Page:Forth Bridge (1890).djvu/66

Rh bottom, allow a horizontal movement of the whole girder round the centre of the pins, while yet allowing no lateral movement of any kind except that which may be due to the mere hair's-breadth of play in the slide-block.

At the Queensferry and Fife ends, however, these blocks are absolute fixtures, steel packings having been placed into the two 12-in. spaces which are left open on the Inchgarvie ends; and while yet the same circular horizontal movement round the centre of the pins is possible, both longitudinal and lateral movements are prevented. Should it therefore happen that one cantilever has to sustain the impact of a heavy gust of wind while the other is not so affected, the deflection thereby produced can take with it one end of the central girder and leave the other in its original position without putting any side stress upon the girder itself.

It should be mentioned that the lower bearings at Queensferry and Fife in which the pins rest, are not bolted down to the cantilever ends, but have play sufficient to allow horizontal circular movement round the centre of the pins in the interlocking arrangement above described.

All expansion joints in the main structure have now been considered, with the exception of that in the cantilever end piers. At this point expansion and contraction to the amount already stated can take place longitudinally, but in no other direction. This is the only occasion where rollers are used for bearings, and the arrangement of these is clearly shown in Figs. 133 and 134. The lateral or vertical movements arising here are prevented by the following means: The steel canting A is a fixture to the under side [sic] of the extreme end of the cantilever, here in form of a box. This casting forms the head of the roller bearing, but also bears a side flange, which is set hard against a heavy cast-steel plate B, which is bolted down hard to the masonry, and carries a side flange similar to that of A. As the same thing is arranged on the opposite side, it is clear that lateral movement can only take place to the amount of play between A and B, which is practically nil. Again, on the top of the bottom end girder of the cantilever, outside the loaded box, a piece of steel C is laid, which touches, or nearly touches, another cast-steel plate D, let into the masonry of the pier, and bolted to it by holding-down bolts. Independent therefore of the counterpoise placed at this end, which prevents the cantilever lifting under any conditions of train load or fixed load, there is provision made to prevent its rising should an unusually heavy gust of storm-wind strike the end of the free cantilever, and thereby try to lift up this. This it cannot do, but the stress causes an upward deflection in the fixed cantilevers within the limits of the elasticity of the metal. Neither of these two arrangements interferes with the longitudinal expansion or contraction, which are facilitated by the arrangement of rollers between the castings E and F. All movements have now been considered in detail, and since they are of such vital importance, it will not be superfluous to recapitulate them here once more in toto.

1. Expansion and contraction produced by changes in temperature provided for by the sliding bedplates, by the rocking posts at one end of each central girder, and by roller bearings in the two cantilever end piers.

2. Lateral deflections, whether due to wind pressure, or to the influence of the sun's rays, provided for

(a) By the play left in the bedplates of the fixed cantilevers. (b) By the arrangement in the keyplates between lower and upper bedplates, which permit a slightly circular movement on the centre of each column, producing torsion in the same.

(c) By the central vertical pins in the interlocking arrangement between ends of cantilevers and ends of central girders.

All vertical deflections are taken up and resisted by the elasticity of the metal in the structure. As soon as the end posts of the cantilevers had been built, the necessary preparations were made to start with the erection of the central girders. The principle upon which this work was to proceed was the same as in the cantilevers, by overhanging, but in the absence of a fixed joint between the two, other arrangements had to be made. Botli ends of the central girders required to be temporarily attached to the cantilevers until they were brought together in the centre of the span, and could there be joined up. The ends could then be released.

In the first instance, heavy temporary platform girders had to be attached to the under side [sic] of the bottom members in cantilevers projecting over towards the central girders, some 25 ft. (See Fig. 135.) Upon these were laid cross-timbers, and a strong platform was formed upon which the first half-bay of the central girder was erected. This had to be done, however, outside, and about 6 ft. away from the end posts, because all rivets inside the end posts, and through those portions of the central girder which would have to be drawn within the end posts, required rivetting up. All these holes were countersunk, and the rivets made flush. The cups which had to receive the rocking posts were meanwhile placed in position, and carefully set and levelled, and next the rocking posts, weighing nearly nine tons each, were hoisted up by special tackle and shipped into their places, and lowered into the cups or sockets. Into the tops of the rocking posts were now placed the half -balls or knuckles with flat tops, upon which the under side of the top members of the central girders would come to rest. Thus far the Inchgarvie ends. The Queensferry and Fife ends were similarly dealt with, except that no rocking posts, but only the horizontal pins, had to be placed. The shipping in and fixing of slide-blocks and vertical pins had also to be done at the same time. As soon as the half-bays of the girder on each side were built, the whole portion, weighing about 42 tons, was shifted back into its place inside the end post, and connected up and also drawn tight by a number of chains and wire ropes passed round. Heavy temporary ties were now attached at the centre of bay 6 in the top members of the cantilevers at one end, and at the centre of bay 1 in the top members of the central girders. (See Fig. 130.) These ties consisted of three layers of plate, each $3/4$ in. in thickness, by 26 in. in depth. There was one such treble plate-tie to each side of a top member, or four altogether. The ends were fixed to large gusset-plates and rivetted up, but the middle joint was bolted up by special turned steel bolts, 1 in. in diameter. There were about seventy-four bolts to each joint.

