Construction of the Victoria Falls Bridge - An Engineer's Aspect


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Tuesday, January 21, 2014

Construction of the Victoria Falls Bridge

The Victoria Falls Bridge is a 152-meter span, two-hinged arch bridge that crosses the Zambezi River just below the Victoria Falls. It has a deck level 122 m above the Zambezi River and is built over the Second Gorge of the falls. The bridge links Zimbabwe and Zambia, Africa.

I created this slideshow about the construction of the Victoria Falls Bridge using the following article, "The Great Zambesi Bridge," as reference. Construction was completed in 1905 and the bridge is still in use today.

"Build the bridge across the Zambezi where the trains as they pass will catch the spray from the Falls."        
            -Cecil Rhodes, former governor of Rhodesia (today the countries of Zambia and Zimbabwe), as quoted in The Great Zambezi Bridge by Chief Engineer G.A. Hobson (1922)

I hope you enjoy this article from Engineering Wonders of the World as much as I did.

Engineering Wonders of the World, Volume 1, Edited by Archibald Williams, 1909, Pages 90-101.

The Great Zambesi Bridge

The erection of this great steel arch across the gorge of the Zambesi, just
below the Victoria Falls, supplied a much-needed link in the Cape to Cairo
Railway project. Besides being one of the loftiest bridges in the world,
the Victoria Bridge is situated in a spot of unique beauty, and for that
reason attracts the tourist as well as the engineer.

DURING his first expedition down the Zambesi in 1855 Livingstone struck the mighty cataract which the natives called Mosioatunga--"smoke sounds there"--a name suggested by the roar of this greatest of the world's waterfalls, and by the columns of fine smoke-like spray which rise ever from the abyss, and, on attaining a height of from 200 to 300 feet above the upper water-level, condense into a perpetual shower of fine rain. In honour of his sovereign the explorer dubbed his discovery the Victoria Falls.

Immediately above the Falls the Zambesi is a broad-flowing stream, more than a mile wide, with well-wooded islands. Suddenly the waters encounter a gigantic fissure of supposed volcanic origin in the black basalt, and thunder vertically downwards through a distance of nearly 400 feet. From this chasm the

water escapes through a narrow opening into a deep cañon, which zigzags southwards in a most extraordinary manner, as seen in our illustrations on pages 92 and 93. The upper surface of the cañon is almost on a level with the upper bed of the Zambesi close to the Falls.

The Falls and the Bridge.

Every traveller in South Africa nowadays includes in his programme, if he possibly can, a visit to these wonderful Falls, which relegate even Niagara to a second place. They are 1,641 miles from Cape Town by rail, and the journey, despite the conveniences of the train de luxe that runs twice a week, is long and tedious. Yet the reward is sufficient to repay weariness and expense. The tourist gazes spellbound on this grand freak of nature; and when his eye is sated with the splendours of the waterfall, he finds fresh food for admiration in the remarkable arch bridge which has been thrown across the chasm below the Falls. This bridge is vested with a romance of its own--first, by its proximity to the Falls; second, by the fact that it is one of the loftiest, if not actu-
ally the loftiest, in the world; and third, because it is the most notable feature of, and the link most difficult to forge in, a notable scheme--the Cape to Cairo Railway. We may add that from the purely engineering aspect it is of the first interest as regards its design and its erection in a locality so remote from a base of supplies.


Alternative Sites for the Bridge.

During the great Boer War the engineers of the Rhodesian section of the Cape to Cairo Railway pushed northwards manfully, heedless of disasters in their rear. Railhead was already within a few hundred miles of the great Zambesi when the relief of Kimberley enabled Mr. Cecil Rhodes to give his consideration to the question of carrying the rails across the river. The choice lay between a long, many-spanned bridge some few miles above the Falls, near Livingstone Drift, and a much shorter arch bridge flung boldly across the cañon below the Falls. Mr. Rhodes desired that travellers on the railway should have on their passage a good chance of seeing and visiting the cataract; and as financial considerations pulled the same way, decision was given in favour of the arch bridge, though voices were heard exclaiming that the intrusion of a giant structure of steel would ruin the natural beauty of the spot.

