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Iron Bridges, and Their Construction

by Edward Rowland

1873

"ASSEMBLING" BRIDGE UNDER SHED.—p. 22.
"ASSEMBLING" BRIDGE UNDER SHED.

In a graveyard in Watertown, a village near Boston, Massachusetts, there is a tombstone commemorating the claims of the departed worthy who lies below to the eternal gratitude of posterity. The inscription is dated in the early part of this century (about 1810), but the name of him who was thus immortalized has faded like the date of his death from my memory, while the deed for which he was distinguished, and which was recorded upon his tombstone, remains clear. "He built the famous bridge over the Charles River in this town," says the record. The Charles River is here a small stream, about twenty to thirty feet wide, and the bridge was a simple wooden structure.

THE LYMAN VIADUCT.
THE LYMAN VIADUCT.

Doubtless in its day this structure was considered an engineering feat worthy of such posthumous immortality as is gained by an epitaph, and afforded such convenience for transportation as was needed by the commercial activity of that era. From that time, however, to this, the changes which have occurred in our commercial and industrial methods are so fully indicated by the changes of our manner and method of bridge-building that it will not be a loss of time to investigate the present condition of our abilities in this most useful branch of engineering skill.

In the usual archaeological classification of eras the Stone Age precedes that of Iron, and in the history of bridge-building the same sequence has been preserved. Though the knowledge of working iron was acquired by many nations at a pre-historic period, yet in quite modern times—within this century, even—the invention of new processes and the experience gained of new methods have so completely revolutionized this branch of industry, and given us such a mastery over this material, enabling us to apply it to such new uses, that for the future the real Age of Iron will date from the present century.

The knowledge of the arch as a method of construction with stone or brick—both of them materials aptly fitted for resistance under pressure, but of comparatively no tensile strength—enabled the Romans to surpass all nations that had preceded them in the course of history in building bridges. The bridge across the Danube, erected by Apollodorus, the architect of Trajan's Column, was the largest bridge built by the Romans. It was more than three hundred feet in height, composed of twenty-one arches resting upon twenty piers, and was about eight hundred feet in length. It was after a few years destroyed by the emperor Adrian, lest it should afford a means of passage to the barbarians, and its ruins are still to be seen in Lower Hungary.

With the advent of railroads bridge-building became even a greater necessity than it had ever been before, and the use of iron has enabled engineers to grapple with and overcome difficulties which only fifty years ago would have been considered hopelessly insurmountable. In this modern use of iron advantage is taken of its great tensile strength, and many iron bridges, over which enormous trains of heavily-loaded cars pass hourly, look as though they were spun from gossamer threads, and yet are stronger than any structure of wood or stone would be.

BLAST-FURNACES.
BLAST-FURNACES.

Another great advantage of an iron bridge over one constructed of wood or stone is the greater ease with which it can, in every part of it, be constantly observed, and every failing part replaced. Whatever material may be used, every edifice is always subject to the slow disintegrating influence of time and the elements. In every such edifice as a bridge, use is a process of constant weakening, which, if not as constantly guarded against, must inevitably, in time, lead to its destruction.

DUMPING ORE AND COAL INTO BLAST-FURNACES.
DUMPING ORE AND COAL INTO BLAST-FURNACES.

In a wooden or stone bridge a beam affected by dry rot or a stone weakened by the effects of frost may lie hidden from the inspection of even the most vigilant observer until, when the process has gone far enough, the bridge suddenly gives way under a not unusual strain, and death and disaster shock the community into a sense of the inherent defects of these materials for such structures.

The introduction of the railroad has brought about also another change in the bridge-building of modern times, compared with that of all the ages which have preceded this nineteenth century. The chief bridges of ancient times were built as great public conveniences upon thoroughways over which there was a large amount of travel, and consequently were near the cities or commercial centres which attracted such travel, and were therefore placed where they were seen by great numbers. Now, however, the connection between the chief commercial centres is made by the railroads, and these penetrate immense distances, through comparatively unsettled districts, in order to bring about the needed distribution; and in consequence many of the great railroad bridges are built in the most unfrequented spots, and are unseen by the numerous passengers who traverse them, unconscious that they are thus easily passing over specimens of engineering skill which surpass, as objects of intelligent interest, many of the sights they may be traveling to see.

ELEVATOR.
ELEVATOR.

