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THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[April, 



With respect to Mr. Taylor's system of paving, it was contended, that the 

 Mount Sorrel gianitc was a very superior material, both as regarded its 

 toughness and durability, and that its natural structure enabled it to be 

 worked very advantageously into the small cubes. The main feature of the 

 system was the selection of the material for the substratum, and the careful 

 preparation, so as to afl'ord a sufficiently rigid, but yet imperceptibly elastic 

 bed, whereon the small cube stones should rest. These stones being well 

 driven down by repeated blows of light rammers, attained a degree of solidity 

 which defied tlie heaviest traffic; and in the towns where the system was 

 employed, considerable economy had resulted. The surface of the paving 

 approached as nearly as possible to tliat of a macadamised road, affording 

 even a safer foothold to the horses, and with less noise of passing vehicles. 

 The surface possessed extraordinary durability, and it might be considered 

 as a solid mass of granite. It was announced that within a few weeks there 

 would be specimens of Mr. Taylor's system of paving laid down at the 

 entrances of llyde Park, where they would be subjected to regular traffic of 

 a destructive nature, and which would be under constant observation. 



A model of an improved Crossing Point was exhibited by Mr. Duncan, of 

 Leeds; the notch in the rail was shown to be done away with, and the two 

 rails in it were so dovetailed together, as to render any vertical motion 

 between them impossible, thus materially strengthening the crossing. 



A piece of brickwork, set in Greavc's blue lias lime, and which had been 

 kept under water for nine days, was also exhibited. This material was com- 

 posed of one-third of lime to two-thirds of burnt clay; and it was stated to 

 have been used with great success in the tunnels on the Great Northern 

 Railway, as well as in many hydraulic works, in which it was as durable as 

 cement. 



March 12. — The paper read was, " On Tubular Girder Bridges." Bv Mr. 

 W. Fairbairn, M. Inst. C.E. 



The author commenced by stating, that the chief points to be taken into 

 consideration were: — First, the application of a given formula, for computing 

 their strength; second, the excess of strength that should be given, over the 

 greatest load that could be brought upon the bridge; and, third, the effects 

 of impact, with the best mode of testing the strength, and proving the 

 security of the bridge. 



In the first place, it had been determined by experiments, that, in order to 

 balance the two resisting forces of tension and compression, in a wrought- 

 iron tubular girder, having a cellular top, the sectional area of the bottom 

 should be to the sectional area of the top, as eleven to twelve; and that 

 until this proportion existed, the usual formula could not be applied; this 

 formula was, that the breaking weight was equal to the total area, multiplied 

 into the depth, and into a constant (80), and divided by the length of the 



girder (w=-j-\. 



Considering the particular ease of the Torksey bridge, the mean sectional 

 areas of the top and the bottom, being respectively 51-08 square inches and 

 54'93 square inches, the latter was in excess of strength over the former, so 

 that a reduction of the area of the bottom from 54'93 to 46'76 square inches 

 Slight have been made with propriety, and would have been in conformity 

 with the formula. By calculation, the ultimate strength of the bridge was 

 found to be 1,1.'>2 tons, whilst the greatest total load, including the weight 

 of the girders, Sic, was only 372 tons; this gave a strength, greater than the 

 heaviest rolling load that could be brought on the bridge, in the proportion 

 of nearly five to one. Although, therefore, the proportion of the girders 

 was not exactly that which the author recommended, he considered that 

 "they were, nevertheless, sufficient to render the bridge perfectly secure." 

 This conclusion was arrived at without taking into consideration, the amount 

 of additional strength derived from the continuity of the girders, across the 

 central pier. The exact proportions recommended were given in two tables 

 extending respectively to spans of 150 feet, and of 300 feet. The depths of 

 the girders of the first class were taken at one-thirteenth of the span, and 

 those of the second class at one-fifteenth of the span. The author then 

 investigated the effects of impact at different velocities. It did not appear 

 that experiment established the fact of increased deflection at high velocities 

 for in several experiments on a large scale, he had found the deflection as 

 nearly as possible the same at all velocities. He concluded by recommending 

 that the tests to be applied should never exceed the greatest'load the bridge, 

 was intended to bear. 



Remarks. — In the opening of the discussion by Mr. Fowler, Mr. Bidder 

 and Mr. Eaton llodgkinson, it was remarked, that satisfactory as it was to 

 have the confirmation of Mr. Fairbairn's authority, for the perfect safety of 

 the bridge for all purposes of traffic, it would have been desirable, that he 

 should have extended his calculations a little further, into the question of 

 the increased strength derived from the continuity of the girder, across the 

 central pier, which augmented the total strength fully one-fourth. It was 

 also argued, that the excessive proportion of the bottom of the girder, 

 although not an economical disposition of material, was in itself an im- 

 portant addition to the strength of the girder. 



The definite i)roportions assigned in the paper for girders were disputed, 

 and the attempt to assign empirical rules for the practice of engineers, in 

 structures of this novel character, was earnestly deprecated. 



