Bridge builder

“ Once across, the traveller might find his path or make it for himself. Parapets are not provided on the old bridges, and the surface is a flight of broad and shallow steps. If you walk unwarily and are drowned in the torrent below, that is no concern of these resolute but unluxurious bridge-builders. Their business is simply to span the stream. ”

“ The earliest of the very distinguished list of American bridge-builders erected great timber structures for the highroads and the post-roads. Some of them went back many centuries and came to the stone bridge, in many ways the most wonderful of all the artifices by which man conquers the obstructive power of a running stream. ”

“ This bridge has three arches, one a good deal larger than the others; bays are driven through the spandrils for spate water to pass through. The masonry consists of stone in big blocks, and the craftsmanship has a very peculiar feature: the voussoirs are notched or joggled one into the other, like those in the Elizabethan bridge at Ross-on-Wye, Herefordshire. This technique is a thing to be remembered: it occurs in no other Roman bridge that is known to me. The notching adds much to the endurance of an arch ring, yet it has never entered into the technical routine of bridge-builders. ”

“ It is in metal bridges alone that we find a virile growth, a genuine evolution; not often artistic, and as sensitive to bombs as card castles are to a touch from your finger; but yet a great evolution because it represents modern times. If we could summon to earth the spirits of the greatest bridge-builders—Caius Julius Lacer, Apollodorus of Damascus, Isembert, Bénézet, Ammanati, and several others—they would learn nothing much from our stonework, whereas a metal bridge here and there could not fail to strike awe into these spiritual beholders. ”

“ Then advertise for bridge-builders to send in plans and proposals. Let the contractors understand that all plans and computations are to be submitted to your engineer, that all materials and workmanship will be submitted to your inspectors, and that the whole structure [Pg 55] is to be made subject to the supervision of a competent engineer, and accepted by him for you. You will find at once, that, under such conditions, all travelling agents and builders of cheap bridges will avoid you as a thief does the light of day. ”

“ It does not follow, because newspapers say, "magnificent bridge," "two million feet of timber," "eighty or one hundred tons of iron," "cost half a million," that there is any merit about either the bridge or its builder; as one span is, so is the whole; and a bridge fifty feet long, and costing only a few hundreds, may show more engineering skill than the largest and most costly viaducts in America. Few bridges require more knowledge of mechanics and of materials than Mr. ”

“ 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. ”

“ In bridge-building it marks an era, not only because of its strength and beauty and the daring of its design, but also because of its many labor-saving devices, the inventions of a thoroughly practical mind. A distinguished engineer calls it "a great pioneer in the art of sinking deep foundations and building spans over wide stretches of space, that astonished in its construction the entire civilized world." ”

“ The contractors have already completed the iron-work for the “first half” of the bridge; with the exception of this face-work, which will be finished somewhat later. It then only remains for the engineer to complete the road and pathway, and open the bridge for traffic. One obstruction will, we fear, remain. The blocks of houses on either side of the river (ultimately destined to be removed) , still look stable as ever—these abut directly upon the new portion of the structure, and will, if not removed before opening the bridge, make a sharp and inconvenient turn in the road necessary. ”

“ 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. ”

“ The foundations for bridges are not always put down by the builders of the bridge proper; that is a work by itself and requires special experience. On the strength and permanency of the foundation depends the life of the bridge. While the foundries and steel mills are making the metal-work the foundations are being laid. If the bridge is to cross a valley, or carry the roadway on the level across a depression, the placing of the foundations is a simple matter of digging or blasting out a big hole and laying courses of masonry ”

“ The aspect of solidity and permanence is remarkable. Even the temporary bridge over the Ussuri, 1,050 feet in length, a trestle bridge of heavy timber to resist the impact of the ice, is so massive as to make the great steel bridge, the handsome abutments of which were already built, appear as if it would be a work of supererogation. ”

“ 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." ”

“ The short-span timber bridges were being displaced by concrete slab or concrete I-beam bridges. The reinforced concrete box culvert also became common. But aside from this, most of the growth consisted of further developments and refinements of existing bridge types. Riveted joints in steel trusses replaced the earlier pin connected joints, and stiffer and more substantial truss members developed. Steel rolled beam and built-up “plate girder” structures became common. The small arch brige changed from stone to concrete and began to grow in span length. Wider bridge roadways were being used. ”

