The Forth Bridge/Building out of the Cantilevers

by Wilhelm Westhofen

Building out of the Cantilevers.

Shortly before the lifting platform had arrived at the point of intersection of the diagonal struts or columns in the central towers, a commencement was made with the building out of the bottom members in the first bay of cantilevers. This was done as yet by steam derrick cranes, but their reach was limited, and other means had to be adopted to forward the work. A cage about 19 ft. square, and consisting of a number of sections each 8 ft. in length, was placed upon the bottom member. (See Figs. 105 and 106, and Plate IX.) It was so constructed that each section could be taken away at the back and placed in front, and this operation was performed by a hydraulic crane placed on the top of the cage itself. This crane has a jib of fixed length, which it could not alter, but it could slide up and down the top of this cage by means of hydraulic rams on each side. It could also slew completely round. The forward part of the cage was used by the platers for building up the tube, the beams and plates and other parts being first brought within reach of the hydraulic crane and then swung round into place. The nearer portion of the cage contained the same rivetting machine which had already done service in finishing the horizontal portion of the bottom members between columns. For a length of about 64 ft. the bottom member was thus rivetted up in order to stiffen it as much as possible, and enable it to carry itself and the necessary plant at the point of it, unsupported for a time until temporary support could be found for it. (See Fig. 105.) As soon as the cage had got beyond the point of junction between the bottom member and the first vertical tie, that is, at centre of bay 1 (see Fig. 121), gussets were attached to the bottom members close to this point and to the vertical columns immediately above the horizontal bracing between the columns, and a chain of Hammersmith links were fixed up between these two points. On these links a temporary staging was suspended, by means of which a heavy plate tie—one on each side of the tube—could be inserted. For these ties also strong gussets were attached to both bottom members and vertical columns. To the gussets on the columns other gussets were attached, and to these were bolted horizontal plate ties, suspended to and immediately below the horizontal bracing girders at the intersection of the diagonal struts. A set of plate ties were also placed at the opposite cantilever, and balance thus established. These plate ties, both horizontal and inclined, were about 2 ft. 6 in. deep and of two thicknesses of ³⁄₄ in. plate, being, in fact, portions of the main webs of the top members in bay 2. Previous to the attachment of these ties to the gussets, the bottom members were lifted up to the extent of several inches by means of hydraulic rams, in order that they might be able to support their own and any further weight which might be put upon them during erection of the first bay. Most of these appliances are shown clearly in Plates, IX., XV., and XVII.

The lower portion of the first vertical tie was then built, and a lifting girder laid across from one bottom member to the other, and a platform built on each side similar to those for the erection of the central towers, only so much smaller and lighter. These platforms were lifted by hydraulic rams in 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. Elevation of internal viaduct.
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 in such manner that this tie could be shortened to a certain extent, its upper end at the crossing being now an absolute fixture. By means of an arrangement of cross-girders and hydraulic rams this joint could now be closed to any desired extent, new cover-plates being, of course, required to make good the joint.

All being thus prepared, a theodolite was placed in the centre of each bottom member, and these were lifted, both at the vertical ties and at the end of the inclined ties 1, by means of the hydraulic rams, until the members had risen to the correct angle and a trifle beyond, when hardwood packings were put between the girders, and wedges driven in, so as to secure the maintenance of their position. Thus placed, the holes in the new cover-plates making up the joints in the booms of the vertical ties were drilled at once, and the joints rivetted up.

The closing lengths of the booms of the inclined ties 1 were also measured, and templets made of them and taken to the shops. These lengths were then cut and drilled, and at once put into their places, and the whole of the ties finished up in every part and detail.

The effect of these operations then was: 1. To secure the absolute correctness of the position of the first bottom junctions as regards the height above water, and their distance from the centre line of bridge.

2. To put upon both the inclined ties 1 and the vertical ties that stress which would correspond with their share of weight of the structure put up, and to give them the elongation proportionate to that weight.

With this there were now, outside the central towers, two points established, the position of which was correct, and which were capable of sustaining the full weight which they would have ultimately to sustain in the completed structure.

The next operation was to raise the top members from the temporary vertical supports carried up from the points of crossing, and this also was done by hydraulic rams. The temporary supports raised from the points of intersection and carried to the top members were used at the centre of every bay out, becoming in each case much shorter and lighter. They had to be removed, owing to want of material for making a larger number, frequently before the top members were rivetted up sufficiently to bear their own weight and that of the staging which was attached to them, and considerable deflection in some of the longer sections was the result.

