The Forth Bridge/Bedplates

by Wilhelm Westhofen

Bedplates.

The lower or fixed bedplates, in size about 37 ft. long by 17 ft. wide, are built up of five layers of plates, the lowest ³⁄₄ in. in thickness, the second 1+¹⁄₄ in., and the third and fourth 1 in., alternately laid longitudinally and transversely, to obtain distribution of fibre in all directions. The fifth layer consists simply of a band, 11 in. wide and ¹⁄₂ in. thick, laid round the edges of the bedplate, partly as a stiffener, partly as a means of retaining the lubricating medium between the lower and upper bedplates. Immediately under the vertical column a recess is formed by cutting out of the two upper 1-in. plates a space varying in form, but in all cases for the purpose of admitting a keyplate of similar form, a portion of which is rivetted to and forms part of the upper bedplate.

The bedplate as above described was put together on a carefully-prepared bed in No. 1 shed, clamped and fastened down securely, and a traveller with several boring spindles passed over the whole of the plate, drilling all holes through the various thicknesses at once.

All holes were 1+¹⁄₈ in. in diameter, and countersunk both top and bottom. The forty-eight holes 3 in. in diameter for the holding-down bolts were drilled in the same place, but by a specially constructed tool described above, and the plate was then taken to pieces and stowed away until wanted.

As soon as the granite piers were ready for the reception of the bedplates they were put together at a height of about 4 ft. above the masonry, and supported for the time being on short pieces of bolts, coupled to the foundation bolts by long nuts and in other places by pieces of cast-iron piping of equal length. The rivetting machine was then placed at one end, and the rivetting commenced. (See Figs. 86 and 87.) The machine consisted of two strong box-plate girders, carried on two side-frames moving on wheels, and kept apart vertically a sufficient distance to admit the bedplate, with a rivetting cylinder attached to the upper and another to the lower girder. The cylinders could by screw movement be moved from one end of the girders to the other, and thus commanded the whole width of the bedplate, while for forward or backward movement the side-wheels had to be pinched. The two cylinders were turned on simultaneously by one valve. The cylinders were 12 in. in diameter, and the water pressure 1000 lb. per square inch. The pressure upon the rivet and the plates was, therefore, about 40 tons from each cylinder. The rivets had a countersunk head at one end, and flat snaps were used on both cylinders. The rivets were heated in an ordinary brick furnace placed on the pier, and brought to a full yellow heat. There were 3778 rivets in each of these bedplates, which gives for the area of 654 square feet about six rivets per square foot.

As soon as the machine had moved away a yard or so from the end of the plate, chippers were set to work to pare down the projecting piece on each rivet-head on the under side of the bedplate, in order to have this perfectly even and flush.

After the men got into the way of properly using the rivetting machine, these bedplates were rivetted up in twenty-eight hours, or 135 rivets struck per hour, which is very creditable work, considering the size of the rivets and that so much shifting of the machine and of the supports for the bedplate had to be done.

After all the rivet-heads had been chipped, the bedplate was lowered into its place by hydraulic rams. Two thicknesses of canvas, one laid in the length, the other in the breadth of the plate, both being well soaked in and painted over with red lead, were interposed between the cement bed

described above (see granite piers) and the plate to

EDGE-PLANING MACHINE FOR LONG PLATES OF LATTICE GIRDERS; NO. 1 SHED.

make a water-tight joint. Nuts were then placed on the holding-down bolts, and screwed down hard with long spanners. As the bolts have no play in the 3-in. holes of this bedplate, the latter becomes thus an absolute fixture upon the pier. The weight of these bedplates is 44 tons each, except those in the fixed cantilevers, where it is 33 tons only.

The projecting parts of the rivet-heads on the upper side of this bedplate had now to be removed, and various means were devised to do this—rose bits, ordinary flat drills, and emery wheels being all tried in succession; but hand-work by chisel and hammer was in the end found to be cheapest.

The various forms given to the recesses in the lower bedplates and to the key-plates fixed to the upper bedplates are shown in the sketch. (See Fig. 88.) In the three piers already mentioned above as the fixed points, a plain circle is shown, and here the key-plate fits the recess exactly without any play. The lined spaces shown in the other piers give an indication of the amount of play between key and recess, and also the direction in which movement can take place.

