1S47.] 



THE CIVIL ENGINEER AND ARCHlTECrS JOURNAL. 



2V.) 



BARLOW ON ARCHES. 



(Continued from page 213. ) 



(Remarks after the Heading of the Paper at the Institution of Civil En- 

 H ineers : ) 



Mr. CliBlTT, V.P., said he felt the propositions in (he paper were so 

 conclusive, tliat they scarcely afforded an opportunity for remark, ninch 

 less for discussion. Tiie great merit of the comnuinicatinn, and of the 

 illustrations, was the adaptation to pradice; in nn'St of the treatises on 

 arches, the theory alone was considered ; Mr. Barlow had, however, very 

 propfrly poin'ed out the possibility of constructing arches of certain forms 

 and diinen ions, strictly within the theoretical rules, so that they siiotild 

 stand well alone; but that when any pressure was imposed on then), they 

 would fail Th- se were points of great importance, which should never 

 be lost sight of by the engineer, and demanded not only great attention to 

 the proportions of the structure, but also to the qualily of the materials 

 employed, the sitciaiion, the nature of the foundation and of the backing 

 snd numerous other considerations, in order to adapt the arch to the use 

 for which it was intended. 



Mr. SorwiTH agreed in the value of the paper. He vieweil it more par- 

 ticularly in its application to the construction of arches in mines, where 

 s ilulity and periHanence were of such importance, on account of the un- 

 equal pressure to which they were suljected. 



Mr. Brunel had endeavoured, during the reading of the paper, and the 

 remarks of the preceding speakers, to find sotue point whereon to found 

 observations, but it was very difticidt, as the author's p'actical experience 

 appeared to have constantly directed his theoretical investigations. He 

 thought, h'lWever, that the compressibility an^i elasticity of materials of 

 construction had not been sufficiently insisted upon. This ilid not gene- 

 rally obtain enough consiiieration, yet it was of great importance to the 

 stability of a structure; all materials, even to granite, possessed an amount 

 of elasticity, and it did not suffice to have the line of pressure fall merely 

 within the mas- ; it should be sufficiently within it to allow for any yield- 

 ing from elasticity, without eudangeriog the building. 



Mr. Pellatt observed, that the valuable information might be rendered 

 available iu the construction of the vaults of furnaces, the dui-ation of which 

 was of great importance iu the glass manufactory. It was desirable Ih.it 

 the crown of the arch of a glass furnace should be so low as to keep the 

 heat well down, and yet if it was too Hat, it was soon destroyed by the 

 ira pinging action of the tlame, or else the expansion of the materials by the 

 heat destroyed the equilibrium of the arch, and it fell. At present the 

 practical judgment of the workman was alone depended upon tor tlie pro- 

 per form, and the consequence was, that although a well-built furnace arch 

 might last 14 years, it might not last longer than 14 months. 



Mr. Inman said that the ruins of ancient buildings wotdd afford many 

 5'riking lessons of the correctness of the principles l.i.d down iu the paper. 

 Numerous examples of remains of arches standing without other support 

 than the stones of which they were coru posed, might, he believed, be fonud, 

 which could corroborate the views of the author, and he recoiumeuded such 

 examples being sought for as illustrations. 



Mr. R. Stephenson wished to express his conviction of the useful clia- 

 r.icter of the paper, which, he was convinced, would ren)ove ruany difficul- 

 ties hitherto felt in examining the subject by the process iaiti down tiy Pro- 

 fessor Moseley. wiiose formula;, though highly scientific, and no doubt very 

 i)eautiful, were much too abstruse for the use of the practical man. Any 

 thing which tended to elucidate these formulae, and render the sultject more 

 popular, must be received with great interest by the civil eiigint-er, whose 

 labours would be materially facilitated by such clear adajitatioiis of theory 

 to practice. It would appear, ttiat the principal iioveliy consisted, in de- 

 scribing by a simple process from two given, or assumed, points, a curve 

 of equal horizontal thrust, falling witliiu such points in the voussoirs, as 

 should demonstrate the stability or instability of the structure. There could 

 be no doubt of the value of such a process ; but he would suggest to Mr. 

 ISarlow the desirableness of giving, in somewhat more precise and simple 

 terms, the mathematical demonstration of that which must be universally 

 admitted in practice. He would suggest whedier iMoseley's term of the 

 *' tine of pressure,*' as contra distinguished to the '* line of' resistance,*' did 

 not convey the meaning of the proposition better than the term ■* curre of 

 horizontal thrust." It was accept.»d as perfecily true, that, as slated in 

 the paper, the horizontal force at any part of the curve was opposed by a 

 horizontal force of equal amount, exerted in an opposite direction, nnil that 

 the horizMilal force or thrust was equal throughout the curve, and hence 

 tlie equilibrated arch ; yet this had not been hitherto clearly and simply 

 laid dov^n, in such a manner as to be practically used. 



Mr. Bidder accorded with Mr. Stephenson in his appreciation of the 

 value of the paper, he had seldom heard one of greater utility, and he 

 trusted so good an example would be followed in the Institution. The 

 proposed mode of describing the curve or line of pressure, showed the im- 

 propriety of constructing brick arches in separate superposed rings ; the 

 i ue would in almost every instance, travel out of the nog in which it com- 

 menced, and in case of fracture, the rings would fail consecutively; but if 

 tiie arch was well bonded together throughout its entire depth, the line, or 

 curve, would be traced within it, and it would possess the requisite strength. 

 All the best brick arches were now built iu that maancr with full boud. 



