The Tower Bridge.

Manawatu Herald, 28 October 1893, Page 2

The Tower Bridge.

Tho following description, of the new bridge over the Thames below London bridge is taken from 'lhe People. The new Tower Bridge, which spans the Thames just beyond the famous Tower of London, and which, in its magnificent proportions is already the wonder of those who pass beneath it, is one of the most remarkable engineering exploits of our time. In its stupendous size, no less than its method of construe tion, it has no parallel in any part of the world. In the first place the two magnificent towers reaching from the ground level to a height of over 200 fret, are the most prominent objects in the eastern approach to the City, and even dwarf the celebrated tower. It will be imagined by the casual observer that these pieces of massive masonry are the real supports of the bridge, and the sweeping tte girders which curve gracefully from the towors on? third of the way across the river to lhe land ends. This, however, is not so, and to understand the principal of construction it must be borne in mind that the masonry is a mere ornamental shell, and is actually supported in each case by four steel columns built inside at the nngles of the square. The columns, architecturally, a"'o of the early Gothic, and in some suggestive details of the massive Norman type, though there i3 a Venetian lightness in the balconied upper storey. The archways beneath, and especially the arch over the approach on the Surrey side, are of the Norman pattern, and the castellated or embattlemented ornamentations are also picturesquely suggestive of the /% early period, and are appropriate to the background formed by the tower. It was necessary that the last of the bridges should be constructed so as to open to allow the passage of vessels, and while this has been accomplished by two " wings," or, drawbridges, it has also been designed so that even when the bridge proper on the roadway level is open so that foot passengers may not be delayed, they will be able to ascend the towers by an inside lift, 14ft by 6ft, and cross over by the raised footbridges about 140 ft above the high water level, so that the bridge provides for constant and uninterrupted traffic both for foot passengers and any class of shipping. The most remarkable part ot the scheme is that which provides for the raising into the air, so as to rest perpendicularly against the sides of the' towers of these ponderous leaves or 4 wings which form the carriage-way or main bridge, meeting in the centre to a fraction of an- inch. ~F!H?§t,~vbe it understood, that the width >©f -this opening span is 200 ft., the width between the parapets 50ft. These can be raised in a couple of minutes by means of the enormous counterbalance of steel and lead built within the masonry of the piers. The counter-balance is set in motion by hydraulic power, the w^ater being pnmped into the accumulators by steam-engines. The weight oi each leaf, including the roadway and counter-balance, is no less than 950 tons. The total length of the bridge, with the approaches, is just half a I mile; opening span, 200 f t; headway above high water when open (this is the height to the upper foot bridges), 139 ft Gin ; headway above high water when shut (this is the height to the

carriage way or lower bridge), 29ffc ! ftin ; eide 3pans arc each 270 ft long with a headway above high water of from 20 to 27 ff* The width h=(w-rn J the parapets of the opening span is i 50fc, and ot tho side spans and ! arctics, 60ft. The steepest gradient j of the approaches is 1 in <iO, while j that of London Bridge is 1 in 27. . The foundation ? extend 27ft below; the bed of the river. Iho sectional area of the water-way is 20,010 ' square feet, while that of London j Bridge is 10,800. The depth of! water in the opening span at high ! water is 85ft 6in and at low water : it is 80ft Gin. The, estimated «|imn- ' tities of material used in tho eonsfcruction of the bridge and ap- • proaches are as follows :— Bricks, • 81,000,000; concrete, 70,500 cubic; .yards; cement, 19,500 tons ; granite J and other stone, 235,000 cubic feet ; ' iron and steel, 15,000 tons ; number of rivets used, 2,000,000. From tho ! ground to the extreme top of the towera, or cresting, is 220 ft. The footpaths on each side of the carriage .way are 12ft wide. The lifts will carry from twenty to thirty persons at a time. The bridge is designed by Mr Wolfe Bairy, engineer-in chief. The iron and steel work, which forms the principal part of the entire structure, is supplied by Sir William Arrol and Co., Limited, contractors, of Glasgow ; and a remarkable and wonderful feature of the work is that the whole of the parts are made in Glasgow and brought round by steamer and iitted here. Considering that some of these ponderous pieces of five and six tons have to fit to the eight of an inch, the accuracy of the measurements has been astonishing. Particularly was this so in the making or building of the upp r foot bridges. Thesa upper bridges w.?re built out simultaneously from either tower foot by foot and inch by inch, until they mot in the centre, and so accurate were the measurements that when the iron lattice work and girders met in the centre 150 feet above the river, they joined, and the ends of the girders were found to be only one-eighth of an inch apart in a temperature of OOdeg. Fahr. The method of building on 1 ; the upper bridges was briefly this : small cranes were in the first place put on top of the steel columns on each of the large masonry piers, so that tho first portion of the footway cou'd be put into position and fixed, a j travelling stage carrying a small ; hand crane which was put on the ' overhanging portion. The lower ; part of this staging formed a platform j on which the workmen could stand ' to bolt up the steelwork as ie was put into its place by Ihe crane above them, piece by piece. The travelling ; crane was drawn forward to bolt another piece in position as ea-h sage was completed. Tho central girders were then placed into position after measuring the riwr width ! with the astonishing result a* ptaterl above. The total weight of the stoUwork in these upper footways is about 650 ton?, and all this stupendous mass of metal was ' •• flung in air " without any staging or exterior support whatever. The total load on the foundations of each pier is estimated at 68,700 ton?, the required area of each foundation is given by making the pier 204£fr. long by 100 ft. wide, thus forming the largest pier of its kind in the ; world, and one which in urea is . equal to the four large piers of the Forth bridge taken together. The work of first casing was commenced in Sept., 1886, and it was not until Jane, 1890, that both piers were completed op to the limits of the contract, viz., 4ft. above T. H. W. Thd total cost of the two piers, including all temporary works, was £111,122, or £2 8s 7d per cubic yard. The total cost of the bridge was to be £750,000, but this amount, owing to delays, strikes and otherdifficulties, is likely to be exceeded. The Tvbole of the work will not be completed until the beginning of next year. There have been in all ' eight fatal accidents, one for each yJEjijr. The number of workman* engaged has varied from 500 to 800.