Fig. 121. Erection of Cantilevers.

The top member crane could now slide forward another section and build, not only the next section of the top member, but also the upper halves of struts 1. These did not take long. They were built from the bottom upward, in the same way as the inclined ties were built from the top downward. For the struts light square stages fenced on four sides were used, these being 15 ft. square and open in the centre to admit the strut. The stages were accessible by means of wooden ladders laid from bottom upwards or by rope ladders hung from the top. The bars and plates were picked off the viaduct by the top member crane, swung into position, and at once bolted on, while the stage itself, when it required raising, was attached to the crane for the moment and lifted up, when it was attached again by chains or wire ropes to the strut itself.

The tie staging was hung to stout rope tackle and let down by the men working on the ties themselves.

As soon as the struts had been built up to a point close under the junctions with top members the latter had their position checked by theodolites, and, if required, they were upon these points also lifted by hydraulic pressure rams and then drifted up, and when in correct position were at once rivetted. Another fixed point was thus secured, and the first bay in cantilevers practically completed.

The plate girder, reaching transversely from one vertical tie to the other, and upon which are supported the girders of the internal viaduct, had meanwhile been put in place, as also the diagonal bracing below it. The viaduct was then advanced over it and carried forward till it reached the first trestle at end of bay 1, which reaches across the first bottom junctions and gives support to the internal viaduct at this point.

So far completed, all members were at their correct elevation so far as this could be secured, but it was possible, and happened frequently, that, owing to continued strong winds (no permanent wind-bracing was as yet fixed) from one quarter, a certain amount of displacement occurred. Thus, although the bottom members might at the end of the first junctions be the proper distance apart from each other, yet the centre of an imaginary line between them might not coincide with the centre line of the bridge, but fall to one side or the other. Precisely the same thing might happen in the case of the inclined struts, and the top members thus lie to one side or the other of the true centre line.

Nor was the wind the only factor that could produce such a result, for the fact whether the sun was shining on the east side or the west side of the bridge made a material difference. On the sun appearing the plates on that side of the tubular members on which its rays fell would expand, while on the other sides the plates remained as they were. This produced, therefore, a bend away from the sun, to an extent not inconsiderable, but, of course, only temporary in its action; for, with the sun turning to south and passing to west, the members not only became straightened again, but became curved in the opposite direction.

Errors in measurements and faults in construction might, of course, also produce similar and permanent results, and it was therefore necessary to take steps to fix the position so far as it could be done.

The diagonal wind-bracings between bottom members, of which the first pair starts from the skewbacks, now required to be dealt with.

Fig. 122. Erection Of Cantilevers.

As neither bottom members nor inclined struts had ever been laid together in the relative position, the wind-bracings could not be built completely, but the last or closing lengths outwards were always left to be measured and templeted in situ. The gussets by which they were attached to the bottom members were already fixed and rivetted to the latter, and it remained, therefore, only to erect the girders in their places and take the templets for the closing ends. While they were being erected they were hung by wire ropes to the internal viaduct. Previous to this, however, the exact position of the bottom members had to be ascertained, and, if not correct, wire rope ties attached between bottom members and girders by means of union screws or timber struts, worked with wedges or small hydraulic rams, had to be employed to draw the bottom members in or push them out, according to requirements. This done, the templets of the booms were taken and the girders completed. The same process had to be repeated at every point of attachment, that is, every half-bay out. The position of the inclined struts with regard to the centre line of the bridge was corrected in a precisely similar way, and by this means was achieved the successful building out of these arms, nearly 700 ft. in length and weighing some 5400 tons, with an error of only 2 in. For an error there is, and, curiously, it exists in nearly all the six cantilevers, and in the same direction, namely, to east. Whether this is due to the prevailing westerly winds, and the fact that the total pressure from the west upon the structure during a twelvemonth must be probably fifty times as great as the total from the east, or whether the fact that the lateral deflections due to the sun's rays are always so much greater from the west than from the east has something to do with it, the writer will not take upon himself to say, but appearances decidedly point in that direction.

With the corrections in the positions of the struts and bottom members, the first bays of the cantilevers were now completed, and, except in so far as the elasticity of the steel came into play, all the points were as fixed as if each was resting on a solid masonry pier.

The duties of the survey department in connection with the erection of the cantilevers were of the heaviest kind. The work had to be carried on in the most exposed positions and in all weathers. To Mr. W. N. Bakewell belongs the credit of a great achievement, and it is not too much to say that to his courage and decision and promptitude in fixing points, is due the saving of much time and much expenditure.

