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so great as to cause the line of resistance to cut the back of the pier at some point above its base, then the pier will be overthrown, and the arch will fall. When the arch falls, the line of resistance is made to cut the intrados at the points in the haunches, Qc£, where before it touched it. Hence, these points are called the points of rupture. The line of resistance thus cutting the intrados of the arch at these points, the direction of the whole pressure is made, at these points, to act beyond the joints of the stones there, so that it causes the stones to turn upon their lower edges, and to open at their upper edges. Besides touching the intrados at the haunches, it is another general characteristic of the line of resistance, in the state of the equilibrinm of the arch, that it touches the extrados over the crown, at A, and that when the arch is falling, it is made to cut the extrados there: so that the pressure, there also, acting beyond the
joints of the stones, causes them to turn, but, in this case, on their superior, instead of their inferior, edges. The arch then opens at the Fig. are. crown, A, at its
intrados, and thus it falls, separating itself into four distinct parts as in Fig. 276. This is precisely what has been observed to be the process of the fall of the arch in experiments made for the purpose. M. Gauthey having occasion to destroy a bridge, caused one of its arches to be isolated from the rest; and the adhesion of the cement being sufficient to counteract the tendency of the pressure to rupture the piers, he caused them to be cut across. The whole then at onee fell, the falling portion separating itself into four parts. Having constructed small arches of soft stone, and without cement, he loaded them until they fell. Their fall was always observed to be attended with the same circumstances. Before the arch finally yielded, the stone was observed to chip at the intrados, about the points Q Q', round which the upper portions of it finally revolved.
Professor Robison also relates some similar circumstances connected with the failure of a considerable arch. It had been built of an exceedingly soft and friable stone, and the arch-stones were too short. About a fortnight before it fell, chips were observed to be dropping off from the joints of the arch-stones, about ten feet on each side of the middle, and also from another place on one side of the arch, about twenty feet from its middle. The masons in the neighbourhood prognosticated its speedy downfall, and said it would separate in those places where the chips were breaking off. At length it fell; but it first split in the middle, and about fifteen or sixteen feet on each side, and also at the very springing of the arch. Immediately before the fall, a shivering or crackling noise was heard, and a great many chips dropped down from the middle, between the two
places from whence they had dropped a fortnight before. The jomts opened above at those new places more than two inches, and in the middle of the arch the joints opened below, and in about five minutes after this, the whole came down. Even this movement was plainly distinguishable into two parts. The crown sunk a little, and the haunches rose very sensibly, and in this state it hung for about half a minute. The arch-stones of the crown were hanging by their upper corners \ when these splintered off, the whole fell down.
Professor Robison caused models of arches to be made in chalk, and loaded them at the crown until the line of pressure cut the extrados, and they fell. The material of the arch would of course be most likely to yield at those points where the line of resistance most nearly approaches the intrados; and in these experiments the chalk was observed to chip and fall off there before the final rupture. Having loaded his arches at the crown until they fell, he observed, however, that the points where the material began to yield were not precisely those where the rupture finally took place. This would necessarily be the case; for any variation in the least force which would support the semi-arch, if applied at its crown, would cause a corresponding change in the position of the points Q and Q', Fig. 275. Now, as the load on the crown is increased, this force is also increased, and the result is a variation in the form of the line of pressure, tending to carry its point of contact with the intrados lower down upon the arch. Accordingly, it was observed that the arch began to chip at a point about half-way between the crown and the point where the rupture finally took place.
