The Grafton Bridge, Auckland.

Progress, Volume III, Issue 7, 1 May 1908, Page 239

The Grafton Bridge, Auckland.

The rapidly-growing city of Auckland is spread over a series of steep hills and narrow gullies. That part of the city which contains, among many residential streets, the fine domain and the public hospital buildings, is separated from the main arteries of traffic by a long and deep gully, the Cemetery Gully, so-called because of the old burial place situated theie. Across this gully goes a small foot bridge in conjunction with hundreds of awkward steps. The only other means of access are the narrow and congested Kyber Pass road, and the steep, and for goods traffic almost inaccessible, Graf ton road. The City Council decided to connect half way between the afore-mentioned roads in a line from the intersection of Karangahape road and Symond street to that of Park avenue and Grafton road A loan of £30,000 was sanctioned by the ratepayers for the purpose of erecting a bridge suitable for heavy traffic at the level of the connected streets. The City Council called for designs and tenders in reinforced concrete and in steel, making the provision that the best design of the unsuccessful material should obtain a prize of £100. The tendering time was extended from Bth May to Bth June, 1907, when two tenders were received — one from the Ferro- concrete Company, of Australasia, for a reinforced concrete design to cost, without pavemnent, £31,890, and one from McLean and vSons, in steel, a typical American trussed girder design on latticed piers, for the amount of £31,000. The tender for the reinforced concrete design was accepted and operations started at the end of July. This design contains some features which make it one of the most important bridges of the day, and we have no doubt will interest our readers. The main dimensions are as follows - - The total length o£ the bridge is 910 ft., the width of roadway 24 ft., with two footways 6 ft. each wide. The roadway is carried by two short spans of 33 ft. 6 in. clear, and four longer ones of 70 ft. 6 in. to the main arch, which has a clear span of 320 ft., and the highest point of which is 140 ft. over the bottom of the gully. The remainder is spanned by two spans of 75 ft 6 in. each. The approach spans are straight giideis 8 ft. 6 in. deep, and lightened by the cuttingout of rectangular panels, leaving a sort of lattice girder without diagonals. The prominent feature of the structure, however, is the arch. The design shows a pair of " three-hinged " aiched rings, carrying the roadway, by means of rectangular piers. The rise of the arch is 60 ft., and the height of the superstructure over the apex of the arch is 30ft. The advantages of the " three hinged " arch are considerable, and some of them we will enumerate. The obviating of unforseen stresses caused by yielding of the abutments or by deformation of the timbering, due to influence of weather and shiinkage , further the ex-

pansion due to rise of temperature is made harmless and does not cause perceptible additions to the stresses in the material as would be the^case were the arch built solid. The hinges which have to transmit the tremendous thrusts are intended to be made of steel castings. The arch rings will be reinforced concrete like the rest of the structure. The road superstructure rests on the arch rings by means of rectangular columns braced where required and joined together by semi-circular arches, somewhat resembling the style of the old Roman aqueducts. The arch is flanked by two big piers on which the approach girders end. These piers are somewhat more elaborately treated than the rest of the structure; which, however, perhaps because of its plainness, produces a very handsome effect, and in such a fine setting as offered by the bush of the gully the bridge can be safely cited as one of the finest anywhere. The size of the arch, when compared with others the world over, proves to be the widest span of any concrete or masonry structure. The nearest in size is a stone bridge near Planen, in Saxony, with a span of 295 ft. The largest concrete arch after the Grafton bridge is the "Walnut I^ane" span, in Philadelphia, U.S.A., 233 ft. in the clear. The nearest approach is a three-hinged system is the Gruntal bridge near Munich, Bavaria, with two spans 25 ft. each, and steel hinges. The Grafton bridge therefore occupies a very important position among the world's bridges, and it must be a matter of congratulation and pride that in Auckland, in our Dominion, such a bold and handsome application of reinforced concrete is being erected. A further matter of congratulation in connection with this bridge is that with this method of construction almost the whole of the work is done in the country, and all the cement, some 1500 tons, is New Zealand made. Of course the steel has to be imported. The concrete foundations were all brought up to ground level before Christmas, 1907. The contract time is two years, and the bridge should be easily completed by then. The engineer to the City Council, Mr. W. B. Bush, has to approve of all the work in connection with this contract, which is being personally superintended by Mr. R. F. Moore, the Chief Engineer of the contracting firm, the Chief Assistant Engineer, Mr. C. Rosegger, being associated with him in the design and the computations for the structure. We intend to give our readers progress reports as the work proceeds.