disabled.[58] Eight men were saved from drowning by rowing boats positioned in the river under the working areas.
Forth Rail Bridge
The Forth Bridge is a cantilever railway bridge over the Firth of Forth in the east of Scotland, 9 miles (14 kilometres) west of Edinburgh City Centre. It is considered an iconic structure and a symbol of Scotland, and is a UNESCO World Heritage Site. It was designed by the English engineers Sir John Fowler and Sir Benjamin Baker.
Construction of the bridge began in 1882 and it was opened on 4 March 1890 by the Prince of Wales.[2] The bridge spans the Forth between the villages of South Queensferry and North Queensferry and has a total length of 8,296 feet (2,528.7 m). It was the longest single cantilever bridge span in the world until 1917 when the Quebec Bridge in Canada was completed. It continues to be the world's second-longest single cantilever span.
The bridge and its associated railway infrastructure is owned by Network Rail Infrastructure Limited.
It is sometimes referred to as the Forth Rail Bridge to distinguish it from the Forth Road Bridge, though this has never been its official name.
Prior to the construction of the bridge, ferry boats were used to cross the Firth.[3] In 1806, a pair of tunnels, one for each direction, was proposed, and in 1818 James Anderson produced a design for a three-span suspension bridge close to the site of the present one.[4] Calling for approximately 2,500 tonnes (2,500 long tons; 2,800 short tons) of iron, Wilhelm Westhofen said of it "and 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."[5]
Thomas Bouch designed for the Edinburgh and Northern Railway a roll-on/roll-off railway ferry between Granton and Burntisland that opened in 1850, which proved so successful that another was ordered for the Tay.[6] In autumn 1863, a joint project between the North British Railway and Edinburgh and Glasgow Railway, which would merge in 1865, appointed Stephenson and Toner to design a bridge for the Forth, but the commission was given to Bouch around six months later.[7]
It had proven difficult to engineer a suspension bridge that was able to carry railway traffic, and Thomas Bouch, engineer to the North British Railway (NBR) and Edinburgh and Glasgow Railway, was in 1863-1864 working on a single-track girder bridge crossing the Forth near Charlestown, where the river is around 2 miles wide, but mostly relatively shallow.[7][8] The promoters, however, were concerned about the ability to set foundations in the silty river bottom, as borings had gone as deep as 231 feet (70 m) into the mud without finding any rock, but Bouch conducted experiments to demonstrate that it was possible for the silt to support considerable weight.[9] Experiments in late 1864 with weighted caissons achieved a pressure of 5 tons/ft2 on the silt, encouraging Bouch to continue with the design.[9] In August 1865, Richard Hodgson, chairman of the NBR, proposed that the Company invest £18,000 to try a different kind of foundation, as the weighted caissons had not been successful.[10] Bouch proposed using a large pine platform underneath the piers, 80 by 60 by 7 feet (24.4 m × 18.3 m × 2.1 m) (the original design called for a 114 by 80 by 9 feet (34.7 m × 24.4 m × 2.7 m) platform of green beech) weighed down with 10,000 tonnes (9,800 long tons; 11,000 short tons) of pig iron which would sink the wooden platform to the level of the silt.[9] The platform was launched on 14 June 1866 after some difficulty in getting it to move down the greased planks it rested on, and then moored in the harbour for six weeks pending completion.[9][11] The bridge project was aborted just before the platform was sunk as the NBR expected to lose "through traffic" following the amalgamation of the Caledonian Railway and the Scottish North Eastern Railway.[9] In September 1866, a Committee of Shareholders investigating rumours of financial difficulties found that accounts had been falsified, and the chairman and the entire board had resigned by November.[12] By mid-1867 the NBR was nearly bankrupt, and all work on the Forth and Tay bridges was stopped.
The North British Railway took over the ferry at Queensferry in 1867, and completed a rail link from Ratho in 1868, establishing a contiguous link with Fife.[14] Interest in bridging the Forth increased again, and Bouch proposed a stiffened steel suspension bridge on roughly the line of the present rail bridge in 1871, and after careful verification, work started in 1878 on a pier at Inchgarvie.[14]
After Tay Bridge collapsed in 1879, confidence in Bouch dried up and the work stopped.[14] The public inquiry into the disaster, chaired by Henry Cadogan Rothery, found the Tay Bridge to be "badly designed, badly constructed and badly maintained," with Bouch being "mainly to blame" for the defects in construction and maintenance and "entirely responsible" for the defects in design.[15]
After the disaster, which occurred in high winds for which Bouch had not properly accounted, the Board of Trade imposed a lateral wind allowance of 56 lbs/ft2.[16] Bouch's 1871 design had taken a much lower figure of 10 lbs/ft2 on the advice of the Astronomer Royal, although contemporary analysis showed it would likely have stood, but the engineers making the analysis stated that "we do not commit ourselves to an opinion that it is the best possible" [design].[17] Bouch's design was formally abandoned on 13 January 1881, and Sir John Fowler, W. H. Barlow and T. E. Harrison, consulting engineers to the project, were invited to give proposals for a bridge.
