The Forth Road Bridge
Created | Updated Sep 4, 2007
Travel between Scotland's capital city, Edinburgh, and the major cities of East Scotland has always been complicated by the difficulties imposed by the necessity of negotiating the estuary of the River Forth.
The main crossing of the Firth of Forth for travellers heading north from Edinburgh was by the ferry, which was established between the villages of South and North Queensferry1 sometime in the 11th or 12th Centuries. This ferry was, especially in early years, at the mercy of the elements, rendering timetables unreliable. In later years it was increasingly unable to cope with the volume of traffic occasioned by the appearance of the motor car. By the mid 1950s, the ferry was the busiest in Scotland, with four ferries carrying 1,500,000 people, 600,000 cars and 200,000 goods vehicles, making over 40,000 crossings each year.
Individual travel between the major cities was well catered for by the opening of the Forth Rail Bridge in 1890, but the first major road link across the Forth did not appear until the completion of the Kincardine Bridge in 1936.
Serious proposals for a road bridge over the Forth near Queensferry had been put forward from as early as the beginning of the 19th Century, but it was not until 1958 that work began on the new bridge.
Before any construction could take place, the bridge had to be designed. This task was given to Freeman, Fox and partners, the chief designer for this project being Sir Gilbert Roberts.
The Dream Becomes Reality
Once the plans were complete, the first major task was to prepare the foundations for the main towers by sinking piles 35m through the clay of the riverbed, to reach bedrock which was levelled by underwater blasting. Upon these piles, two towers were erected in sections. A total of some 6000 tons of high-tensile steel was required for each tower.
Once the two towers were completed, the next stage was to spin the main suspension cables between them. This operation was achieved by hanging a cable supporting the cable spinning wheel between the towers and spinning the cables in situ. Each cable consists of 11,618 parallel high-tensile steel wires, amounting to 610mm in diameter. Despite the machine's capability to spin up to 50 miles of cable in each day (in good weather conditions), it took almost nine months for this part of the operation to be completed.
Similar cables led from each main tower to side towers, one on each shore, and from these to massive anchorages tunnelled into bedrock and wedged into position. Suspender cables were then fitted to the main cables, and the trusses to support the main deck could begin to take shape.
The main deck is actually split into four separate carriageways: the outside pair for pedestrians and cyclists, the central pair for vehicles, with gaps between each carriageway. These gaps, and the open latticework design of the supporting trusses, were carefully designed to achieve aerodynamic stability and prevent a repeat of the Tacoma Narrows Bridge disaster.
At the time of construction it was the largest suspension bridge in Europe, and together with the approach viaducts is over 2.5km long.
The bridge has a central span of over 1km between its two main towers. The side-spans, which carry the deck to the side towers, are each 408m long.
The main towers reach 156m above mean river level, and the sag of the cables between the towers is approximately 91m. The clearance for shipping below the deck of the main span ranges from 46m close to the towers to 50m at mid-river.
Some 39,000 tons of steel and 115,000 cubic metres of concrete were used in the construction of the bridge.
The total cost of the bridge including the approach roads was £19,500,000.
On 4 September, 1964, Her Majesty Queen Elizabeth II declared the bridge officially open. The combination of elegance and slenderness allied to a sense of fitness of purpose has resulted in a beautiful balance with its more massive neighbour, the Rail Bridge.
To the Present Day and Beyond
On 3 April, 2001 the Forth Road Bridge was listed as a category 'A' building. Since the bridge was opened, the permitted weight of commercial vehicles using the roads in Scotland has been increased from 24 tons to 44 tonnes.
In the first full year of operations (1965), the bridge carried 4,600,000 vehicles, but by 1996 (the last year of two-way tolls) this had jumped to 20,300,000 per year. One-way figures for 2001 indicate 11,300,000 northbound vehicles2.
In order to accommodate both the heavier vehicles and the large increase in the number of vehicles using the bridge, certain parts of the structure have been strengthened in recent years. Extra protection has also been provided to the main tower piers at river-level, and all of the hanger ropes supporting the bridge deck have been renewed.
Forecasts of future traffic increases have led to claims that the present bridge will be unable to cope with demand for much longer, and that another road link across the Forth will soon be required. Whatever is finally decided, one thing is certain - any designer of a future Forth bridge will have a very difficult task to live up to the standard established by the present examples.