W. & A. K. Johnston, Edinburgh.
The Hutcheson Bridge was completed in 1832. The masonry of the piers was laid at the level of seven feet below the bed of the Clyde, on a platform of timber, on piles eighteen feet in length. I found by a section made in 1845, after a lapse of thirteen years, that the level of the river had been lowered, in consequence of the deepening of the river Clyde by the Navigation Trustees, no less than eleven feet, and even with that amount of scour the bridge was, and might long have remained, a safe structure. But immediately above its site there is a weir which dams up the Clyde and forms a lake, or almost still pool, in the river’s bed for several miles. It was determined, in the interests of navigation, to take powers to remove the weir, and on its removal the bridge could, no longer be pronounced safe; it was also resolved to take powers to replace the Hutcheson by the new Albert Bridge, designed by Messrs. Bell and Miller.
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Mr. Stevenson has also left behind him some traces of originality of design in bridge-building.
In 1826 he gave a design to the Corporation of Newcastle for raising on the existing bridge another roadway, on a high level, to communicate with the higher parts of the town, as shown in [Plate VIII.], being the idea since so successfully carried out on a large scale by the late Mr. Robert Stephenson in his justly celebrated “high-level railway viaduct.” Mr. Stevenson’s design, as will be seen, consists of piers of masonry raised on the piers of the old bridge supporting a roadway of cast iron. The upper bridge being continued across the quays on either side of the river, and joining the roadways leading towards the south and north by easy gradients, avoided the circuitous and dangerous route of the old post road through Newcastle.
For timber bridges Mr. Stevenson also proposed, in 1831, a new form of arch of a beautiful and simple construction ([Fig. 16]), in which what may be called the “ring-courses” of the arch are formed of layers of thin planks bent into the circular form and stiffened by kingpost pieces, on which the level roadway rests. This form of bridge was afterwards very generally employed for railway bridges before the discovery had been made that for such works, structures of iron were, in the end, more economical than timber.
Fig. 16.
In 1820, he proposed to the Cramond District of Road Trustees, with a view mainly to lessening the cost of the work, a form of suspension bridge applicable to spans of moderate width, in which the roadway passes above the chains, and the necessity for tall piers is avoided. The suspension bridge over the Rhone at Geneva, and other bridges, have since been constructed on this principle.
In 1821 Mr. Stevenson wrote an article on Suspension Bridges for the Edinburgh Philosophical Journal; and as it contains a description of this new form of construction, as well as some historical information relative to bridges on the suspension principle, a few extracts from the paper may not be without interest:—