GLENFINNAN VIADUCT carries the Mallaig extension of the West Highland Railway (LNER) across Glen Finnan. Built of concrete throughout, the viaduct is 416 yards long and is formed by twenty-one spans of 50 feet. The maximum height is 100 feet. The viaduct is built on a curve with a radius of twelve chains. All but two of the piers are 20 feet long and 6 feet thick at the top.
SCOTLAND has always been a country of bridges. Such structures as those spanning the Firths of Forth and Tay need no introduction, but right back through the ages the country has provided opportunities for the bridge builder. At Bothwell, Lanarkshire, may still be seen a bridge dating from the Roman occupation, and there are several beautiful structures dating from the late Middle Ages. The eighteenth and early nineteenth centuries saw numbers of new road bridges, one of the most notable being that at Kelso, Roxburghshire, built by Rennie much on the lines of his Waterloo Bridge in London.
Great as is their variety, the bridges of the Highlands have an interest that is all their own. General Wade, a man who understood the Highlands and the Highlanders more than any other Englishman at the time, built a remarkable system of roads for military and trade purposes through the mountains in the early years of the eighteenth century. His sturdy yet picturesque stone bridges across many a brawling Highland stream may be seen to this day. Two generations later came the great Thomas Telford, who built miles of. magnificent roads through the difficult mountain places. Among Telford’s bridges in the Scottish Highlands there is no better example than that of Craigellachie, across the River Spey, a few miles south of Elgin. The Spey has a tremendous current and Telford decided to bridge the river with a single span of 150 feet.
Telford completed his Craigellachie Bridge in 1815. It is an iron arch with abutments of masonry, and was the first in which the advanced theory of using braced arched ribs was applied. The bridge was thus the forerunner of many fine modern bridges of the present day.
Though not large, it is a beautiful piece of work, reaching the high aesthetic standard which its designer set on the much larger Menai Suspension Bridge. To-day the Craigellachie Bridge carries heavy motor traffic such as nobody could have foreseen in the days when it was built.
Neither Telford, nor Wade before him, built any really large bridges in the Scottish Highlands. It was the dawn of the Railway Age that brought the big bridge and the long viaduct into the fastnesses of the ancient hills. With a railway the engineer cannot take his line sharply down the mountainside and over the bed of an intervening valley, merely bridging the river in the middle of it, and the methods of Wade and Telford had to go by the board.
The railway builder has to bridge the entire valley in mountainous country, and there are many such valleys in the Highlands. Moreover, on the wild and rugged West Coast there are long sea lochs and firths which need to be spanned to avoid long detours round the heads of these inlets of the sea. One such detour may involve the route being taken fifty miles or so out of its direction.
Railways came late to the Highlands. In their infant days no ordinary locomotives could have been expected to master the gradients involved by a main line across the Highland tableland. It was not until 1861 that a start was made with the laying of a railway - the Highland Railway, now L.M.S. - between Perth and Inverness. This line was to cross the Grampians through Drumochter Pass at an elevation of 1,484 feet above sea level. Joseph Mitchell, of Inverness, was the engineer, and he and his assistant, Murdoch Paterson, had a difficult task to carry the tortuous route across the successive Highland streams. Where long spans were essential they used iron lattice girders resting on stone piers. Across ravines they built a series of graceful stone viaducts, all of which survive to this day, despite the considerable increase in the weight of the trains.
The most southerly of the iron bridges of Mitchell and Paterson is that across the Tay between Dalguise and Guay, north of Dunkeld, Perthshire. This bridge has two spans of 209 feet and one of 141 feet. Across the big ravine at the head of the Pass of Killiecrankie they built a fine stone viaduct of ten 35-feet arches, the height at the deepest part of the ravine being 54 feet.
Outside Blair Atholl they spanned the River Tilt with another iron girder bridge, having a single long span. A few miles farther north they had to cross the River Garry near Struan.
The gorge is exceedingly narrow, and the only possible site for a bridge was already occupied by the old stone arch carrying the Kinloch Rannoch road. So the engineers carried their railway bridge diagonally across road bridge and river together, the railway bridge clearing the road bridge in a single stride of its central arch.
In the north the builders of the Highland Railway had more heavy bridge work before them. Near Dunphail, Moray, they had to span the Glenerney Ravine with a big stone viaduct, and in the final stretch along the coast from Forres to Inverness they had to cross the Nairn and Findhorn Rivers. Though none of the bridges on the old Highland line are large, they were the first examples of major bridge work in the Highlands.