From the first bottom junctions struts 2 were started upwards, and from the first top junctions ties 2 were started downwards, in repetition of the operations already gone through in the first bay. The bottom members were built out and the internal viaduct brought forward. Upon the latter were now erected—upon a staging sliding on the rail troughs—two steam cranes with movable derricks standing side by side, each being worked by a steam winch placed some distance behind. Wire ropes were exclusively used by these cranes, which, owing to their position, were called the twins. As the Jubilee crane, resting on the top members, built all the work from near the viaduct level upwards, so the twin cranes built everything from that level downwards.

Up to this time most of the material of which the members were built was lifted by the hoists in the central towers, and brought forward on temporary lines of rails to within reach of either top member cranes or cranes on the viaduct. From this point forward only the light material, such as bars, angles, and other things, were sent up the hoists, while all the heavy booms and plates were placed on board one of the steam barges, and hoisted up by one of the twin cranes to the viaduct level, and thence were taken by the Jubilee crane to the top. Somewhat later, again, when the top members had still nearer approached to the water, the material for these, and for the upper portions of struts, and ties, was lifted by the Jubilee crane out of the barge, and swung into place at once, thereby saving much time and labour.

In the erection of the bottom members in bay 2 the temporary attachments necessary to hold them up in position were carried partly to the point of intersection of struts 1 and ties 1, partly to the first top junction, and there attached to heads of struts 1.

Hammersmith Bridge links were used for the first and wire ropes for the second, no plate-ties being required. Later on still wire ropes were capable of dealing with most of the lifting required, union screws being used in the place of hydraulic cylinders.

The erection of bays 3, 4, 5, and 6 was simply routine—work repetitions of the work gone through—and so much more easy for the reason that not only had the distances, both vertically and horizontally, between the members become so much less great, but also because the men had become so skilful and so accustomed to their tasks, that what appeared at one time to be insurmountable difficulties and hazardous undertakings, had now become mere child's play, and was done in those exposed positions as easily as if the men were standing upon the floor of an ordinary workshop.

The rivetting of the work followed the erection so closely, that many squads of rivetters were working upon the extreme ends side by side with the erectors. In the case of the tension members all the main joints were rivetted as soon as possible after erection, and where such could not be done, all joints were bolted up with specially prepared turned steel bolts.

In the bottom members the hydraulic rivetting machine was carried forward to beyond the end of the third bay, after which all rivets were put in by hand, particular care being taken to have all the work thoroughly well bolted up, and to put the best and most trustworthy hands to the job. All the tension members were almost exclusively rivetted by hydraulic rivetters, while the struts were rivetted by hand only. The main booms of the ties were rivetted up in the yard as far as could be done by machines, and little more than the joints and bracing bars were left to do after erection. Of the internal viaduct also as much as could conveniently be got at was done by hydraulic machines, and the rest by hand.

The booms of the wind-bracings were also rivetted together in the yard by hydraulic machines, and as they decreased in weight further forward they were rivetted, at times in the full square, at others tops and bottoms, thus leaving the side bracings only to be completed.

THE "JUBILEE" CRANE ON TOP MEMBERS OF CANTILEVERS.

In the same way the lattice box girders between the struts were also dealt with; they could be lifted straight into their places, the joints only and the points of intersection requiring to be rivetted.

The top member was rivetted in all parts by hydraulic machines wherever it was possible to get the machine applied. For this purpose two or three light timber stages followed at the back of the Jubilee crane, and here two or three rivet-heating furnaces were kept going to supply the various machines going below, the hot rivets being dropped down a long pipe, the end of which was stuck into a pail with ashes at the bottom. These furnaces were heated by oil and compressed air. It was thus necessary to bring along in the first instance from the deck up to the tops of the towers, and then to each side down the cantilevers, not only the pressure pipes for the water working the hydraulic rivetters, but also for the supply of compressed air and oil to the furnaces. The oil was brought in pipes connected with a tank on the top of the central tower, and run down to the furnace tank by gravity.

With the last intersections built in bay 6, and struts and ties, top members and bottom members carried past them, it only remained to put up the end posts to complete the cantilevers.

The weight which had now been raised upon the supports may be shortly stated as follows:

Inchgarvie.

Queensferry and Fife.

or, on the three points, a grand total of 49,050 tons, not counting the approach viaduct spans at either end.

