An invaluable link in the communications of Southern Rhodesia is the Birchenough Bridge, across the River Sabi. This imposing steel structure in the heart of Africa is the third longest single-arch span in the world
A SPAN OF 1,080 FEET carries the road across the River Sabi, near the towns of Chipingi and Umtali, Southern Rhodesia. The Birchenough Bridge shortens the journey from Chipingi to Bulawayo by 150 miles, and has made it possible to travel by road from Capetown to the Congo in all seasons. Before the bridge was built a detour of 600 miles was necessary.
AFRICA can always be relied upon to produce the unexpected, be it lions in the long grass, or a mile-wide river that suddenly drops down a precipice. But in Rider Haggard’s country, in the valley of the River Sabi, in Southern Rhodesia, there is a man-made marvel which even that great and imaginative novelist could not have foreseen. Rising high above plain and forest, spanning the mighty Sabi River, there stands a giant steel arch lifting to tropical skies - a rainbow immobilized, slender and beautiful, but strong with the skill of man, serving his various purposes.
The Birchenough Bridge spans the Sabi River near the towns of Chipingi and Umtali. The bridge is about five miles south of the junction of the Sabi with the Odzi River, and provides direct communication by main road between Victoria and the Melsetter district to the east of the two rivers.
Before the building of the bridge the only outlet of the district of Melsetter, during most of the year, to the towns of the west was along the mountain road to Umtali, where the Beira Railway joins the Rhodesian line to Salisbury.
Because it was impossible to cross the River Sabi, the journey from Chipingi to Bulawayo, the commercial centre of Southern Rhodesia, formerly necessitated a detour by road and railway of 600 miles. The bridge has shortened that journey by 150 miles, and has provided access to the markets of Salisbury and Bulawayo for cattle and produce from the Melsetter district. In addition, it is now possible to travel by road, in all seasons, from Capetown to the Congo.
A road vehicle in that part of the world is generally a long-distance lorry or coach, carried on six wheels and driven by a diesel engine. Such are the vehicles that use the Birchenough Bridge. The great passenger coaches take visitors from South Africa to wonderful mountain scenery round the town of Melsetter, which stands at 5,000 feet above sea level.
Those, briefly, are the reasons for the building of the Birchenough Bridge, far out in the jungle and over 100 miles from the nearest large town. The bridge takes its name from the late Sir Henry Birchenough, Bart, GCMGr, as chairman of the Beit Railway Trustees, who provided the necessary funds.
It was designed by Mr. Ralph Freeman, MInstCE, MAmSocCE, who assisted in designing the bridge over the. Zambezi at the Victoria Falls (see pages 411-416) and who designed the bridge over Sydney Harbour. Similar methods were adopted in building all three bridges, and there are many interesting associations that link these wonderful feats of engineering. The weight of steel in the Birchenough Bridge is exactly the same (1,500 tons) as that used in the bridge at Victoria Falls. But the arch span at Victoria Falls is only 500 feet as against more than twice that distance in the Birchenough Bridge. That difference in span, for the same weight of steel, represents thirty years of progress in metallurgy.
The structure is the third longest single-arch bridge in the world, second only to the bridge over Sydney Harbour and to the Bayonne Bridge, New York. The total length of the Birchenough Bridge is 1,240 feet and the great arch that rises 280 feet above the river measures 1,080 feet between the foundation bearings. The depth of the arch crown is 37 ft 6 in and the end posts are 46 feet high. The width between the arch trusses is 45 feet, and the top and bottom chords are provided with lateral diagonal bracing to resist wind pressure. The arch trusses comprise twenty-seven panels, braced N-fashion, so that the upper and lower chords are diagonally braced.
The main cross girders of the deck, placed 40 feet apart, are suspended at either end from the arch by steel cables of 2¾-in. diameter. These cables were used as anchorages during the building of the arch and previously in the building of Sydney Harbour Bridge, the hangers of which comprise steel girders and not cables. Thus the great steel ropes that travelled more than half-way round the world and back have found a permanent resting place in the heart of Africa.
COMMUNICATIONS IN SOUTHERN RHODESIA have been greatly improved by the spanning of the River Sabi with the Birchenough Bridge. It provides easy access by road to the markets of Bulawayo and Salisbury for the cattle and produce from Chipingi and the Melsetter district near the border of Mozambique, or Portuguese East Africa.
