The Victoria Falls Bridge, one of the finest bridges in South Africa, was erected during 1904-1905. It spans the Zambesi Gorge about 700 yards below the Victoria Falls, within sight of the Falls. It consists of a 500ft spandrel braced arch, with short approach spans of the deck truss type - 87 ft 6in on the right bank and 62ft 6 in on the left bank.
It was designed to carry two lines of railway, and was made strong enough to sustain, besides dead load, the following:-
(1) A train on each line consisting of two engines weighing 1,75 tons per foot.
(2) Temperatures stresses caused by a variance of 60 degrees F. above or below the mean.
(3) Wind stresses due to a wind pressure of 30lbs per square foot on the train and bridge, or 45lbs. per square foot on the bridge alone.
During the past 25 years the weights of engines and rolling stock have steadily increased, so that the present loads of trains are far higher than were dreamed of when the bridge was built. It was therefore decided to strengthen the bridge to enable it to carry trains with 20-ton axle loads, thus leaving a considerable margin for future increases in the weights of engines and rolling stock.
At the time of the building of the original bridge, road traffic was negligible, and it was therefore considered not necessary to provide a bridge to carry road vehicles. This has now been changed by the tremendous progress made in the development of motor vehicles. In designing, therefore, the alterations now being made to the bridge, provision was made for the inclusion of a motor road, and this road over the Falls Bridge will form an important link in the Great North Road.
To obtain a road wide enough to take two streams of traffic and to allow pedestrians ample footway free from danger, it has been necessary to increase the width of the deck by about 13 feet, the added width being obtained by an overhang on either side of the bridge.
The old deck consisted of cross girders 12ft 6in apart, which had the disadvantage of placing a load midway between the panel points of the bridge, thus introducing a bending load on the top chord of the arch. The new deck will do away with this, the cross girders being spaced on the main span 25 feet apart and coinciding with each panel point. This needs a much deeper type of girder for the rail bearers, and the new cross girder is therefore raising the rail level 4ft 7in.
Before the erection of the new steelwork could be commenced, the track over the bridge had to be raised on temporary structures to the new level and also moved to a new alignment at as near the centre of the bridge as possible. The new design of the bridge deck permits one line of railway track only.
The contract for the supply and erection of the new steelwork was obtained in competition by the C B and E Company Ltd, of D, Eng, who were the contractors for the building of the original bridge. The contractors have undertaken to erect a 25ft bay of deck each week, being allowed 14 hours one day a week during which they may hold up traffic. Actually traffic is delayed from 8 to 10 hours on these days.
On the 16th July the material required for the initial bay arrived at site from Beira. On the 23rd July work on the erection of the deck was commenced, and since that date one bay per week has been erected. Occasionally an extra bay has been put in, the Transportation Department having been able to give a few extra periods of 9 hours to the contractors, who are anxious to get as much of the erection completed before the commencement of the rains.
The contractor's plant consists of a Smith's 7-ton locomotive steam crane, and a large steam boiler which supplies steam for the running of a steam-driven air compressor. This compressed air is used for working the pneumatic riveting hammers and a drill. Steam is also provided to run an electric generator, which supplies current for working two electric drills and, in case of need, supplies electric light. They also have an oxy-actylene plant for burning through certain parts of the existing deck to facilitate removal. As a stand-by they have a portable air compressor driven by an internal combustion engine.
Previous to the day appointed for the erection of a 25ft bay the different sections of new deck are assembled on the bank and riveted together before being placed on the bridge. There are eight sections to each 25ft. bay, and these sections are as follows:-
(1) A cross girder composed of a plate girder, with each end tapered to allow for the fixing of two bracket-shaped trusses, one of which carries the road and the other the footway. In addition a square frame is fixed to the road bracket carrying the road curb, parapet and 2ft 6in footway, and to this is fixed the upright of the parapet. There is a bracket for the downstream curb of the roadway fixed to the cross girder direct, a bracket to carry the upstream curb of the footway, and a further bracket for the footway parapet. These are all riveted in position before erection.
