22 Design for a Bridge [Jan. 



Now as there are 100 suspending rods, this will give 18 tons 

 of weight to each rod. Supposing the greatest load that can 

 possibly come on the road to be 100 six horse waggons weighing 

 12 tons each, this will give an additional 12 tons to each rod ; 

 thus 12 and 18 = 30 tons may, therefore, be considered as all 

 the weight which such rods can possibly, or in all probability, 

 become liable to sustain ; but which weight, as will presently 

 appear, is totally independent of the strain produced by position. 



As it has been ascertained* from experiment that an iron bat 

 will sustain, without injury or derangement to its particles, half 

 the weight which will tear it asunder, we may conclude that a 

 rod requiring 70 tons to break it will be sufficient for No. 1, in 

 Fig. 11, the lowest situation, being that where there is little or 

 nothing more than the perpendicular weight to be sustained. 



But on the rods 13, supposing the short sides or ends of the 

 parallelograms to indicate 30 tons, the long side will indicate 

 100. This, therefore, will demand a rod which would require 

 220 tons to break it. 



Taking as a standard that it requires 28 tons to break a rod 

 of one inch diameter, + then a round rod that will require 70 

 tons to break it must be 1-^ diameter, and one not to break 

 under 220 tons must be 2-^- diameter. An average so as 

 to calculate their total weight is thus easily obtained. The 

 strain on the basis chains of that part of the road between 

 the fulcrums accumulates from one towards 13, and balances 

 itself at 13 ; that is, the strain from 1 at the i one fulcrum 

 to 13 at the centre, is counterpoised by the strains from 1 

 at the other fulcrum to 13 in the centre. The total amount 

 of this strain (by measurement), the bridge being fully loaded, 

 is 660 tons to each bundle of 26 rods. Supposing this strain to 

 be resisted by the two longitudinal rods of the basis at 13 in the 

 centre, they would require to be of 4-^V diameter in that part. 

 To render this unnecessary, advantage is taken of the wrought- 

 iron sides of the frames Fig. 7, which, being all closely and 

 strongly screwed together, and to the basis of rods, are pulled 

 lengthwise with the rods, and sustain part of their strain. As 

 they are at an average six inches deep by half an inch in 

 thickness, it seems needless attempting to prove their adequacy 

 for this purpose. 



Half the same amount of strain would have to be provided 

 for at each pier or abutment, were it not thrown off among the 

 rods, c, c. Thus, by being divided among 20 rods, c, c, and the 

 two basement rods, D, D, at each abutment, it is easily over- 

 come. Did it all centre in the longitudinal basement rods, 

 D, D, they would require to be of 4-^\ in diameter ; and the 

 difficulty of fixing them, keeping them fixed, and repairing 

 them when tney become deranged, would be inconceivably 

 great. 



• By Brunton, Middlefofi, and Co. and various others. 

 + Dr. Thomson's Annals. 



