CHAP. XVI.l THE INTERTED ARCH. 331 



CHAPTER XVI. 



THE INVERTED AECH SUSPENSION SYSTEM. 



183. The inverted arch forms the supporting member of 

 chain or cable suspension bridges. Whether the cable be com- 

 posed of chains, links, or wires, we suppose them so flexible 

 that they can perfectly assume the curve of equilibrium. As, 

 therefore, disregarding the dead weight, any partial load would 

 cause a change of shape, the cables must be stiffened in order 

 to prevent the motion which would otherwise take place. 



We may stiffen the chains, as shown in PI. 26, Fig. 106, by trian- 

 gular bracing, thus making a rigid system ; or we may have two 

 parallel chains and brace them to each other, as shown by Fig. 

 90 inverted ; or we may introduce an auxiliary truss, the office 

 of which is not to add in any degree to the supporting power 

 of the combination, but simply to distribute a partial load over 

 the whole span, so as to cause it to take effect as a distributed 

 load, and thus prevent change of shape. 



As in the first and last cases the structure is commonly 

 hinged at the centre in order to eliminate the effects of tem- 

 perature, the method of resolution of forces explained in Arts. 

 8-13 will, in general, be applicable for the determination of 

 the strains. 



In the second case, we can apply the principles of Arts. 158- 

 161. 



The rear chains, anchorages, and stiffening truss deserve, 

 however, special notice. 



184. Rear Chains and Anchorages. The greatest ten 

 sion in the main chains occurs, of course, for full load. To 

 find the tension at top of tower, as also the horizontal pull, we 

 have simply to lay off half the whole load vertically from o to 

 d [PI. 26, Fig. 106], and then draw O o horizontal and O d 

 parallel to the last side at tower. Then O d is the tension in 

 that side, and o O the horizontal pull. This pull is neutralized 

 by the opposite and equal pull of the rear chain leading to the 

 anchorage ; provided, as should always be the case, it makes an 



