Glasgow Bridge
There have been three bridges on this site since the medieval period. The bridge is also known as the Jamaica Street Bridge.
The first bridge
The first bridge on this site was a seven-arch masonry bridge built by brothers William and Robert Mylne between 1768-72.
c.1772 The Clyde Trustees deposited a number of large stones around the piers of the Jamaica-street Bridge, later called the Glasgow Bridge, in order to strengthen the piers of the bridge, which had shown signs of instability, arising chiefly from the deepening that had for sometime previously been made of the river and harbour by the Clyde Trustees immediately below or to the west of the bridge.[1]
Telford's bridge
1829 An Act authorised the Bridge Trustees to take down the old bridge and to remove the weir.
Thomas Telford designed a replacement bridge (his last) in masonry, also with seven arches.
1835 This bridge was completed in 1835 by contractor John Gibb and opened to traffic on 1st January 1836. The bridge had gently curving extrados rising less than 900mm from the banks to the bridge centre; the spans of the arches ranged from 15.9m to 17.9m wide.
Gibb's great-grandson, Sir Alexander Gibb, called it "perhaps the most beautiful of all Telford's bridges … a fitting crown to his creative life".
The progressive effects of navigational dredging weakened the foundations of each of these bridges in turn. As the city’s population and industrial capacity increased, so did the need for a wider, stronger bridge. A higher bridge would also have helped shipping to pass without lowering masts or funnels.
Replacement of Telford's bridge
1892 Engineers Blyth and Westland proposed a granite arch bridge to replace Telford's bridge; this would have had four high spans, 30.5m wide between parapets, and was estimated to cost £240,000. Glasgow Corporation declined the proposal because, apart from costing more than rebuilding Telford’s bridge, the old bridge was held in high regard by Glasgow's citizens.
So, the third bridge on the site was a re-build of Telford's bridge by Blyth and Westland. Work began in 1894 and Telford’s bridge profile was retained. Much of the original Aberdeen granite was reused.
The bridge is now 24.4m wide and has much deeper foundations than its predecessor. Instead of timber piles, the piers are founded on 4.6m diameter steel cylinders sunk into the river bed using pneumatic pressure, which were then filled with concrete. The piers are founded at a minimum depth of 23m below the arch springings, though some of them are more than 30.5m below this level.
Manual excavation for the piers was carried out inside a 2.75m high chamber filled with compressed air. However, at the greatest depth, an air pressure of 296.5kN per sq m (almost three times atmospheric pressure) was required in order to repel the water — virtually impossible working conditions for the labourers.
The contractor was Morrison and Mason, of Glasgow; steelwork was by Sir William Arrol and Co Ltd
Work was completed in 1899, at a cost of £129,500. The gas lights that had adorned the parapets of Telford’s bridge were replaced by electric lamps along the central reservation.
See Also
Sources of Information
- ↑ The Engineer 1894/09/21
- Engineering timeline [1]