But, apart from the permanent work, many hundred tons of weight in the shape of cranes, temporary girders, winches, steam boilers, rivet furnaces, rivetting machines, miles of steel wire ropes and of gangways, and acres of solid timber staging were suspended from these cantilevers. A heavy shower of rain would in a few minutes put an extra weight of a hundred tons, and the storm would try its worst against these immense surfaces, but with no result. Only divided by a gap of some 350 ft. it was now possible to take up a position in a gale of wind, and by fixing a point on the opposite cantilever end and another point fully half a mile further back on the shore, try to see what the lateral deflection might be. But, whatever its amount, it was too small to be noticed by the naked eye; nor could any movement be felt except a slight vibration whenever an extra heavy gust of wind would hurl itself against the solid face of steel plates.

In setting out the centre lines in the vertical sense of the bottom members while building out plate by plate, allowance had been made at every junction for the natural and unavoidable deflection in the whole cantilever as a mass. This was, of course, also done in setting out the internal viaduct.

HYDRAULIC LIFTING ARRANGEMENT FOR BOTTOM MEMBERS OF CANTILEVERS.
Fig. 127. Expansion joint for rails at ends of central girders; inchgarvie, north and south.

It was intended that there should be in the free cantilevers a camber of 10 in. at the end posts when the cantilever was completed—that is, the line of rails at the end post would be 10 in. higher than in the central towers when no load was on the bridge. But to get this it was necessary to set each section higher by so much as it would deflect by the addition of the remaining sections further forward. The point aimed at was, therefore, set another 10 in. up, or 1 ft. 8 in. altogether. This was, of course, entirely a matter of calculation and of judgment, and in the end the cantilevers arrived at the position in which they were desired to be.

It remains now to tell how the central girders were erected and the final connections made.

The end posts are hollow boxes about 4+¹⁄₂ ft. deep and 3 ft. wide, by 40 ft. in height, and are closed on three sides, the fourth side towards the central girder being open. The bottom members project right to the end of the post, while the top members stop short at the closed or inner side, except the webplates, which, in the shape of large gussets, are also carried full to the open end. So far the four free cantilevers are exactly alike, but in the further arrangements they differ considerably, as will presently be described.

The end posts of the two fixed cantilevers where they rest in the cantilever end piers are different from the above. Here the posts are replaced by a large box about 8 ft. long and extending over the whole width and height of the end of the cantilever, out of which an arched way has been cut to allow the passage of the trains. An end elevation of this box is shown in Fig. 23 on a preceding page. The object of these boxes has been already explained, and will be again referred to in connection with the expansion movements provided for at this point. They are filled with cast-iron bricks and punchings and other scrap all laid in asphalt poured in hot and firmly set, thus preventing shifting and at the same time making the box water-tight. About 1000 tons of dead weight is placed at these points over and above the weight of the steelwork.

The central girders have already been described as having a slightly raised or curved top member of polygonal form; that is to say, it is straight from one support to another, a kink taking place at the point of support, in the same manner as the bottom member in the cantilevers. The bottom member is straight. The two are connected by eight pairs of cross-bracings on each side, intersecting each other at the centre and consisting of struts and ties. The girder is 350 ft. in length, 40 ft. high at the ends, and 50 ft. high in the centre. It is divided into eight bays of slightly unequal length. The lop members are braced together by 16 sets of diagonal lattice bracings, while the bottom members are connected by solid plate girders acting as cross-bearers, one at the centre and one at the end of every bay, in addition to those forming the ends of the girders. Vertical ties are attached to each intersection of struts and ties and carry the bottom members between the bottom junctions. The bottom members are trough-shaped

and about 3 ft. high by 2 ft. 6 in. wide;

DETAILS OF CONNECTIONS OF CENTRAL GIRDERS AND INTERNAL VIADUCT.

the top members are inverted troughs of the same dimensions. Each pair of opposite struts is connected by two pairs of diagonal wind bracings carried down as low as the passage of trains will admit. All struts and ties are of box shape of varying strength and section. The joints in top booms and bottom booms were so arranged that each half-bay could be built out in succession, the length of boom just reaching beyond the junctions or beyond the vertical ties. The permanent way and the flooring in the central girders are made up much about the same as in the cantilevers, with four rail troughs and two footpaths, and a wind fence on each side. The 6-ft. way—the space between the two troughs of each line of rails and the footpaths—were made up of buckle plates supported on T bars from trough to trough.

Previous to the exact lengths of the central girders being fixed the distance between the cantilevers had been measured carefully and frequently by wire or steel tape measurements. These measurements were taken at various times, under different conditions of temperature, and were carefully checked.

