lol9."l 



THE CIVIL EXGINEER AND ARCHITECTJ'S OURNAL* 



381 



Society, warranted bim in stating that the thanks of the Society were most 

 especially due to Iheir excellent Secretary, to whose unwearied and labo- 

 rious exertions they owed so very much ; also to their Treasurer and 

 liditor of Trnnsaflions. 



The following conin)uni<*ations were made : — 



STRENGTH OF MATERIALS. 



"At the request of the Council an Experimental Exposition teas given, con- 

 taining his concluding obset'vations on the ^Strength of Materials* as applic- 

 able to the construction of Cast or If'rought-Iron Bridges, and on the Con- 

 way and Britannia Tubular Bridges" (Part I.) By George BncHANAN, 

 Esq., F.R.S.E., late Pres. R.S.S.A. 



In this exposition, Mr. Buchanan, after apologising for the length to 

 which he had been imperceptibly drawn in these comniunicalions, com- 

 tnenced by recapitulating the general principles which had formerly been 

 laid down regarding the tensile and compressive strains of materials, and, 

 in addition to the results of former experiments, made at Ihe request of 

 the Society, on the stones from different quarries in the neighbourhood, 

 gave now the results of others which had since been carefully made on the 

 harder materials of Caithness and Arbroath pavement, along with while 

 marble and whinstone, as follows — viz. :^ 



Tensile. Compressive. 



Whinslont 14691b 827015. 



Arbroath pavement 1261 7H84 



Caithness do 1064 fi4a3 



Warble 723 6431 



In all these experiments the peculiar nature of the two strains is dis- 

 tinctly exhibited ; the specimens exposed to the tensile strain showing a 

 clean fracture and no fragments ; those exposed to the compressive being 

 generally crushed to powder, and the fragments flying in all directions 

 by lateral divergence ; and generally, when any considerable fragment 

 remains, showing the appearance of a pyramid from which the sides of Ihe 

 square had been broken— a form which has also been observed in the com- 

 pression of cast-iron. 



In regard to the transverse strength, he repeated the principles and 

 general rule for calculation formerly explained by adopting what he termed 

 a unit of strength, which differs in each material ; but being once deter- 

 mined by actual experiment, affords a (?a/«m for calculating the stiensth 

 of that material in every case, whatever be the dimensious of the masses 

 acted on. This unit expresses the strength of a cubic inch of the material 

 — i.e., a bar 1 inch square, supported on bearings 1 inch apart, and loaded 

 in the middle till it breaks. The strength of such a unit for cast-iron had 

 been given on a former evening at 11 tons. In regard to timber, he had 

 himself made various experiments on Memel fir, and had found the unit 

 4,0001b. Oak and beech, by other experimenters, was found 6,000 lb. ; 

 ash, 8,0001b. 



In regard to the tranverse strength of stones, few experiments, he said, 

 had been made on our building materials, although it was a strain they 

 were much subjected to in stairs, balconies, covers of conduits, &c. He 

 proposed, therefore, to try several specimens which were now before the 

 meeting — viz., Hailes pavement, Craigleith, and Arbroath. Each of 

 these specimens was 3 inches thick, 9 inches broad, laid flatways and sup- 

 ported at each end between two upright pillars, Ihe distance between 

 the bearings being exactly 3 feet. These specimens were loaded by 

 weights successively laid on a scale hung from the centre of Ihe pavement, 

 until it broke. The Hailes was first tried, and, after carrjing succes- 

 sively 4 cwt. and 5 cwt, for a little time, at last it gave way with 7 cwt. 

 10 lb. A specimen of the same rock and dimensious previously tried gave 

 nearly the same result, being 7J cwt. The Craigleith carried considerably 

 more. After bearing 7 cwt. and 8 cwt. for some time, it gave way at last 

 with 10^ cwt. The Arbroath pavement was found still greatly ahead even 

 of the Craigleith. After carrying 12 cwt. and 14 cwt. for some time, it 

 went on bearing 16J cwt. This it bore for a short interval ; and while an 

 additional weight was m the act of being put on it gave way. These 

 experiments are important, and appeared to excile much interest. From 

 these the unit of strength is easily calculated. 



The transverse strength and the forms of cast-iron girders for spanning 

 wide openings were formerly explained, and the application of malleable 

 iron in the form of hollow tubes or girders ; and, connected with this 

 subject, he explained a plan which had lately been propossd by I\lr. 

 Beardmore, C.E., London, who had favoured him with the results of some 

 interesting experiments made by him. The plan consisted in constructing 

 tire-proof or other floors by girders, consisting merely of thin plates of 

 sheet-iron running parallel to each other at intervals, like ordinary joists 

 resting on the walls at each end ; these plates strengthened and united to 

 angle-irons on the top, and to a thin plate below, running Ihe whole way 

 between the girders. The interval between them is filled up with a mass 

 of concrete, the use of which is chiefly to keep the thin plate girders in 

 their place, so that being incapable of bending, the full effect of the section 

 of Ihe iron is obtained, whereby the strength of such flouring, considering 

 the thinness of the metal employed, is remarkable. In one experiment, 

 where the girder consisted of sheet-iron. No. 14 gage, or ^'^ih of an inch 

 thick and 13 inclies deep, and placed 13 inches apart, and the length or 

 span between ilie walls or bearings :!3 feet — also the total seciinniil area 

 Gj inches, while that of the concrete was 331 inches. This was loiideil 

 8,000 lb., which is nearly doulile the weight of any number of persons 

 tlial could have room to stand on the beam, and only ilcSected ^ inch. 

 With 12,000 lb. it deflected about Jim h, which was considered the proba- 



ble limit of safe deflection. With 13,670 1b. it deflected 1 inch; it was 

 not loaded farther, hut the calculated breaking weight was 25,0ii0 lb. Mr. 