The various processes by which the iron is prepared to be used in bridge-building are many of them as new as is the use of this material for this purpose, and it will not be amiss to spend a few moments in examining them before presenting to our readers illustrations of some of the most remarkable structures of this kind. Taking a train by the Reading Railroad from Philadelphia, we arrive, in about an hour, at Phoenixville, in the Schuylkill Valley, where the Phoenix Iron-and Bridge-works are situated. In this establishment we can follow the iron from its original condition of ore to a finished bridge, and it is the only establishment in this country, and most probably in the world, where this can be seen.

THE ENGINE-ROOM.
THE ENGINE-ROOM.

These works were established in 1790. In 1827 they came into the possession of the late David Reeves, who by his energy and enterprise increased their capacity to meet the growing demands of the time, until they reached their present extent, employing constantly over fifteen hundred hands.

RUNNING METAL INTO PIGS.
RUNNING METAL INTO PIGS.

The first process is melting the ore in the blast-furnace. Here the ore, with coal and a flux of limestone, is piled in and subjected to the heat of the fires, driven by a hot blast and kept burning night and day. The iron, as it becomes melted, flows to the bottom of the furnace, and is drawn off below in a glowing stream. Into the top of the blast-furnaces the ore and coal are dumped, having been raised to the top by an elevator worked by a blast of air. It is curious to notice how slowly the experience was gathered from which has re suited the ability to work iron as it is done here. Though even at the first settlement of this country the forests of England had been so much thinned by their consumption in the form of charcoal in her iron industry as to make a demand for timber from this country a flourishing trade for the new settlers, yet it was not until 1612 that a patent was granted to Simon Sturtevant for smelting iron by the consumption of bituminous coal. Another patent for the same invention was granted to John Ravenson the next year, and in 1619 another to Lord Dudley; yet the process did not come into general use until nearly a hundred years later.

CARRYING THE IRON BALLS.
CARRYING THE IRON BALLS.

The blast for the furnace is driven by two enormous engines, each of three hundred horse-power. The blast used here is, as we have said, a hot one, the air being heated by the consumption of the gases evolved from the material itself. The gradual steps by which these successive modifications were introduced is an evidence of how slowly industrial processes have been perfected by the collective experience of generations, and shows us how much we of the present day owe to our predecessors. From the earliest times, as among the native smiths of Africa to-day, the blast of a bellows has been used in working iron to increase the heat of the combustion by a more plentiful supply of oxygen. The blast-furnace is supposed to have been first used in Belgium, and to have been introduced into England in 1558. Next came the use of bituminous coal, urged with a blast of cold air. But it was not until 1829 that Neilson, an Englishman, conceived the idea of heating the air of the blast, and carried it out at the Muirkirk furnaces. In that year he obtained a patent for this process, and found that he could from the same quantity of fuel make three times as much iron. His patent made him very rich: in one single case of infringement he received a cheque for damages for one hundred and fifty thousand pounds. In his method, however, he used an extra fire for heating the air of his blast. In 1837 the idea of heating the air for the blast by the gases generated in the process was first practically introduced by M. Faber Dufour at Wasseralfingen in the kingdom of Würtemberg.

In this country, charcoal was at first used universally for smelting iron, anthracite coal being considered unfit for the purpose. In 1820 an unsuccessful attempt to use it was made at Mauch Chunk. In 1833, Frederick W. Geisenhainer of Schuylkill obtained a patent for the use of the hot blast with anthracite, and in 1835 produced the first iron made with this process. In 1841, C.E. Detmold adapted the consumption of the gases produced by the smelting to the use of anthracite; and since then it has become quite general, and has caused an almost incalculable saving to the community in the price of iron.

The view of the engines which pump the blast will give an idea of the immense power which the Phoenix company has at command. Twice every day the furnace is tapped, and the stream of liquid iron flows out into moulds formed in the sand, making the iron into pigs—so called from a fancied resemblance to the form of these animals. This makes the first process, and in many smelting-establishments this is all that is done, the iron in this form being sold and entering into the general consumption.

The next process is "boiling," which is a modification of "puddling," and is generally used in the best iron-works in this country. The process of puddling was invented by Henry Cort, an Englishman, and patented by him in 1783 and 1784 as a new process for "shingling, welding and manufacturing iron and steel into bars, plates and rods of purer quality and in larger quantity than heretofore, by a more effectual application of fire and machinery." For this invention Cort has been called "the father of the iron-trade of the British nation," and it is estimated that his invention has, during this century, given employment to six millions of persons, and increased the wealth of Great Britain by three thousand millions of dollars. In his experiments for perfecting his process Mr. Cort spent his fortune, and though it proved so valuable, he died poor, having been involved by the government in a lawsuit concerning his patent which beggared him. Six years before his death, the government, as an acknowledgment of their wrong, granted him a yearly pension of a thousand dollars, and at his death this miserly recompense was reduced to his widow to six hundred and twenty-five dollars.

