marykirk, annan, stirling, and hutcheson stone bridges—high-level bridge for newcastle—timber bridge of built planks—winch chain bridge—american bridges of suspension—runcorn bridge—menai chain bridge—new form of suspension bridge.
mr. stevenson’s stone bridges over the north esk at marykirk, and the nith at annan (plate vi.), are good specimens of road bridges of moderate extent; and his bridge over the forth at stirling, and hutcheson bridge over the clyde at glasgow (plate vii.), are structures of a larger class.
of the latter, mr. fenwick, of the royal military academy, woolwich, in the preface to his work on the mechanics of construction, published in 1861, says,—“the london and waterloo bridges, in the metropolis, which rank among the finest structures of the elliptical arch, and stevenson’s hutcheson bridge at glasgow, which is one of the best specimens of the segmental arch, together with many others, have supplied me with a variety of problems for illustration.”
plate vi.
annan bridge
1824.
marykirk bridge
1811.
w. & a. k. johnston, edinburgh.
plate vii.
hutcheson bridge, glasgow.
1828.
stirling bridge.
1829.
w. & a. k. johnston, edinburgh.
plate viii.
design for high level road bridge
at newcastle on tyne.
1828.
transverse section.
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 by161 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.
* * * * *
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, in162 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 of163 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:—
“winch chain bridge.—the earliest bridges of suspension of which we have any account are those of china, said to be of great extent; major rennell also describes a bridge of this kind over the sampoo in hindostan, of about 600 feet in length. but the first chain bridge in our own country is believed to have been that of winch bridge over the river tees, forming a communication between the counties of durham and york. this bridge is noticed and an elevation of it given in the third volume of hutchison’s antiquities of durham, printed at carlisle in 1794. as this volume is extremely scarce, owing to the greater part of the impression having been accidentally destroyed by fire, the writer of this article applied for a sight of it from the library of his friend, mr. isaac cookson of newcastle-upon-tyne. the following account is given by hutchison at p. 279:—‘the environs of the river (tees) abound with the most picturesque and romantic scenes; beautiful falls of water, rocks and grotesque caverns. about two miles above middleton, where the river falls in repeated cascades, a bridge suspended on iron chains is stretched from rock to rock over a chasm nearly sixty feet deep, for the passage of travellers, but particularly of miners; the bridge is seventy feet in length, and little more than two feet broad, with a hand-rail on one side, and planked in such a manner that the traveller experiences all the tremulous motion of the chain, and sees himself suspended over a roaring gulf, on an agitated and restless gangway, to which few strangers dare trust themselves.’ we regret that we have not been able to learn the precise date of the erection of this bridge, but from good authority we have ascertained that it was erected about the year 1741.
“american bridges of suspension.—it appears from a treatise on bridges by mr. thomas pope, architect, of new york, published 164in that city in the year 1811, that eight chain bridges have been erected upon the catenarian principle, in different parts of america. it here deserves our particular notice, however, in any claim for priority of invention with our transatlantic friends, that the chain bridge over the tees was known in america, as pope quotes hutchison’s vol. iii., and gives a description of winch bridge. it further appears from this work that a patent was granted by the american government for the erection of bridges of suspension in the year 1808. our american author also describes a bridge of this construction, which seems to have been erected about the year 1809, over the river merrimack in the state of massachusetts, consisting of a catenarian arch of 244 feet span. the roadway of this bridge is suspended between two abutments or towers of masonry, thirty seven feet in height, on which piers of carpentry are erected which are thirty five feet in height. over these ten chains are suspended, each measuring 516 feet in length, their ends being sunk into deep pits on both sides of the river, where they are secured by large stones. the bridge over the merrimack has two carriage-ways, each of fifteen feet in breadth. it is also described as having three chains which range along the sides, and four in the middle, or between the two roadways. the whole expense of this american work is estimated to have been 20,000 dollars.