The site finally selected was surveyed in the years 1900-1901. It is situated in the first arm of the cañon, about 700 yards below the cataract, and inside the zone in which spray falls for several months of the year, so that the wish of Mr. Rhodes that the trains should catch the spray as they passed has been fulfilled, though the author of the wish unhappily did not live to see its fulfilment.


The Site Chosen.

At the point chosen the chasm has a top width of about 750 feet, and narrows downwards to 400 feet at water-level. On the north bank the cliff is practically sheer, while the southern face has a shelf about half-way up, and is generally more easy of access.

The design of the bridge was entrusted to Mr. G. A. Hobson, a member of the firm of Sir Douglas Fox and Partners--a firm that has been prominently connected with engineering enterprise in Rhodesia and other parts of South Africa.

A Description of the Bridge.

The plans finally chosen were for a two-hinged spandrel-braced arch bridge. the words "two-hinged" imply that allowances for movements due to alterations in temperature and load are concentrated on the points of support at the bottom of the end posts. Spandrel-bracing makes the horizontal top chord, over which the track is laid, the partner of the curved arch in the matter of bearing the strains and stresses to which an arch bridge is subjected, and also affords certain advantages in the erection of the members.


It should be mentioned that the bridge has, in addition to the main arch, two short end spans of 62 1/2 and 87 1/2 feet respectively, supported by the banks and the end posts of the arch. The arch itself is 500 feet long between the centres of the end posts, which are 105 feet high. The rise of the crown is 90 feet, so
that at the centre the bridge is 15 feet deep. The vertical girders have an upward taper, or "batter," of 1 foot in 8; and the booms of the arc approach one another towards the centre of the bridge, so that the distance between them diminishes from 53 feet 9 inches at the main bearings to 27 1/2 feet at the crown. This gives sufficient room for a roadway, 30 feet broad between the parapets, designed to carry two tracks of rails.


The total weight of the bridge is about 1,500 tons. With the full "live load" added, and the stresses due to the horizontal thrust of the arch, to temperature, and to wind pressure, each of the four bearings on which the arch rests is called upon to sustain a maximum thrust of some 1,600 tons. The combination of ease of movement at these four points with power to resist great pressure was one of the chief problems confronting the designer, and it received a masterly solution.

The Bearings and Skewbacks.

Beginning at the bottom, we find at each point of support a solid foundation of concrete, reinforced with steel bars. To this is affixed by four huge bolts, 3 inches in diameter, a massive base plate, carrying an equally massive pedestal built in six sections of thick steel plates. At the top of the pedestal is a steel forging, in which from end to end is cut a semicircular channel 1 foot in diameter. In the groove rests a huge steel hinge pin--also 1 foot in diameter--5 feet 10 inches long, pierced by a central bolt hole. Pressing on the top of the pin is a second channelled forging, named the "saddle," supporting the skewback, a beam of immense strength, in which meet an end post, the main boom of the arch, and two members of the vertical and lateral bracings.

This arrangement allows the saddle of the skewback to move circumferentially on the steelwork above. to prevent the pins shifting endways, each has an annular projection in the middle, engaging with corresponding recesses in the channels of the pedestal and saddle. Furthermore, at each end is a circular plate kept tight up against the pin by a bolt passing through the centre of the pin.

Contract let for the Steelwork.

The contracts for the construction of the steelwork and for the erection of it at the site were let to the Cleveland Bridge Company of Darlington in May 1903. Exactness to one thirty-second of an inch was specified for some of the members; and to facilitate the assembling of the parts provision was made for pinning them together as erected, prior to riveting. Before the steelwork left the builders' yard it was assembled in sections, so that any inaccuracy might be detected and remedied.