ROTARY SQUEEZER.
ROTARY SQUEEZER.
BOILING-FURNACE.
BOILING-FURNACE.

When iron is simply melted and run into any mould, its texture is granular, and it is so brittle as to be quite unreliable for any use requiring much tensile strength. The process of puddling consisted in stirring the molten iron run out in a puddle, and had the effect of so changing its atomic arrangement as to render the process of rolling it more efficacious. The process of boiling is considered an improvement upon this. The boiling-furnace is an oven heated to an intense heat by a fire urged with a blast. The cast-iron sides are double, and a constant circulation of water is kept passing through the chamber thus made, in order to preserve the structure from fusion by the heat. The inside is lined with fire-brick covered with metallic ore and slag over the bottom and sides, and then, the oven being charged with the pigs of iron, the heat is let on. The pigs melt, and the oven is filled with molten iron. The puddler constantly stirs this mass with a bar let through a hole in the door, until the iron boils up, or "ferments," as it is called. This fermentation is caused by the combustion of a portion of the carbon in the iron, and as soon as the excess of this is consumed, the cinders and slag sink to the bottom of the oven, leaving the semi-fluid mass on the top. Stirring this about, the puddler forms it into balls of such a size as he can conveniently handle, which are taken out and carried on little cars, made to receive them, to "the squeezer."

THE ROLLS.
THE ROLLS.

To carry on this process properly requires great skill and judgment in the puddler. The heat necessarily generated by the operation is so great that very few persons have the physical endurance to stand it. So great is it that the clothes upon the person frequently catch fire. Such a strain upon the physical powers naturally leads those subjected to it to indulge in excesses. The perspiration which flows from the puddlers in streams while engaged in their work is caused by the natural effort of their bodies to preserve themselves from injury by keeping their normal temperature. Such a consumption of the fluids of the body causes great thirst, and the exhaustion of the labor, both bodily and mental, leads often to the excessive use of stimulants. In fact, the work is too laborious. Its conditions are such that no one should be subjected to them. The necessity, however, for judgment, experience and skill on the part of the operator has up to this time prevented the introduction of machinery to take the place of human labor in this process. The successful substitution in modern times of machines for performing various operations which formerly seemed to require the intelligence and dexterity of a living being for their execution, justifies the expectation that the study now being given to the organization of industry will lead to the invention of machines which will obviate the necessity for human suffering in the process of puddling. Such a consummation would be an advantage to all classes concerned. The attempts which have been made in this direction have not as yet proved entirely successful.

In the squeezer the glowing ball of white-hot iron is placed, and forced with a rotary motion through a spiral passage, the diameter of which is constantly diminishing. The effect of this operation is to squeeze all the slag and cinder out of the ball, and force the iron to assume the shape of a short thick cylinder, called "a bloom." This process was formerly performed by striking the ball of iron repeatedly with a tilt-hammer.

COLD SAW.
COLD SAW.

The bloom is now re-heated and subjected to the process of rolling. "The rolls" are heavy cylinders of cast iron placed almost in contact, and revolving rapidly by steam-power. The bloom is caught between these rollers, and passed backward and forward until it is pressed into a flat bar, averaging from four to six inches in width, and about an inch and a half thick. These bars are then cut into short lengths, piled, heated again in a furnace, and re-rolled. After going through this process they form the bar iron of commerce. From the iron reduced into this form the various parts used in the construction of iron bridges are made by being rolled into shape, the rolls through which the various parts pass having grooves of the form it is desired to give to the pieces.

HOT SAW.
HOT SAW.
RIVETING A COLUMN
RIVETING A COLUMN.

These rolls, when they are driven by steam, obtain this generally from a boiler placed over the heating-or puddling-furnace, and heated by the waste gases from the furnace. This arrangement was first made by John Griffin, the superintendent of the Phoenix Iron-works, under whose direction the first rolled iron beams over nine inches thick that were ever made were produced at these works. The process of rolling toughens the iron, seeming to draw out its fibres; and iron that has been twice rolled is considered fit for ordinary uses. For the various parts of a bridge, however, where great toughness and tensile strength are necessary, as well as uniformity of texture, the iron is rolled a third time. The bars are therefore cut again into pieces, piled, re-heated and rolled again. A bar of iron which has been rolled twice is formed from a pile of fourteen separate pieces of iron that have been rolled only once, or "muck bar," as it is called; while the thrice-rolled bar is made from a pile of eight separate pieces of double-rolled iron. If, therefore, one of the original pieces of iron has any flaw or defect, it will form only a hundred and twelfth part of the thrice-rolled bar. The uniformity of texture and the toughness of the bars which have been thrice rolled are so great that they may be twisted, cold, into a knot without showing any signs of fracture. The bars of iron, whether hot or cold, are sawn to the various required lengths by the hot or cold saws shown in the illustrations, which revolve with great rapidity.

