Bridge (structure)

“ At a point where opposite portions of these rocky walls jut out so as to render the space between extremely narrow, the little arch of the bridge springs boldly across the void, the jutting portions of rock forming piers more substantial and durable, barring earthquakes, than any artificial structure for the same purpose in the kingdom. ”

“ The flooring of the bridge is suspended from the arch by innumerable pendants of iron, so that the weight of a body, in crossing the bridge, is brought to bear gradually upon the structure, and when it reaches the center, where common bridges are the weakest, under this invention it reaches the strongest part, for it is directly beneath the arch. ”

“ The truss bridges which have been mentioned as types of the modern railway bridge are erected by the use of false works of timber, placed generally upon piling or other suitable foundation, between the piers or abutments, and made of sufficient strength to carry each span of the permanent structure until it is completed and all its parts connected, or, as is technically said, until the span is swung. ”

“ Girders may be of various materials; wrought iron, cast iron, and wood are chiefly used. Professor Fleeming Jenkin describes with apt brevity the essential difference between the three classes of bridge. “In all forms of the suspension bridge the supporting structure is extended by the stress due to the load; in all forms of the arch the supporting structure (i.e. the ring of voussoirs) is compressed by the stress due to the load; and in all forms of the beam or girder the material is partly extended and partly compressed by the flexure which it undergoes as it bends under the load. ”

“ It is a matter of notoriety that railroad bridges are built upon solid abutments of mason-work and resting on piers of stone between the abutments, thus giving strength and security to the frame above. It is not easy to see wherein such a structure differs from an ordinary bridge, except that, as it is to endure a greater burden, it is more solid and substantial. ”

“ The new high-level bridge, carrying the railway only, was opened by King Edward VII. in 1906; it consists of four steel spans on granite piers. The hydraulic swing bridge, on the low level, was built to replace a stone structure erected in 1781 on the site of a bridge dating from 1250, and destroyed by a flood in 1771. The Roman bridge, the Pons Aelii, is said to have spanned the river at the same point. The hydraulic bridge (1876) consists of one large centre pier, two midstream piers and two abutments; and its foundations are iron cylinders resting on the solid rock, 60 ft. ”

“ The bridge flooring rests on the supporting members, and is of very various types according to the purpose of the bridge. There is also in large bridges wind-bracing to stiffen the structure against horizontal forces. The substructure consists of (a) the piers and end piers or abutments, the former sustaining a vertical load, and the latter having to resist, in addition, the oblique thrust of an arch, the pull of a suspension chain, or the thrust of an embankment ”

“ Every situation that confronts the bridge builder requires different handling; at one time he may be called upon to construct a bridge alongside of a narrow, rocky cleft over a rushing stream like the Royal Gorge, Colorado, where the track is hung from two great beams stretched across the chasm, or he may be required to design and construct a viaduct like that gossamer structure three hundred and five feet high and nearly a half-mile long across the Kinzua Creek, in Pennsylvania. ”

“ The bridge was equipped with a swinging span, supported to a turntable resting on a base of stone and piles driven into the bottom of the river, leaving a space for the passage of vessels on either side of the supporting structure. ”

“ The structure was held together by pegs of hard wood. No mortar, cement, bolts, or nails were used. The marvel is that so many of them have stood up for more than a century. In their way they were masterpieces of construction.
A third feature of interest is that while most of the old bridges followed the same lines of construction, no two were precisely alike. Many writers have emphasized the fact that each bridge has a certain individuality. ”

“ The idea of a suspension bridge was dismissed as inapplicable; a degree of rigidity and strength, greater than could be secured by any bridge constructed on the principle of suspension, being considered an indispensable condition of the proposed structure. ”

“ Evidently a difference should be made in determining the load for London Bridge [Pg 22] and the load for a highway bridge upon a New-England country road in a thinly settled district. A bridge that is strong enough is just as good and just as safe as one that is ten times stronger, and even better; for in a large bridge, if we make it too strong, we make it at the same time too heavy. The weight of the structure itself has to be sustained, and this part of the load is a perpetual drag on the material. ”

“ Instead of the arched roof and masonry side-walks of the ordinary underground, there is a rectangular structure with a framework of steel beams riveted together, concrete enclosing the erection completely at the top and sides, and forming the bottom, rows of steel columns helping to support the roof between the tracks—in other words, a kind of Britannia Bridge let into the surface of the earth. ”

“ If the structure is too weak to satisfy this modified condition, it may be possible to bring it within the stress limit by a reduction of ballast or other removable dead weight. If this expedient does not promise to be satisfactory, or the bridge shows actual signs of [111] weakness, or palpable defects, it will be necessary to deal with the question of repair, strengthening, or reconstruction.
The repair of built up bridgework resolves itself largely into a matter of replacing loose rivets by cutting these out, rhymering the holes, if desirable, and again riveting. ”

“ Water running upon the surface of a member gravitates to its lower end, and, if there be a joint or other connection, settles there, to be productive of lasting mischief. From this cause, together with a very common deficiency of bearing surface relative to the forces to be met, the joints soon develop some movement; working of the structure commences under passing loads, its final destruction being then a question of time only. Each joint is, in fact, in timber bridge construction a source of serious weakness to a degree which has no parallel in well-designed metallic bridges. ”

“ Where timber staging is used to carry the permanent way before dismantling an old structure, it is convenient to begin by placing stout balks of timber under the sleepers from end to end of the bridge, or directly under the rails if space is limited ”

