STO

460

STO

of the arch. It can seldom be greater, and consequently canseldom require, an addition of more than one-fifth of thatbreadth, which would be sufficient were there no immersion.

“ But although the total immersion, even of a lofty pier,will seldom require any great alteration in the thickness,there is yet another circumstance which well deserves atten-tion. Bridges are often built, especially in a tideway, withthe arches springing below the high waters ; we have in thatcase a diminution from the weight of the arch itself ; butunless the keystone be under water, the horizontal thrust isunchanged ; we must, accordingly, in our calculation, makethe same diminution for that part of the arch which is thusimmersed, as we did in the above example for the piers.The result will oblige us still more to increase the thicknessof pier.

“ On the whole, we may conclude from this investigationrespecting the piers, that the increase of breadth which maybe, and usually is, given to the pier, is of much less import-ance, on account of the weight that is thereby gained, thanby its increasing the length of that arm of the lever, wherebythe weight of the whole resists the effect of the horizontalthrust oversetting it.

“ Instead, therefore, of building up the pier with perpen-dicular sides, we should think it more advisable to begin thefoundation of the pier on a base much wider than usual, andfrom thence, by regular recesses, or otherwise, gradually todiminish it, until, at the springing of the arch, it does notexceed the depth of the two arch-stones, while the outline ofthe pier may be a curve of any shape that is most pleasing.Many advantages would, in our opinion, be obtained by thisconstruction : the water - way will be enlarged ; the pierequally strong; the stability equally great, nay, much greaterthan usual ; and the chance of the foundations being hurt infloods will be greatly diminished; and all this with a smallerquantity of materials.”

It is manifest, that before an arch of equilibration can bedestroyed, the pressure applied must be so great as to over-come the resistance offered by the friction of the adjoiningvoussoirs, or the adhesive power of the cements ; and be itremembered, the greater the pressure, the greater also is thefriction. We give the following as a method of discoveringthe extent of lioence allowed in practice, on account of thisquality in the materials employed.

To find the quantity of friction in individual instances,—Place the stone to be used upon a platform of the same mate-rial, and raise up one end of the platform, until the leastadditional elevation would cause the stone to slide down, andmeasure the angle of elevation so found.

Now the effect of friction is this, that instead of beingcompelled to place the joints of the voussoirs in the positionassigned them by theory, we may place them in any positionon one side of it, provided the joints fall within an angle fromtheir theoretical position, not greater than the angle of eleva-tion, determined as above; or if the joints of the voussoirsare maintained in their original position, we may allow aproportionate variation in their weight. On this accountvery great latitude is allowed in construction, for we maydepart very far from the rules laid down from theory, beforewe can endanger the stability of the edifice.

Dr. Robison is of opinion, that the voussoirs may be con-sidered as a solid mass, and that the principal object to beattended to, is, that the arch should be made so flat, as toadmit the same straight line being drawn in such a manneras to pass through some point in every voussoir on eitherside of the keystone, and where this is impracticable, he recom-mends that the straight line should be carried through asmany arch-stones as possible, and that the arch should be

carefully loaded at the intersection of such lines, cC ®® 1

passed through by one

line,

that

as

cases, of the nature of the materials employed

In accordance with the intention expressed by u ? ,j on ot

article on Bridges, we now proceed to give

descrip 1

occupy,

adv* 1

, n ce'

:<h

the masonry of the piers was commenced:gradually sinking in the water as the masonry " r ;ve l ’until at length they finally settled on the bed othe sides of the caissons were then removed, to n( j gto^where, and the piers, which were built of . ° r e jglit, 8 .°jin blocks, each of which was at least a ton in * &^ r,e emany of them weighing as much as five tons, vv ^ g to' ] ^up to the springing of the arches. These bloc ^ e \ng c0l>were cemented together with Dutch tarras, besides^ lead’nected with each other by iron cramps fastened w 1 ,, e yes r ’The building of this bridge occupied nearly ‘ ■ t j l6 (o° a

owing to the plan that had been adopted indation ; in consequence of which, one of the P^ gtrua 111 ^much, as seriously to endanger the stability 01 p ] a ce S 1and it was found necessary to put another m * pC tion> 11circumstance greatly enhanced the cost of c °” s „ g^gOO- „Q tthe various sums expended amounted to * ^ £iOt u t

appears that materials of this bridge, to the vaiue^ th® | C jiare constantly under water. The ca^son on w wpier was erected, contained 150 ' -.ads of of sol'd st f ee tre laid 3,000 cubic feet, or nearly three ton , g 44Westminster Bridge is 1,220 feet l° n °’ oP e ith er 8l gIfl flll

were

Westminster Bridge is

wide, including the foot-path for passengers \ tyf o a t

~ ■ “ large, aI sP T ' lt>e tai.

The ard^ f^culsallsef^se 1 }

the carriage-way ; it consists of 13arches, 14 piers, and two abutments,about 2 feet above low-water mark, they are ■

decree

The centre arch is 76 feet span, and the ot ^, 1 ' S t i 1 e b®"

tb e

either side by 4 feet, so that the arches nei i es ides,the river are only 52 feet span. There are, cU t-

small arches each of 25 feet span. j na te 1 wh' c ^

The piers are 70 feet in length, and ter r ^ oS e ^ gr ywaters, pointing up and down the stream^. de. < as *support the centre arch are each 17 f eel .„ a dth, a °successive pier is diminished a foot in r wld 0 * 1

id»>,

leave a water-way in the clear of 870 feet, | og tra'

There was originally a handsome stone a 11 ^ jpn 1 *

formed the parapet of the bridge on either ^ " r ’ t ^ lIl giil |irevery pier there was a bay or rece.-s, covei e ^ re® 1 *of half cupola; while over the central ai° ■>

deriflg

th®

each number of voussoirs

separate block of stone. Generally, he supposes, lin epressure at the crown, is communicated in a stral °j, r oUg^’through as many stones as one straight line will passAll suggestions, however, on this subject, must be j^ostwith considerable doubt and difficulty, as we are j ca tioflentirely ignorant of the method by which the cominuand distribution of pressure is regulated, and even,

oU r

ai citric kjii no uun ~ - rnPS

the principal stone bridges erected over the lha ’ 0 fjrstmeneing with Westminster, as being the oldest. ggtbistone for that bridge having been laid on January ^ e lng>1739, by the Earl of Pembroke, and the bridge itse*entirely completed on the 10th of November, 175 • j e3 ign s >Westminster Bridge was built according to t n au>®dand under the superintendence of, a Swiss archi ’ t ; c e o 1Labeyle, who, in its construction, adopted the P ngtea d °*forming the foundations by means of caissons,driving piles into the ground, or making use ot , j n tl> eaccording to the general custoM. This he effec 0 f afollowing manner:—Large timber cases, consis ^ pjafl®strong platform, or raft, surrounded by sides, (Q gild 1water-tight, and floated on the surface of the ^^ a ® tua lly t0places as the piers of the intended bridge were ev oS \tio°’In these, when firmly moored in P r0 P® e pa jsso" 3