CONNECTIONS OF CENTRAL GIRDERS AND CANTILEVERS.

The lengths of the girders were then decided upon with a view of leaving sufficient play between them and the cantilevers, under the fullest extension by summer heat and sun heat.

At the mean temperature the end faces of the bottom booms are about 1 ft. apart from the ends of the cantilevers, and no weight-carrying connections exist between these two, all the weight of the central girders being carried by the top booms and bracings. The top booms project into the end posts of the cantilever and are there supported in a manner presently described.

The central girders—so far as longitudinal expansion is concerned—are fixed at the Queensferry and Fife ends, and to all practical purposes form parts of these two cantilevers. On the other hand, both at the Inchgarvie north and the Inchgarvie south ends, longitudinal movement is provided to the extent of 2 ft. at each end by means of an arrangement of rocking posts, slide-blocks, and expansion joints in rails.

Upon the ends of the cantilevers within the hollow end posts are placed steel castings in the shape of large cups or sockets. (See Figs. 128 and 129.) Into these are fitted knuckles or half-balls, fixed to the bottom of square posts or columns built of stout steel plates in box form. Upon the upper extremities of the posts are fixed steel castings of cup shape, the same size as those below, and into these are fitted half-balls which are fixed to the under side of the top booms of the central girders projecting into the end posts. Thus the weight of the central girders at these ends is transmitted from the top members of the central girders through these rocking posts to the bottom members of the cantilevers, and full freedom is given to the longitudinal movements.

At the Fife and Queensferry ends of the central girders there are steel castings also placed on the ends of the bottom members within end posts, but here the castings are in shape of half-bearings, to receive cylindrical pins 9 in. in diameter, and about 3 ft. long, set horizontally but at right angles to the centre line of the bridge. (See Figs. 130 and 131.) Here the girder ends are brought square down from the ends of the top members of the central girders, and are cut out to receive a casting which is placed on the top of the pin and is the exact counterpart of the one under the pin.

Longitudinal movement is therefore impossible here, but the attachments at both ends admit of a small amount of lateral deflection, which may be caused by wind pressure and by the heat of the sun's rays on either side. These are provided for in the following manner:

From the cross-girder which connects the ends of the cantilevers, both between top members and bottom members, strong plate brackets project and pass right through openings arranged in the corresponding end girders of the central girders. (See Fig. 132.) These brackets are somewhat stronger in the lower than in the upper girders. Between the two projecting brackets passes a vertical slide-block about 17 in. square, which fits exactly, and the slide-block is held in position by a vertical pin, 9 in. in diameter, the ends of which are fixed in the cross-girder of the central girder. In the top girders the slide-block is 19 in. square and the pin 6 in. in diameter.

At the Garvie ends these slide-blocks are in mean position at mean temperature, and have play at each end to the amount of 12 in. for longitudinal movement, while the vertical pins top and 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 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 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.

This joint having been made at the time of the highest temperature of the day, when the cantilevers and girders were of the greatest length owing to expansion, it follows that as soon as temperature decreased, the cantilevers and girders would shorten, and the only gap left in the bottom booms, namely at the Inchgarvie end, where the wedges were placed, would open. Had the change between the maximum temperature of day and the minimum temperature of night been considerable the wedges would thereby have been released, and, if free, would have dropped out. Precautions had, however, been taken, and had the wedges been ever so slack they could not have moved from their places. In addition to this, hydraulic cylinders had been attached to the ends of the wedges, by means of which they could when necessary be drawn out from between the plates. The change was, however, only about 3 deg., and with the tremendous compression on these wedges could make but little difference.

In this condition the girder was left overnight, but carefully watched, and all changes of temperature and of movement in the upper booms ascertained and noted down.

At 6 A.M. on October 11 a start was made with the drawing of the wedges first at the Queensferry end, and then at the Inchgarvie end of the girder, and they were drawn down with little effort. (See Fig. 143 at C C.) At the same time the wedges or key-plates which closed the top booms were placed in position and driven down hard into their places at A A. The top boom connections were somewhat different in their nature, the stresses, after connection had been made, being exactly the opposite of those in the bottom booms, namely, compressive instead of tensile.

