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 37⁄8 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.
Rivetting.
An early estimate has fixed the number of rivets in the Forth Bridge at 5,000,000, but this was evidently insufficient and the figure has risen to 6,500,000. It is, however, doubtful whether even this covers the total amount, for on the central or Inchgarvie pier, where an exact record of rivetting was kept, the number closed there amounts to near upon 2,700,000 alone, and a very large quantity of material was sent across from the shops and the field which had already been rivetted up, and such rivets are not included in the above total. The rivets varied as to diameter from 11⁄8 in. for the heavy tubes and the skewbacks down to 3⁄4 in. for the buckle-plates; and, as to length, from 111⁄4 in. (measured without the head) down to 111⁄4 in. The greatest thickness of plates rivetted together was 9 in., and this occurred in the top junctions at the head of the vertical columns on Inchgarvie—the least thickness was 1⁄2 in., in the flooring of the viaduct.
The various hydraulic rivetting machines, by which about one-half of the rivetting was done, have already been described in the places where they carried on their work, and need not be further enlarged upon here.
At the commencement, ordinary furnaces fired with coal were used for heating, but it soon became evident that these could not be taken on the outlying platforms owing to their weight, the weight, of fuel, and last, though not least, to the danger of fire caused by hot ashes left on timber staging.
Various kinds of furnaces were designed and tried, all heated by oil, and in the end the difficulty was solved by turning the burner of an ordinary Lucigen lamp, in a somewhat modified form, into a small furnace and setting fire to it by a piece of burning waste drenched in oil. This was tremendous advantage, for these little furnaces, though made of iron and brick-lined, weighed little more than half a ton, and could be handled and shifted about with the greatest ease. All they required was a small pipe-lead to supply compressed air, and a small tank with oil, and a crane would pick them up and put them into any place where they were most handy. A boy could work them and turn out 200 rivets an hour easily, all heated evenly to a bright yellow heat in perfect condition for the hydraulic machines, three or four of which were fully kept going by one of these little furnaces. They were taken inside the tubes, and the smoke, if care was taken in adjusting the burner, could not molest any one.
Larger furnaces were set up in No. 2 shed for heating angles and tees, and they were very successful owing to the regularity and evenness of the flame and the facility with which they could be lighted and kept up.
For the hand-rivetters in the struts and lattice-girders, ordinary small forges were in use, with bellows worked by a treadle or by the hand. But here also the oil-furnaces came in usefully, for not only were the rivets pre-heated in the furnace if one happened to be anywhere near, but the boys contrived to make a connection somewhere with the compressed air pipes, and thus obtained a constant blast for their forges and saved themselves the trouble of working the bellows.
In places where so large an amount of timber staging was required, and where a fire on such staging might have been accompanied by the most disastrous results to portions of the permanent work, such furnaces as above described are of inestimable value, for there is nothing left behind that could cause a fire after the men left work. As soon as the supply of air and oil are turned off all flame at once disappears, and in five minutes the inside is black and cold.
The ordinary small furnaces used in the tubes and on the staging were about 11 in. wide by 8 in. high and 4 ft. long on the inside, with two doors for charging and drawing of rivets. They consumed about two gallons of oil per hour, with which they could heat 200, and on an emergency 250 rivets per hour, or even 300, if of smaller size.
The actual size of this kind of furnace is however a matter regulated entirely by circumstances, and it is not possible to lay down hard and fast rules for their construction. The furnaces and fittings shown in the illustrations are:
A furnace for heating the ends of angle bars, tees, or narrow plates. (See Figs. 144 to 146.) An early form of oil furnace with grate at bottom to keep a body of glowing coal to assist combustion. This however is not necessary. (See Figs. 147 to 149.) The latest form of small rivet furnace as used on the stagings and inside the tubular members. (See Figs. 150 to 152.)
Figs. 153 and 154 show the disintegrator, or spray producer, the action of which explains itself. In its present form it is much simplified, and if any obstruction in the small jet occurs, it is easily cleared by screwing back the mouthpiece.
Temporary Work in Connection with the Erection.
If the proud boast is perfectly justified that in building the two 1710 ft. spans across the Forth, and erecting the 11,600 tons of steel massed therein, without having placed a single stick of timber into the river, it is yet equally certain that this mode of erection entails an immense amount of auxiliary and temporary work, which is both costly and wasteful with regard to time. There were many days