The roadway is 18 feet wide and is formed by 5 in of reinforced concrete, surfaced with a layer of “Bitumastic” material. The footways on either side of the roadway are of Rhodesian teak, 2 in thick, and the timber was treated with arsenic, to discourage the activities of white ants. African white ants are capable of devouring in time a complete wooden footway, leaving behind merely a steel skeleton. Beneath the roadway and running the whole length of the deck is a huge horizontal truss, 48 feet wide, which maintains the suspended portion of the structure rigid against the pressure of the wind.
During four months of the year the shade temperature at the bridge site rises, on occasions, to 112 and even 115 degrees, and special attention had to be paid to expansion allowances in the steelwork. Accordingly, at the west end of the roadway an expansion joint was inserted, permitting a movement of 6 in, equivalent to a range in temperature of 100 degrees Fahrenheit.
In addition to this lengthening of the roadway, the great arch rises in the daytime and sinks again during the hours of darkness. The increase in height of the arch is one inch for every ten degrees that the temperature rises. The height of the roadway, however, is little affected because the suspension cables expand and contract correspondingly, and so the movements of arch and cables partly cancel out. The height of the roadway above the river bed is 60 feet.
At either end of the bridge are steel approach spans that link the deck roadway with the concrete abutments. The eastern span is 60 feet long; on the other side of the river there are two spans, each of 50 feet, resting at the centre on a pier of concrete which is 30 feet high.
Flanking the roadway at either end of the bridge are rectangular pylon towers of reinforced concrete, rising 16 feet above road level. The towers are not solid but walled, and are provided with doors and windows so that they can be used as storerooms or for other purposes. They are flanked by concrete walls 30 feet long and 12 feet high.
The pylons carry bronze memorial plaques, 8 feet high and 5 feet wide, which face the bridge approaches. The right-hand plaques on the east and the west abutments show a portrait in bronze of the late Sir Henry Birchenough, Bart, GCMG, and those on the left of either approach read as follows: “This bridge was erected by the Beit Railway Trustees out of funds bequeathed by the late Mr. Alfred Beit and at the request of the people of Rhodesia was named The Birchenough Bridge in recognition of the services rendered to the country by Sir Henry Birchenough, A.D. 1935. Ralph Freeman - Engineer”.
The arch is supported on four steel pins with a diameter of 14 in, which rest in bearings carried by massive concrete skewbacks set in the solid rock. The thrust of the enormous arch amounts to 1,200 tons on each bearing - equivalent to a pressure of 25 tons per square foot on the concrete foundations. The use of the remarkable “Chromador” steel, with a strength 50 per cent greater than the mild steel ordinarily used in bridge construction, has resulted in a structure of tremendous carrying capacity despite its size and slender appearance. This special steel, containing percentages of chromium, manganese and copper, has great rust-resisting properties - a matter of considerable importance.
The design of the bridge provides for two 15-tons and two 10-tons lorries in any position on the roadway with an additional load of 100 lb per square foot on the whole of the remaining area of road.
The steel used in the Birchenough Bridge is about 20 per cent stronger than that used in the building of the bridge over Sydney Harbour. The design of the bridge was influenced by the new steel available to the builders, because the additional strength, weight for weight, permitted of a single-arch span at a moderate cost. During their discussions of other possible designs the Beit Railway Trustees found that a single-arch bridge, using the special steel, was less costly than a bridge of several spans on piers.
That estimate was based on the assumption of reasonably satisfactory foundations for the masonry and concrete work.
Examination of the river bed, however, revealed the fact that it was treacherous and that the shifting soil would have entailed enormous difficulty in the building of piers. On all accounts the choice of the single-arch span for this magnificent structure seems to have been the best possible solution in view of all the circumstances.
Although the methods of construction adopted for the building of the Birchenough Bridge were similar to those at Victoria Falls and Sydney Harbour, there were some interesting deviations in detail, particularly in connexion with the transport of materials and the use of anchorages during the raising of the arch. Because the bridge across the River Sabi was destined for road traffic it seems fitting that roads should have provided the means of transport, although sea and rail also played their part in supplying the needs of the builders.