It was important also to remember, that the large proportion of the 

 hottom of the beam brought into action a corresponding quantity cf the 



upper part of the side plates, in aid of the top. Thus it appeared, that if 

 the subject had been pursued further, the proportion of five to one bv 

 which the proportional strength of the beam, over the rolling load, was 

 represented, would have been, from various causes, materiallv increased. 



March 19. — The subject of Mr. Fairbairn's paper was resumed. Messrs. 

 M'ild, Pole, Ilennie, Scott Russell, Eaton Hodgkinson, Walker, GIvnn 

 Bidder, Professor Willis, General Pasley, and Captain Simmons, K.E.' 

 examined the question at great length, and under all views, illustrating their 

 position by diagrams and models, used in the experiments and in the mathe- 

 matical investigation. 



It was stated, that after the remarks made at the last meeting, it was 

 merely requisite to describe the experiments alluded to, and before doing so, 

 to briefly describe their object. 



In the Report of the Government Inspector, the limiting strain required 

 for the public safety was defined, and the Torksey bridge had been con- 

 demned for not complying with those conditions. A calculation, therefore, 

 had been made to ascertain the actual strain on the bridge. It appeared, 

 however, that it was really less than the limit prescribed by the Government 

 Inspector. The experiments instituted were for the purpose of testing these 

 contrary results. It was also stated, that in the paper there were manv 

 objectionable points, but particularly one that was positively dangerous. 



The author bad not only omitted the effect of the continuity of the 

 Torksey girders, but stated, that it was sa/er to do so. Now all writers 

 upon the subject, and all who had considered the matter, agree that in a 

 continuous beam the effect of continuity was most important, and that in a 

 perfectly continuous beam, the strain over the supports was even greater 

 than elsewhere. It was therefore submitted that this was not the part, the 

 consideration of which it could be "safer to omit." 



The form taken by a continuous beam, when uniformly loaded, was 

 convex over the supports, and concave between the points at which the con- 

 vexity ended; at these points of contrary flexure, the horizontal strains were 

 null, and the beam might then be severed, without altering its condition. 

 The virtual length of the beam, in the Torksey bridge, was determined by 

 the distance between the exterior support and the point of contrary flexure; 

 and it was to determine this point practically that the experiments were 

 instituted. It was shown that this point was 21| feet from the centre 

 support, and that lience the length of the beam was reduced from 130 feet 

 to 108J- feet. 



The compressive strain upon a girder of this length, loaded as prescribed, 

 was 4| tons per inch, being less than the limit defined. Consequently, it 

 was asserted, that the Railway Company to whom this bridge belonged, had 

 been deprived of its use, not in consequence of any omission on the part of 

 their engineer, but in consequence of the inability of the Government autho- 

 rities to appreciate the strength that had been provided. 



In reference to the application of formulE to the calculation of the 

 strength of the girders, it was considered desirable, in such an important 

 case, not merely to form a general approximate notion of the strength of the 

 bridge, but to ascertain, with all possible exactness, the nature and amount 

 of the strains to which the structure was exposed; and this could only be 

 done, by using a comprehensive process of calculation, which should 

 embrace all the elements affecting the strength of the bridge. 



The effect of the continuity of the girders over the two openings, was 

 carefully considered, and the nature of its effect upon the strain was 

 explained, as deduced from the application of the most modern mathe- 

 matical investigations, and it was demonstrated that the strength of the 

 beam was thereby augmented above one third. 



It was then shown, how the rules for estimating the strength of elastic 

 beams, were rendered applicable to the case of the Torksey bridge, and the 

 results proved, that when the bridge was weighted with the load prescribed 

 by the Government authorities as a test for its strength, the strains of com- 

 pression and extension were only one half of what competent authorities had 

 stated might be safely applied. 



The diagrams exhibited, shewed the results of mathematical calculation, 

 as applied to the Torksey bridge girders, and the remarkable coincidence of 

 these, with experimental results obtained by other investigators in an 

 entirely different manner, was insisted on, as a proof of the correctness of 

 the conclusions arrived at. 



It was stated, in reply to a remark upou the increased deflection due to 

 velocity, that the result of the experiments tried by the "Cast-Iron Bridge 

 Commission," proved, that "this increase was wholly insignificant in beams 

 of the length and stifl'ness of those of the Torksey bridge." 



The discussion was summed up by its being stated, that, with one 

 exception, all those who had spoken during both evenings, agree that the 

 formula given in the paper was empirical and not trustworthy; that the 

 effects of percussion and increased velocity were practically only shadowy 

 visions; and as it was admitted, that in the calculations of the Government 

 Inspector, the effect of continuity was neglected, and as it had been proved 

 that the strain was less on the bridge than that assigned as requisite for the 

 public safety, and that it was, in fact, amply strong, it was evident, that the 

 public had been wrongfully deprived of the use of the bridge, and the Com- 

 pany had been prohibited from gaining the just return for the capital 

 invested, in consequence of an incomplete investigation, and the assumption 

 of untenable formula;. 