Delivery of the parts at the Falls was delayed by unforeseen difficulties in railroad construction, owing to which railhead did not reach the Zambesi until May 1904. The first material delivered was that for a cableway to be erected across the gorge near the line of

the bridge, to transport to the eastern bank one half of the steelwork, and also rails, sleepers, and plant for the immediate extension of the railway northwards towards Broken Hill.

Erecting a Cableway across the Gorge.

A rocket was shot across, carrying the end of a cord, which served to pull a wire over the gorge. This in turn helped a small steel rope across, to bear a temporary conveyor. Mr. C. Beresford Fox, nephew of Sir Douglas Fox, was the first person to make the apparently perilous passage of the chasm, 400 and more feet above the boiling torrent.

The temporary conveyor transported the materials for the eastern tower of the permanent cableway, for which was provided a rope capable of withstanding a 275-ton strain. As soon as the tower had been erected, one end of the rope was drawn across, passed over the tower, and firmly anchored; the other end being attached to a counter-weighted sheer-legs on the western bank, designed to keep the tension on the cable uniform for all positions of the travelling conveyor.

The Conveyor.

The conveyor itself weighed 5 tons, and was self-moving, picking up current for its motors from a copper trolley wire slung close to it. Its driver, who also operated the hoisting mechanism, was accommodated on a railed platform at one end. Critics asserted that nobody would be found willing to drive the carriage to and fro over the abyss; but this fear was entirely unjustified, and as a matter of fact the aerial journeys became very popular with the employees.

A load of 10 tons could be taken across the gorge by this contrivance, which proved invaluable both to the bridge builders and the railroad constructors. During use the 870-foot steel rope stretched eight inches, but did not show any serious signs of wear until it had carried loads totaling something like 100,000 tons, inclusive of the travelling carriage.

Foundations for the Bridge.

The first item of bridge construction was the placing of the foundations for the four main bearings in excavations previously made by the railway company. Excavating in the north bank was dangerous work, as the face of the cliff was there almost perpendicular, and one of the staff had a narrow escape at this place, being saved from a fatal fall by the branches of a

friendly tree. On the south bank operations were easier, but more protracted, as the solid rock had a thick coating of debris which must be removed.

About 110 feet below rail-level, on the ledges prepared, the masons laid a thick bed of concrete for the pedestals and bearings, reinforced top and bottom by iron rails. This was allowed to set for several weeks before any weight was placed upon it.

Meanwhile began the construction of the bridge proper. As scaffolding or other direct support from below was out of the question, it was necessary to build the main span out from both banks, on the bracket or cantilever principle, until the two parts of the arch should meet and become self-sustaining.

The engineers gave their attention first to the two shore spans, resting at their land ends on abutments built into the rock. the main girders of the trusses were partly supported on temporary timber baulks and trestles, sufficiently strong to bear the additional weight of the jib cranes, which, when the shore spans were completed, lowered the materials for the skewbacks and end posts of the arch. As soon as these were up the shore spans were lowered on to them, and the temporary supports removed.

The next thing was to provide an anchorage at each end to sustain the main span during its cantilever stage. The plan adopted--a novel one--was as follows:--

Anchoring the Cantilevers.

Some distance back from the edge of the cliff two shafts were sunk to a depth of 30 feet in line with
THE EARLY STAGES OF A CANTILEVER. (Photo, Cleveland Bridge Company.)

the top of the end posts of the arch. At the bottom they were connected by a short tunnel. A number of wire ropes, specially provided for the purpose, were then attached at one end to an end post, carried down one shaft, through the tunnel, up the other shaft, and affixed to the other end post. Each rope had separate attachments and adjusting apparatus, so that it might be made to bear exactly its fair proportion of the total strain. This gave the cantilever a large amount of Mother Earth--or rather rock--to pull on; but to make safety doubly sure 400 tons of rails were piled on the ground between the two shafts.

When once the anchorages were in, the work proceeded rapidly. The cranes, running forwards as the cantilevers grew, lowered the parts of the steelwork to the assemblers, who quickly pinned them at the junctions. At their heels came the riveters with their forges and mechanical closing tools.