Having completed the Sydney Harbour Bridge, the builders, Dorman Long and Company, of Middlesbrough, shipped back to England the anchorage cables used in erecting that structure. Later these cables were again put on board ship to make yet another long sea voyage, this time to Africa for use in building the Birchenough Bridge.
Lorries Through Africa
Work began in June 1934, and the carefully prepared plans of action were put into operation as and when occasion arose. At the eastern end of the bridge an assembly yard was laid out, to which men, materials and machinery came from Umtali after the journey by rail from the port of Beira on the Indian Ocean. The steelwork was rolled and fabricated in the Dorman Long works at Middlesbrough and shipped to Beira. The journey from Umtali to the site, through some eighty miles of bush, was accomplished by heavy lorries drawing specially designed trailers. Some of the heavy steel sections weighed 10 tons and were 45 feet long, but the road service was maintained throughout without mishap or delay. After the arrival of the engineers and their equipment, the first job was excavation for the foundations. On either side of the river large areas of rock were laid bare to receive the concrete foundations of the bridge bearings. Other excavations were made for the foundations of the pylons and the pier for the double span at the east end of the bridge.
MASSIVE CONCRETE SKEWBACK set in the solid rock for the steel pins on which the arch of the Birchenough Bridge is supported. The thrust of the arch amounts to 1,200 tons on each bearing, equivalent to a pressure of 25 tons per square foot on the concrete foundations.
During this part of the work, precautions were pushed forward for securing the anchorage cables. Well behind each of the four skewbacks, tunnels were driven into the solid rock. The tunnels were about 5 feet square and 50 feet deep. They were driven at an angle of 45 degrees pointing upwards towards the river, and they were not cross-connected at the lower ends. The tunnels were thus separate cavities, and deep in them were concreted great steel beams to hold the anchorage cables.
The next problem was to provide a rapid means of transport for plant and materials across the river. It was decided to erect a cableway, suspended from two steel towers, across the river, The length of this cable way was about 1,400 feet and it was operated, as were also the cranes used on the bridge, by electricity. Power was supplied by two portable generating sets driven by high-speed diesel engines. When the bearings had been accurately fixed and the end posts were in position, the work of erecting the arch by adding panel to panel was begun. The panel sections were placed in position by “creeper” cranes, on each half-arch, running on the top chord of the structure. These cranes weighed 50 tons each and were capable of lifting 5 tons at a radius of 50 feet. The steelwork, after preparation in the assembly yard, was picked up by a steam-driven locomotive crane and transferred to the cableway. The carrier on the cableway then carried the steel sections to within reach of the creeper cranes on either side of the river. As panel was added to panel and the half-arches grew out from the river banks the creeper cranes were hauled up the steep slope of the upper chord.
The first seven panels of each truss (with their cranes) were built with an anchorage of seven cables a truss, fourteen cables on either side of the river. The cables were attached to the anchorage girders and led direct to pins at the ends of the upper chords. At this stage, with the creeper cranes surmounting the seventh panels, the anchorages had reached the limits of endurance for which they had been designed. Accordingly six additional cables were attached to each truss, twelve on either side of the river, at points immediately behind the cranes. The original anchorage cables were then removed and used as part of the second system. The cables of the final anchorage were not taken direct from the upper chord to the rock tunnels, but were led over the tops of steel towers, 80 feet high, which were inclined away from the arch. The arrangement may be compared with a fiddle string, stretched tightly over the instrument’s bridge.
THE STEEL ARCH of the Birchenough Bridge is braced laterally to resist wind pressure. The depth of the arch crown is 37 ft 6 in and the width between the arch trusses is 45 feet-The arch trusses form twenty-seven N-shaped panels.
These great masts served to place the line of the cables’ pull in the most advantageous direction, but they had also another important function. With the completion of the last panels on either side of the arch-centre it was necessary to lower the half-arches to close a prearranged gap of 2 ft 6 in. The lowering was accomplished by hydraulic gear at the tops of the masts, which were in effect slightly shortened to impart a certain amount of slack to the cables.