“ Then, of course, there are sky-tickling metal bridges, and these spindle-shanked devotees of peace are popular also in Canada. All this work is nothing but industrial engineering, like the mighty bridges at New York, though these do try to look somewhat architectural. One specimen, indeed, a vast structure called the New Manhattan Bridge, has marvellously long suspension cables which do not go through a tower or gateway; they actually pass over their supports in a logical manner. ”

“ Whether Roman or Romanesque, the structure of the arches has great interest, and a large photograph is sold by Neurdein, 52 Avenue de Breteuil, Paris.
[Pg 408] Rivers, how their violence has given lessons to bridge-builders, 181.
Roads, ancient British, 22; Roman, 139, and footnote; they and bridges circulate all the strife in the overland enterprise of mankind, 4, 14-52; types of society are as old as their systems of circulation, just as women and men are as old as their arteries, 13 ”

“ You are standing aloft, balancing yourself upon tiny planks at the steadily advancing end of the bridge, as it forces itself over a stream of formidable width. Overhead, a gigantic, ungainly traveller, equipped with steel derricks at every corner, is advancing foot by foot as the bridge advances foot by foot. Underneath, through the thin network of planks, of girder and of supporting false work, you can see the surface of the river a full hundred feet below. ”

“ So the design is a clever feat not merely of repetitive decoration, but of repeating solids and voids that oppose each other in a harmony of contrasts; for the empty spandrils in their form oppose the leaf-shaped openings made by the arches, and all the curved solids of the bridge are foiled in a rugged manner by the upright columns, as well as by the long horizontal lines of the covered roadway. In the contrast between cold granite and warm brick there is colour also, and it suits the pulsating light and heat of Italy. ”

“ Hence each arch is complete and independent in itself, the piers having simply to sustain their vertical pressure. The arches are six in number, of 125 feet span each, the two approaches to the bridge being formed of cast-iron pillars and bearers in keeping with the arches.
The result is a bridge that for massive solidity may be pronounced unrivaled. It is one of the most magnificent and striking of the bridges to which railways have given birth, and has been worthily styled "the King of railway structures." ”

“ They have a rise of sixty feet, sufficient to permit the passage of steamers under them, even at high water. The piers are sunk through the sand to the bed-rock, a distance of from ninety to one hundred and twenty feet, the work having been accomplished by means of iron wrought caissons and atmospheric pressure. Each span consists of four ribbed arches, made of cast steel. The bridge is two stories high, the lower story containing a double car track, and the upper one two horse-car tracks, two carriageways and two foot-ways. ”

“ Professor Fleeming Jenkin. If any reader wants to continue the study of timber bridges, let him turn to Colonel Emy and to the huge volumes compiled and edited by Hosking. But it is clear enough that timber bridges belong to the past; in these days they are ludicrously out of joint with the needs of social life, owing to the rapid advance which “progress” has made in artillery, in high explosives, in airships, and in aeroplanes.
[52] These date from about the year 1816, when Galashiels Bridge was constructed. ”

“ After a single rib of bricks has been made, other bricks are dabbed against the first set, more being added at the abutment ends than in the centre of an arch; and so, as the work goes on, the arch grows to be self-supporting, like a cantilever bridge. When the middle part of the span has been covered over, the remaining courses at each side are completed with bricks set at right angles to the others. ”

“ Finally it was decided to build the railway along a hanging bridge. And this bridge is surely one of the most curious ever erected. From the cliff-face on either side, iron girders spring at an upward slant, like an inverted V, and from the point at which they meet, steel rods descend. These are securely fastened to the river-side of the bridge. The other side of the bridge is built into the cliff-face. Thus it is neither a suspension bridge nor an arch bridge, but is sustained by the strength of the overhead girders. ”

“ Previous to the 19th century, the greatest bridges being constructed mostly of solid masonry piers and arches, no demand for a bridge of this kind existed; but after the use of wrought iron and steel became extensive in bridge making, and as these apparently light and airy frames may be extended, piece by piece across the widest rivers, straits, and arms of the sea, a substitute for the great, expensive, and frequent supporting piers became a want, and was supplied by the system of trusses and truss arches. ”

“ We thus see that by increasing very slightly the amount of material used in the construction of the "bridge we have an open, clear span in the center three times the width that would be possible with the ordinary framed truss, and we have what is known and has lately become famous as the cantilever bridge. ”

“ Sometimes a hanging bridge has to be crossed, spanning a horrid chasm, at the bottom of which roars a foaming torrent—the bridge itself, composed of ropes and brambles, all the while swinging like a hammock under the tread of the affrighted traveller! ”

“ Modern drawbridges across rivers, canals, the entrances of docks, &c., are generally made to open horizontally, and the movable portion is called a bascule, balance, or lifting bridge, a turning, swivel, or swing bridge, or a rolling bridge, in accordance with the mode in which it is made to open. ”

“ The simplest bridge that can be built, is a single beam, or stick of timber, spanning the opening between the abutments—but this is only of very limited application— (only for spans of 20 feet and less) owing to the rapid increase in sectional dimensions which is required as the span becomes greater. ”

“ Not only is there evidence that bridges of the simplest forms have been used from prehistoric times, but the engineering world has been frequently surprised at the discovery, in semi-barbarous lands where there was evidently no scientific knowledge of bridge construction, of a bridge which, in its mechanical analysis, is a rude example of some one of the more complicated types now in use. ”

“ The next step was the contrivance of arched beams or bowstring girders, firmly held together by horizontal ties to resist the thrust, instead of abutments. Numerous excellent specimens of this description of bridge were erected by Robert Stephenson on the original London and Birmingham Railway; but by far the grandest work of the kind—perfect as a specimen of modern constructive skill—was the High Level Bridge, which we owe to the genius of the same engineer. ”