 Buchanan then showed a model of a floor on this principle, consisting of 

 the thinnest tin-plate iron gilders, 3 feet long, 1 ^ inch deep, and 2 inches 

 apart, and the spaces filled-in with plaster of Paris. Even this slender 

 material carried with safely a person standing in the middle, and gave 

 way with 3 cwt., chiefly owing to the joints in the bottom plate not being 

 soldered, hut merely laid over. 



In regard to the application of hollow girders, or tubes, and the won- 

 derful discoveries on this subject which the progress of engineering works 

 had recently brought to light in Ihe construction of the Conway and Bri- 

 tannia Tubular Bridges, he had formerly given a particular account of 

 these, and I ad only now further to add, that he had the pleasure recently 

 of visiting these structures, and nas in every respect highly gratified with 

 the result, and with Ihe progress and state of the works, which were all 

 pointed out to him and explained in the most litjeral manner by Mr. Edwin 

 Clarke, the very able and accomplished engineer on whom the active 

 charge of the principal department in this undertaking had been devolved 

 by Mr. Stephenson. 



The Conway Bridge, as we know, has been long since finished, and the 

 trains on the Chester and Holyhead Railway are seen daily passing and 

 repassing. It ceases already to be any longer a wonder in the neighbour- 

 hood, yet the stranger pauses to gaze with admiration on this extraordinary 

 triumph of science and engineering skill, as Ihe train enters the tube, and 

 again emerges under Ihe walls of the magnificent remains of Conway Castle. 

 In passing through the tube the sound of the train is peculiar, but not 

 greatly louder than in ordinary tunnels. No sensible tremor or vibration 

 is expurienced, and Ihe heaviest trains, when observed externally, do not 

 produce any visible deflection. The line of the under surface of the tube 

 is quite horizontal. The upper surface rises with a gentle curve towards 

 the centre. The under surface had also when constructed a slight rise or 

 camber in Ihe centre of about seven or eight inches; but when the snp- 

 poits were removed from beneath, leaving it siamling on Ihe two extremi- 

 ties, it sunk in the middle by its owu'weight into a straight line, and this 

 exactly as was intended by the engineers; showing the accuracy of Ihe 

 principles and data on which such nice calculations could previously be 

 made of Ihe probable deflection, and this chiefly from Ihe experiments on 

 the model lube by Mr. Fairbaini, described ou a former evening. 



The Britannia Bridge, to which he next proceeded, is a still greater 

 work even than Ihe Conway, and connected with many circumstances cal- 

 culated to impart interest to this structure, and everything connected with 

 it. The curiosity and wonder excited by the famous Bridge of Suspensioa 

 over llie iSIenai Straits were great: and he well recollected visiting this 

 work during its progress, and the vast operations, as they were then con- 

 sidered, of fabricating, connecting, and finally lifting the enormous chains 

 of which it is composed, each of which atier all, hardly exceeded 100 

 tons, between rock and rock, and the central portion, which alone had to 

 be lifted by one purchase, not above 30 or 40 tons. The principle of sus- 

 pensiou also was not quite new, but had previously been exemplified in 

 structures of considerable magnitude; still this was considered, and 

 justly, an astonishing etiort of skill, and remains a monument of the genius 

 of Telford. What must we think, then, of the structure now in progress, 

 and already seen partly spanning Ihe same Straits, which is not only new 

 in principle, and untried before Ihe great experiment of the Conway, but 

 where Iht- entire bridge itself, nearly 2,000 tons in weight, requires not 

 ouly to lie floated en ilie water from the place of its construction, but Iheu 

 raised nioie than 100 feet in perpendicular altitude in one mass, and by 

 one mighty purchase, to its seal ou the lop of the towers prepared for it. 

 What extraordinary strength of materials — of chains, bolts, bars, and con- 

 nections, does not this imply ! What amazing resources of nieclianical 

 power and comliinalion in the lifting machinery ! M'hat cou=umniate 

 arrangements, in lauuching the gigantic mass into the truuljied waters of 

 these Straits, and steering it with safely to Us destination I All these cir- 

 cumstances lend to raise a singular degree of interest in this structure, 

 and Ihe operations connected with it, and the result full) realised his anti- 

 cipations, of which, however, he could only give but a faint idea by de- 

 scription. 



The first view which the traveller obtains is in crossing the Straits by Ihe 

 present Suspension Bridge. Looking about a mile to the westward, the 

 towers of the Britannia Bridge are .seen rising with imposing efl'ect — the 

 centre one, as it were, from out of Ihe water — Ihe two exterior ones from 

 the edge of the waters, and the terminal pillars or abutments on the top of 

 Ihe high ground ou eacii side. But the object to which the attention above 

 all becomes riveted, is the appearance of an extended wall or roadway 

 spanning the 460 feet opening between the Biitauuia lower and the Angle- 

 sea shore. This is the first of the four lubes which has been raised to its 

 elevation of 103 feet above the waters. This is, in fact, The Bridge, 

 standing without any appearance of support, and totally uulike any of the 

 great works of this kind which ba\e hitherto formed Ihe pride of the 

 architect and engineer. No more the noble arch rising, as we often see, so 

 magnificently troni the level of Ihe opposite shoies — nur the light and ele- 

 gant curve of suspension hanging witli such regular and airy proportions 

 between the lofty towers ou each side, but a figure perfectly horizontal 

 and nearly rectilineal, spanning the opening and resting on the opposite 

 towers — a figure certainly unequal to the others in beauty, but yet raising 

 in the mind a sensation irresistibly striking, as exhibiting on such a mag- 

 nificent scale the successful development of a new, granu, and simple idea 

 lu mechanical science. 