FURNACE AND HYDRAULIC DIE.
FURNACE AND HYDRAULIC DIE.

For the columns intended to sustain the compressive thrust of heavy weights a form is used in this establishment of their own design, and to which the name of the "Phoenix column" has been given. They are tubes made from four or from eight sections rolled in the usual way and riveted together at their flanges. When necessary, such columns are joined together by cast-iron joint-blocks, with circular tenons which fit into the hollows of each tube.

To join two bars to resist a strain of tension, links or eye-bars are used from three to six inches wide, and as long as may be needed. At each end is an enlargement with a hole to receive a pin. In this way any number of bars can be joined together, and the result of numerous experiments made at this establishment has shown that under sufficient strain they will part as often in the body of the bar as at the joint. The heads upon these bars are made by a process known as die-forging. The bar is heated to a white heat, and under a die worked by hydraulic pressure the head is shaped and the hole struck at one operation. This method of joining by pins is much more reliable than welding. The pins are made of cold-rolled shafting, and fit to a nicety.

The general view of the machine-shop, which covers more than an acre of ground, shows the various machines and tools by which iron is planed, turned, drilled and handled as though it were one of the softest of materials. Such a machine-shop is one of the wonders of this century. Most of the operations performed there, and all of the tools with which they are done, are due entirely to modern invention, many of them within the last ten years. By means of this application of machines great accuracy of work is obtained, and each part of an iron bridge can be exactly duplicated if necessary. This method of construction is entirely American, the English still building their iron bridges mostly with hand-labor. In consequence also of this method of working, American iron bridges, despite the higher price of our iron, can successfully compete in Canada with bridges of English or Belgian construction. The American iron bridges are lighter than those of other nations, but their absolute strength is as great, since the weight which is saved is all dead weight, and not necessary to the solidity of the structure. The same difference is displayed here that is seen in our carriages with their slender wheels, compared with the lumbering, heavy wagons of European construction.

VIEW OF MACHINE-SHOP
VIEW OF MACHINE-SHOP.

Before any practical work upon the construction of a bridge is begun the data and specifications are made, and a plan of the structure is drawn, whether it is for a railroad or for ordinary travel, whether for a double or single track, whether the train is to pass on top or below, and so on. The calculations and plans are then made for the use of such dimensions of iron that the strain upon any part of the structure shall not exceed a certain maximum, usually fixed at ten thousand pounds to the square inch. As the weight of the iron is known, and its tensile strength is estimated at sixty thousand pounds per square inch, this estimate, which is technically called "a factor of safety" of six, is a very safe one. In other words, the bridge is planned and so constructed that in supporting its own weight, together with any load of locomotives or cars which can be placed upon it, it shall not be subjected to a strain over one-sixth of its estimated strength.

NEW RIVER BRIDGE ON ITS STAGING.
NEW RIVER BRIDGE ON ITS STAGING.

After the plan is made, working drawings are prepared and the process of manufacture commences. The eye-bars, when made, are tested in a testing-machine at double the strain which by any possibility they can be put to in the bridge itself. The elasticity of the iron is such that after being submitted to a tension of about thirty thousand pounds to the square inch it will return to its original dimensions; while it is so tough that the bars, as large as two inches in diameter, can be bent double, when cold, without showing any signs of fracture. Having stood these tests, the parts of the bridge are considered fit to be used.

BRIDGE AT ALBANY.
BRIDGE AT ALBANY.

When completed the parts are put together—or "assembled," as the technical phrase is—in order to see that they are right in length, etc. Then they are marked with letters or numbers, according to the working plan, and shipped to the spot where the bridge is to be permanently erected. Before the erection can be begun, however, a staging or scaffolding of wood, strong enough to support the iron structure until it is finished, has to be raised on the spot. When the bridge is a large one this staging is of necessity an important and costly structure. An illustration on another page shows the staging erected for the support of the New River bridge in West Virginia, on the line of the Chesapeake and Ohio Railway, near a romantic spot known as Hawksnest. About two hundred yards below this bridge is a waterfall, and while the staging was still in use for its construction, the river, which is very treacherous, suddenly rose about twenty feet in a few hours, and became a roaring torrent.