It will be seen by a reference to Fig. 275, that above the points Q and Qv the direction of the line of resistance is such as to indicate a direction of the pressure which would produce in the arch-stones a tendency to slide downwards upon one another, while below that point the tendency is to slide upwards. Hence, it might be expected that when the centre, or wooden frame, used for supporting the arch-stones during the erection, was removed, the motion of the arch-stones1 would be slightly downwards, in reference to those voussoirs which are above the points Q and Q , and upwards in reference to those which arc below those points. This motion of the voussoirs among themselves, on the removal of the centre, produces what is called the settlement of the arch, and this settlement is observed to take place precisely under the circumstances above described. At the bridge of Nogent-sur-Seine, before removing the centre of the arch, Perronet caused three lines to be cut upon the face of it, (Fig. 277) one horizontally, immediately above the crown, and the other two lying obliquely from the extremities of this on either side, towards the springing of the arch. After the striking of the centre, these lines were observed greatly to have altered their
(1) The arch-stones would admit of some degree of motion among one another, by the yielding of the cement, or by reason of the closer degree of contact into which the additional pressure would bring them;
immediately above the key; thus indicating a downward motion in all the voussoirs on which this line was traced. The oblique lines, too, had on either side deflected from their first position towards the jntrados of the arch, or downwards, up to certain points, corresponding to Q and Cj; beneath these points the deflexion was from the intrados of the arch, or upwards. Thus, among the voussoirs on which the oblique lines were cut, there was shown to be a downward motion, in respect to those above the points corresponding to Q and q\ and an upward motion in respect to those beneath those points. The same phenomena were observed to attend the settlement of other great arches constructed by Perronet, especially those of the Pont de Ncuilly.
An arch may be so loaded about its haunches as entirely to alter the direction of its line of resistance; in which case it will be overthrown as effectually as by the too great height or insufficient weight of its piers in respect to the load it bears on its crown. An excessive load on its haunches may entirely alter the line of resistance, so as to flatten it at the top, and give it two elbows on cither side of the crown, causing it to cut the intrados instead of the cxtrados at the crown, and the extrados at two points a short distance on either side of the crown; the points where it touches the intrados being by this process thrown much lower down upon the arch. The arch will in this case fail by the rising of its crown and the falling in of its sides. The great art of arch-building consists in so loading the arch as to secure it against cither of these contingencies.
Section III.—On The Practice Op BmdgeBuildikg.
The practice of bridge-building requires a large amount of knowledge on the part of the engineer or bridge-architect, and has reference to the situation, the design and the materials of the bridge, which of course may vary according to circumstances. The bridge itself has reference to the foundations, the piers and abutments, the centres, the arches, the spandrels and wings, the parapets and the roadway, which are also subject to great variation, according to the taste and skill of the architect, and the means at his disposal.
1. The situation of a bridge must be determined by local circumstances, as in a town by streets and in
the country by the adjacent roads. In laying out a new line of road through a country, choice may often be made of the points for crossing rivers, and as a bridge must always be a very costly portion of the road, it may be expedient to go out of the direct line to places at which crossings may be most conveniently effected. In general, the best point upon a river for a bridge, is where the stream is narrowest, and where the banks are raised sufficiently to form natural approaches, and are solid enough to form natural abutments. There may, however, be exceptions to this. "Where a river is narrowest it is deepest and most rapid, and where the banks are high and consistent enough to form approaches and abutments to a bridge, the water will rise higher in floods than at any point lower down, where it may have more room laterally. Unless, therefore, the crossing can be effected in one span, reach or bay, so as to avoid a pier or piers, it may be less expensive, and more certain to build a longer bridge over a wider part, and to form artificial approaches and abutments, than to construct a sufficient and perfect bridge where nature has provided both approaches and abutments, and where the traject is shorter. It does not often occur moreover, that both banks of a river are high in the same place; and again, where one bank of a river is elevated or cliff-like and of good consistence, it is very likely to have been the means of reflecting or throwing the water off to act upon the opposite bank, and so have itself become the head of a reach, or the concave face of a bend, where the stream is very generally widest. When circumstances admit of a bridge over a river being in one span or reach, or of one bay, it is comparatively unimportant whether it be placed upon a bend of the river or not, if it be so contrived as to give sufficient head-way for craft where the channel is, or, in other words, where the water is deepest, which in a bend is most frequently on the concave side. When piers in the water way are determined to be necessary for an intended bridge, the site should be chosen where the course of the river is straight, so that the bridge may be placed at right angles to the thread of the stream, and that the piers may thereby intercept and divide the water in the least objectionable manner. In a running river, the longest part of a straight reach may be left above the bridge with advantage, whilst in a tidal river the bridge should be placed in such a manner as to give the stream at both ebb and flow all the advantage that can be obtained from the piers lying in the direction of the currents; and where, as it must be in most cases, the down stream is the strongest, and the ebb of longer duration, the proportion of the reach above and below a bridgeshould bedetermined accordingly." 2. The design. When the situation of a bridge has been determined, a careful survey must be made of it, and a plan be laid down of the channels and adjacent banks, and of the streets and roads to be connected at each end of the bridge. The plan should present an outline or representation upon a horizontal
(1) Professor Hosking's Essays and Treatises on the Practice and Architecture of Bridges, inserted in Mr. Weale't work, Od the Theory and Practice of Bridges.