It will be remembered that the weights of the two halves of the central girders were held up by the four plate ties connecting them with the cantilevers at D D, Fig. 143. The expansions in the cantilevers, after the bottom booms were connected, would have the effect of drawing up the centre portion of these booms in the same way, as if, instead of a central girder, there had been a simple rope attached to these ties. On both cantilevers contracting or shrinking, the rope would have been tightened and lost some of its downward deflection or sag at B. This was precisely what occurred to the bottom booms of the girder, for the top booms were still gaping and could not prevent the further opening of the gap. A temporary sliding connection was, however, arranged in the top booms in such manner that nothing could prevent their moving nearer together or further apart with the changes in temperature. (See Figs. 140, 141, and 142.) The gaps left between the webs of the booms, which were here thickened to 2 in., were, as before stated, from about 10 in. at top down to 6 in. at bottom of the web. (See Fig. 143.) The gaps at A had been templated and the key or wedge-plates made to these templates. It will now be understood that, in order that the bottom booms of the girder should retain their camber or upward deflection, it was essential that these wedge-shaped spaces should be closed at the lowest possible temperature, for, when once in their places, the top boom was closed like an arch in which the keystone had been placed.

Fortunately the setting up and holding up of the girder halves during construction had given the bottom boom as much camber as was required, and it needed no assistance from the drawing of the ties, otherwise some days of delay might have occurred, for the temperature kept high and the contraction in the cantilevers was not sufficient to draw the ties much.

The key-plates were then driven down hard and the heavy web-covers on each side were at once drilled through and bolted up. At the same time T bars of heavy section were placed at the top and bottom to form a temporary connection of the flange plates. Timber struts from side to side to keep the booms apart, and wire rope ties to hold them together, were also put on.

The remaining halves of struts 4 and ties 4 of the Queensferry half of the central girder were also templated now to the required size, and put in place as soon as possible.

The gaps in the top booms having now at the lowest temperature been filled by the key-plates, the effect of a rise in temperature and consequent expansion of the cantilevers would be to buckle the temporary plate-ties between central girder and cantilevers at D D, Fig. 143. As soon as this movement was felt, the bolts at the central joints of these ties (see Fig. 130, at M) were gradually withdrawn and the ties altogether detached. At the same time the furnaces were set going, and the plate-ties made so hot that, whether a rise or fall in temperature should take place, these ties could no longer restrain any movement, but would yield to either influence. With the removal of these ties the central girder entered upon its full function as a connecting link between the two cantilevers, and the south span was thereby successfully completed. A careful levelling on October 12 showed a camber of 3+⁷⁄₈ in. in the girder, and no deviation laterally from a straight line drawn between the centre of the ends of the two cantilevers.

The north central girder had in the mean time been built out in a precisely similar manner, and by October 15 it was sufficiently advanced to allow a gangway 65 ft. long to be laid across. This enabled the directors of the company to walk across the bridge from end to end the chairman of the company being actually the first person to cross the north span.

By October 28 the last booms were put in, and by November 6 everything was ready to connect this girder also. The temperature on that day did not rise, however, sufficiently high to make the joint, but in the night a sudden rise took place, and by 7.30 in the morning the bottom booms were joined together for good.

It now required a good fall of the temperature to get the top booms connected, for the two halves of this girder had been set less high at starting, and there was now practically no camber in the bottom booms. But the weather remained obstinate and the temperature very high, and it was not until the morning of November 14 that the key-plates could be driven in and the final connection made. An episode, of which much has been made in the papers, occurred on this occasion, and the facts are simply as follows: After the wedges at the bottom ends had been drawn out and the key-plates driven in, a slight rise of temperature was indicated by the thermometer in the course of the morning, and orders were given to remove the bolts in the central joints of the connecting-ties and to light the furnaces. Whether the thermometer indicated wrongly or whether the cantilevers had not had time to fully expand under the rise of temperature, or whether a decrease of the same took place it is not now possible to prove, but when only about 36 of the turned steel bolts remained in the joints, and before the furnaces could get fairly started, the plate-ties sheared the remaining bolts and parted with a bang like a shot from a 38-ton gun. Something of a shake occurred in the cantilevers which was felt at the opposite ends, and caused some little commotion among the men. No mishap occurred, however, and nothing in the way of a fall of the girders took place as stated in the papers—simply the work of the furnaces and the task of knocking out 36 bolts was saved, and the girder swung in its rockers as freely as if it had been freed in the most natural manner.

And thus the Forth Bridge was completed—for the remaining work was simply to replace temporary connections by permanent ones, to rivet up those which were only bolted, and do the thousand and one things which always remain to be done after everything is said to be finished.

The thrilling portion of the story is done, and the novelist would wish to leave off with so dramatic an incident as that just told. But there are yet some details which belong to the history of the bridge, and which could not very well be left unrecorded.