Almost Perfect Alinement
On June 17, 1935, the bridge builders prepared to link the upper chords of the half-arches, poised in mid-air with their 50-tons cranes overtopping the mighty structure. Imperceptibly the hydraulic jacks paid out the anchorage cables and the 30-in gap diminished until there came the order to halt. The steel edges of the chords were found to be only ⅜-in, out of alinement. The thickness of a pencil represented the total error in 1,000 feet, in 1,500 tons of steelwork. The setting out of the main bearings, the calculations of the arch deflections and distortions, the fabrication of the girders in far-off England, the building of the mighty arch under a broiling sun had been accomplished to perfection.
Little more than the turn of a spanner now sufficed to put the finishing touch to that lofty chord, converting the structure into a three-hinged arch independent of the anchorages. The cables that had served as anchorages were then removed and cut into appropriate lengths for their final use as suspension cables for the bridge deck. The next operation was the forcing apart of the lower chord ends by hydraulic jacks. This was done to impart the necessary stress in the lower chord before final completion of the structure as a two-hinged arch.
The importance of this operation calls for some further explanation as to its necessity. An analogy that has previously been used in describing the properties of a two-hinged arch is that of a steel sword. Assume that the hilt is attached to one hinge and that the point is embedded in the other hinge with the blade curving upwards across the stream.
Then temperature variation will cause the blade to rise or fall slightly as it becomes longer under the effect of heat or shorter as the metal cools. It must be borne in mind that the blade is not hinged anywhere except at the ends.
Had the bridge been left in the form of a three-hinged arch, the result of expansion would have been an unsymmetrical lifting of the arch crown, and the whole of the strain would have been thrown on the upper chord containing the central hinge. This strain came in to operation immediately the anchorage cables were released. The ends of the lower half chords were forced apart to a carefully calculated extent, and it was then possible to insert steel packing pieces and finally unite the two arch members. It will be appreciated that as the arch rises under the heat of the sun the cables from which the deck is suspended lengthen correspond-ingly and the roadway is maintained at a virtually constant level.
Now began the retreat of the two creeper cranes from their meeting place on the crown of the arch. As the cranes moved step by step down opposite sides of the upper chords they manoeuvred into position the great transverse deck beams, which were duly secured to the suspension cables hanging from the lower chords of the arch. On reaching their starting point over the arch end posts, the creeper cranes were dismantled. Apart from rivets in fabricated girders, some 90,000 rivets were driven at the site by pneumatic riveters supplied with compressed air from petrol-driven compressors.
FROM EITHER SIDE OF THE RIVER the arch of the Birchenough Bridge was built out, panel by panel. Creeper cranes on the completed panels handled the steelwork for the next panel. As the half-arches grew out over the river the creeper cranes were hauled up the slope of the upper chord.
The staff comprised some thirty Europeans, and half that number were skilled steel erectors and crane drivers from England. Nearly 600 natives were employed as labourers on the work. Despite the climatic conditions at the bridge site, not a single case of fever or Serious illness occurred during the whole period of the work.
The building of the Birchenough Bridge was carried out in the remarkably short space of twenty months. Excavation for the foundations was begun in April 1934, and by the following November all preparations had been completed for the erection of the steelwork.
Completed in Twenty Months
The halves of the arch were joined on June 17, 1935, and by the end of September of the same year the concrete roadway had been almost completed. Then there remained only certain constructional work of an auxiliary nature, including the painting. On November 10,1935, the bridge was handed over by the contractors ready for use.
It has been noted that the Birchenough Bridge is primarily intended to expedite communications across the Sabi River for the improvement of trade and commerce. There is another aspect, however, of the utility of this wonderful structure, and that is in connexion with the rapidly increasing tourist traffic. It is perhaps somewhat-difficult to visualize large diesel-engined motor coaches running on regular routes between feathery palms and grotesque baobab trees. Such routes include the road to Umtali, eighty miles to the north, and to Fort Victoria, over a hundred miles to the west. These coaches, which roll swiftly across the Birchenough Bridge, are divided into three compartments. The first class, for Europeans, is in front; mails and merchandise are carried in the middle compartment; native passengers are accommodated in a compartment at the back of the coach.
The cost of the bridge was approximately £150,000, and the building of the road approaches was undertaken by the Rhodesian Government. The Birchenough Bridge was opened on December 20, 1935, by Sir Herbert Stanley, GCMG, then Governor of Southern Rhodesia.
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