would give a much greater length than a plan upon a horizontal section of lines raised vertically from the surface of the ground to the straight horizontal line A B. Such a section must show the irregularities of the ground, the various substances of which the ground is composed, and the thickness vertically of each substance as far as it is necessary to ascertain it; that is, until a proper substance upon which to work has been found. This may be obtained by boring. In addition to the transverse section of the river, which is the direction to give the longitndinal elevation of the proposed bridge and its approaches, a longitndinal section of the river transversely of the bridge must be taken to show the declivity of its bed, and to exhibit any unsound strata that may lie near the proposed site. Upon both sections of the river, the ordinary depth of water in it, the highest level il ever attains, and the lowest to which it falls, should be ascertained and recorded, as also the velocity of the stream and the quantity of water passing down the river at all times and seasons of the year.
3. The materials of which bridges are constructed arc various, such as timber, the art of combining which is known as Carpentry; stone, the working and setting of wliich is called Masonry; brick, the composition of which is termed brick-work; iron, which is prepared by the founder, and fixed and fitted by the smith, and known as iron-work. "Although a bridge may be built almost entirely of timber, as indeed bridges arc often built,—except as to the smith's work in the form of shoes, rings, hoops, straps, bolts, nuts, washers, spike and other nails, which the carpenter finds essential to the proper and efficient combination- of his principal material;—the best timber bridges are those in which solidity and evenness of pressure, with power of resistance and retention, are given by piers and abutments of masonry or brick-work; and in like manner, what is called an iron-bridge, may be said to require that its piers and abutments, or other points of support, shall be of masonry, or of a combination of mason's work and brick-work. Bridges are built and in some cases most efficiently so, of brick-work alone; but, that a brickbridge may be durable and sightly, it is almost always necessary, and it is always desirable, that some of the more exposed parts should be of stone. Masonry may stand alone in the composition of a bridge, but neither brick-work nor masonry alone, nor the two in combination, can be made to effect such objects as tmv be attained by the aid of the iron founder and smith, and indeed, of the carpenter with his timber."
But the chief material of a bridge must be determined by local circumstances, as one place may furnish one kind of material and not others; but timber must nevertheless be most extensively and variously applied, not only as a bridge-building material, but as an auxiliary in the erection of a bridge with other materials. Yet of all the materials used in bridge-building, timber is the most perishable, although modern science has contrived methods of protecting it from many of the causes of decay. Iron as a bridge-building material possesses many valuable properties. Its tenacity and power of resistance in a comparatively small bulk, allow works to be constructed and effects to be produced, which belong to no other available material. It is however greatly affected by change of temperature, and is hence unstable in framing. It is also injurious to the material which it thrusts against or is connected with, and it wastes away by oxidation and other chemical processes. As an auxiliary in bridge-building iron is second only to timber, and both iron and timber are necessary in bridge-building, although neither may enter into the composition of a structure. Stone, however, is preeminently the material for bridges, whether we regard its grandeur of effect, its power of resistance and endurance, its unyielding nature, its massiveness, its capability of being cut or wrought to any form, size or figure, and its retentiveness of the form given to it; it has scarcely any tendency to change in bulk from changes in temperature, and it is inaccessible to moisture. Its practical want of elasticity prevents it being disturbed by concussions, but being readily frangible, it can only be used and applied where it shall not be subjected to transverse strains. Hence it is unfit for beams or for bearing across over a void, and for situations where it is liable to disturbance from concussions. The inelasticity and frangibility of stone prevent a combination of parts by framing, as is done with timber and iron; but if the stone be cut up into pieces of certain forms previously arranged, as in the arch, it will carry a much greater weight than if thrown across the same space in the form of a beam or lintel; but the parts so arranged require the restraint of an extraneous tie, or a loading of the abutments and haunches not necessary to the level bearing across, and which a susceptibility of being framed would have obviated.
