326 



THE CIVIL ENGINEER AND ARCHITECrS JOURNAL. 



[October, 



BRITANNIA AND CONWAY TUBULAR BRIDGES. 



Tlie Britannia and Conu-ay Tnhnlur Bridyex ; with General In- 

 quiries on Beam.1, and on the Prn]>erti<;s of Materials used in Con- 

 itruetinn. By Edwix Cr.AitK, Resident Jln^iiieer. Published 

 witli the sanction and under the supervision of Robkkt SrEPUEN- 

 soN. London: Day and Son. 18J0. 



In tlie notice we first ^ave of this excellent «(uk", we confined 

 ourselves to remarks on the book itself, and the influence which 

 tubular bridges will exercise on eng-ineering, and from the length 

 to which those remarks extended, we were precluded from giving 

 iiny extract from the book, and which we promised to do, knowing 

 the interest our readers feel in this one of the most important 

 works of the age, and one vvhicli will not be least sought after by 

 visitors to this island in the coming year. 



In consequence of the requirements of the Admiralty, it became 

 necessary to design a new bridge over the Menai. The first plan 

 for this is thus described by Mr. Robert Stephenson himself: — 



"Previous to the erection of the suspension-bridge by Telford, 

 ill 1826, various modes and points of crossing had been proposed 

 by Rennie and Telford. Their reports, plans, and opinions, were 

 carefully studied, wliich led to the adoption of the site known by 

 the name of the Britannia Rock, about a mile to the south of 

 'i'elford's suspension-bridge. Tliis spot is peculiarly eligible for 

 the purpose, the rock being nearly in the centre of the channel, 

 rising just to high-water mark, and of sufficient area to admit of 

 tlie easy erection of a pier upon it. The channel is here also 

 entirely free from sunken rocks, and the current unbroken during 

 tlie ebb and flow of the tide. These peculiarly favourable circum- 

 stances were considered highly advantageous, not only for 

 facilitating the erection of a bridge, but for rendering such a 

 structure unobjectionable to the navigation of the Straits. It was 

 ]u-oposed to construct the bridge of two cast-iron arches, each 350 

 feet span, with a versed sine of .50 feet, the roadway being 105 

 feet above the level of high-water at spring-tides. 



"The span here proposed was the same as that which had from 

 the first been designed for crossing the Conway River. 



"Such wiis the state of the engineering problem in reference to 

 the Conway and Britannia Bridges when the company obtained 

 the first Act of Parliament in July, 18 It. It was jiroposed to 

 construct a bridge consisting of one arch of the unusual span of 

 ,S5U feet over the Conway River, at 20 feet aliove high-watermark, 

 and another over the Menai Straits at the Britannia Rock, con- 

 sisting of two arches, each of similar span, but at the elevation 

 of 105 feet above high-water spring-tides. 



"The rise of tide in both cases is nearly the same, the channels 

 nre also very similar, being from 50 to 60 feet deep, with a rocky 

 bottom, and a rush of tide reacliing five miles an hour at Conway, 

 and seven miles an Iiour in the Straits. 



"These conditions, togetlier with the necessity of keeping the 

 channels open at all times for the purposes of navigation, rendered 

 it ]>erfectly clear that none of the methods heretofore adopted in 

 the erection of cast-iron arches could be brought to bear in either 

 of these localities. The inordinate cost of centering, even if 

 other arrangements had admitted of its application, was at once 

 fatal to its adoption; and it soon became evident that some means 

 external to the arch should bo employed to suspend the voussoirs, 

 or ribs, until the arch was keyed in. 



"A contrivance of tliis kind had at one time been considered 

 by Telford for the suspension of centering, upon which he proposed 

 to frame and connect the voussoirs, or ribs, of a cast-iron arch; 

 and a slight drawing of such a project is given in thj account of 

 the Menai Bridge. Witliout going into the merits of this proposal 

 in the form suggested, or into its applicability to the present case, 

 it is sufficient to say that it was discarded, and a modification, as 

 in-ought forward some years ago by Sir Isambard Brunei, for con- 

 structing lirick arches without centering, taken up as more suitable. 

 Sir Isambard's idea, which was experimentally carried out to a great 

 extent, a]>pearcd unexceptionable, and led to the following design 

 for the erection of the cast-iron arches at the Britannia Rock. 

 Instead of the two arches being erected upon two abutments and 

 one pier, it was proposed to treat the abutments as piers also. 



"Tlie erection of the arch was to be jiroceeded with by placing 

 equal and corres|)onding voussoirs on 0]iposite sides of the pier at 

 the same time, tying tliem together liy horizontal tie-bolts. 