Thus, in the remaining three piers on Inchgarvie, longitudinal movement alone can take place and a slight amount of circular movement round the centre of each individual pier. In the Queensferry north-west, and Fife south-west, these movements are precisely the same as those on Inchgarvie, while in the Queensferry south-east and south-west, and the Fife north-east and north-west, both longitudinal and lateral movements are provided for. The former is for expansion and contraction between the fixed circular piers and the cantilever end piers, the latter for any lateral deflection in the same length due to wind-pressure acting at right angles, or nearly so. In the cantilever end piers lateral movement is prevented, but longitudinal movement provided for.

It will be noticed that the key-plates, with the exception of those belonging to the fixed cantilevers, are shown in three parts, an oblong centre-piece and two segments of a circle. Of these, the two segments are only 2 in. in thickness, and therefore simply fill up the recess in the lower bedplate, while yet, however, they admit and facilitate, as well as control, a circular movement round the centre. The central part, or oblong key-plate, is 2+³⁄₄ in. in thickness, and enters therefore to the extent of ³⁄₄ in. into the upper bedplate, to which it is rivetted. This, however, is somewhat different in the three fixed piers, for here both the two segments and the rectangular centre-plate are all alike 2+³⁄₄ in. in thickness, and the whole key-plate, 12 ft. in diameter, enters into the upper key-plate. The central piece is the only one attached by rivetting, the wings simply are laid loose into the recesses.

The square key-plates in the bedplates of the fixed cantilevers are fixed to the upper bedplate, and enter the lower bedplate to the extent of 1 in. only, the whole bedplate being much lighter than in the case of the free cantilevers.

HYDRAULIC CRANES.
RIVETTING MACHINE ON PIERS FOR BEDPLATES.
ARRANGEMENT OF BEDPLATES ON GRANITE PIERS

The upper bedplates form the under part or foot of the main junctions or skewbacks. These plates are, like the lower bedplates, built up of several layers of plates of varying thicknesses, according to position. The lowest course of plates, placed longitudinally, is ³⁄₄ in. in thickness, and in this is cut the recess which receives the key-plate of whatever shape it may be. The next course is laid transversely—1+¹⁄₄ in. in thickness—and extends, like the first, over the whole area. The third course, laid longitudinally, again is ³⁄₄ in. in thickness, but extends only for about two-thirds of the area on the side next to the horizontal connecting bottom member. This plate also takes the attachments for the bottom booms of the horizontal cross bracing and the horizontal diagonal bracing. (See Fig. 91.) A further thickness of plates, as a fourth course, occurs in the case of the fixed piers, to make up for the enlarged recess in these, and this course is formed under the base of the vertical columns only.

These plates are stiffened by twelve girders of I section—about 11 in. deep, and running longitudinally—consisting of web-plates and four angles. The third and fifth girders on each side are omitted, the main webplates of the skewback taking their places. (See Fig. 90 ) Transversely these girders are combined and stiffened by cell-plates, flanged on all four sides in a die. There are eleven rows of these, and upon them are set the main diaphragms, which reach from the bedplates to the crown of the skewbacks. The remaining spaces on each side of the inner webplates, and between these and the outer web-plates, are filled by similar diaphragms, and at the same distances apart.

The sections and plans shown in Figs. 89 to 94 will fully explain these parts, which are of the utmost importance, because through them all the stresses thrown upon or against the structure, must pass on to the supports. The lower half of the skewback is square in form, the upper half for about two-thirds is rounded, the remainder being squared in order to receive the attachments of the top booms of bracing girders mentioned above. The inner main webs are J in. thick on Fife and Queensferry and 1 in. thick on Inchgarvie. and these are carried through from end to end of the skewback at a distance of 3 ft. 6 in. to each side of the centre line. These plates form the main points of attachment for the diagonal struts and for the bottom members, both within the central towers and in the cantilevers. The outer webplates, 1 in. thick on Fife and Queensferry and 1+¹⁄₄ in. thick on Inchgarvie, receive the main thrust of the vertical columns, and are carried on each side into the bottom members by a change from the rectangular into the circular form. (See Figs. 92 and 93.) They are considerably stiffened in these parts by horizontal diaphragms and by doubling plates of great strength, and the skeleton work to which they are attached is gradually merged into the main beams of the regular section of the bottom members.

The attachments of the five lattice wind-bracing girders, although most ingeniously contrived, does not call for any special remark.