Brick must be regarded only as a substitute for stone, where stone cannot be obtained. Brick resembles stone in being practically incompressible and inflexible, but being prepared in small, regular and equal forms, it must be combined with cement or mortar, which, from its nature and mode of preparation, is yielding and liable to change until it has set and become perfectly dry. An arch turned with uncut bricks depends entirely on the mortar with which the bricks arc packed, and is hence liable to change its form and become insecure while the mortar remains incompressible or less hard and unyielding than the substance of the brick. It is, however, not uncommon to strike the centering of brick arches before the mortar is properly set, in order, as is supposed, to allow the materials to settle and consolidate by mutual pressure. This practice has probably led to the destruction of many of the large brick arches recently constructed on some lines of railway. A combination of brick and stone produces excellent results in bridges, the stone being introduced in chains and strings to assist in binding the brick-work, and in springing, blocking and coping courses and upon salient and exposed parts generally, and to receive and distribute pressure where it is greatest.
It may be stated as a general rule, that wherever the object to be attained in the use of a bridge can be effected by timber, the same end may be answered better and more effectually, both for use and durability with iron; and where the object proposed in the use of a bridge can be attained by the use of brick, the same thing can be done more effectually with stone; while iron and stone can often be applied where timber or brick could not be used. Probably in all cases where there is a choice between iron and stone, the latter will deserve the preference; but the art of constructing a bridge in the readiest, cheapest and simplest form will greatly depend on the facility of procuring materials. Bridges, like roads, canals and other means of transit, arc often necessary to give value to the products of industry, and must be adapted in many cases, as in that of an infant community, a new colony, for example, to supply the want with the smallest expenditure of labour.
We come now to notice the practice of bridge building chiefly with reference to stone bridges, taking the several parts of the bridge in the order enumerated at the head of this section.
1. The foundations of a bridge consist of the underground work of the piers and abutments, and these require to be constructed with the utmost care in order to produce firm and solid bases whereon to carry up to any required height the various pedestals of support for the arches of the bridge. Alberti's distinction between the structure above-ground and the foundations of any building is applicable to bridges. He considered the foundation, not as a part of the structure itself, but as an artificial support on. which the latter is to be placed; and remarks, that if the natural site of a building consisted of rock or other stratum equally hard with the material of which foundations are constructed, these would be unnecessary, and the building might be commenced and carried up without previous preparation of the bottom. Gauthey also remarks that the solidity of a bridge depends almost entirely on the manner in which its foundations are laid. When these are once properly arranged, the upper part may be erected either with simplicity or elegance, without impairing in any degree the durability of the structure. Experience has proved that many bridges either decay or are swept away by sndden floods, by reason of the defective mode of fixing their foundations, while very few suffer from an unskilful construction of the piers and arches.
In constructing the foundation of bridges which are to pass over roads or railways, the absence of water removes the greater part of the difficulty which is met
with in spanning a wide and rapid stream; for if the bottom be unsound, the stability of the work may be ensured by driving piles, or by using concrete. In crossing narrow streams, the abutments may often be founded on dry land, or the course of the river may be turned and the piers constructed in the dry bed; but where none of these facilities are afforded, the bridge architect or engineer will have to contend against all the difficulties of operating in the midst of water.
In the ancient practice of bridge building, two methods were adopted for constructing the foundations. For shallow rivers stones were sunk to the bottom in strong baskets made of the pliable boughs or branches of trees; when this rnde basket or caisson was sunk, stones were added until they were piled up to within a foot or 16 inches of the lowest water surface. In later times this plan was improved upon by forming wooden chests st rongly hooped with iron, and constructing in them above-ground masses of masonry, which were sunk into the bed of the river. This method was practised by Labelye in the construction of Westminster, and by Mylnc in the construction of Blackfriars bridges.
