Page:Forth Bridge (1890).djvu/62

Rh 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.

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