When, towards the close of the last century, the old Highland Railway Company decided to shorten the northern part of its route by building a short cut across the Monadhliath Mountains between Aviemore and Inverness, Murdoch Paterson had two great works before him. These involved spanning the valley of the Findhorn, near Tomatin, and the equally formidable valley of the Nairn between Daviot and Culloden Moor.
For these bridges, Murdoch Paterson called into consultation Sir John Fowler, famous for his work in connexion with the London Underground and the Forth Bridge. Fowler’s influence is particularly apparent at Tomatin, where the viaduct across the Findhorn is strongly reminiscent of the great approach viaducts to the Forth Bridge. The viaduct consists of a steel girder superstructure resting on piers of granite, and has nine spans of 130 feet. The rails are 145 feet above the surface of the Findhorn. The viaduct is built on a curve having a radius of 40 chains, and it rises on a gradient of 1 in 60 towards Aviemore. It was completed in 1896.
IN THE SCOTTISH HIGHLANDS long arms of the sea extending inland for many miles are great obstacles to direct communication. In certain places these sea lochs have been spanned by bridges, such as the large cantilever bridge across Loch Etive at Connel Ferry, near Oban. In other places railways must be carried across entire valleys ; this necessitates the building of such structures as the Glenfinnan Viaduct on the West Highland Railway to Mallaig, or the Nairn Viaduct, near Inverness. Many bridges built by engineers such as Telford and Rennie are still in service to-day. Telford’s bridge across the River Spey at Craigellachie is an outstanding example.
For spanning the Nairn Valley, near Culloden Moor, Paterson returned to his old love, the stone viaduct. The Nairn Viaduct is one of the most beautiful structures of its kind. It consists of twenty-eight arches of 50 feet, and one great semicircular arch, with a span of 100 feet, over the river itself. The whole viaduct is built of Old Red Sandstone, and its deep rose-red length extends right across the wide green valley. Paterson’s life was coming to its close when he began the Nairn Viaduct. He had been Chief Engineer to the Highland Railway for nearly a quarter of a century, as successor to Joseph Mitchell. He died just before the viaduct’s opening, at the end of 1898.
The last decades of the nineteenth century saw the extension of railways and the building of notable bridges in other parts of the Scottish Highlands. Most remarkable was the work carried out in the course of extending the West Highland Railway (now L.N.E.R.) from Fort William to Mallaig in the ’nineties. One of the most interesting features of the bridges and viaducts on this West Highland extension is the fact that the engineers built nearly all their viaducts of concrete. In more recent years the builders of the new Glencoe Road and other great Highland highways have made extensive use of concrete, but those responsible for the Mallaig railway were covering entirely new ground. Moreover, they erected structures which have not been approached in size by the builders of the new roads.
Where the River Morar, in the extreme west of Inverness-shire, flows out of its beautiful parent loch into the sea over a series of waterfalls, the builders of the Mallaig line erected across the gorge a concrete viaduct of four spans, the largest being one of 90 feet. At the time nobody had attempted such a span in concrete, but the bridge was a complete success and stands unchanged to this day. There were other fine concrete viaducts between Arisaig and Loch Ailort, but the biggest feat of the West Highland engineers was the great viaduct across Glen Finnan. It is remarkable that one great viaduct should stand at the place where Prince Charles Edward raised his standard in 1745, and that another, Murdoch Paterson’s last great work, should span the valley near Culloden Moor, where the rising came to its tragic end in 1746.
At Glen Finnan the engineers had to span the valley with a viaduct 416 yards in length, connecting one steep mountainside with its opposite neighbour. They used concrete throughout, as at Morar and elsewhere, and raised twenty-one spans of 50 feet. The maximum height of the viaduct is 100 feet. All but two of the tall piers are 20 feet long and 6 feet thick at the top, the sides being built with a batter of 1 in 50.
Spanning Arms of the Sea
The two remaining piers are 21 feet long and 15 feet thick at the top, the batter of the sides being the same. The eastern abutment and six of the piers are founded on solid rock, the others resting"on a bed of firm gravel and boulders carried down the glen by some prehistoric glacier. Glenfinnan Viaduct is built on a curve, the radius being no more than 12 chains. As it carries only a single track, it is slender in appearance and from a distance resembles some strange ivory gateway to the wild interior of the glen.