LA SALLE BRIDGE.
LA SALLE BRIDGE.

The method of making all the parts of a bridge to fit exactly, and securing the ties by pins, is peculiarly American. The plan still followed in Europe is that of using rivets, which makes the erection of a bridge take much more time, and cost, consequently, much more. A riveted lattice bridge one hundred and sixty feet in span would require ten or twelve days for its erection, while one of the Phoenixville bridges of this size has been erected in eight and a half hours.

The view of the Albany bridge will show the style which is technically called a "through" bridge, having the track at the level of the lower chords. This view of the bridge is taken from the west side of the Hudson, near the Delavan House in Albany. The curved portion crosses the Albany basin, or outlet of the Erie Canal, and consists of seven spans of seventy-three feet each, one of sixty-three, and one of one hundred and ten. That part of the bridge which crosses the river consists of four spans of one hundred and eighty-five feet each, and a draw two hundred and seventy-four feet wide. The iron-work in this bridge cost about three hundred and twenty thousand dollars.

The bridge over the Illinois River at La Salle, on the Illinois Central Railroad, shows the style of bridge technically called a "deck" bridge, in which the train is on the top. This bridge consists of eighteen spans of one hundred and sixty feet each, and cost one hundred and eighty thousand dollars. The bridge over the Kennebec River, on the line of the Maine Central Railroad, at Augusta, Maine, is another instance of a "through" bridge. It cost seventy-five thousand dollars, has five spans of one hundred and eighty-five feet each, and was built to replace a wooden deck bridge which was carried away by a freshet.

BRIDGE AT AUGUSTA, MAINE.
BRIDGE AT AUGUSTA, MAINE.

The bridge on the Portland and Ogdensburg Railroad which crosses the Saco River is a very general type of a through railway bridge. It consists of two spans of one hundred and eighty-five feet each, and cost twenty thousand dollars. The New River bridge in West Virginia consists of two spans of two hundred and fifty feet each, and two others of seventy-five feet each. Its cost was about seventy thousand dollars.

The Lyman Viaduct, on the Connecticut Air-line Railway, at East Hampton, Connecticut, is one hundred and thirty-five feet high and eleven thousand feet long.

These specimens will show the general character of the iron bridges erected in this country. When iron was first used in constructions of this kind, cast iron was employed, but its brittleness and unreliability have led to its rejection for the main portions of bridges. Experience has also led the best iron bridge-builders of America to quite generally employ girders with parallel top and bottom members, vertical posts (except at the ends, where they are made inclined toward the centre of the span), and tie-rods inclined at nearly forty-five degrees. This form takes the least material for the required strength.

SACO BRIDGE
SACO BRIDGE.

The safety of a bridge depends quite as much upon the design and proportions of its details and connections as upon its general shape. The strain which will compress or extend the ties, chords and other parts can be calculated with mathematical exactness. But the strains coming upon the connections are very often indeterminate, and no mathematical formula has yet been found for them. They are like the strains which come upon the wheels, axles and moving parts of carriages, cars and machinery. Yet experience and judgment have led the best builders to a singular uniformity in their treatment of these parts. Each bridge has been an experiment, the lessons of which have been studied and turned to the best effect.

PHOENIX WORKS.
PHOENIX WORKS.

There is no doubt that iron bridges can be made perfectly safe. Their margin is greater than that of the boiler, the axles or the rail. To make them safe, European governments depend upon rigid rules, and careful inspection to see that they are carried out. In this country government inspection is not relied on with such certainty, and the spirit of our institutions leads us to depend more upon the action of self-interest and the inherent trustworthiness of mankind when indulged with freedom of action. Though at times this confidence may seem vain, and "rings" in industrial pursuits, as in politics, appear to corrupt the honesty which forms the very foundation of freedom, yet their influence is but temporary, and as soon as the best public sentiment becomes convinced of the need for their removal their influence is destroyed. Such evils are necessary incidents of our transitional movement toward an industrial, social and political organization in which the best intelligence and the most trustworthy honesty shall control these interests for the best advantage of society at large. In the mean time, the best security for the safety of iron bridges is to be found in the self-interest of the railway corporations, who certainly do not desire to waste their money or to render themselves liable to damages from the breaking of their bridges, and who consequently will employ for such constructions those whose reputation has been fairly earned, and whose character is such that reliance can be placed in the honesty of their work. Experience has given the world the knowledge needed to build bridges of iron which shall in all possible contingencies be safe, and there is no excuse for a penny-wise and pound-foolish policy when it leads to disaster.