THE TAY BRIDGE.

Kumara Times, Issue 1030, 20 January 1880, Page 4

THE TAY BRIDGE.

The following are further particulars respecting the above bridge, the fall of a portion of which caused the death of over ninety persons : The Tay bridge was the longest in the world, its length exceeding two miles. IndeedTt has. been well described as a “ stupendous railway over the sea,” for the estuary of the Tay which it crosses is more an arm of the sea than a mere river. It was regarded by English engi neers, with just pride, as one of the greatest triumphs of British engineering and one of the wonders of the world. The Tay River—or rather estuary—at the point where it is crossed by the famous and fatal bridge, is a little over two miles broad. Rocks, which on the Fife side rise out of the water, and on the Dundee side approach the surface, disappear as the middle of the river is neired, until at length they are found too far below the river-bed to be of much use as foundations for the piers of the bridge. But there is above the rocks a stratum of gravel 18ft thick, of which an engineering writer remarks, that it is “ quite able to bear the weight put upon it, and so serves the purpose for foundations although not of the same solidity as the firm and enduring rock.” Whether this, after all, proved a source of weakness to the structure has yet to be learned. The bridge itself is of no fewer than 87 spans, suppcr'ed by 85 piers, the total length being 10,612 ft or two miles and 62ft. Of these.B7 spans 74 are of varying length, raging from 69ft to I7ofr. But over the middle of the river, the navigable channel, there are 13 spans, e>ch 246 ft long, supported by piers so lofty that at the highest tide there is clear waterway of 88ft perpendicular, which gives abundant space for the steamers and small vessels which ply between Perth and Newburg. Tnere seems no doubt that tbese ; were the thirteen spans, which are recorded to have given away. Extending as they did over the navigable channel in the middle of the estuary, they would be exposed to the full fury rf the tempest, which would have all the more poser Owing to the enormous unsupported length of each span (245 ft), and the resistance offered by the passing train. How it came about that ail the thirteen spans gave way together has yet to be explained. The bridge underwent the most severe and stringent tests by the Government authorities before it was allowed to be used for public traffic. One consisted in crossing it with six goods engines belonging to the North British Railway, These are the heaviest and most powerful goods engines in the kingdoro, each weighing with its tender no leas than 71 tons, and were built expressly for the exceptionally steep gradients of the Waverly portion of the line. The six engines weighing in the aggregate 426 tons—more.than double the weight of an ordinry train, and four times that of the one which' was ass .dated with the disaster— were not only run across the bridge, but were rested for a time on each of the 87 spans, and the deflection was practically inappreciable. This renders the collapse of a structure so severely and successfully tested all the more remarkable unless the foundations, which, as we have shown, were not considered all that could be desired, proved unequal (to the continued strain and gave away on its beingenchanced by the pressure a furious storm meeting th» resistance of the passing train. It is idle, however, to indulge in mere conjectures, and we must be content to await the arrival of full explanations by mail. Proceeding with out account of the ill fated bridge we may mention that it was projected by the directors of the North British Rail way Company to save a considerable amount of time, distance, expense, and trouble on the journey between Dundee and the South. It is estimated that they effected a clear saving of at least ,£40,000 per annum in working expenses through the erection of the bridge, while gaining many, other advantages. The contract was taken in 1871, but unforseen delays arose, and operations were not actually commenced till August, J 875, the contract having been transferred (on the death of Mr De Bergne, the original contractor) to Messrs Hopkins, Gillkes, and 0«»., Mr A. O.E. (from whose report we glean the present particulars) being the P?®° a .glpg engineer. In two years it was completed, and the first train crossed it on 25th September 1877, although it was not formerly opened for traffic until 31st May, 1878. The special principle which was adhered to throughout, and which Mr Grothe, thinks “ was of the greatest bearing on the success of the work,” was to dispense with the usual staging and scaffolding, and instead to erect all the piers and girders on shore subsequently floating them to their des tinalion. “No matter of what material the parts were constructed,” says Mr Grotl.e, “ whether they were iron receptacles for concrete, huge lumps of brickwo’rk weighing above 200 tons, or iron girders of 190 tons, they were all finished on shore, and floated to their destination, the principle was adhered to as the only which could produce good results in a tidal river, subject to such vicissitudes, and with continually shifting sandy bottom. Some of the piers were of brickwork, set cemen * ; j others iron cylinders the lower portion filled with concrete or bftckwork. The bridge itself was constructed on the lattice girder principle, it course being a long curve with the convexity to the westward. The girders were raised to their position by hydraulic power at the rate of 20ft per day. Every possible care and forethought seems to baye been exercised on this gigantic bridge, and _ alt available 'inodern engineering skill appliances

brought to bear in its construction. That, nevertheles, it should have callapsed so disastrously, and with consequence so fatal, will tend somewhat to shake the popular faith in the infallibility of modern engineers, whose readiness to undertake the most stupendous works that the human imagination can conceive has grown into what has been well called “ a splendid audacity.”