Of all the bridge work in the Highlands, with its great variety, there is none more noteworthy than that in the west of Argyll, carried out at the turn of the nineteenth and twentieth centuries. At the end of the ’nineties work was started on another railway extension, from the existing Callander and Oban line at Connel Ferry to Ballachulish on Loch Leven. Where the engineers had had glens to cross before, they now had to carry their way across two considerable fjords running in from the Firth of Lome and from Loch Linnhe. These arms of the sea, extending far inland among high mountains, are Loch Etive and Loch Creran.
The line crosses Loch Creran near its head, and the crossing is relatively short. Over the loch, between Barcaldine and Creagan, the engineers were able to build a two-spans lattice girder structure of fair size but of simple design.
Loch Etive, however, was another matter. It is long and has swift tidal currents where it enters the sea near Connel Ferry. Connel Ferry, the site of the bridge, is at the narrowest part of the loch, but those narrows contain an extraordinary series of tidal rapids known as the Falls of Lora. In the ebb of a spring tide, over a stretch of about 350 feet, the level of the water drops 4 feet, falling over a series of natural rock steps. The force of the current, with the ebb and with the flow of the tide, is extreme. At full flood tide, however, small steamers are able to pass safely over the site of these strange tidal falls. The Connel Ferry Bridge, therefore, was to be one which allowed both for the rapids and for the demands of coastal shipping.
ACROSS THE RIVER FINDHORN at Tomatin, Inverness-shire, the railway viaduct was built by Murdoch Paterson working in conjunction with Sir John Fowler, designer of the Forth Bridge. The approach spans of the Forth Bridge bear a strong resemblance to the 130-feet spans of the Findhorn Viaduct. The viaduct consists of a steel girder superstructure resting on piers of granite. The rails are 145 feet above the river and rise on a gradient of 1 in 60 towards Aviemore. The viaduct is on the direct Aviemore - Inverness line of the Highland Railway (now L.M.S.).
The building of piers in the rapids would have been attended with great difficulty. Moreover, such piers, if they had been built, would have been prone to scour and would have aggravated the already severe currents. Only two types of bridge were possible across the Falls of Lora - the suspension bridge or some kind of cantilever bridge, either of which could be erected without disturbance from the tidal currents. The engineers acting for the railway company (the former Caledonian Railway, now L.M.S.) unhesitatingly chose the cantilever bridge.
The channel has a width of 600 feet, and the designers of the bridge had to place their two main piers, supporting the two great cantilevers, 524 feet apart from centre to centre. This gave the bridge a clear span of 500 feet, a width eclipsed in Europe, at the time it was built, only by the Forth Bridge. The underside of the central girder span supported by the two cantilevers is 50 feet above high-water level, and the height to the tops of the cantilevers is 125 feet.
The designers of the Connel Ferry Bridge adopted a peculiar arrangement of the cantilevers, the visual impression of each being that of a huge steel triangle with its apex on a pier in the water. One side of each triangle rises upwards and outwards over the loch, so that they appear to be leaping towards each other over the intervening stretch of water. Another side of the triangle slopes in the opposite direction and abuts against the massive masonry piers of the approach viaducts. Both these sides
of the triangle are struts, that is, they are in compression. The third side of the-triangle is a tie, that is, it is in tension. The cantilever trusses are all above the deck of the bridge, unlike those of the better-known Forth Bridge, the deck in this instance being wholly suspended. The centre portion of the bridge is a plain span supported at either end from the cantilevers.
Tilt Counteracted by Anchors
To counteract the tendency of each triangle to tilt over into the loch, the engineers anchored the landward ends to the two masonry abutments forming the ends of the approach viaducts. These abutments are 45 feet wide and 20 feet long. The anchors consist of huge girder members embedded in masonry. The weight of the masonry round and above each anchor was estimated to equal about twice the pull exerted by the triangular cantilever. The anchors are situated 50 feet below the level of the rails crossing the bridge, and are accessible for painting and inspection by subways leading from doors in the sides of each abutment.