It will be noticed in connection with the skewbacks that the centre of the vertical columns does not coincide with the centre of the circular granite pier, but that it is set back 5 ft. to the inside of the pier, the centres between the vertical columns being, therefore, 10 ft. less than the distance between piers. This was done with a view to the better distribution of the downward stresses, which are greatest in the bottom members of the cantilevers; and as it was for structural reasons not practicable, or, at any rate, expedient, to bring the centres of the main compression members to one point, it was thought best to make the distribution as even as possible.

DETAILS OF SKEWBACKS OVER PIERS.

To return again to the upper bedplate. This was put together with all its longitudinal I girders and transverse cell-plates upon the pier, resting upon supports similar to those used for the lower bedplates. The same machine was used for rivetting the bottom portions of the plate, and all rivets were countersunk on the under side, while for the upper end a bent snap had to he used in order to get round the flanges of the girders and cell-plates. On the under side all projecting rivet-heads had to be removed as before. The key-plates were now placed in the recess of the lower plate to ascertain whether they fitted and had good bearing all round; and after these were found to be satisfactory, the rectangular portion of the key-plate was raised up and rivetted to the upper bed-plate. This was then lowered down into its place to see how it fitted, and whether all parts of the key were in proper contact. After thus lowering and raising it several times (a weight of about 57 tons), the surfaces were carefully cleaned and thick brown oil of a special character was poured into the key recess and on the whole area of the lower bedplate, and the upper bedplate lowered down upon it for the last time. Nothing else but oil was placed as lubricating medium between the two surfaces.

The movements described in connection with the keyplates require that a certain amount of play be given in the holes of the upper bedplates through which the holding-down bolts pass. The amount of play varied, of course, in the same manner as with the key-plates.

The lower bedplates being fixed and held down firmly by the bolts, it was necessary that the nuts should be screwed down upon them. These nuts, shown in Fig. 95, were therefore arranged with a long neck, circular in form, and with an enlarged head of hexagon form. The holes in the upper bedplate had to be therefore made of such size and shape that with the nuts screwed down tightly upon the lower bedplates, with a considerable stress upon the holding-down bolts, the upper bedplate could yet move in any desired direction without hindrance.

This was done by means of a washer of oblong form placed upon the carefully-levelled sides of the cells in the bedplates, which had bolts passing through them. The nuts were first put on and screwed down by means of a hydraulic spanner—that is, a heavy box spanner with a short lever on the top of it, to the end of which a hydraulic ram was applied with a given pressure. This assured an even stress to all the bolts. The distance between the side of the cell and the under side of the hutul of the nut was then carefully callibered for each nut, and the washers made to that measurement, one-sixteenth of an inch being allowed for play. The sixes and shapes of the holes in the upper bedplates are shown by the diagram here given, the holes for Queensferry bedplates being, of course, the same as those for Fife, but in reversed position. (See Fig. 96.)

The building of the skewbacks could now be proceeded with, and this was done in such manner that the work as erected was at once rivetted up by machine. The most difficult places to get to were the lower edges of the inner and outer webplates, with their double angles already rivetted to the bedplates. All the work in these cells which could not be done previous to the put ting in of these plates had to bo done by small hydraulic rivetters. These were simply cylindrical rams of 4 in. to 6 in. diameter, and in length from 6 in. to 10 in., and these were worked by a pressure of 3 tons per square inch. The cylinders were placed to either side of the rivet to be struck, facing each other, and backed by hardwood packings against the sides of the cells, and the pressure water was supplied to them through small copper pipes about ³⁄₁₆ in. in bore. The rivets were heated on the outside, and passed through holes in the webplates; one cylinder was set against the rivet-head, and the other then closed upon the free end, and formed a fiat head, the hole on that side being somewhat countersunk. To produce the 3 tons per square inch pressure, a multiplier was used, consisting of an 11-in. ram, which was acted upon by an ordinary accumulator pressure of 1000 lb. per square inch, and which had at its end another ram attached, 4 in. in diameter, which was forced into a cylinder already charged with 1000 lb. pressure water. The 4-in. ram forced the water to a small accumulator, loaded to produce a pressure of 3 tons per square inch. It needs no saying that this was a tedious kind of work, and took a long time to accomplish. On many days not more than half a dozen rivets were done—at no time more than about a hundred in the twelve hours, and the average would not probably amount to more than twenty-five per day.

