202 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Ji;ne, 



In the framed system the ribs, beams, Sec, are generally very 

 heavy, require much workmanslup in their coiietruction, are 

 difficult to cast, and after good castings have been obtained are 

 very liable to be damaged before being put in their places; thus 

 causing the reconstruction of other castiiiirs, and conseiiuently 

 adding much to the expense, on account of the risk and delay. 

 By the proposed system the castings become of an ordinary nature, 

 re(|uire less workmanship, are easier constructed, very light and 

 easily handled, and run less risk of bciriir damaged; and even 

 when a number of the voussoirs were daniaued, the contingent 

 expense would be very little compared to that arising from the 

 loss caused by the damage of a large and massive beam. Such 

 castings are conscijuently cheaper executed, the cost per ton for 

 the same kind of castings being frequently not much more than 

 one-half that of the other; besides, by a judicious arrangement and 

 economy, no more, or at least very little additioniil, metal need be 

 required by the proposed than by the framed system. 



Both in ancient and modern times, hollow bricks have been 

 used in the construction of arches, especially where lightness was 

 required, and no great weight to be sustained; as these bricks were 

 liable to be easily crushed. AVith cast-iron this is, however, not 

 the case, it being to a very great extent inccmipressible, the 

 crushing weight for a square inch of cast-iron being 1-I0,000lb., 

 while good stock bricks require only 12,000lb. to crush it, and in 

 stone the crushing weight varies, according to the qualit}', from 

 31()()lli. to 62.^0!b. per square inch. Since hollow bricks have been 

 successfully employed, it is easy to conceive that hollow voussoirs 

 formed of such a hard and incompressible material as cast-iron, 

 may likewise successfully be employed, not only for arches of a 

 small but also for those of a very large span. 



It is now an established principle, that when the materials of 

 which an arch is composed are hard enough to resis Icon press on, 

 and the al)utments sufficieiitly strong to resist being crushed or 

 forced aside, there is no particular limit to the extent to which, 

 if properly constructed, the span may not be carried. Of course, 

 no substance being incompressible, it follows that there must be 

 a limit beyond which the arch would destroy itself, but that limit 

 will be greater or less according to the hardness of the material 

 employed in the construction. An arch constructed of granite is 

 capable of being carried to a greater span than one of good free- 

 stone, and still moi-e so than one of freestone of an inferior 

 quality, or of brick not sufficiently fired. And following out the 

 same principle with hollow cast-iron voussoirs, a still greater span 

 could he accomplished than with any of these other materials. 



Besides the advantage of cast-iron voussoirs, on account of its 

 extreme hardness it possesses another advantage, that of lightness, 

 these voussoirs being capable of being made sufficiently lighter than 

 the same constructed of stone, and still retain sufficient strength 

 to resist the required pressure. The weight of material in a cast- 

 iron arch would be from ^th to ^th that of a stone one, supposing 

 the depth of the voussoirs was made the same in each, whicli 

 however would not always he necessary, as when constructed of 

 iron less depth would be sufficient, on account of its extreme hard- 

 ness, the weight being so considerably diminished, and the pressure 

 being more uniform over the entire surface of the joint : the surface 

 of castings being much smoother and evener than that of an arch 

 stone, which except in very particular cases is generally only neatly 

 hammer-dressed. 



Again, in the framed system usually adopted, the pressure is 

 thrown on a very small surface, which is not the case in the pro- 

 posed system; likewise the use of malleable iron is entirely avoided, 

 it being purely a cast-iron arch, every part of which contributes its 

 due proportion of resistance; forming a firm and compact mass, and 

 possessing all the advantages of a stone arch. 



Taking into consideration these many advantages — namely, the 

 extreme luirdness of the material employed, the decrease of weight 

 and the superiority of the joint compared to stone arches, and the 

 large extent of bearing surface compared to that of the framed 

 system, it is surely iu)t unreasonable to say that an arch on this 

 principle may not only be carried to a greater extent than any 

 hitherto constructed of stone, but equally as far, and perhaps 

 further, than any that have yet been constructed of iron on 

 the framed system. In the tirosvenor-bridgc, across the River 

 Dee at Chester, a stone arch has been succcsfuUy thrown over a 

 span of 200 feet. And in the Southwark-bridge, across the 

 Thames at London, which is formed of cast-iron on the framed 

 ])rinciple, the centre arch is carried to the extent of 2tO feet; hut 

 with hollow cast-iron voussoirs, an arch equal and even exceeding 

 either of these spans may be executed with safety. 



In the construction of an arch upon this principle, it is proposed 



to have a raised ]iiece cast on the side of each voussoir, fitting into 

 a corresponding hollow in the one adjoining. By this means the 

 whole becomes more firmly joined together, forming, as it were, a 

 series of joggles throughout the whole structure, and entirely pre- 

 venting any tendency of the arch to rise at the haunches, or of 

 any of the voussoirs to slide. This is a very important advantage, 

 and one which, in an iron arch, can be easily obtained with little 

 or no additional expense. 



{J 



Fig. 1. — Trariftver^e Sec- 

 tion if Vou-soir. 



Fig. 2. — Transverse Section of Arch. 



The form of the voussoirs may either he made similar to those 

 in stone bridges, with the addition of these projections and hollows 

 (see figs. 1, and 2), or, where additional strength is required, they 

 may be executed according to fig. 3. 



1 



1 



Fig. ."J.— Transverse Sect on of Arch. 



On account of the voussoirs being all firmly fixed to each other 

 by means of the joggles already mentioned, it would not, on all 

 occasions, be necesssary that they be placed close to each other at 

 the ends, hut kept a little separate, as shown in fig. -t. By this 

 means, while the arch could still he made sufficiently strong, a 

 considerable saving of material would be effected. 



Fig. 4.— Plan of part Arch. 



As to the thickness of metal required for small and medium 

 spans, the average may be from ^ to J-inch, and for large 

 spans one inch would he sufficient for the average, care being 

 taken that the ends of the voussoirs be made thicker than the sides. 

 In order still further to strengthen the ends without requiring 

 additional metal, the sides may be made slightly open. 



In places where stone cannot be easily obtained, bridges could be 

 constructed on this principle at a very moderate cost, while they 

 at the same time would be both substantial and durable. The 

 spandrils and abutments may be constructed of such materials as 

 could be most readily obtained, and which was considered suitable; 

 as such bridges admit of being finished similar to a stone one or 

 otherwise, acording to the taste of the projectors and resources of 

 the locality. 



REVIBTVS. 



An Elementary Course of Geology, Mineralogy, and Physical Geo- 

 graphy. By David T. Ansted, M.A. F.R.S. London : Van 

 Voorst, 1850. 



We are precluded from writing an article on Professor Ansted's 

 new book, because on former occasions, prompted by him, we have 

 gone over the whole subject of geology and engineering; and 

 because he has so fully carried it out asto leavens no ground forcavil, 

 and only the opportunity of expressing strongly our approval of tlie 

 volume now before us. ' So far from being a mere reproduction of 

 the Professor's former works, this is a complete manual of the 

 several allied sciences, most carefully treated according to the last 

 discoveries in the important domain of philosophy to which these 

 newest offspring of science belong. 



What was in the first instance a few pages has now become a 

 regular section on practical geology and its application in engineer- 

 ing, and it has ceased to be a matter of question whether geology 

 is an essential part of professional education. 



