,500 



BRIDGE. 



Tin 



PLATE 

 LXXXI11 



fonn of roadway. All that is necessary, is to com- 

 pare the densities in the Table above given for a 

 circular arch, with the relative height between the 

 back of the archstone and the bottom of the roadway 

 in the given design ; and this will, we are sure, be 

 more readily, and more satisfactorily done by the 

 common builder with his sector and compasses, than 

 by giving him equations for any number of figures 

 of extrados. 



We have arrived at a theory which is very nearly 

 consonant with what lias become, among our modern 

 builders, the most approved way of distributing the 

 weight over the haunches of an arch. It was cus- 

 tomary, in the construction of bridges, to fill up the 

 haunch with solid matter, such as gravel, earth, or 

 the like, until a roadway of a proper slope was pro. 

 cured. Where the arches were small, this might not 

 be attended with any perceptible bad effect, provided 

 the archstones were of a good depth. But the ne- 

 cessity of lightening the haunches, has been forced 

 upon the attention of builders, whenever large arches 

 have been attempted. A more remarkable instance 

 of this we cannot have than in the bridge of Ponty- 

 pridd, in Wales, built by William Edwards, a simple 

 country mason of Glamorganshire. It is one of the 

 boldest arches in Britain, perhaps even in Europe, 

 being 140 feet span, and 35 feet rise; a portion of a 

 circle of 87^ feet radius, and the depth of archstones 

 is only 3 feet. 



In each haunch there are three cylindrical open- 

 ings running through from side to side : the diameter 

 of the lowest is nine feet, of the next six feet, and of 

 the uppermost three feet ; and the width of the bridge 

 is about eleven feet. To strengthen it horizontally, it 

 is made widest at the abutment, from whence it con- 

 tracts towards the centre, in the old and unartificial 

 way, by seven offsets, so that the roadway is one 

 foot nine inches wider at the extremities than at the 

 middle ; and is also very steep. 



That this mode of lightening the haunches is ef- 

 fectual, we have undoubted proof in the case before 

 us. It is not ungraceful, but were it thought so, 

 the tunnels might be concealed by the side walls. 

 Indeed these tunnels might even become useful, by 

 affording additional passage for the waters in danger- 

 ous floods, as seems to have been intended in the an- 

 cient bridge of Merida, the Pont St Esprit over the 

 Rhone, and in many other similar structures both 

 ancient and modern. Nevertheless we cannot approve 

 of this mode. It seems to press unequally on the 

 arch, and only at a number of detached points ; and 

 though the widest tunnel may be placed just where 

 the greatest evacuation is necessary, yet this ill agrees 

 with the gradual approximation to solidity, which 

 we should find in passing down the back of the arch. 

 To make correct workmanship in these tunnels- is 

 troublesome and expensive. The following mode, 

 which has now become the customary practice of our 

 most experienced bridge builders, is much preferable. 



T!iin longitudinal walls are built over the flank 

 of the arch parallel to the sides of the bridge, and 

 about three or four feet asunder. The spaces between 

 are covered at top with thin flat stones, or arched over 

 by pointed or circular arches ; or they are covered by 

 regularly projecting courses in the way of an Egyp- 



tian arch ; and in any case a platform is thus formed, Thco 

 upon which the gravel may be laid for a roadway. '" V 



In all probability, the first inventors of this mode 

 of building, besides employing it with the view of 

 equilibrating the arch by lightening the part over 

 the haunches, had also an idea of steadying it by the 

 lateral abutment. They appear to have considered 

 these spandrel walls as a sort of hoops, that would 

 keep the parts of the arch together, and hinder any 

 stone from moving, by their great friction, iner- 

 tia, and mutual abutment. Hence various ingenious 

 modes have been employed for locking them into 

 the back of the archstones, propagating the pressure 

 through, and securing them from sliding away at the 

 bases. 



They indeed act in this way ; nevertheless the 

 equilibration of the arch should be attended to in 

 their construction, that every unnecessary strain may 

 be avoided. The thickness of these walls may be 

 varied indefinitely, and the vacant spaces made in 

 any proportion to the solid parts. The walls ought 

 to be near each other, that their effect may be felt 

 over the whole arch, and perhaps they should spread 

 out towards the bottom ; but this is not so very ne- 

 cessary, for the courses of archstones break joint 

 with each other, and the inequality of pressure in 

 one course is immediately corrected by being propo- 

 gated to the succeeding. We may determine readily 

 the thickness proper for these walls, by the help of 

 the Table last given, provided we know the thickness 

 of the arch, and of the roadway, (including the small 

 arcade below it,) and the breadth of the whole struc- 

 ture. For example, let the breadth of the soffit be 

 20 feet, the thickness at crown T ' ff of the radius, and 

 the archstone alone -3 ; being the same proportions 

 as for the numbers in the example to last Table. 

 Then from that example we find the thickness of 

 all the masonry in the spandrel must, near the crown, 

 be , or 5 feet j at 30 from the crown, it must be 7 

 feet; at 40, 9 feet 4* inches; at 50, 13 feet 1 1 inches ; 

 and at 57 or 58, the whole must be solid masonry. 

 Suppose, in the next place, that the side walls are 

 18 inches thick, and the spandrels 3 in number, of 

 course there will be four openings. The thickness 

 of each wall must be at 50 from the crown 3 feet 

 8 inches; at 40, 2 feet 1* inches; at 30, 1 foot 

 4 inches ; and diminishing from thence to half that 

 thickness. Perhaps eighteen inches is too thin for 

 the side walls, but they may be thickened towards 

 their bases, diminishing the thickness of the span- 

 drels in proportion. On the other hand, nine inches 

 appears too little for the spandrel wall, when we 

 consider that an arch is to be built on it ; but the 

 height near the crown will be so small, that a little 

 additional thickness there will be of no moment; 

 nay, it will enable the arch the better to resist any 

 overload at the crown. 



We have now determined a method of constructing E 

 an equilibrated arch for sixty degrees on each side of tioii < 

 the vertex ; and this method, so far from having any micir 

 thing unusual, is even strictly analogous to that which arclie 

 is adopted by the practical Uuilder. Why then can- 

 not we keep pace with him throughout, and give a 

 construction for the entire semicircle ? No difficulty 

 is felt by the mason in that case. He construct* 



