The Forth Bridges (Forth Rail Bridge & Forth Road Bridge)

Submitted by George Farquhar on Sat, 2011-04-23

Forth Railway Bridge is the world’s first major steel bridge with 2.5 km. (1.5 mile) in length and 521 m. (1710 ft.) of gigantic girder spans makes it as one of the great feats of civilization.

The construction of the bridge was began at 1883 and formally completed on 4 March 1890 when HRH Edward Prince of Wales tapped into place a ‘golden’ rivet.

The Forth Rail Bridge was constructed by Tancred–Arrol robustly designed by the civil engineers Sir John Fowler and Benjamin Baker in the aftermath of the Tay Bridge disaster. Balanced cantilever principle was adopted while constructing the bridge.

The main crossing comprises tubular struts and lattice-girder ties in three double-cantilevers each connected by 105 m. (345 ft.) ‘suspended’ girder spans resting on the cantilever ends and secured by man-sized pins. The outside double-cantilever shoreward ends carry weights of about 1000 tonnes to counter-balance half the weight of the suspended span and live load.

The bridge continues to form a vital artery in Network Rail’s East Coast railway system considered as Scotland’s biggest ‘listed’ building which carries 180 – 200 train movements per day.

Forth Road Bridge

The Forth Road Bridge, together with the approach viaducts is over 2½ km. (over 1½ miles) long was the largest suspension bridge in Europe during the time of its opeing on 4 September 1964 by Her Majesty Queen Elizabeth II

Between its two main towers he bridge has a spectacular central span of over 1 km. (3300 ft.). The side spansare are each 408 m. (1340 ft.) long which carry the deck to the side towers and flanked by approach viaducts.

The supporting cables at the side towers turn down towards the anchorages which are, essentially massive wedges built by tunneling into the rock. The cables are 610 mm. (2 ft.) in diameter and were spun on site using 11618 parallel high tensile steel wires 5 mm. (0.196 ins.) in diameter.

The main towers are extend above mean river level by 156 m. (512 ft.) and the sag of the cables between the towers is approximately 91 m. (300 ft.). Close to the towers the clearance for shipping below the deck of the main span is 46 m. (150 ft.) which increases upto 50 m. (163 ft.) at mid-span.

The mentioned above dimensions of span, sag, tower height and shipping clearance of the bridge give it basic sense of fitness for purpose which result in a structure of slenderness and elegance combined with strength.

During the construction of the bridge some 39,000 tons of steel and 115,000 cubic metres (150,000 cubic yards) of concrete were used. The total cost to construct the bridge was £19,500,000 (~£250,000,000 today) including the approach roads consisting of 13 kilometres of dual carriageway, 13 kilometres of minor access roads and 24 minor bridges.


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