THE ARCH RIB COMPLETED. Observe the great safety net stretched under the work. (Photo, Cleveland Bridge Company.)

The Safety Net and Nervousness.

To give confidence to the workmen, a huge net was slung under the points where building was in progress. Fortunately it had to catch nothing heavier than bolts and tools, and eventually it was removed, as the men complained that, instead of making them feel more secure, the sight of it caused nervousness. It may be remarked here that the experienced bridge-builder never gets dizzy, and foolhardiness is a greater danger than nervousness. Without hesitation he will walk across a beam only a few inches wide, even when a high wind blows gustily and his foothold is made precarious by ice and snow. The new hand soon gets accustomed to positions the perilousness of which really depends on his nerve. Thus when a "skyscraper" is in its earlier stages he may feel great reluctance to cross a broad plank, but by the time he has helped to build it to a height of 500 feet above the street he experiences no qualms whatever.

TRIAL ERECTION OF THE CENTRE PANEL OF THE ARCH. (Photo, The Cleveland Bridge Company.)

The Cantilevers joined.

Progress became more rapid as the cantilevers advanced, and the amount of steelwork in each panel--that is, section of bridge between two upright posts--diminished. The last eight panels at the centre of the arch (out of twenty-six in all) were put together in twenty-six days, and on April 1, 1905--less than six months from the start--the great 3-foot square booms of the arc were joined. The rapidity of the work bears witness to the efficiency of the workmen and the designer, and to the precision with which the parts of the steelwork had been made.


In order to give the top chord its proper share of the final strain, a slight gap was left in it until the arch was complete. Hydraulic jacks forced the ends apart to create the required strain, while packing-pieces were inserted and riveted up. Then the jacks were removed, and the anchorage ropes slackened off, allowing the main span to ride free on its four bearings.

The decking of the roadway and the laying of the rails call for no special remark. It is interesting to note, however, that the steelwork received liberal coatings of a gray paint of such a tint that a patch of red dust would show up against it conspicuously by contrast. This colour has the further advantage of harmonizing with the landscape.

Painting the Bridge.

The painting was done by native workmen, who, as Mr. Hobson points out, were ready to
GENERAL SIDE ELEVATION OF THE BRIDGE. By courtesy of Mr. G. A. Hobson, M.Inst.C.E., designer of the bridge.

follow the white man whithersoever he would give them a lead. Until the advent of the railway the natives kept clear of the Falls, of which they had a superstitious dread, and for some time afterwards they would not approach them without first flinging up into the air a handful of grass to propitiate the demons of the gorge. But when they found that no harm resulted from a closer association, and that good wages could be earned, they came in their hundreds--some from the remote districts of Central Africa--and proved very valuable workmen. More conservative was an old Barotse chief, who watched the building of the bridge with the greatest interest, but predicted that so slender a construction could not bear the weight of a man. Even when trains began to pass over it he maintained that not its own strength but the finger of God held it up.

The stiffness of the bridge was tested by sending over it a 612-ton train. At the crown the downward deflection was less than an inch with the train moving at 15 miles an hour, and only half an inch with the train at rest.

For several months in the year the bridge is wet perpetually with the spray of the Falls, and for this reason it was of the utmost importance that the designer should make provision
for enabling the painters to get at every part of the steelwork easily, and avoid water-holding and unventilated areas. Rust is one of the deadliest enemies of those who have to deal with steel construction, and if not carefully combated may nullify the finest work.

THE OPENING OF THE BRIDGE. The bridge was formally opened to traffic by Professor Darwin on September 12, 1905.

Now that the bridge is up the critics have been silenced. So far from detracting from the picturesqueness of the scenery, the great parabolic arch serves rather as a standard measured by which the immensity of the chasm comes home to the spectator, who, while admiring the natural features of the great gorge, wonders at the scientific skill that has made a secure path for the locomotive as far above the boiling waters as the cross of St. Paul's is above the busy pavement.


NOTE.--Some of the illustrations to this article
were kindly supplied by the British
South Africa Company.