HOLDING-DOWN BOLTS FOR BEDPLATES

Meanwhile staging had been erected between the piers on trestles of certain height, upon which were put together the horizontal tubes connecting the skewbacks and the cross-girders and diagonal girders. All these were now connected to the skewbacks, and preparations made to rivet them up. All the erection of the lower portions of the central towers was done by 3-ton or 5-ton steam derrick cranes placed on platforms at some height from the deck and in convenient position to pick the material off the bogies and at once place it in position. For the rivetting of the latticed girders the ordinary fixed or jointed machines were used, which are extremely simple in their construction and their action. (See Figs. 97, 98, 99, 100, the three first representing fixed machines, the last a jointed machine.) It depended entirely upon the places where rivets had to be struck whether one kind of machine or the other was used; but the direct-acting machine was surer in its action, because the double lever arrangement was apt to twist, and thereby bend the unclosed portion of the rivet before the latter was properly staved. Of course the longer the arms the greater becomes this tendency, and as some of the jointed machines had to be used with the arms fully 4 ft. 6 in. in length, great care had to be exercised to get good work done.

The horizontal tubes, built up of heavy plates 1+¹⁄₄ in. thick, required a special machine for rivetting, and as this machine is the same, or at all events typical of others, which were used for the rivetting of the bottom members, the vertical columns and the diagonal struts in the central towers, a brief description will not be amiss.

BEDPLATES ON INCHGARVIE AND FIFE PIERS

In the main it consisted of two circular girders, the inside diameter being about 1 ft. larger than the outside diameter of the tube. They were placed some 24 ft. apart and tightly wedged by hardwood blocks all round the tube. A box girder of a strong section, and about 25 ft. long was placed upon the ring girders in such manner that it could be pushed completely round the tube, and for this purpose hydraulic rams, one at each end were attached in such a manner that they could lie fleeted forward or backward as desired. Upon the box girder a cylinder was placed facing the tube, being connected to a saddle which was capable of sliding from one end of the girder to the other. The saddle was worked by a ratchet and pinion, the latter working upon a rack winch ran along the whole length of the girder. Finer adjustment could be given by two screws and handwheels on the saddle, like that of a lathe. By this means the centre of the cylinder could be brought opposite any point of the surface of the tube within the length of the girder. On the inside of the tube a mandrel or central hollow shaft was formed of the same length as the outside girder, and this was supported at both ends by frames fixed and wedged to the skeleton frame of the tube. On this mandrel a toothed rack was fixed, and a saddle worked by ratchet and pinion could move from end to end. Upon this saddle a cylinder of the same size as the outside one was placed with a long snap or dolly reaching to the plates. The supply of pressure water was so arranged that both cylinders would be turned on simultaneously, but by means of a check valve placed upon the outside supply pipe, the motion of the outside ram was slightly retarded. The girder outside was now placed in line with a row of rivets in the tube, and the inside cylinder was set opposite the same row. The rivets were heated in small furnaces or forges on the inside and heated to a good yellow heat. As soon as a rivet was put into place the inside dolly was set against it, the outside ram being also set down, a tap given to indicate that all was ready, the pressure was turned on, and the rivet closed. The two cylinders were then moved forward, one pitch to the next hole and so on. When a whole row extending over about 18ft. in length had been done, the girder was raised or lowered as the case might be, to the next row, and this continued until the whole section of the tube was rivetted. The machine was then moved 18 ft. forward and the process repeated. No drawing of these rivetting machines is given here, but a machine similar in construction, and used for rivetting the vertical columns, is shown further on.

Between 600 and 700 rivets could be put in by this machine per day, all rivets here being 1+¹⁄₈ in. in diameter. Only the rivets passing through the thick outside plates and beams were done by the machine. Of these there were about 1550 in each section rivetted, or nearly 100 per foot run. The other rivets, in diaphragms and beam-covers, were done by hand. The least number of hands to work this machine were three men outside and two men and a boy inside, with a lad for heating the rivets and a boy to carry them.