"This system, it is confidently believed, may be successfully 

 carried out to a far greater extent than would have been required 

 in the case of the Britannia Bridge. 



"It will appear evident, on a little reflection, that as every suc- 



ceeding step of voussoirs is secured by the tie-bolts, the tension of 

 the last bolt, as well as all the previous ones, will be relieved by an 

 amount equal to the whole of the horizontal thrust due from the 

 voussoirs last placed. 



" If the voussoirs could be constructed or weighted, so that an 

 arch of equilibrium could be formed, all the horizontal tie-bolts 

 might bo removed, except the last one, for in such an arch the hori- 

 zontal thrust is every where equal. It is not meant that such a 

 method of proceeding as that of removing all the bolts could be 

 carried out practically — it is merely alluded to here to show how 

 largely the bolts would have been relieved from strain as the arch 

 jirogressed into a form which might appear to endanger the 

 stability of the structure. 



"Had this plan been carried out, it was not intended to have 

 keyed the arches at the crown, but to have left ample space 

 between the culminating voussoirs to admit of expansion and 

 contraction taking place freely. The bridge would, therefore, 

 have been simply a double-jibbed crane, perfectly balanced on 

 each pier. A connection at the apex of each arch would be neces- 

 sary, but so contrived as not to interfere in the least with the 

 expansion and contraction, and yet to counteract any tendency to 

 tilt, consequent upon the variable pressure of the passing loads. 



"This mode of construction, although decided upon for the 

 Britannia Bridge, was found unsuited for that of Conway. There 

 only one span was required, and the springing of the arch would 

 have been below the high-water line, and from a natural mass of 

 rock on both sides, which, at the east extremity, rose nearly to the 

 permanent level of the railway. 



"It was, consequently, impossible conveniently to treat the 

 abutments in the light of piers, as has been just described. More- 

 over, the great additional expense of this method, where one arch 

 only is required, formed a serious objection to it, as it necessarily 

 involved the use of double the weight of material requisite for one 

 simple arch, the weight of each overhanging wing being equal to 

 half the weight of the arch itself. 



"The objection on the score of expense did not apply to the 

 Britannia, for there the overhanging wings were a useful portion 

 of the bridge, and formed a substitute for the extension of masonry, 

 which would have been nearly as costly. Both the expense, 

 therefore, and the peculiarity of the site of the Conway Bridge, 

 pointed out the necessity of some other method being devised for 

 the erection of the arch. Various modes for erecting and sup- 

 porting a fixed centering were considered, but none appeared 

 satisfactory or safe; whilst the formidable difficulty of stopping 

 tlie navigation, and seriously interfering with many vested interests 

 for probably two years, remained in all its force. 



" This state of things led to the idea of building the arch com- 

 plete on centering supported entirely upon, and framed into, a 

 series of pontoons kept afloat during the whole time of con- 

 struction. 



" The rise and fall of the tide was such as to admit of its being 

 brought immediately above the springings and lowered into its 

 place by the falling tide, or by admitting water into the pontoons 

 at the top of the tide, before the velocity of the elib stream had 

 increased so as to interfere with the accurate adjustment of the 

 descending mass. This method of fixing arches I have since learned 

 was proposed many years ago by Mr. Dixon, of Darlington. He 

 made designs for a cast-iron bridge across the River Tees at Stock- 

 ton, and, instead of erecting centres on the permanent site of the 

 arch, he proposed to use pontoons, precisely in the manner which 

 has been described. These plans were not carried out, in conse- 

 ((uenoe of the Stockton and Darlington Railway Company having 

 determined to try a suspension bridge for railway purposes instead 

 of the cast-iron arch. For a brief description of the particulars of 

 the novel proposal of Mr. Dixon I have been favoured with a com- 

 munication from Mr. R. B. Dockray, who resided at Darlington 

 at the time when .Mr. Dixon made tiie design. I have also learned 

 fiom Sir Jolin Rennie that this was tlie method adopted for placing 

 the centering of the \\'aterloo and London Bridges; the centres 

 being constructed on pontoons and floated and lowered into their 

 proper position." 



We very much regret that this ingenious plan of Mr. Stephen- 

 son was not adopted, in consequence of the hostility he had to 

 encounter on the part of the government; but we hope the oppor- 

 tunity will present itself for its realisation under his direction. 



In reference to one of the original forms of the tube, the circular, 

 .Mr. Kdwin Clark makes some interesting I'emaiks. 



"It is to be regretted tliat circular tubes, with thicker plates, 

 were not experimented upon; ;is subsequent experience has shown 