(2) The railway floor section, composed of two plate girders, as stringers, 3ft. 6in. deep, spaced 5ft 6in. apart, and held by two sets of cross bracings built up of 3in x 3in angles. On the stringers 5in deep steel troughing is riveted, the troughs running at right angles to the track. Ballast plates are riveted to the ends of the troughs by means of angle cleats. Angle cleats are riveted to the troughs in pairs. This serves to fasten down the hardwood running timbers, a bolt being passed through one cleat, then through the running timbers and through the second cleat of the pair.
(3) The roadway floor section, composed of three rolled steel joists, 16in c 6in x 50 lbs., spaced 3ft 10 1/2in. apart. One these, 3in deep steel troughing is riveted at right angles to carry the road. Curb girders are riveted to the ends of the troughs which, when erected, rest on the brackets mentioned above and form the bearing for the ends of the roadway troughs.
(4) The footway floor section and downstream parapet - This is made up of a parapet girder composed of angles and flats, with a channel forming the footway curb at the bottom. Similar troughing to that used on the roadway is riveted between the channel and a curb girder, the latter being riveted to the other end of the troughs. The channel and curb girder, when erected, rest on the brackets fixed to the cross girder, and they support the footway troughing.
(5) The upstream parapet girder, similar to the downstream.
(6), (7) and (8). Chequer plates.
The actual work of putting in a new bay is carried out in the following order:- As soon as the contractors secure possession of the bridge, the track is broken and a length of rails removed. The crane lifts out the temporary timber and cribbing put in to raise the track, and loads it direct into a truck. This truck is removed and a second truck replaces it. The crane then proceeds to removed the old deck in sections and to load these sections on to the second truck. The expeditious removal of the old deck is made possible by the previous substitution of rivets by bolts. The old parapet has to be burned through each time by means of oxy-acetylene flame owning to its joints being inconveniently placed. The two old cross girders are then taken out and loaded on to the truck, after which the truck is removed to the rear.
The crane then bridge up the cross girder for the new bay and places it on its new bearing plates. The upstream end of the girder is fastened by means of a bolt and the downstream end swung round towards about a foot off its bearing plate to enable the railway floor section to be got in position. The cross girder is secured in this position by means of a raking strut fastened back on to a part of the old deck.
The railway floor section is then brought up by the crane and lowered into position so that the near end of the stringers can be bolted up to the cross girder erected with the previous bay. The crane continues to hold up the railway floor section while the new cross girder is pulled on to its downstream bearing and the far end of the stringer secured to it. The track is then laid on this section.
A special 'making up' span, 15ft. long, is used to connect the new work with the old. This is taken out for the erection of each new bay and replaced each time, one end is bolted up to the new cross girder and the other rests on the old deck. When this has been placed the track can be re-connected.
Before reopening the bridge to traffic, the other sections mentioned above are placed in position. The crane brings up the roadway floor section and drops it into position on its four brackets, where it is bolted fast. The footway section follows in a similar manner. The upstream parapet is then fastened between its two uprights which, as previously described, were riveted on to the end of the cross girders. The three sections of chequer plates are then put down. Once forms a 2ft 6in footway on the upstream edge of the roadway; the next, which is also 2ft 6in footway on the upstream edge of the roadway; the next, which is also 2ft 6in wide, covers the space between the roadway and the railway floor section and will later carry the handrailing between the road and the railway. The third section of chequer plate covers the space between the railway floor section and the footway. This section is 4ft 3 1/2in wide and, apart from carrying the handrailing between the railway and the footway, it will form a gangway for railway gangers.
The trough floor of the roadway is to be filled in with concrete reinforced with expanded metal, over which will be laid a road surface of asphalt macadam 2in thick. The footway troughs will be filled with concrete and finished off with one inch of granolithic. The parapets will be completed with panels of galvanised wire mesh.
A special footway leading from the new deck to the lower level of the old deck has been provided for convenience of pedestrians during the work of re-construction. After the erection of each new bay, it is moved forward and its handrail connected with the new parapet on one side and with the old parapet on the other.
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