Earlier proposals Forth Bridge

prior construction of bridge, ferry boats used cross firth. in 1806, pair of tunnels, 1 each direction, proposed, , in 1818 james anderson produced design three-span suspension bridge close site of present one. calling approximately 2,500 tonnes (2,500 long tons; 2,800 short tons) of iron, wilhelm westhofen said of , quantity [of iron] distributed on length have given light , slender appearance, light indeed on dull day hardly have been visible, , after heavy gale no longer seen on clear day either .

for railway age, thomas bouch designed edinburgh , northern railway roll-on/roll-off ferry between granton , burntisland opened in 1850, proved successful ordered tay. in late 1863, joint project between north british railway , edinburgh , glasgow railway, merge in 1865, appointed stephenson , toner design bridge forth, commission given bouch around 6 months later.

it had proven difficult engineer suspension bridge able carry railway traffic, , thomas bouch, engineer north british railway (nbr) , edinburgh , glasgow railway, in 1863–1864 working on single-track girder bridge crossing forth near charlestown, river around 2 miles wide, relatively shallow. promoters, however, concerned ability set foundations in silty river bottom, borings had gone deep 231 feet (70 m) mud without finding rock, bouch conducted experiments demonstrate possible silt support considerable weight. experiments in late 1864 weighted caissons achieved pressure of 5 tons/ft on silt, encouraging bouch continue design. in august 1865, richard hodgson, chairman of nbr, proposed company invest gb£18,000 try different kind of foundation, weighted caissons had not been successful. bouch proposed using large pine platform underneath piers, 80 60 7 feet (24.4 m × 18.3 m × 2.1 m) (the original design called 114 80 9 feet (34.7 m × 24.4 m × 2.7 m) platform of green beech) weighed down 10,000 tonnes (9,800 long tons; 11,000 short tons) of pig iron sink wooden platform level of silt. platform launched on 14 june 1866 after difficulty in getting move down greased planks rested on, , moored in harbour 6 weeks pending completion. bridge project aborted before platform sunk nbr expected lose through traffic following amalgamation of caledonian railway , scottish north eastern railway. in september 1866, committee of shareholders investigating rumours of financial difficulties found accounts had been falsified, , chairman , entire board had resigned november. mid-1867 nbr bankrupt, , work on forth , tay bridges stopped.

bouch s proposed bridge (top) along other proposals on same principle

the north british railway took on ferry @ queensferry in 1867, , completed rail link ratho in 1868, establishing contiguous link fife. interest in bridging forth increased again, , bouch proposed stiffened steel suspension bridge on line of present rail bridge in 1871, , after careful verification, work started in 1878 on pier @ inchgarvie.

after tay bridge collapsed in 1879, confidence in bouch dried , work stopped. public inquiry disaster, chaired henry cadogan rothery, found tay bridge badly designed, badly constructed , badly maintained , bouch being blame defects in construction , maintenance , entirely responsible defects in design.

after disaster, occurred in high winds bouch had not accounted, board of trade imposed lateral wind allowance of 56 lb/sq ft (270 kg/m). bouch s 1871 design had taken lower figure of 10 lb/sq ft (49 kg/m) on advice of astronomer royal, although contemporary analysis showed have stood, engineers making analysis stated not commit ourselves opinion best possible [design]. bouch s design formally abandoned on 13 january 1881, , sir john fowler, w. h. barlow , t. e. harrison, consulting engineers project, invited give proposals bridge.