W. J*. Troicbridge — Design for a Bridge. 265 



each, or 102 rods A T and 102 rods B T. The rods for the outer tubes 

 are 2f inches diameter, and those for the inner tube 3^ inches diame- 

 ter. These latter being heavier, because double weight, or half the 

 weights of both roadways will be borne by the middle system of rods. 



The rods are all kept from deflection under the action of their own 

 weight by braces, shown best in sketch 2. The object of this will be 

 explained in the j^roper place. 



It will now be seen that the structure A T B, consisting of the lower 

 chords, the towers and the stay-rods (all of wrought iron) constitute 

 a homogeneous structure entirely independent of the pier, but resting 

 upon it — the pier, througli the anchoring rods, forming the counter- 

 weight, Avhich i^revents the overturning of the half-span when it is 

 loaded. 



This structure is first to be examined xmder the action of its own 

 weight. 



1. The horizontal chord from A to D is sustained by the stay- 

 rods A T, and there will be developed in this chord neither a bending 

 movement nor shearing force, or rather the shearing force will be dis- 

 tributed equally along the chord at the points of suspension, and the 

 only strains or stresses that need be taken into account in this chord 

 are the thrusts which increase uniformly from A to D. In a strut of 

 this length the yielding under pressure is apt to take place by bend- 

 ing. The bending cannot take place laterally, because the three hor- 

 izontal chords are firndy braced by diagonals in this direction. 

 Neither can the bending take place downwards at any point ; and 

 the only yielding that can occur will be from the rising of the middle 

 of the chord. To coixnteract this tendency, a light truss shown in 

 the drawing is placed upon each chord, forming part of it. These 

 trusses form at the same time the side railings or guards of the bridge. 



The tension or stay-rods A T will evidently sustain all the perma- 

 nent load, including their own weights, this load being transferred to 

 the pier through the action of the counter-i-ods B T and the anchoring- 

 rods. The tensions of these rods will all be equal, if we neglect the 

 weights of the rods ; and the stresses upon the lower chords, the tower, 

 and any stay-rod, will be relatively as the sides of the right-angled 

 triangle formed by the stay-rod, the chord at bottom, and the tower. 

 For the longer rods the upper joints or sections should be increased 

 slightly in diameter, since they have to bear, as a part of the perma- 

 nent load, their own weights. Under this condition of things no 

 cross strain can come upon the tower, and the thrusts will diminish 

 uniformly from the top to the bottom. 



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