The members of entirely circular form were the vertical columns, the horizontal tubes or bottom members between skewbacks, and the bottom members in cantilevers from the skewback to the end of bay 4. In design and in construction they were all the same—differing only in diameter—in the breadth and thickness of the plates, and in the depth and strength of the longitudinal beams. All plates are made, except in special places or for closing lengths, of a uniform length of 16 ft., and the tube is formed of ten plates, lap-jointed, and therefore consisting of five outer and five inner plates. All plates break joint at 8 ft., or half length, with an absolutely close butt, and the butts are covered inside and outside by plate-covers. The lap-joints are covered on the inside by ten continuous girders of T section, the T head being rivetted in with the lap-joint, and two continuous angles are rivetted to the other end of the web, thus making the girder of double T or H section. (See Figs. 68, 73, and 94.) The beams are made in lengths of about double that of the plates, and break joint in different places, and all their joints are covered both in the webs and flanges. At every butt—that is, every 8 ft.—is placed a circular girder or diaphragm, which consists of ten wings placed between the beams and rivetted to the shell, the wings projecting beyond the beams to the extent of about 3 in. An angle-iron ring running right round takes up all the wings to its web, while its heel is rivetted to each of the beams in succession, and thus combines the whole into a stiffening girder. At considerable distances apart, heavy plate-diaphragms are inserted which only leave a manhole in the centre to pass through. All these diaphragms are primarily for the purpose of preserving the true circular form of the tubes, and to prevent flattening; and it is a curious fact that the stiffest of these tubes, although bolted together in the most careful manner tened before rivetting as much as 3 in., and had to be constantly strutted with timbers to keep it in form.

The plates forming the shell of the vertical columns varied in thickness in the case of the Fife and Queensferry central towers, from ¹⁄₂ in. at bottom to ³⁄₈ in. at top; and, in the case of Inchgarvie, from ⁵⁄₈ in. at bottom to ⁷⁄₁₆ in. at top. The beams decreased in strength proportionately.

The diagonal columns or struts in central tower: are of different shape, namely, circular top and bottom, but flattened on both sides in order to facilitate their intersection with each other and their entering into junctions top and bottom, when they are gradually changed into rectangular section. It has been pointed out in the general description of the bridge, that the effects of the live load upon the Inchgarvie cantilever are different from those in the Fife and Queensferry piers. The diagonal struts in the former case are therefore very much stronger than in the latter. The bottom and top plates are 1¹⁄₄ in. in thickness, while the corner and side plates are 1 in. thick. The intersection of these two struts at centre of the tower is very heavy, some 80 tons of metal being massed therein. The inside beams are of corresponding strength. These diagonal columns were rivetted in the case of Inchgarvie from top to bottom by a special machine of similar design as that for the vertical columns, but more difficult to work on account of the angle at which the machine was placed.

Meanwhile, steam derrick cranes were erected on platforms raised some 30 ft. to 40 ft. above deck, commanding the whole of the skewbacks, and these were now built with portions of the diagonal struts, the vertical columns and the struts 1 in cantilevers, to as great a height as could be reached by means of these cranes. All this work was put together as carefully as possible, the larger number of holes being drifted up, but it was only bolted together pending the necessary checks and corrections being made by means of theodolites. This setting out was a work of no small difficulty, seeing that the members had to be set, not only to an inclination towards the point of intersection, but had at the same time to follow the uniform batter of the vertical columns, and had therefore a strong tendency to lean to the centre of the tower. So soon as the vertical columns and struts had been built to a height of about 50 ft. above deck, to which, in the case of Inchgarvie, had to be added the central ties, preparations were made for the construction of the lifting platforms, by means of which the central towers were raised to their full height.

PORTABLE HYDRAULIC RIVETTERS

A staging, reaching on each side, north and south, from column to column, about 25 ft. in width, was raised from the deck to a height of about 38 ft. above the level of the staging, and upon this a pair of longitudinal girders were built. These girders were about 190 ft. long in the case of Fife and Queensferry, and 350 ft. long in the case of Inchgarvie. (See Figs. 101 and 102.) There were four girders—one to each side of the vertical columns—placed 18 ft. 6 in. apart, with angle cross bracings top and bottom, and diagonal bracing between top booms on one side, and bottom booms on the other side, alternately. The main booms, D D, Fig. 101, and most of the vertical angle bracing, were permanent work, being, in fact, the booms belonging to a portion of ties 1 in the cantilevers borrowed for the occasion. This platform and all its details are shown in a large number of the text illustrations as well as plates.

During the time these girders were being put together, a strong box girder C C was built on a staging between vertical columns east and west, which girder passed through openings left in both columns, and projecting outside of each, to the extent of about 4 ft. or 5 ft. For this purpose one plate was left out on each side of the vertical columns during the time the girder passed aloft. This girder in its turn was supported upon a slide-block inside the vertical column, which block was attached to a cross-girder B B. This cross-girder was of double box section, all in plate, and in its turn was supported by a number of pins about 1³⁄₄ in. in diameter, which passed through girder and through holes drilled for the purpose in four of the H beams of the vertical columns. Immediately below this box girder, and at a distance of about 1 ft. from it, was a second box girder of the same dimen- sions A A, and held up in the same manner by a number of pins. Upon the lower girder was placed, on a rocking underside, a hydraulic ram G G 14 in. in diameter, with a stroke of a little over 12 in., the cylinder and ram passing through the upper box girder. The head of the ram acted by means of a swivelling cap immediately upon the cross-girder C C passing from column to column. The working of these girders and rams was as follows:—The lower box girder was held by a sufficient number of pins to carry the total weight placed upon it.

