The New Zealand Railways Magazine, Volume 5, Issue 3 (July 1, 1930)
Reconstructing Railway Bridges — A Review of the Works Now Proceeding
To the Maintenance Branch of the railways is entrusted the design, construction and maintenance of the railway track and structures, no small part of the operations of the Branch being directed to the upkeep of our fifty-three miles of bridges. In the following article some interesting particulars are given of bridge building reconstruction work now in progress in the North Island.
The Conditions of Safety
Our railway bridges owe their safety largely to their being kept in good condition; also, they were costly to build, and are even more costly to replace, so that this portion of our equipment requires the greatest care in inspection and maintenance, as is well known to those engineers, foremen, inspectors, and bridge-men, to whom the safekeeping of bridges is entrusted. On the New Zealand Railways a high standard of bridge maintenance has always been observed, and its economy is now amply demonstrated in the excellent condition of many of our old bridges which still have a long remaining useful life, whereas with less careful maintenance they might easily have been reduced, before this, through corrosion, to an unserviceable condition.
Careful maintenance, however, cannot do more than check the loss of strength with age. While often giving the impression of permanency, bridge structures, like railway rolling stock, or any other industrial plant, are all the time depreciating and becoming obsolete. Their timbers begin to decay, and steel and ironwork becomes corroded, thus losing strength. Obsolescence will often be a powerful though indirect factor in determining the life of a bridge. The utility of a bridge becomes restricted by its load-carrying capacity, and when traffic conditions call for increasing axle loadings, a stage is reached where economy demands an increase of strength in the bridge, and this may necessitate complete reconstruction.
Modern Structures of Steel and Concrete.
In the Railways Statement, 1929, the requirements and policy with regard to bridge strengthening and reconstruction has been outlined, and in accordance with this policy several important bridge reconstruction works have been put in hand during the past financial year.
With one exception (Whenuakura bridge) all the reconstructions now in hand are on the Auckland-Wellington Main Trunk line, so that the policy has been followed as far as possible of concentrating first on the elimination of weaknesses on the most important sections. The object of this is to obtain, as soon as possible, savings in operating expenses in return for the expenditure involved in reconstruction. (The removal of restrictions on the running of some of the heavier types of engines will have this effect.)
The Ngaruawahia bridge crosses the Waikato River at Ngaruawahia, near Frankton. The new bridge will have three 120ft. main spans crossing the river, and three short approach spans. The superstructure will be of steel plate girders in the short approach spans and steel trusses for the main spans, which will be of the “through” type, to afford sufficient headroom for steamer traffic on the river. The main shore piers are of mass concrete construction on piled foundations, and the high piers in the stream will be constructed by sinking pairs of 10ft. diameter reinforced concrete cylinders to a depth of about 40ft. below water level.
As the new bridge is being constructed on a site alongside the existing bridge, a deviation of the line is required at each end to form the approaches to the new bridge. These approaches have been constructed by railway workers.
A contract for the construction of the bridge has been let to Mr. B. V. Rope, and good progress is being made with the foundation work, all of which is practically completed, with the exception of one of the cylinder piers in the stream. Cylinder sinking at this pier is now proceeding.
The steelwork for both the Ngaruawahia and Whenuakura bridges, weighing about 460 tons, is being fabricated at the works of Messrs. A. and G. Price, at Thames. The steelwork materials are being shipped from Great Britain to Auckland, and carried by rail to the Thames shops. After fabrication the steelwork will be sent out in parts of suitable size for erection.
The Whenuakura bridge crosses the Whenuakura River between Rangikura and Patea, on the Marton-New Plymouth section.
Massive Concrete Piers.
Another view of the new Whenuakura bridge shewing the foundation work nearing completion.
The new bridge will have a main steel truss span of 142ft. and three 60ft. steel plate girder spans. The main piers will be constructed by sinking 10ft. diameter reinforced concrete cylinders and driving piles inside the cylinders. The shore pier at the south end is to be of mass concrete also on a piled foundation, while the abutments are a combination of mass concrete and reinforced concrete construction, piling being dispensed with, and the foundation load distributed over a large area by means of a reinforced concrete raft.
The formation of the approaches to the new bridge has been completed, and the contractors for the construction of the bridge, Messrs. Bird and Codling, are making good progress with the foundation work, being now ready to proceed with the erection of the superstructure.
Oroua River, Otaki River, and Waikanae River Bridges.
These bridges are all on the Auckland-Wellington Main Trunk line between Wellington and Marton. The type of bridge adopted in the reconstruction is similar in all three cases, viz., 60ft. deck plate girder spans on mass concrete piers supported on reinforced concrete piles.
The Oroua bridge will have ten spans, the Otaki nineteen, and the Waikanae three. The construction of the steel plate girders (approximately 700 tons of steelwork) is now being carried out at the Hutt Railway Workshops, the steel plates and shapes being imported from Great Britain.
The Waikanae bridge is being reconstructed on the site of the present bridge, and it was, therefore, impracticable to consider letting a contract for the work, which has to be carried out under traffic conditions. Railway gangs have now been at work for some weeks on the construction of the new foundations.
The Otaki and Oroua bridges are each being reconstructed a short distance upstream from the existing bridge. In order to obtain easy curves on the approaches, so that maximum speeds can be run on the deviations, the new centre lines are slightly divergent from the lines of the present bridges.
The construction of the embankments on the approaches to both these bridges has been completed. At Otaki a steam shovel and work train were employed, and the work was carried cut very expeditiously. For the purpose of facilitating the transport of materials and carthwork, and to enable the girders to be run out on trucks from the shops into position alongside the new piers, and thence lifted by page 38 cranes direct from the trucks into their final positions, temporary tracks, with light bridging over the main channels, were laid across the wide riverbeds. This arrangement will make for fast and cheap construction.
The concrete foundation work is in the hands of contractors. Rapid progress cannot be made with this work, however, because of the hard driving encountered in putting down the reinforced concrete piles in the shingle riverbeds. The contractors for the Otaki bridge foundations are Messrs. Christiani and Nielsen, and for Oroua, Mr. C. Wesley.
In the South Island, during the past year, the strengthening of the Teremakau River and Crooked River bridges, on the Westland section, was completed, and two weak bridges (Nos. 10 and 13) on the Addington-Waiau line were also strengthened up.
From the foregoing it will be seen that quite a comprehensive programme of bridge-work has been carried on during the last twelve months.