The piers themselves are of granite and rest on the solid rock through which, in the course of countless ages, the waters of Loch Etive have carved out their central channel. Visible proof of the hardness of this rock exists in the survival of the tidal Falls of Lora. The engineers built their piers by means of cofferdams set in shallow water, where the current is small even with the full ebb and flow of the tide. Each pier is 110 feet long, that is, measured at right angles to the line of the bridge, and 24 feet wide at the top. On the summit of each pier, carrying the main supporting legs of the cantilever, are two bases of steel, each of which measures 14 feet by 10 feet and is capable of bearing a maximum load of approximately 1,500 tons. From these, the legs rise upwards and inwards, similarly to those of the Forth Bridge except for their inclination towards the middle of the waterway. The girder spans in the middle and in the two cantilever trusses are, however, all built in a perpendicular plane.
CONNEL FERRY BRIDGE has a clear span of 500 feet across the entrance to Loch Etive from the Firth of Lorne. The two main piers which support the great cantilevers are 524 feet apart from centre to centre. The peculiar arrangement of the cantilevers gives an impression of two huge triangles, each with an apex resting on a pier in the water.
The legs of the cantilevers at Connel Ferry are 43 feet apart at their bases and support the tracks by horizontal girders 21 ft. 6 in. across from one side to the other. Where the outer booms came into contact with the main supports, the designers inserted rubbing pieces of brass, each rubbing plate on the booms being in contact with a corresponding plate on the supports. Measured over the exterior stiffening angles, the legs are 6 ft. 7 in. by 4 ft. 6 in. and the back struts 5 ft. 6 in. by 4 ft. 6 in. in cross-section. The central suspended span is 232 feet long, as compared with 350 feet in the Forth Bridge suspended spans. Throughout the length of the bridge, except on the approach spans, the flooring is supported by 21-in. plate girders set at intervals varying from 14 feet to 16 feet.
The bridge, with the line of railway which it serves, was sanctioned by an Act of 1895 and work began on the masonry approach piers in 1898. May 1900 saw the initial operations on the steel superstructure. Arrol’s Bridge and Roof Company, the builders, did most of the riveting in their yards at Germiston Ironworks, Glasgow, the remainder being carried out on the spot with the girders in position. Once the base plates on the masonry piers had been finished, the engineers were able to proceed with the erection of the main girders and back struts. The use of timber staging was possible here, though not after construction had proceeded beyond the main piers. By November 1901 the builders had the back struts in position and the main legs built up to what was to be the level of the floor. A year later the shoreward sides of the trusses were up and a start had been made with cantilevering out over the loch.
Accurate Within ¼-inch
The big crane used for this job (there was one on either side) ran on an 80-feet trolley. This trolley projected in advance of the work to be done to the extent of about 40 feet. The jib of the crane extended 70 feet, thus giving it plenty of scope for carrying the girders forward and holding them steady before they were fixed in position by pneumatic riveters. At the landward end of each trolley there was mounted a 3-tons hand crane, used for lifting the lighter girders and holding them in position. The steam crane was capable of taking a load of 10 tons with its jib fully extended.
By the beginning of March 1903 the contractors had completed the essential features of the southern, or Connel Ferry cantilever. At the same time the North Connel cantilever was rapidly approaching completion. After that work proceeded at a rapid rate as the two halves of the great structure drew stage by stage nearer to each other. When the last section, in the middle of the central span, alone remained to be inserted, the exact measurement of the gap showed that it came to within a quarter of an inch of the initial estimate. This final section was placed in position on May 9, 1903. Loch Etive had been spanned at last. The bridge at first carried a railway, but no road. The rails were laid on longitudinal sleepers of jarrah wood and had a footway beside them.
The first instance of motor traction being introduced to the bridge was when a kind of petrol bus running on the existing rails was placed in service for the purpose of carrying non-railway passengers across between Connel Ferry and North Connel. This, however, still kept the motorist at a complete disadvantage. In 1903, however, motoring was in its infancy and was still looked upon as an unusual form of transport. The practice of “ferrying” cars across in flat wagons attached to the ordinary trains was a poor substitute for a proper roadway, and at length the Caledonian Railway decided to make the structure a road toll bridge, in addition to serving its original purpose as a railway bridge. The arrangement is in use to this day.
As the structure was not originally intended to take both rail and road vehicles, the clearance is small and no car is allowed on the bridge during the passage of a train. As, however, trains are few in the West Highlands, this does not cause any undue inconvenience or congestion at the approaches. A single carriageway for cars runs along at the side of the metals, which are bounded on either side by elevated guard rails.
