I 



THE FORTH BRIDGE. • 423 



tageous width for the superstructure. Since the fall of the Tay Bridge, 

 engineers generally, and the Board of Trade in particular, have vividly 

 realised the fact that the severest wrench to which a railway viaduct is 

 subject arises not from the vertical stress due to the loading of both lines 

 of rails with locomotives throughout, but to the diagonal stress due to 

 the combined action of the ordinary rolling load and a violent hurricane. 

 In the case of the Forth Bridge this stress would act at an angle of about 

 45°, so that, were it not for the dead weight of the structure, the 

 required strength would be the same horizontally as vertically, and the 

 economical width would be the same as the economical depth. Although 

 the dead weight modifies this conclusion, it was obvious that the bridge 

 should be a continuous girder of varying depth on plan as well as on 

 elevation, and investigation showed the economical width of superstruc- 

 ture to be about 32 feet at the centre, and 132 feet at the piers. 



It was open to consideration whether the wind stresses should be 

 resisted by bracing together both the top and bottom members of the 

 girder, or the bottom members alone. The author, however, never had 

 any doubt that, as the stresses must sooner or later be brought down to 

 the masonry piers, they had better be brought down at once by the 

 shortest route along the bottom members only. The top members are 

 therefore spaced at the distance of from 33 feet to 27 feet apart, centre to 

 centre, and are unconnected by wind-bracing. Each of the main vertical 

 and diagonal struts consists of a pair of tubes spread out at the base like 

 a bridge pier, and the wind stresses on the bracing between the tubes 

 are much reduced thereby. In like manner are the wind stresses on 

 the bracing of the bottom member reduced by the spreading out of the 

 legs of the cantilevers, and the general stresses on the web members 

 by the tapering depth from the piers towards the ends of the canti- 

 levers. 



Having thus blocked out the general outline of the girder so that the 

 shearing stresses from the diagonal action of the wind and load should 

 be largely taken up direct by the, main members, the next point was to 

 determine the number of bays, and the angle of the web bracing. It was 

 not infrequently assumed that 45° was the most economical angle, but 

 this was true only when the admissible stress was the same in compres- 

 sion as in tension, which was not the case either with wrought iron or 

 steel, or where, as in the instance of wind-bracing, the diagonals were 

 subject to alternate compressive and tensile stresses. American bridge 

 builders, as a rule, disposed their long slender struts vertically, and their 

 diagonal ties at an angle of 45°, and as the question of competition 

 entered into the problem, it might safely be assumed that economical 

 considerations dictated this arrangement. The general slope of the web 

 membei's of the Forth Bridge was something between the angle of 45° 

 and the vertical, and, although the author was not prepared to contend 

 that the disposition adopted was the most economical attainable, yet he 

 was satisfied that it reasonably approached that limit. 



Whatever the angle of the bracing, it was quite clear that, in a girder 

 of 1,700 feet span, exceptionally long struts would have to be provided, 

 and it was a matter of much importance, therefore, that the struts and 

 compression members generally should be of the most economical and 

 efficient form. The advantages offered by a circular form of cross-section 

 were self-evident. A flexible sheet of drawing paper, simply rolled upon 

 itself, became transformed into a stiff column, as everyone accustomed to 