As soon as a small amount of pressure water was allowed to enter the ram, the latter lifted the cross-girder C, and with it the box girder B, to a small extent. The pins could then be withdrawn from the girder B, and the girder lifted 6 in. or 12 in., as the case might be. The pins were then driven through the sides of the girder B into holes newly exposed, and the beams and the weight let down upon that girder. The pressure was then allowed to pass on the top of the ram, and the cylinder was drawn up to the ram, bringing with it the lower box girder A. When the holes in girder and beams had come opposite, the pins were driven in, and the same process could be gone over again.

The arrangement as just described was placed in each of the four vertical columns, and in the case of Inchgarvie into the central ties as well, thus giving six points of support there, instead of four. The longitudinal girders were now placed upon slides on the top of the cross-girders passing from side to side, and the tops of the girders covered over with cross-timbers, and decked with planking 3 in. thick. Thus there were two platforms about 25 ft. wide by 200 ft. long, or 350 ft. long in the case of Inchgarvie, embracing completely and projecting to some distance beyond the vertical columns. Figs. 103, 104, and 105 show various positions and details of platforms.

HYDRAULIC RAMS AND GIRDERS FOR LIFTING RAMS IN CENTRAL TOWERS

At the starting-point just above the skewbacks these platforms were some 110 ft. apart, centre to centre, approaching each other, of course, with every few feet of lifting to the extent of the batter of the vertical tubes, which is 1 in 7+¹⁄₂ about. (See Plate IX.)

With every few feet of lift these platforms had to be pushed together, which was done by hydraulic rams also, and this accounts for the number of slide-blocks mentioned above.

The ordinary routine in lifting was as follows:

From a level position of the platforms the cross-girder in the north or south columns was lifted first 6 in; next the other end was lifted 1 ft.; then the opposite end 1 ft.; and so on until sixteen lifts had been made, when a last lift of 6 in. at the end which commenced the lifting made the platform level again.

When the platforms had in the first instance been lifted to a sufficient height, the rivetting cages were suspended from the under side of the platforms, and in subsequent lifts they were always drawn up with the platforms. (See Plate IX., rivetting cages on Inchgarvie.) This cage simply consisted of a stout circular wire cylinder being placed round a rivetting machine, similar in construction to the one described for the horizontal tubes, but hung vertically. Inside this cage the men worked with perfect safety as regards falling themselves or dropping things down on those working below. (See Figs. 106 and 107.)

The portions of the rivetting machine inside the vertical column were not drawn up by the lifting of the platform, but were lifted separately subsequently. The length of the plates in the vertical columns being 16 ft., the lifts of the platforms were of the same length.

The vertical columns were always built above the platforms to any height which could be reached with the cranes or other means at disposal, while the diagonal struts were at times built above, at times below the platforms.

On the inside of the platforms at each end were hoists worked by wire ropes from winches placed on deck, and all plates, beams, and other material was brought up by means of these hoists and distributed over the platforms. (See Figs. 104 and 105.) Two-ton and 3-ton derrick hand cranes were found the most kandy for the erection of the different members, but a traveller, or Goliath, worked by hydraulic power, was also used.

When a lift had first been made, the first work, as a rule, was to put in the two closing plates to each column which had been left out to allow for the passage of the cross girders. The inner part of the rivetting machine was then drawn up, the whole machine fixed, and rivetting was at once commenced ; while above the platform a section was added to the vertical tubes and the diagonal struts. The platform on Inchgarvie being of so much greater length, and weighing fully 700 tons, had to be provided with two additional supjKirts, and these, of course, were carried upward the same as the vertical columns.

