the vertical ties, and by the same agency along the vertical columns, and while lifting these in stages of about 16ft., the two struts 1 in cantilever and the vertical ties were built up. The use of lifting-platforms was abandoned after this; all the work of erection was done far more quickly and efficiently by the cranes specially designed for this work.
Meanwhile, the top members had been completed, and a 3-ton hand-crane was set up on the top of each vertical column. (See Fig. 121.)
The viaduct girders in the central towers had also been put together by now, and a new platform was thus secured about half-way up the erection. The viaduct girders were now built out by overhanging into the cantilevers, and a length of top member as well, these two last being built by the 3-ton cranes on top, and by winch and tackle wherever such could be used. All the material for the viaduct level was hoisted to it, and for the top members, it was lifted right to the top.
The viaduct girders were strong enough to carry themselves, overhanging for a length of 100 ft., and even then to carry at the forward end the weight of a 3-ton crane and its load, but, as a matter of safety, wire ropes were carried from the outer ends up to the top junctions on vertical columns.
The top members also had to carry their own weight unsupported for a length of nearly 100 ft.
The lifting platform in the first half of bay 1 was only carried up to about 20 ft. above the level of the viaduct, and there it remained. (See Plates XV. and XVII.)
The first inclined ties in cantilevers were now brought down from the top junctions, being held up and in position by means of timber struts between them and the vertical columns, and laterally apart by lattice girders from one to the other. (See Fig. 122.)
The first struts had also been built up to some 20 ft. above the platform, and when the ties had been brought down to the point of intersection with the struts, preparations were made to make good their junctions. Previous to this the positions of struts, ties and vertical ties, were carefully checked, both as regards their meeting in the true centre as well as being the right distance apart from the centre-line of the bridge. Large gusset-plates, joining all three members together, were then placed in position and the necessary holes drilled in situ. This junction was then at once rivetted up, and thus a new fixed point secured from which to proceed further out. (Plate XV.)
In the meantime, the top member, or Jubilee Crane, as it was popularly called (because it was invented, or at any rate designed, about the time of the Queen's Jubilee in 1887), had been erected on the top of top member.
This crane consisted of a square frame supported on two girders reaching over all four booms of the top members from side to side. The girders were of different heights, in order to get the platform on which the crane was placed level, and the girders were placed on slides, so as readily to allow them to move down the incline of about 1 in 4 when required. The crane itself had a horizontal jib with a reach of 34 ft., and could slew round, by means of a circular rack and pinion, to about 220 deg., or three-fifths of a full circle. It was worked by a pair of reversible steam engines, and had a large barrel capable of winding up about 400 ft. of wire rope. It carried its boiler with it, and was thus quite self-contained. Suspended from the two main girders of the frame was a platform, carried some 4 ft. to 5 ft. below bottom booms of the top member, 64 ft. long about 36 ft. wide, mainly supported on four light lattice girders.
_Fig._119,_Page_51.png)
Fig. 119. Elevation of internal viaduct.
_Fig._120,_Page_51.png)
Fig. 120. Section of internal viaduct.
The planking of this platform was so arranged that it could readily be taken up in every place where required, the four lattice girders being so placed as to pass all struts and ties and vertical supports in succession, the flooring only requiring to be taken up. The sides of the platform under the top members projected to within about 6 ft. of the end of the jib. In building the top members, the booms of which were in lengths of about 24 ft., the crane could lift them from the level below and place them at once in position, the platform allowing safe ground for the men to guide them into place and bolt up the joints and cover-plates. The vertical bracings followed next, and then the top booms, and all other necessary work, and when both sides of the top member were built and well bolted up, the crane was pushed forward 24 ft. on to the newly built section.
The crane with platform and all necessary gear weighed about 64 tons, which, placed at a distance of about 80 ft. from the centre of the top junctions, put a tremendous strain upon the two top members, which were then still unsupported, and though they bore their load well it is evident that the building of this first half of bay 1, was a work of great anxiety, lest a heavy gale should cause some serious distortions.
As the top members were never intended to carry any load except their own weight, the vertical side bracings were not sufficiently strong to carry the crane without a risk of bending the bars. These bracings were consequently not only doubled by reverse bars running the full length with them, but the reverse bars were of a section double and treble that of the ordinary bracing bars. Templates for these bars were taken as soon as each section of the top member, was built up, and they remained in their places until the weight of the crane had passed beyond the next main support. Generally speaking this crane built all the members above the level of the viaduct, and of course below that level also if necessary. Its maximum lift was 3 tons, and with rare exceptions no portions of the work were made heavier than about 50 cwt. at the outside.
As soon as the crossings between struts 1 and ties 1 had been made secure—a vertical support—a box lattice girder, was raised from, that point square upwards in continuation of the vertical tie below, in order to give the top member the necessary support. As this support formed no part of the finished structure it was made of iron only, but of fully sufficient strength.
It required, however, both longitudinal and lateral support, and this was provided by carrying lattice girders at a point about two-thirds of the height between the crossings and the top members on each side, from the ties 1 to the vertical supports, and transversely from one support to the other. Cross ties of wire rope were also carried up between the supports. As soon as the supports had reached up to within a foot or two of the top members, a further length was built to the latter, which reached right across the supports. By this time and with so much weight on them the top members had deflected about 9 in. to 10 in., and hydraulic rams were now arranged to lift them up from the vertical supports, and give them their proper position, and place them as much higher as would allow for the probable compression in the vertical supports.
Meanwhile the lower portion of the ties 1 had been built from the point of intersection downwards, the bottom member had been brought forward, and the junction of bottom members with struts and ties at the end of the first bay, built in. But the bottom members also had deflected, owing to their own weight and to that of the rivetting machines—the cages and the bottom junctions. It was, therefore, necessary to lift the bottom members up at this point, and to do this the following plan was adopted (See Figs. 123 and 124):
Four heavy angles were attached near the ends of ties 1 to far as they had been brought down; these angles were of such length as to project some 6 ft. to 8 ft. below the under side of the bottom member, two heavy box girders were now passed under each of the bottom members, the lower one being fixed to the ends df the four angle-bars, while the upper girder was brought up to hardwood packings, which gave it a full bearing against the under side of the bottom members. Between the two girders, hydraulic rams about 10 in. in diameter were placed. The action of the hydraulic rams in being forced out was, therefore, to push the upper girders hard up to the bottom members and the lower girders downwards, thereby putting a corresponding tensile stress on the ties 1.
In the vertical ties 1, in the centre of bay 1, a joint in each of the foui booms had been left open