It is 8,094 feet (2,467.05 m) in length, and the double track is elevated 150 feet (45.72 m) above the water level at high tide. It consists of two main spans of 1,700 feet (518.16 m), two side spans of 680 ft (207.3 m), and 15 approach spans of 168 ft (51.2 m).[1][19] Each main span consists of two 680 ft (207.3 m) cantilever arms supporting a central 350 feet (106.7 m) span truss. The weight of the bridge superstructure was 50,513 long tons (51,324 t), including the 6.5 million rivets used.[19] The bridge also used 640,000 cubic feet (18,122 m3) of granite.[20]
The three great four-tower cantilever structures are 361 feet (110.03 m) tall,[1] each tower resting on a separate granite pier. These were constructed using 70 ft (21 m) diameter caissons; those for the north cantilever and two on the small uninhabited island of Inchgarvie acted as cofferdams, while the remaining two on Inchgarvie and those for the south cantilever, where the river bed was 91 ft (28 m) below high-water level, used compressed air to keep water out of the working chamber at the base.
The bridge is built on the principle of the cantilever bridge, where a cantilever beam supports a light central girder, a principle that has been used for thousands of years in the construction of bridges.[22] In order to illustrate the use of tension and compression in the bridge, a demonstration in 1887 had the Japanese engineer Kaichi Watanabe supported between Fowler and Baker sitting in chairs.[23] Fowler and Baker represent the cantilevers, with their arms in tension and the sticks under compression, and the bricks the cantilever end piers which are weighted with cast iron.
The bridge was the first major structure in Britain to be constructed of steel;[25] its French contemporary, the Eiffel Tower, was built of wrought iron.
Large amounts of steel had become available after the invention of the Bessemer process in 1855. Until 1877 the British Board of Trade had limited the use of steel in structural engineering because the process produced steel of unpredictable strength. Only the Siemens-Martin open-hearth process developed by 1875 yielded steel of consistent quality.
The original design required 42,000 tonnes (41,000 long tons; 46,000 short tons) for the cantilevers only, of which 12,000 tonnes (12,000 long tons; 13,000 short tons) was to come from Messrs. Siemens' steel works in Landore and the remainder from the Steel Company of Scotland's works near Glasgow.[26] When modifications to the design necessitated a further 16,000 tonnes (16,000 long tons; 18,000 short tons), about half of this was supplied by the Steel Company of Scotland and half by Dalzell's Iron and Steel Works in Motherwell.[27] About 4,200 tonnes (4,100 long tons; 4,600 short tons) of rivets came from the Clyde Rivet Company of Glasgow.[27] Around three or four thousand tons of steel was scrapped, some of which was used for temporary purposes, resulting in the discrepancy between the quantity delivered and the quantity erected.
The Bill for the construction of the bridge was passed on 19 May 1882 after an eight day enquiry, the only objections being from rival railway companies.[29] On 21 December of that year, the contract was let to Sir Thomas Tancred, Mr. T. H. Falkiner and Mr. Joseph Philips, civil engineers and contractor, and Sir William Arrol & Co..[30] Arrol was a self-made man, who had been apprenticed to a blacksmith at the age of thirteen before going on to have a highly successful business.[31] Tancred was a professional engineer who had worked with Arrol before, but he would leave the partnership during the course of construction.[32] The steel was produced by Frederick and William Siemens (England) and Pierre and Emile Martin (France), following advances in the furnace designs by the Siemens brothers and improvements on this design by the Martin brothers ,the process of manufacture was thus that it enabled high quality steel to be produced very quickly.