“proposed bridge at runcorn.—perhaps the most precarious and difficult problem ever presented to the consideration of the british engineer was the suggestion of some highly patriotic gentlemen of liverpool, for constructing a bridge over the estuary of the mersey at runcorn gap, about twenty miles from liverpool. the specifications for this work provided that the span of the bridge should measure at least 1000 feet, and that its height above the surface of the water should not be less than sixty feet, so as to admit of the free navigation of this great commercial river. the idea of a bridge at runcorn, we believe, was first conceived about165 the year 1813, when the demand for labour was extremely low, and a vast number of the working classes of lancashire were thrown out of employment. a variety of designs for this bridge were procured by a select committee of the gentlemen who took an interest in this great undertaking. the plan most approved of, however, was the design of a bridge of suspension; and mr. telford the engineer, and captain brown of the royal navy, are understood pretty nearly to have concurred in opinion as to the practicability of such a work. mr. telford has reported fully on the subject, and has estimated the expense of his design at from ￡63,000 to ￡85,000, according to different modes of execution. though as yet little advancement has been made in carrying this enterprising design into execution, yet the novelty and magnitude of an arch of 1000 feet span is a subject of so much interest that we have thought it proper in this place to mention these circumstances.
“menai chain bridge.—the straits of menai, which separate the island of anglesea from caernarvonshire, have long formed a troublesome obstruction upon the great road from london to dublin by holyhead, by which the troublesome ferry of bangor might be avoided. many plans for the execution of this undertaking have also been agitated, chiefly in cast iron, including a range of estimate from about ￡128,000 to ￡268,000; but that which is now acted upon is a bridge of suspension upon the catenarian principle, the extent of which between the piers or points of suspension is to be 560 feet, the estimate for which is only about ￡70,000. this by many has been considered a work of great uncertainty; but the union bridge on this plan has already been executed on the tweed, to the extent of 361 feet.”
mr. stevenson then goes on to mention several wire and chain bridges erected in scotland, and gives the following description of his design for cramond bridge:—
166
fig. 17.
“fig. 17 is a section and plan designed for crossing the river almond on the great north road between edinburgh and queensferry. the extent of the span between the points of suspension is laid down at 150 feet. the chief circumstances which particularise this design are a mode of fixing the chains to the abutments of suspension on each side of the river, by which the main chains can be distributed equally under the roadway. the main chains are likewise made to collapse or turn round the abutments of masonry, as will be seen from the section, in which the parts of the work are so contrived that access can be had to the chains by an arched way on each side. in this design the two ends of the chains are formed into great nails or bolts, with countersunk or conical heads made to fit into corresponding hollow tubes of cast iron built into the masonry of the abutments.
“from this description the reader will readily form an idea of the simplicity and effect of this mode of fixing the chains, being such, also, that any particular chain may be withdrawn and replaced without deranging the fabric of the bridge. the roadway, instead of being suspended from the main chains, is made up to167 the proper level upon the chains by a framework of cast iron, prepared for the reception of a stratum of broken stones for the road.
“the making up of the roadway of this bridge, however, and the enlarged angle of its suspension, may be considered as limiting the span or extent of bridges of this construction to about 200 feet. the structure represented by fig. 17 appears to possess many advantages for bridges of that modified extent, and the manner of fixing the chains is applicable to all bridges of suspension; it is likewise new, so far as we know.”
in the close of his paper mr. stevenson says:—
“to what extent suspension bridges may be carried is very uncertain, and he who has the temerity to advance sceptical or circumscribed views on this subject would do well to reflect upon the history of the steam-engine. when the marquis of worcester first proposed, by the boiling of water, to produce an effective force, no one could have conceived the incalculable advantages which have since followed its improvement by our illustrious countryman, watt.”
a prophetic announcement, which has had its full realisation in the suspension railway bridge of 821 feet span at niagara falls, and in the still bolder design now in execution for connecting new york and brooklyn by a steel wire suspension bridge, having a clear opening between the piers of no less than 1600 feet.