According to the second ancient method where strong currents existed, and it was necessary to construct the piers on dry ground, the plan adopted was to form the piers at some convenient distance from the river, in a range with the general direction of the stream, and at right angles with some good and direct approach previously selected, leading to and over the bridge, and when the bridge was completed to turn the course of the river by a new channel through the water way of the bridge. In Pig. 279 we have the plan of a bridge to be built in
a neck of land formed by a bend of the river, whicii must afterwards be diverted from its course in the direction of the dotted line A B, and the roadway can be readily embanked across the old channel, which will be laid dry. Even where the course of the river is nearly straight, the difficulties and expense of constructing the piers in the water may be so great as to warrant the engineer in slightly turning the river as in Fig. 280. which shows the appearance of the diversion, the bridge having been built on dry land on one side of the old channel, thus affording the advan
In the old bridges, if the bottom proved unsound, piles, 3 or 4 feet apart, were driven down over the site of the proposed piers; and instead of cutting off the pile heads evenly, and planking them over with timber, as is now done, it was customary to fill in between them with a species of coarse concrete,1 which being brought to a level surface, formed the bed on which the first course of stone was placed. Another ancient method of using piles for building in water, and still used by the French, who call it eneaissement, consisted in driving in main piles with sheet piling between them, secured and bound round with waling as in the modern coffer-dam, but only one row of piles was driven all round the space of the pier or abutment to be founded, and not a double row of piles to be filled with clay, as in a coffer-dam. The encaisscment being formed, and the loose soil, &c. got out, a mass of concrete or dry rubble stones is thrown in until a sufficient foundation is formed reaching to the level of the water. After being allowed some time to settle, the dressed masonry is laid in courses upon the foundations thus formed. A design by Mr. Semple will illustrate this mode. In a river 6 feet deep he proposed to drive sheeting piles about 10 feet in length, to a depth of 4 feet in the ground all round the site of each pier, and to fill the space or coffer thus formed with a bed of concrete 6 feet in depth. The top of the concrete being level with the surface of summer low-water mark, the masonry was to be commenced therefrom and carried up to springing height clear of the water.
Mr. Hughes remarks that " on examining the foundations of old bridges, particularly in this country, they are all found to be extremely massive, and the piers were even carried up above water of a thickness quite incommensurate with the necessity of the case. On inspecting the masonry of their foundations, however, it is found that no very great attention has been paid to the regularity of courses, or to the perfection of beds and joints. Some of the strongest specimens
(1) Called by the French b:lan. It seems now to be generally admitted, that the use of a concrete mixture composed of lime and coarse gravel, was common among the Romans. The use of concrete in modern times on a large scale, was introduced by Mr. Peter Semple, arehitect of a bridge over the Lificy at Dublin.
of ancient masonry existing in this country, consist of a kind of building little superior to rubble walling, with this most important qualification, that the mortar is always of an excellent description, and in most cases by no means inferior in hardness and cohesiveness to the stone itself."2 The mortar usually contained a great number of small stones or pebbles, some of them equal in size to a pigeon's egg, and was altogether much coarser than that used at the present day. Another point of difference between ancient and modern bridges is, that in modern times engineers form their bridges with as few arehes as possible; the ancient structures " consisted of a long low series of culverts, hardly deserving the name of arehes, with intervening piers often of greater thickness than the span of the arehes they were built to support." The weight of such a bridge distributed over a great many points, such as the 20 or 30 piers sometimes built in the old bridges, each pier had scareely more to support than its own weight. "The ancients in their bridges throughout the whole structure, substituted quantity for quality, that is to say, large masses of rough undressed masonry, or rubble, instead of the firm, compact and elegant piers of moder n bridges, which, above the bed of the river at all events, are invariably built with the most durable stone, of wellsquared dressed ashlar fronts, and suitable filling within." In some cases, however, there is no objection to these coarse and massive subaqueous structures; and provided they do not obstruct the free course cf
Fig. 2S1. Tux Beidge Of Fabeicius At Rome.
the river, they may be of great advantage, especially in projecting sea piers and similar structures.
In modern times, the invention of the coffer-dam has enabled the engineer to secure a firm foundation in the bed of the river. Suppose a bridge is to be built over a river with 5 feet depth of water at the lowest summer floods; that the breadth of the waterway is great; and that it is impracticable to lay dry the bed by turning the course of the river. In such a case, the best method is to drive a coffer-dam all round the space which is to be occupied by the piers and abutments. The depth of water being 5 feet, suppose that of the unsound bottom to be 25 feet: the piles must not be less than 45 feet long, and must be driven into the solid ground as far as they will go, which may be from 8 to 10 feet. For such a depth of water, a double dam, with three rows of piles, will be required. The coffers between tho rows of piles should be 6 or 7 feet apart, and filled with a retentive
(2) A series of papers on the Foundations of Bridges. By T. Hughes, Civil Engineer,—inserted in Mr. Wcale'j work on the Theory, Practice, and Arehitecture of Bridget.