The water pressure for lifting was supplied by a special set of pumps which produced a pressure of 35 cwt. to the square inch. No accumulator was used, but the pumps simply forced up the rams gradually. Under this great pressure the cup-leathers in the hydraulic rams and the leather washers in the pipe-joints, frequently gave way and caused much delay. Frosty weather also did a great deal in the way of freezing pipes to hinder the work, but after once the men had got into the way of working the plant, things went more smoothly. The first lift of the Inchgarvie platform took, with one thing and another, nearly eighteen days in the months of January and February, 1887; while the last lift, on the 9th of August of the same year, was accomplished in five hours. With the batter which the vertical columns and other side members have towards each other, it is easy to understand that, independent of any weight which was imposed on them, they would be liable to deviate from their true position. It was, therefore, necessary from time to time, that is every 30 ft. or 40 ft., to secure the members by the interposition of temporary struts or ties either in the shape of lattice girders or of timber balks, and even timber trusses. Thus, after two lifts had been made with the platforms, the diagonal struts in the central towers had not only deflected downwards by their own weight, but had also deflected towards the centre line of the bridge. It was not possible to go any further without putting them right, and this was done by girders placed both longitudinally and transversely between the struts, and using hydraulic rams to push them apart. In the case of the vertical columns the same difficulties arose, and here strong timber struts were interposed. For the same reason, the first diagonal bracing between these columns was kept up as close as possible to the platform. (See Plate X.)

Nor was this the only matter requiring care, for although these columns might be kept the proper distance apart from each other, yet might they also both deflect in one direction, be this east or west, north or south. With the heavy platforms carried on these comparatively unconnected members, a strong wind from one side or the other could produce serious distortions and deflections, and the rivetting up of the vertical columns following so close upon the rise of the platforms, made them so stiff that it was not easy to deal with them. In checking the columns, reference was always made to the centre line of the bridge, which was thrown upon the cross girder which carried the platforms, and measurement from this centre was taken to each side. That side which deviated least was first dealt with and pulled or pushed to the right position; there it was held by wire rope ties or timber struts, and the other corrected subsequently. This cross-girder itself was used for pushing the columns apart or in, one side being fixed for the time and the other left loose.

The first thorough correction was made at the top of the first vertical wind-bracing between the columns, where a solid plate girder passes right across, which carries the internal viaduct girders. This is shown in Plate X. The large gussets were fixed to the columns, and the booms of the latticed girders brought up to them. But the gussets were not yet drilled, and only after the position of one column had been ascertained and corrected, these holes were drilled, and the corner booms fixed. A fixed point being thus obtained, the other column was either pushed out or drawn in as the case might be, and the booms fixed in the same manner.

By this time the platforms had already approached each other to half the original distance, and all the temporary girders became much shorter and more easy to handle. The men also had become so familiar with the work that they knew exactly what was required, and made little account of the height at which the platforms had now arrived, namely, some 200 ft.

ERECTION OF TOWERS

At the next halt a great deal of work required to be done, namely, to build in the crossing of the diagonal struts in the centre of the pier. This for obvious reasons could not very well be done above the platform, but some 20 ft. below. In the case of Inchgarvie this crossing contains 80 tons of steel on each side, and at the crossing of the vertical tie at centre, and the horizontal bracing occurs at the same point, a very intricate piece of work required to be done.

Here the vertical columns could be corrected for position north and south by means of the horizontal bracing which runs longitudinally from one vertical column to another, intersecting the crossing of the diagonal struts. A pair of horizontal bracings are also placed here between the four columns as shown in Fig. 4, Plate III. A strong framework, similar to the one immediately above the circular granite piers, thus closed the lower half of the central tower, and a new start upwards could be made from a fresh fixed base. At this stage a prodigious amount of work was done, immense quantities of material were drawn up the hoist and distributed on the top of the platforms where the platers, working on tall ladders, bolted up beams and plates often 25 ft. above the platforms. Between the top of the platform and the bottom, men were at work removing and replacing temporary bracing, which came in the way of the diagonal struts as they were built up. Inside the columns, the hydraulic men were busy preparing for the next lift and examining the caps, leathers, and pipe joints. Below these in the cages and inside the tubes, the gangs of machine rivettors vied with each other which could get done the quickest, a premium being paid to the squad that had done its section first. Again, below the cages, men were replacing diaphragms and other details of the structure which had been removed to allow the lifting girders to pass, and still lower down, squads of hand-rivetters were rivetting up beams and diaphragms, and putting in such rivets as the machine had not been able to do. Besides these a host of other men carpenters to put up stages, gangways, timber stiffeners, floors, and all sorts of things; men attending to the rivet furnaces and trying to keep the machines going which put in their eighty to ninety rivets every hour, electric lightmen shifting lamps, putting in carbons and lixing fresh cables, and finally the hundred and one men who never seem to do anything, and yet cannot somehow be spared.