LATTICE GIRDERS, resting on massive piers of native stone, form the bridge which carries the Connel Ferry-Ballachulish line across the head of Loch Creran, Argyll. This bridge is typical of many of the smaller bridges in the Scottish Highlands.
At the North Connel end easy access from the main road is afforded by an inclined way. At the south, or Connel Ferry end a zigzag roadway on a steep incline had to be built. The Connel Ferry Bridge is one of the most remarkable structures in Europe, though as a Scottish bridge it has always had to play second fiddle to the great bridge spanning the Firth of Forth. With its two cantilevers rising to 125 feet, and its single span of 500 feet, the Connel Ferry Bridge contains about 2,600 tons of steel. Its construction gave employment to some 200 men simultaneously.
Provision had to be made for the accommodation of these men, as the resources of a small Highland village such as Connel Ferry were inadequate. Considerable labour camps grew up on either side of the loch, the camps being fully equipped to make the men as comfortable as possible.
The weather conditions in which the men worked were often wild and tempestuous in the extreme, sometimes forcing a complete stoppage of operations on the superstructure. In windy weather, and when frost made the girders slippery, men were particularly liable to fall off the half-finished trusses, and more frequently implements of various kinds would slip off. To save these, the Resident Engineer rigged up huge nets which were suspended from the undersides of the crane trolleys and progressed with them as they advanced towards the middle.
Early in August 1903 Colonel Yorke inspected the bridge on behalf of the Board of Trade. The test involved the use of nine locomotives and four loaded wagons, totalling about 1,000 tons in weight. These were driven across the bridge, backwards and forwards, first at full speed, then at varying speeds. Finally, they were halted, successively, on different parts of the structure and careful measurements were taken of any yield which followed. The bridge responded admirably to its tests, and on August 21, 1903, it was formally opened, with the railway it served, for all traffic. To-day its great grey girders still span the turbulent Falls of Lora in their strange grandeur, as gaunt and unchanging as the surrounding hills and islands themselves.
Wrought Iron Lattice Girders
Though the Connel Ferry Bridge and those on the Perth-Inverness and West Highland railway lines are the finest examples of their various types in the Scottish Highlands, there are many other notable bridges in various parts of the north-west. In 1868, when the Sutherland Railway was being built between Bonar Bridge and Golspie, its builders were faced with the necessity of spanning the Kyle of Sutherland. This is an arm of the North Sea, forming the estuaries of the River Shin and the River Oykell, which flow down from the mountains of Assynt and the great inland lake of Loch Shin.
It was not practicable to sink piers into the bed of the Kyle. The engineers therefore threw a single great lattice girder span across between Culrain, on the south side, and Invershin on the north. The length of the span is 230 feet, the lattice girders are of wrought iron and the floor of the bridge is 65 feet above the waters of the Kyle. This structure, and a similar one crossing the River Spey at Keith (a considerable distance down the river from Telford’s famous Craigellachie Bridge), had the longest spans of all the bridges on the old Highland Railway system.
Across the Caledonian Canal swing bridges have been necessary, as seagoing craft make the passage of the canal from end to end. Of these swing bridges the finest examples are the one at Inverness, carrying the main railway line northwards out of the city across the canal at Clachnaharry Docks, and the similar structure at the other end of the canal, taking the Mallaig extension line over it at Corpach.
Many of the Highland bridges of recent construction rank among the best examples of bridge engineering. Another attractive feature of the study of bridges in this region is that there are so many structures of historic interest. One of these is the Bridge of Balgownie, across the River Don near Aberdeen. It is a beautiful structure embodying a single Gothic arch with a span of 67 feet. It dates from 1281, being attributed to the energetic Bishop Cheyne.
Though Scotland’s - and, indeed, Great Britain’s - largest bridges are situated in the Lowlands, the Highland hills embody some of the richest and most interesting diversity in bridge building to be found anywhere in the country. No visitor, whether he travels by railway or by motor, can fail to notice and appreciate these bridges, as well as the skill of their designers.
ACROSS THE NAIRN VALLEY near Culloden Moor, the historic battlefield, is the beautiful stone viaduct built by Murdoch Paterson. Built of Old Red Sandstone, the viaduct contains twenty-eight arches of 50 feet and one great semicircular arch with a span of 100 feet across the river.