Offices and stores erected by Arrol in connection with Bouch's bridge were taken possession of for the new works, and would be expanded considerably over time.[35] An accurate survey was taken by Mr. Reginald Middleton, to establish the exact position of the bridge and allow the permanent construction work to commence.[36][37]
The old coastguard station at the Fife end had to be removed to make way for the north-east pier.[38] The rocky shore was levelled to a height of 7 feet (2.1 m) above high water to make way for plant and materials, and huts and other facilities for workmen were set up further inland.[38]
The preparations at South Queensferry were of a much more substantial character, and required the steep hillside to be terraced.[38] Wooden huts and shops for the workmen were put up, as well as more substantial brick houses for the foremen and tenements for leading hands and gangers.[38] Drill roads and workshops were built, as well as a drawing loft 200 by 60 feet (61 by 18 m) to allow full size drawings and templates to be laid out.[38] A cable was also laid across the Forth to allow telephone communication between the centres at Queensferry, Inchgarvie, and Fife, and girders from the collapsed Tay Bridge were laid across the railway to the west in order to allow access to the ground there.[38] Near the shore a sawmill and cement store were erected, and a substantial jetty around 2,100 feet (640 m) long was started early in 1883, and extended as necessary, and sidings were built to bring railway vehicles among the shops, and cranes set up to allow the loading and movement of material delivered by rail.[38]
In April 1883, construction of a landing stage at Inchgarvie commenced.[38] Extant buildings, including fortifications built in the 15th century, were roofed over to increase the available space, and the rock at the west of the island was cut down to a level 7 feet (2.1 m) above high water, and a seawall was built to protect against large waves.[38] In 1884 a compulsory purchase order was obtained for the island, as it was found that previously available area enclosed by the four piers of the bridge was insufficient for the storage of materials.[38] Iron staging reinforced wood in heavily used areas was put up over the island, eventually covering around 10,000 square yards (8,400 m2) and using over 1,000 tonnes (980 long tons; 1,100 short tons) of iron.
The bridge uses 55,000 tonnes (54,000 long tons; 61,000 short tons) of steel and 140,000 cubic yards (110,000 m3) of masonry.[39] Many materials, including granite from Aberdeen, Arbroath rubble, sand, timber, and sometimes coke and coal, could be taken straight to the centre where they were required.[39] Steel was delivered by train and prepared at the yard at South Queensferry before being painted with boiled linseed oil before being taken to where it was needed by barge.[39] The cement used was Portland cement manufactured on the Medway.[40] It required to be stored before it was able to be used, and up to 1,200 tonnes (1,200 long tons; 1,300 short tons) of cement could be kept in a barge, formerly called the Hougomont that was moored off Queensferry.[40]
For a time a paddle steamer was hired for the movement of workers, but after a time it was replaced with one capable of carrying 450 men, and the barges were also used for people carrying.[39] Special trains were run from Edinburgh and Dunfermline, and a steamer ran to Leith in the summer.
The three towers of the cantilever are each seated on four circular piers. Since the foundations were required to be constructed at or below sea level, they were excavated with the assistance of caissons and cofferdams.[40]
Six caissons were excavated by the pneumatic process, by the French contractor L. Coisea.[41][42] This process used a positive air pressure inside a sealed caisson to allow dry working conditions at depths of up to 89 feet (27 m).[42][43]
These caissons were constructed and assembled in Glasgow by the Arrol Brothers, namesakes of but unconnected to W. Arrol, before being dismantled and transported to Queensferry.[41][44] The caissons were then built up to a large extent before being floated to their final resting-places.[45] The first caisson, for the south-west pier at Queensferry was launched on 26 May 1884, and the last caisson was launched on 29 May 1885 for the south-west pier at Inchgarvie.[45] When the caissons had been launched and moored, they were extended upwards with a temporary portion in order to keep water out and allow the granite pier to be built when in place.[45]
Above the foundations each of which is different to suit the different sites, is a tapered circular granite pier with a diameter of 55 feet (17 m) at the bottom and a height of 36 feet (11 m).
The rock on which the two northern piers at Inchgarvie are located is submerged at high water, and of the other two piers, the site of eastern one is about half submerged and the western one three-quarters submerged.[47] This meant work initially had to be done at low tide.[47]
The southern piers on Inchgarvie are sited on solid rock with a slope of around 1 in 5, so the rock was prepared with concrete and sandbags to make a landing-spot for the caissons.[48][49] Excavation was carried out by drilling and blasting.
Once the positions of the piers had been established, the first task at Fife was to level the site of the northernmost piers, a bedrock of whinstone rising to a level of 10 to 20 feet (3.0 to 6.1 m) above high water, to a height of 7 feet (2.1 m) above high water.[40] The south piers at Fife are sited on rock sloping into the sea, and the site was prepared by diamond drilling holes for explosive charges and blasting the rock.
