NATURE 



THURSDAY, JULY 8, iSSo 



THE TA V BRIDGE 



THE Report of the Court of Inquiry appointed to 

 investigate the circumstances of the fall of the Tay 

 Bridge last December has now been made public. There 

 appears to be some difference of opinion amongst the 

 members of the court respecting the scope of the inquiry 

 and the duties placed upon them by the Board of Trade, 

 in consequence of which two separate reports appear 

 together, one by Col. YoUand, Chief Government In- 

 spector of Railways, and Mr. Barlow, President of the 

 Institution of Civil Engineers, and the other by Mr. 

 Rothery, the Wreck Commissioner. The former report 

 describes in detail the design and method of erection 

 adopted in the bridge, giving also a description of the 

 various alterations in the plan which were rendered 

 necessary as the worlc progressed. 



The bridge was 3,465 yards in total length, divided into 

 86 spans, and it was the central portion, of 3,149 feet in 

 length, which fell on the evening of December 28. As 

 originally designed, this central portion was to consist of 

 lattice girders of 200 feet span, carried by brickwork piers 

 somewhat over So feet in height from [high-water level, 

 but as the river bottom turned out to be different from 

 what was expected from the borings, and the difficulty of 

 obtaining a secure foundation greater, eleven spans of 245 

 feet and two of 227 feet were substituted, and braced iron 

 piers were adopted in the place of brickwork, as imposing 

 a less weight on the foundations. It is these piers which 

 at the inquiry chiefly received attention, as there can be 

 little doubt that they were the immediate cause of the 

 catastrophe. The process of floating out and sinking the 

 caissons for these piers has already been described in 

 these columns, and so successfully was this — certainly the 

 most difficult and hazardous part of the imdertaking — ■ 

 accomplished, that no suggestion of insufficient strength 

 has been made, and in the Report it is stated that there 

 is nothing to indicate any movement or settlement in 

 the foundations of the piers which fell. 



The caissons were lined with brickwork and filled with 

 concrete, on which was built a hexagonal pier of masonry 

 carried up to 5 feet above high-water mark. Upon this 

 pier was built up six cast-iron columns secured by holding- 

 down bolts to the masonry at the angles of the hexagon. 

 The columns were made up of lengths united by flanges 

 and bolts, and connected with each other by horizontal 

 struts and diagonal ties. The up-stream and down- 

 stream columns were each 18 inches in diameter, the 

 remaining four, 15 inches; all were inchned 12 inches 

 inwards at the top. The piers thus formed were from Si 

 to 83 feet in height from the top of the masonry to the 

 under-side of the girders. The diagonal bracing consisted 

 of flat bars attached to the columns by means of " lugs '' 

 cast on them, being secured at one extremity by a screw- 

 bolt passing through the lugs and bar, and at the other 

 by a strap provided with a gib and cotter for tightening 

 up. The horizontal struts consisted of two channel-bars 

 bolted back to back to a single lug on each column. 



It Avill thus be seen that all vertical load must be borne 

 entirely by the columns, and with the exception of the 

 Vol. XXII. — No. 558 



small transverse resistance of the latter the whole of any 

 lateral pressure must be transmitted by the bracing. 



Whether as designed the bridge would have been 

 strong enough for its work if the materials and workman- 

 ship had been good throughout is very doubtful, but, as 

 carried out, the evidence shows distinctly that it was not 

 sufficiently substantial for the heavy traffic and severe 

 gales to which it was exposed. When everything was 

 tight and in good order the bridge, at the time of its 

 inspection by General Hutchinson in February, 1S78, 

 showed great rigidity under the tests imposed by him, 

 but by October of the same year so much slackness had 

 made its appearance in the bracing that, besides the 

 ordinary keying-up by driving the cotters, more than 100 

 packing-pieces about three-eighths of an inch thick had 

 to be introduced in different parts. 



Respecting the immediate cause of the accident the 

 Court states — " In our opinion the weight of evidence 

 points out the cross bracing and its fastening by lugs as 

 the first part to yield." This we believe the calculations 

 of Dr. Pole and Mr. Stewart, taken in connection with 

 the experiments of Mr. Kirkaldy, are quite sufficient to 

 establish. With a wind pressure of 30 lbs. to the square 

 foot on the windward girder and train, and half this 

 amount on the leeward girder, the stress on the tie-bar 

 most severely strained, would be i6'8 tons, or lo'iS tons 

 per square inch ; again, with a wind pressure of 40 lbs. to 

 the square foot the stress on the tie-bar would be 22'4 

 tons. Now, as Mr. Kirkaldy's experiments, made by 

 order of the court on some of the tie-bars removed from 

 the bridge, showed that they broke with a load of from 

 19 to 23 tons, and the corresponding lugs with a load of 

 23 to 25 tons, it is pretty certain that the ultimate strength 

 of this part of the structure would be reached by a wind 

 pressure of 40 lbs. to the square foot. And in addition to 

 this more variation is to be expected in the strength of 

 the lugs, as some at least were admitted to be of bad 

 manufacture, and when the pier was most severely 

 strained it would be some of the worst lugs in the lower 

 tiers that would be the first to yield ; thus the samples 

 taken for testing would not be likely to embrace speci- 

 mens of the lowest strength, as these would probably have 

 already given way. 



Again, it does not appear necessary- to assume a wmd 

 pressure of 40 lbs. per square foot to ensure the destruc- 

 tion of the pier ; the stresses above mentioned are due 

 merely to the statical pressure, and it can hardly be 

 denied in the face of the evidence respecting the details 

 of the structure that there would be a great deal of motion 

 due to backlash over and above the elastic yielding of the 

 material. Thus a much lower pressure would produce 

 the elTects calculated for one of 40 lbs. per square foot. 



The principal conclusions arrived at by the court are 

 that there is no indication of settlement [in the founda- 

 tions, that the wrought iron employed was of fair strength, 

 though not of high quality as regards toughness, that the 

 cast iron was fairly good, that the main girders were of 

 sufficient strength, and that the iron piers, though strong 

 enough to sustain the vertical load, were insufficient to 

 resist the lateral action of heavy gales from the weakness 

 of the cross bracing and its fastenings ; that the railway 

 company did not enforce the recommendation of General 

 Hutchinson by limiting the speed of trains over the 



