870 KEPORT— 1887. 



SATURDAY, SEPTEMBER 3. 

 The following Papers were read: — 



1. The Forth Bridge Works.^ By A. S. Biggart, 



The erection of the main steel piers of the Forth Bridge is now practically com- 

 pleted. The only exception from the lines indicated in my paper two years ago 

 requiring to be noticed is the case of the internal viaduct, which was built in 

 position instead of being raised in a completed state. 



The working of the plant proved satisfactory in all respects. On some occa- 

 sions the piers and platforms were raised as much as 48 feet within eight days. 



A point of vital importance is the starting of the various members of the piers 

 at their proper angle. A slight movement at the point of fixture may be found to 

 produce an initial stress as great as the full working stress. Hence, in setting out, 

 not only had great care to be exercised, but many points involving careful calcula- 

 tion and supervision arose. 



Erection of Cantilevers. — This was commenced before the piers had attained 

 their full height. The bottom member was built on the overhang system, at first, 

 by means of an ordinary crane. Resort was afterwards had to a cage placed at 

 the end of each tube within which the men can work. This cage is secured to rings 

 encircling the tube, and as it is built of exactly similar sections, it can easily be 

 moved forward by removing the two back sections and placing them in front. 

 Above the cage is placed a hydraulic crane for manipulating the beams and plates 

 brought out on a tramway running alongside the tube. 



When fully 100 feet out support was given to the tube by carrying up temporary 

 ties to the main piers. Two kinds of ties are employed, the main object of the one 

 being to assist in the erection of the other, and afterwards serve to carry it. On the 

 completion of the ties the platforms for erecting Bay No. 1 were built immediately 

 over the bottom member, and afterwards raised into position. Subsequent raisings 

 are accomplished by hydraulic jacks. From these platforms the struts, ties, and 

 other parts of the Ijridge are being built. 



A most interesting and important part of the work consists in observing and pro- 

 viding for the vertical movements and stresses of the various members of the canti- 

 levers. Those of the bottom member are alike luimerous and intricate. Fhst there is 

 a deflection due to the tube's own weight and that of the plant : then an upward rise 

 from the pull of the linlc ties ; an upward again, occasioned by the weight of the 

 other tie and its supports hanging on the link tie ; another upward caused by the 

 hydraulic rams raising the tube ; a fall due to the weight of permanent material of 

 the cantilever being built in position ; a rise when the permanent ties will be con- 

 nected, and finally a gradual fall as the cantilevers are built out. These form seven 

 clearly marked movements to be considered and provided for, the ultimate aim 

 being to leave in the tube only the normal initial stress. 



The experience gained from the work of erection already completed has assisted 

 greatly in enabling us to prepare the plant for the future' work. Thus, after full 

 consideration. Sir John Fowler, Mr. Baker, and ;\Ir. Arrol feel warranted in 

 adopting another principle for the erection of the next bay of the cantilevers. 

 Cranes, some at high altitudes above their work, and light cages and platforms, 

 will be the distinguishing feature of the new method. 



2. The City of London and SouthwarJc Stibway.'- 

 By J. H. Greathead, M.Inst.C.E. 



The Subway Company was incorporated by an Act of Parliament in 1884, and 

 authorised to construct a double line of subway from King William Street to the 



' Published i/i extenso in Industi-ics. 



" Published in the E?iginecr, Oct. 7, 1887 ; and in Industries, Oct. 21, 1887. 



