1864.] 



121 



February 4, 1864. 



Major-General SABINE, President, in the Chair. 



The following communication was read : — 



"Experiments to determine the effects of impact, vibratory action, 

 and a long-continued change of Load on Wrought -iron Girders/' 

 By William Fairbairn, LL.D., F.U.S. Received January 20, 

 1864. 



(Abstract.) 



The author observes that the experiments which were undertaken, nearly 

 twenty years ago, to determine the strength and form of the Tubular Bridges 

 which now span the Conway and Menai Straits, led to the adoption of cer- 

 tain forms of girder, such as the tubular, the plate, and the lattice girder, 

 and other forms founded on the principle developed in the construction of 

 these bridges. It was at first designed that the ultimate strength of these 

 structures should be six times the heaviest load that could ever be laid upon 

 them, after deducting half the weight of the tubes. This was considered 

 a fair margin of strength ; but subsequent considerations, such as generally 

 attend a new principle of construction with an untried material, showed the 

 expediency of increasing it ; and instead of the ultimate strength being 

 six times, it was in some instances increased to eight times the weight of 

 the greatest load. 



The proved stability of these bridges gave increased confidence to the 

 engineer and the public, and for several years the resistance of six times 

 the heaviest load was considered an amply sufficient provision of strength. 



But a general demand soon arose for wrought-iron bridges, and many were 

 made without due regard to first principles, or to the law of proportion ne- 

 cessary to be observed in the sectional areas of the top and bottom flanges, 

 so clearly and satisfactorily shown in the early experiments. The result of 

 this was the construction of weak bridges, many of them so ill-proportioned 

 in the distribution of the material as to be almost at the point of rupture 

 with little more than double the permanent load. The evil was enhanced 

 by the erroneous system of contractors tendering by weight, which led to 

 the introduction of bad iron, and in many cases equally bad workmanship. 



The deficiencies and break-downs which in this way followed the first 

 successful application of wrought iron to the building of bridges led to 

 doubts and fears as to their security. Ultimately it was decided by the 

 Board of Trade that in wrought-iron bridges the strain with the heaviest 

 load should not exceed 5 tons per square inch ; but on what principle 

 this standard was established does not appear. 



The requirement of 5 tons per square inch did not appear sufficiently 

 definite to secure in all cases the best form of construction. It is well 



VOL. XTII. L 