The four Queensferry caissons were all sunk by the pneumatic method, and are identical in design except for differences in height.[41] A T shaped jetty was built at the site of the Queensferry piers, to allow one caisson to be attached to each corner, and when launched the caissons were attached to the jetty and permitted to rise and fall with the tide.[45][50] Excavation beneath the caissons was generally only carried out at high tide when the caisson was supported by buoyancy, and then when the tide fell the air pressure was reduced in order to allow the caisson to sink down, and digging would begin anew.[43]
The north-west caisson was towed into place in December 1884, but an exceptionally low tide on New Year's Day 1885 caused the caisson to sink into the mud of the river bed and adopt a slight tilt.[48] When the tide rose, it flooded over the lower edge, filling the caisson with water, and when the tide fell but the water did not drain from the caisson, its top-heaviness caused to tilt further.[48] Plates were bolted on by divers to raise the edge of the caisson above water level, and the caisson was reinforced with wooden struts as water was pumped out, but pumping took place too quickly and the water pressure tore a hole between 25 and 30 feet (7.6 and 9.1 m) long.[48] It was decided to construct a "barrel" of large timbers inside the caisson to reinforce it, and it was ten months before the caisson could be pumped out and dug free.[48] The caisson was refloated on October 19, 1885, and then moved into position and sunk with suitable modifications.
The approach viaducts to the north and south had to be carried at 130 feet 6 inches (39.78 m) above the level of high water, and it was decided to build them at a lower level and then raise them in tandem with the construction of the masonry piers.[51] The two viaducts have fifteen spans between them, each one 168 feet (51 m) long and weighing slightly over 200 tonnes (200 long tons; 220 short tons).[51] Two spans are attached together to make a continuous girder, with an expansion joint between each pair of spans.[51] Due to the slope of the hill under the viaducts, the girders were assembled at different heights, and only joined when they had reached the same level.[52] Lifting was done using large hydraulic rams, and took place in increments of around 3 feet 6 inches (1.07 m) every four days.
The tubular members were constructed in the No. 2 workshop further up the hill at Queensferry.[53] To bend plates into the required shape, they were first heated in a gas furnace, and then pressed into the correct curve.[53] The curved plates were then assembled on a mandrel, and holes drilled for rivets, before they were marked individually and moved to the correct location to be added to the structure.[53] Lattice members and other parts were also assembled at South Queensferry, using cranes and highly efficient hydraulic rivetters.
The bridge was completed in December 1889, and load testing of the completed bridge was carried out on 21 January 1890. Two trains, each consisting of three heavy locomotives and 50 wagons loaded with coal, totalling 1,880 tons in weight, were driven slowly from South Queensferry to the middle of the north cantilever, stopping frequently to measure the deflection of the bridge. This represented more than twice the design load of the bridge: the deflection under load was as expected.[21] A few days previously there had been a violent storm, producing the highest wind pressure recorded to date at Inchgarvie, and the deflection of the cantilevers had been less than 25 mm (1 in). The first complete crossing took place on 24 February, when a train consisting of two carriages carrying the chairmen of the various railway companies involved made several crossings. The bridge was opened on 4 March 1890 by the Prince of Wales, later King Edward VII, who drove home the last rivet, which was gold plated and suitably inscribed.[20] The key for the official opening was made by Edinburgh silversmith John Finlayson Bain, commemorated in a plaque on the bridge.
At its peak, approximately 4,600 workers were employed in its construction. Wilhelm Westhofen recorded in 1890 that 57 lives were lost; but in 2005 the Forth Bridge Memorial Committee was set up to erect a monument to the briggers, and a team of local historians set out to name all those who died.[55] As of 2009, 73 deaths have been connected with the construction of the bridge and its immediate aftermath.[56] It is thought that the figure of 57 deaths excluded those who died working on the approaches to the bridge, as those parts were completed by a subcontractor, as well as those who died after the Sick and Accident Club stopped.[56] Of the 73 recorded deaths, 38 were as a result of falling, 9 of being crushed, 9 drowned, 8 struck by a falling object, 3 died in a fire in a bothy, 1 of caisson disease, and the cause of five deaths is unknown.[57]
The Sick and Accident Club was founded in the summer of 1883, and membership was compulsory for all contractors employees.[58] It would provide medical treatment to men and sometimes their families, and pay them if they were unable to work.[58] The Club also paid for funerals within certain limits, and would provide grants to the widows of men killed or the wives of those permanently disabled.[58] Eight men were saved from drowning by rowing boats positioned in the river under the working areas.