At last the tops of the platforms have reached their final position; and they can go no further, although the superstructure itself has to rise some 20 ft. more. (See Plate XII. and Fig. 108.) The two platforms by now have come so close together that the inner hoist-frame on one side had to be removed previous to the last lift. The long box girder now projects a long way outside each column, and stands quite clear above them; the whole platform looks as if a heavy gust of wind would lift it up and throw it over the side. The last length of beams and the closing lengths of the plates require to be measured before they can be cut to length. Once more the columns are carefully set to the centre line of the bridge and to north and south, and the levels of all the beams in the columns taken.

While these are being got ready, the gap of about 9 ft. between the two platforms is covered over, and only one platform exists now of more than double width.

The longitudinal platform girders were stiffened and trussed by link chains stretched from support to support. These chains were portions of the Hammersmith Suspension Bridge, bought up by the contractors, and largely made use of here in various ways. But the workmen would not consent to use the whole title due to them, for some of them called them Hammer links, and others called them Smith's links; but it required a Cockney by birth or adoption to get the combination.

These links were passed under double timbers, right under the two girders of each platform, and an initial stress was put upon them by interposing hydraulic cylinders between the timbers and bottom booms of girders, and filling the space made with hardwood blocks.

Blocks were now laid down on the top of the platform in the centre line of the top members, and the vertical webs of the bottom booms, the flange angles of the same laid down, and so soon as the top junctions were somewhat advanced, they were connected up at each end. The object was to relieve the platforms as soon as possible of any weight from the top members. The vertical bracings were then put on, and in these the webs of the top booms placed. The girders could now carry themselves, and rivetting was commenced as soon as the top junctions had been fixed and securely joined to the vertical columns. As there was a great deal of work to do upon these top junctions, they were completely surrounded by timber frames covered with boards, and provided with windows and a large number of electric lamps. Thus protected the work could be carried on day and night, and it was necessary, for every beam and every plate had to have its correct measurement and shape taken before it could be made. The ends of the diagonal struts, changing from a flattened round into a square, had to be templated in every plate and beam to make an exact fit. It will give an idea of the magnitude of the work when it is stated that these wooden buildings, of which there are four to a pier, erected at a height of 360 ft. above the water, were about 24 ft. square and fully 35 ft. high, with three floors at different heights.

HYDRAULIC TUBE RIVETTING MACHINE.

The top junctions, although not of such importance as the skewbacks, are yet fully worthy of more than a passing remark. (See Figs. 109 to 115.) Here are united two tubular members—the vertical column and the diagonal strut—and five latticed members—the top member in the centre tower, the top member in the cantilever, the first inclined tie in the same, the highest of the four wind-bracings between columns, and the horizontal cross-girder between top junctions (the two latter not shown in the engravings).

The main strength of this junction lies in a number of very large webplates carried on each side of the centre in direct continuation of the webs of the top members. Inside these are the starting-plates, or horns, as they were popularly called, of the diagonal struts, and outside them those of the first inclined tie in bay 1 of (he cantilever. Diaphragms, supported by stiff beams with a gradual change from round to square, connect it with the vertical column. The flanges of the top booms of the top members are carried right through, but the flanges of the bottom booms are replaced by extra plates and heavy angles. Most of the rivetting of the main webs was done by ordinary hydraulic rivetters, but a large number inside the cells, by small rivetters at the 3-ton pressure of similar construction as those used in the cells of the skewbacks. The top junction is shown in Figs. 109 and 110 in elevation and plan, and requires no further explanation. The rivetting of the top member between vertical columns was now pushed on as much as possible, and, as soon as completed, the working platform was transferred to the very top of these girders, and the removal of platform girders and all hydraulic lifting gear, commenced. This was all the more necessary since it will be remembered the platform girders were made out of portions of the first inclined tie of the cantilevers, and was now at once required to fill its proper place in the structure. Many of the plates and illustrations show details in connection with the lifting platforms and the work carried on upon them, but the space here is not sufficient to enter into description of them all.

The removal of these platform girders, many hundred tons in weight, with all the odds and ends of a working platform upon them, was a work which caused no little anxiety; for upon nearly every portion of the structure below, work was carried on by scores of men to whom the fall of a small bolt or nut from that height might cause danger to life or limb. The year in which the central towers were erected shows the greatest number of fatal accidents in any one year, namely, 17, while the average over the seven years is only 9.