To the ends of the bottom members, both in cantilevers and central girders, thick cast-steel plates were fitted, leaving a wedge-shaped space between them, the wide end downwards, and this space was carefully templated, and steel wedges, about 9 in. wide, two to each side, were fitted in and set up hard. (See Figs. 128, 130, and 131.) Everything was now ready for continuing the building out of the girders, and half-bay by half-bay was speedily added, some delay being caused on account of all the joints in top and bottom booms having to be rivetted before much weight was added forward. The top member cranes, which had by degrees made their way downhill, doing all the work since the fifth bay had been completed, as the cantilevers had by then narrowed so much that there was no room to bring the twin cranes any further, were now relieved of their heavy platforms underneath the top member, and of other temporary appliances they carried. Thus made lighter, they were moved on the temporary plate-ties, and thence on the top booms of the central girders, where they now began to climb up the curved booms towards the centre of the span. (See Plate XIX.) These cranes now built the whole of the work, lifting all the material out of the barges in mid-channel, except the heavy floor girders, which weighed nearly four tons each, and were hoisted by special tackle with wire ropes and steam winches set up near the ends of the cantilevers.

It was not long before the Jubilee cranes, which had originally started from the tops of the Inchgarvie tower, and had proceeded, the one towards the south and the other towards the north, met face to face, the two others, which had come down from the tops of the Queensferry and Fife towers respectively, and with the putting in of the last lengths of booms, their functions practically came to an end. (See Plate IV.)

Much, however, remained yet to be done, for the connections had to be made, and the making of these depended largely upon the weather, or at any rate upon the temperature. As it was not convenient to arrange the joints exactly in the centre of the girder, where they would have coincided with the junctions of the last pair of struts and ties, the joint was laid in each instance some 6 ft. nearer towards Fife and Queensferry. The four bays of the Inchgarvie halves were, therefore, completed, and half a tie and half a strut belonging to each of the other two halves were left incomplete. (See Fig. 136.)

In setting out from the ends of the cantilevers the wedges had been set in such manner as to give the bottom booms an inclination upwards, which, if no deflection had occurred, would have given the girder a camber of 12 in. The deflection, however, arising from the increasing weight overhanging, reduced this to exactly the camber prescribed—namely $3 1/2$ in.

The lengths of the bottom booms of the girder were fixed so as to leave at the temperature of 60 deg. still a gap of 4 in. clear between the ends. This was necessary, on account of the possibility of the temperature rising considerably above 60 deg., which in the middle of September, when the south central girder had been expected to be connected, was quite possible. For the north central girder, which was to have been connected a month later, the same gap was left for a temperature of 50 deg. only.

In the top booms a somewhat larger space was left, tapering in shape from about 10 in. at top down to 6 in. at bottom of the webplates; for here wedges had to be inserted.

As it happened, however, delays occurred, and the south girder was not connected till October, and the north girder in November.

The connection of these central girders was an operation of a most interesting character; for here a greater use was made of natural forces than in any other portion of this bridge.

As it was essential to have the temporary plate ties, which still held up the weight of the two halves of the central girders, fully under control, a small brick furnace, heated by oil and compressed air, in the same manner as the rivet-heating furnaces, was built round each of the four plate ties at each end, just above the end posts, and everything made ready for instant lighting of the same. Arrangements were also made to draw together by hydraulic pressure the two bottom booms at the gaps in the joints in centre of girders, in order to be able to give every chance to the work being done expeditiously.

The temporary connections between the booms, which, however, allowed full play to expansion and contraction, were as follows: (See Figs. 137, 138 and 139.)

To the bottom flanges of the bottom booms, large and thick plates had been bolted on each side of the gap, extending over the whole width of the bottom flanges, and projecting to some 7 in. on each side of them, and upon these plates other plates, which reached across the gap. They were fixed on the Queensferry and Fife sides, but could move in and out at the Inchgarvie ends. For the temporary connection there large slot-holes were provided, into which bolts $1 7/8$ in. in diameter were fitted, but were not put in until the time of making good the connection, the slot-holes being 4 in. long, while the holes in the plates, which were parts of the booms were simply round, of $1 7/8$ in. diameter. Once these bolts were driven in, the gap between the booms could not be increased, but in case of increase of temperature the slots would allow the gap to decrease and prevent buckling in the booms. The arrangement is clearly shown in the illustration.

The temperature had been watched for several days, and the changes in the length of the boom ends noted, and after everything was ready for making the connection, the temperature yet failed for several days to rise to within 6 deg. to 8 deg. of the 60 deg. required, and the large bolt-holes were barely half open. The application of hydraulic pressure only gave about $3/8$ in., equal to 3 deg. of temperature, and there was nothing for it but to wait patiently. At last, on the afternoon of October 10, with the sun shining brightly from the south-west, the temperature being 55 deg. generally, the west boom came together near enough to give a full bolt-hole; but the east boom wanted a quarter of an inch, in spite of all the pull that could be got out of the hydraulic rams; so a quantity of waste soaked in naphtha was put into the bottom booms for about 60 ft. to either side of the gap and set on fire, and in a few moments the boom had expanded to the full amount required. All the bolts, twenty-three in number, were at once put into the holes and screwed down handtight. The web-covers across the gap to each side of the booms, and the corner angles, were put on, and one side, which had been kept blind, was drilled through at once, and the covers bolted up.