“ 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. ”

“ The increasing number of trained engineers at first affected highway bridge construction not so much in the materials used but in the way they were assembled. In a bridge designed by a self-taught constructor, the cheapness of wood made it entirely feasible to proportion the members by enlarging them to the point where there could be no question as to their structural adequacy. ”

“ On the lips of the abutment, the builders set two stone blocks, smooth and wide, and cut places in them for the bridge timbers. It was a piece of excellent judgment, since the great stones could not be broken from the abutment, and they were mighty enough to bear the weight of a mountain. The bridge rests on three sills, each a log that, unhewn, must have taken a dozen oxen to drag it. ”

“ Instead of arches resting on piers and abutments, the bridge has a single arch, reaching from one side of the cutting to the other, and springing from the slopes, which it helps in some measure to support. A flying bridge of large dimensions near Weston-super-Mare carries a road across the cutting at a height of 60 feet above the line of rails, with a clear span of 110 feet. ”

“ 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. ”

“ In trail and road construction it is very often necessary to build bridges. Sometimes a very simple log bridge meets the need, but in bridging many large mountain torrents, which become very high and dangerous in the spring, large bridges are necessary. Cable suspension bridges and queen and king truss bridges are built where occasion arises for them, but only after being planned in detail and after the District Forester has approved their design and method of construction. ”

“ THE ARLINGTON MEMORIAL BRIDGE DEVELOPMENT, LOOKING TOWARDS THE LINCOLN MEMORIAL
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At the center of the bridge is a drawspan, each leaf of which has a length of 92 feet, the height of an 8-story building. One minute is required for opening and closing the drawspan, which is operated by electricity. Each leaf weighs 6,000 tons. It is in itself an interesting achievement in bridge engineering. ”

“ In the construction of the new bridge it became necessary to dismantle the draw of the old one, and to erect temporary supports while the timbers and draw of the new bridge were being put in place. To prevent the stoppage of its trains while this building was going on, the company constructed a temporary bridge adjoining the old one, for their transportation; expecting to have the new bridge completed before the winter rise, which usually began near the close of December, should render the river navigable. ”

“ 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. ”

“ Thus the railroad crossing the Delaware at Trenton has, in this way, typified step by step every stage of the development of American bridge-building.
The timber bridges developed the steel truss bridge, the typically American construction, of to-day. In an earlier day the timber bridges were the glory of the engineer. Sometimes you see one of these old fellows remaining, like the long structure that Mr. Walcott built across the Connecticut River at Springfield, Mass., in 1805, and which still does good service; but the most of them have passed away. Fire has been their most persistent enemy. ”

“ Myself, I hate such a strumpet of a bridge as the Hoogesluis at Amsterdam, with her ornate spandrils, and her embossed masonry, and her balustraded parapet surmounted by a row of obelisks around which lamps are bracketed. Also I hate such a suspension bridge as the one at Conway Castle, where the metal rods that support the roadway pass through a brace of turrets on each of the embattled gateways. The effect is not only comic but ludicrous. No engineer with any sense would have put a metal viaduct within a few yards of Conway Castle. ”

“ This bridge, a view of which is given in the cut below, is about two and a half miles below the falls, and spans the river near the head of the rapids, above the whirlpool. From pier to pier it is eight hundred feet long, and in breadth eight feet. It is suspended on eight wire cables, four on each side, which pass over towers fifty-four feet high, built of heavy timber. The present structure is only the scaffolding for constructing a larger bridge, intended for the passage of railroad cars. The towers for the large bridge will be of solid masonry, each eighty feet high. ”

“ 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. ”

“ A bridge can be made safe against scour either by 133 giving deep foundations to the piers and abutments or by adding a floor and, if necessary, pitching. The former course is usually adopted and is the best. But in a case in which the discharge of a stream is to be increased or has been underestimated, it is often far easier to add a floor to an existing bridge than to increase the span of the bridge. ”

“ This bridge of nearly two thousand years ago appeared to be built of baulks of timber fastened together in very much the same manner as that adopted by the engineering units of the great armies of to-day.
Every observant person has noticed how tall poles and short sticks tied together with ropes can be fashioned into the firm, strong scaffolding from which workmen can in safety raise great tall buildings. That mode of construction can always be used to form a bridge. ”