Publication Account
Date 1986
Event ID 1017681
Category Descriptive Accounts
Type Publication Account
Permalink http://canmore.org.uk/event/1017681
Designed by I K Brunel, with ironwork by R Brotherhood of Chippenham, the plate-girder bridge spanning the River Dee near the entrance to Balmoral Castle is possibly the earliest of its type of construction in Scotland. Commissioned by the Royal Family as part of the improvements to the Balmoral estate, it did not meet with their wholehearted approval on its completion in 1857, owing to its severely plain appearance and marked elasticity under passing traffic. But in engineering terms it is a distinguished work, incorporating roller-bearings and one of Brunei's novel girder sections devised to resist buckling and compressive strains; and it has the refinements of a gently cambered profile and cut-out diaper patterned web-plates.
Flanked by curved approaches, the bridge rests on 207 18 ft-high (5.49m) abutments built of coursed rock-faced blocks of local granite. The two principal girders, which also form the parapets, are each 130ft (39.62m) by 6ft 6in (1.98m) deep and have a clear span of 124ft (37.80m) between bearers. Spaced 15 ft (4.57m) apart, the webs are connected transversely at 5 ft (1.52m) intervals by 12 in-deep (0.31m) plate cross-beams, which are decked with longitudinal pine planking and originally accommodated raised narrow side-walks and a 10 ft-wide (3.05m) carriageway, surfaced with gravel. The boxed ends of the girders are mounted on cast-iron bed-plates averaging 3 ft (0.91m) square, which are fixed inertly onto the w abutment but ride on rollers on the other to permit free longitudinal movement. The roller-bearings consist of upper and lower bed-plates with a wrought-iron framework between them containing a set of five rollers of 4in (102mm) diameter.
The girder components include a cambered upper flange with fascia-plates, a broader lower flange slightly dished downwards, and a vertical web-plate interconnecting them, the whole being stiffened transversely by diaphragm plates dividing the girders into five-bay units . Built up entirely of rolled wrought-iron members, the girders consist of flat plates butt-jointed together in short regular lengths and strengthened by a complex system of bracing-strips, angle-irons and cover-plates, all carefully chain-riveted to form a rigid structure. The 1/2 in-thick (12.7mm) plates forming the upper flange are doubled in thickness over the three central sections, while those of the lower flange are of two layers throughout, totalling 13/16in (20.6mm), and are disposed so as to break joint on either side of the webplate. Web- and diaphragm-plates, respectively of 17/32 in (13.5mm) and 3/16 in (4.8mm) in thickness, are stiffened with cross-bracing and angle-irons, lapped or spliced where necessary and levelled off with packing-strips. In general, the principal butt-joints are connected with single or double splice-plates and close-riveted in one or more rows on either side.
Three of the four original makers' plates remain on the bridge affixed to the parapet fascia , inscribed R. BROTHERHOOD/ CHIPPENHAM/ WILTS. 1856. In his initial calculations, Brunei assumed that the bridge would be capable of supporting a load of 86 tons (87.38 tonnes) 'or about 1200 people'; it continues to serve the needs of present-day traffic, though the wooden decking was renewed in 1971.
Information from ‘Monuments of Industry: An Illustrated Historical Record’, (1986).
