The New Zealand Railways Magazine, Volume 2, Issue 5 (September 1, 1927)
The renewal and strengthening of existing structures and bridges without interruption to traffic, proves an interesting part of railway working and design. The following lines have been written to show how Waiteti Viaduct on the North Island Main Trunk Line was strengthened.
Waiteti Viaduct is situated 127 miles south of Auckland in between two stations Te Kuiti and Puketutu, two miles south of the former and seven miles north of the latter. The railway rises from the former to within a mile of the latter on a grade of 1 in 70 with curves from 8 to 20 chains radius throughout. Traffic over this section is very heavy, approximately 26 trains passing each day and these are spread over the twenty-four hours.
The viaduct (424 feet in length) was built in 1888, and is of wrought iron consisting of four spans of 106 feet, lattice deck trusses of two per span placed on two abutments, and three wrought iron piers. The height from the rail to the bed of the gully is 120 feet-twenty feet higher than any building in New Zealand. It is satisfactory to note that the wrought iron piers and the eight wrought iron trusses are still in excellent condition, although practically 40 years old; in consequence, all material was used again in the strengthening process.
Owing to the increase in the traffic on this line, with subsequent increase in weights of engines and speeds, it became necessary just before 1914 to strengthen the viaduct. A scheme was arrived at for halving spans 1 and 4 by intermediate concrete piers, and halving spans 2 and 3 by steel rocking piers. The concrete piers were erected up about 60 feet and there stopped. Shortly after the steel contract was let the war came on and of course put an end to the work for the time being. Thus the position remained until prices resumed a reasonable level. The plans (in 1926) were altered somewhat, and this year it was decided to proceed with the job, which was carried out in the following manner.
The 106 ft. spans of Nos. 1 and 4 were halved into 53ft. spans, utilising the concrete piers built in 1914. The four 106ft. trusses thus relieved were shifted to spans 2 and 3, making an increase of 100 per cent. strength without any new material.
The four spans of 53 ft. plate girders arrived in parts at Te Kuiti early this year; and a start was made by Mr. Drummond, Foreman of Works, Ohakune District, with a special gang stationed permanently at the site. An Ingersoll-Rand air plant machine was used to rivet up the girders complete, 3 1/3 miles from the Viaduct (work which it did economically) each span of steelwork when completed, weighing 19 1/2 tons. (The idea of rivetting up complete beforehand is to save as much work as possible in mid-air and to avoid trains, -both being items of importance which affect the costs of the job). The end bracing at the abutments was partly removed page 10 and the concrete brought up in between the trusses (which are at 9 feet centres) thus allowing a complete span of girders, as shown later, to be dropped between the trusses. The concrete piers were also brought up between the existing trusses ready for the plate girders.
The method of placing this 19 1/2 tons of steelwork is shown briefly as follows:-
Two overhead timber gantries, each suspending two Morris pulley blocks of 5 tons capacity and designed to sit on the 106ft. lattice trusses, were built on the shore. By means of these blocks two men can easily lift, or lower, 5 tons; the 19 1/2 tons was, therefore, an easy lift for the eight men concerned with it. At the desired moment (daylight on Sunday morning), the 7 1/2 ton crane placed the two gantries out on the bridge; the wagon with the span was run up with each end under a gantry, the men then lifting up the span off the wagons, which were run off the bridge out of the way. The rails, sleepers, crossbeams, etc., were then removed, and the new span lowered into position between the old two trusses. The sleepers and rails were immediately replaced upon the new span (the crane lifting off the gantries)and traffic was resumed. This cycle of operations was repeated for the four spans of 53 feet each; thus half the bridge was completed.
The problem then was to shift the trusses away to their new positions. Each single lattice truss weighed 13 1/2 tons, so had to be handled carefully. During the week two trusses (one on each side of the track) were pushed out sideways till each was six feet four and a half inches from the centre line of the track, and was securely held there. On the Sunday morning, trucks with an overhanging beam at right angles to the rails, were brought up beside the trusses; the two inch diameter steel bolt through each end of the beam down to the truss was then made fast, and four men on the end of a large spanner tightened the nut till the truss lifted off its support. By this means the four bolts were tightened up and so the twenty-seven tons was hanging in mid-air and evenly balanced. A long rope made fast at the shore leading out to the truck and back to the crane, gave a very steady pull and so the loads moved slowly to their new position. When at their destination, they were temporarily secured by chains, the nuts were slacked back, and the trucks, etc., taken away. This completed the Sunday's work.
The position now was that the span contained four trusses instead of two, each one not being the proper distance from its neighbour. To hold the track up and rearrange the four trusses, then brace the structure again without any false work or support from the ground, was a good day's work for the Sunday and had to be carefully watched.
The procedure is perhaps rather difficult to write out free from technicalities, but briefly, four long bolts, fifteen feet by two inches in diameter and threaded right along with eight nuts to each bolt, were used. These were threaded (two at each end, one at the top, and one at the bottom) through the four trusses. Each truss was clipped between two nuts. In this way each truss could be moved sideways by means of the nuts and spanners, but was rigidly and securely held at any period. The weight of the track was transferred from one pair of trusses to the other and back by means of wedges, as occassion offered.page 11
The weather was not all that could be desired for such work, and one Sunday a storm broke over at the latter part of the day. Working with lamps in heavy rain, thunder, and lightening, was most unpleasant (especially high up) and the men deserve credit for the energy and eagerness they displayed throughout the job.
The Viaduct is now 100 per cent. stronger than before, so shortly there will be seen heavier and more powerful engines working this hilly and difficult section of the line.