An arch had recently been so built by Messrs. Grissell and Peto over the 

 River Lea, with a span of 87 feet, and a rise of G feet; the centres were 

 struck within an unusually short time after the arch was keyed; but it. 

 stood perfectly, and with >ery little subsidence. He was tempted to con- 

 sider an arch constructed of rectangular bricks set in a matrix of cement, 

 as a bent trussed girder, the tension rods of which were represented by the 

 abutments of the arch. Very Bat arches, such as the Maidenhead Bridge, 

 were examples of what he meant. 



Mr. Bkunel could not agree with Mr. Bidder's comparison, or what he 

 might be permitted to term his amusing theory ; on the contrary, he must 

 contend that there was no analogy between the arch and a tru-sed girder, 

 in the former the main force was pressure, in the latter the force exerted 

 was tension ; the abutments of the one had to resist a horizontal thrust, at 

 a given angle, wliilst the wing walls, under the other, had to support only 

 a ve.'tical pressure; any tendency towards horiztmtal thrust, which might 

 have arisen fioiu deflection of the beam or girder, wis prevented by the 

 tension rods which connected the opposite extremities. If an arch could 

 be considered as a bent trussed girder, it must follow, that it would stand 

 equally well whether the curve was upwards or downwards, which cer- 

 tainly did not accord with his notions of the properties of an arch. 



Mr Bidder replied that his views were misapprehended ; what he con- 

 tended was, that a brick arch being formed of rectangular pieces, set in a 

 matrix of cement, having great adhesive properties, upon which it in a 

 great measure depended, it should be considered as a homogeneous mass, 

 assuming the nature of a curved trussed girder, the resistance of the abut, 

 meiits acting as the tension rods of a girder. He must still contend for his 

 position, and that the bridges of great span and small rise, erected by Mr. 

 Brunei, were excellent examples of the construction he meant, 



Mr. R. Stephenson considered thai Mr. Bidder only meant to put for- 

 ward the po-ition for the sake of argument. It was certain, that tlie arch 

 and the trussed girder, being supposed to be formed of the same materials, 

 the former wiuild be supported by the resistance of the abutmeuts to com- 

 pression, and the latter by the tension of the lie-rods ; the adhesive proper- 

 ties of the materials not being in either case taken into consideration. The 

 arch, per se, should always be considered as composed of separate masses, 

 not set in a matrix; but combined in a certain form, the only adhesion 

 being the friction of the surfaces. It would be desirable if iMr. Barlow 

 would give a more perfect mathematical formula for describing the curve ; 

 the rule which he had given had too much the character of being empirical 

 and of being made to tit given cases. 



Mr. W. H. Barlow was unable to perceive any deficiency in his de- 

 finition, or in the method by nhich he ascertained the curve. The line of 

 thrust, as obtaiiie'i by the construction given in the paper, was practically 

 given in the models. — It was not a necessary condiiion of stability, that the 

 line of pressure should intersect the surfaces of contact at right angles, it 

 was sufficient that tl^* direction of the pressure should meet the surfaces of 

 contact, within the limiting angle of friction. The same condition was ex- 

 emplitied in a column ; there the line of pressure was a vertical line, but 

 the surfaces of contact of ihe stones might be inclined, without occa-;ioning 

 tlie upper part to slip, provided the inclination was within the angle of 

 friction of the material employed. — -Mosi-ley's forrauhe, although tiieoreti- 

 cally perfectly accurate, were too complicaied, and involved loo much ma- 

 thematical knowledge for the general Uae of practical men. — .\ deep or 

 thick arch contained more than one 'Mine of pressure ;" the line of pres- 

 sure to be dealt with in practice was, in effect, the centre of a surface of 

 pressure. 



Mv. R. Stephenson said, thai mathematicians always considered the line 

 of pressure to be at right angles with the supporting surfaces or the abut- 

 ments. It would appear, however, from Mr. Barlow's explanation, that 

 ins ead of drawing a series of lines at right angles to tlie surfaces through 

 given points, thus forming what might be termed the polygonal theory, he 

 described a correct curve through the same given points. Mr. Stopheuson 

 coul'l not understand how the voussoir could give a line differing from the 

 line of force treated of by nialheiuaticians. 



Mr. Brcnel said, the subject was one of great difficulty, as it embraced 

 so many considerations ; it might, however, he thought, be rendered simple, 

 by considering an arch not as composed of separate voussoirs bound to- 

 gether by cement, thus involving other principles, but as a liomogeueous, 

 and, he might almost say, an elastic mass. If viewed in that light, the 

 pressure would be found to extend more or less over the whole surface. 

 The *' centre line,'' or '■ neutral axis," might in sucli case receive the de- 

 nomination of the " line of pressure." If this idea were followed up, there 

 ^■oiild be less difficulty in explaining the principles laid down by Mr. 

 Harlow. 



Mr. W. H. Barlow said, that Mr. Brunei's " line of neutral axis" ex- 

 pressed more nearly what he understood by Ihe "line of pressure," and 

 that line described by the impinging points of Ihe curved surface of the 

 voussoirs of the model. Mr. Barlow thought, that Mr. Bidder's experience, 

 as to arches turned in one entire bond, being stronger than those composed 

 of sepa' ate rings, bore out Ihe deductions of the paper. The rings could 

 not separately contain the curve of equal horizontal thrust ; but when 

 bonded they did so. An arch turned in separate rings depended too much 

 on the adhesive strength of the cement or mortar. 



Mr. CiiBiTT, V.P., said, it appeared to him that the whole question was 

 contained iu the proposition demonstrated by the model with curved 



34 



