BRIDGES

When the habitations of man first began to multiply upon the banks of the water courses, the profession of the bridge-builder was born. The first bridge was probably a felled tree spanning some modest brook. But from that first bridge came a magnificent development. Bridge-building became an art and a science. Men wrought gigantic structures in stone, long-arched viaducts, with which they defied time. Then for two thousand years the profession of the bridge-builder stood absolutely still.

With the coming of the iron and steel age it moved forward again. The development of a fibre of great strength and without the dead weight of granite gave engineers new possibilities. They began in simple fashion, and then they developed once again, with marvellous strides. Steel, the dead thing with a living muscle, could span waterways from which stone shrank. Steel redrew the maps of nations. Proud rivers at which the paths of man had halted, were conquered for the first time. Routes of traffic of every sort were simplified; the railroad made new progress; and economic saving of millions of dollars was made to this gray old world.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. But the building of stone bridges took time and money, and time and money were little known factors in a new land that had begun to expand rapidly.

So at first the railroad followed the course of the highroad and the post-road, and took the timber bridge unto itself. In some cases it actually fastened itself upon the highroad bridge, as at Trenton, N. J., where a faithful wooden structure built by Theodore Burr in 1803 was strengthened and widened in 1848 to take the first through railroad route from New York. It continued its heavy dual work until 1875 when it was superseded by a steel bridge. A dozen years ago the railroad tracks were moved from that structure to a magnificent and permanent stone-arch built near-by. 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. Within the past two years fire destroyed the staunch toll-bridge at Waterford on the Hudson, just above Troy. The bridge was a faithful carrier for one hundred and four years. In many ways it was typical of those first constructions. It consisted of four clear arch spans—one 154 feet, another 161 feet, the third 176 feet, and the fourth 180 feet in length. It was built of yellow pine, wonderfully hewn and fitted, hung upon solid pegs; and save for the renewal of some of the arch footings, the roof, and the side coverings, it was unchanged through all the years—even though the heavy trolley-cars of a through interurban line were finally turned upon it.

About the same time, the once-famed Permanent Bridge across the Schuylkill River at Philadelphia was built. It had two arches of 150 feet each and one of 195 feet. In its day it was regarded as nothing less than a triumph. A very old publication says:

In after years, the Permanent Bridge was also entrusted with the carrying of a railroad. It has, however, disappeared these many years.

The early railroad builders did not neglect the possibilities of the stone bridge. Two notable early examples of this form of construction still remain—the Starrucca Viaduct upon the Erie Railroad, near Susquehanna, Pa., and an even earlier structure, the stone-arch bridge across the Patapsco River at Relay, Md., which B. H. Latrobe, the most distinguished of all American railroad engineers, built for the Baltimore & Ohio Railroad, in 1833-35. The Thomas Viaduct, as it has been known for three-quarters of a century, was the first stone-arch bridge ever built to carry railroad traffic. It was erected in a day when the railroad was just graduating from the use of teams of horses as motive-power. In this day, when locomotives have begun to reach practical limits of size and weight, that viaduct is still in use as an integral part of the main line of the Baltimore & Ohio. It is built on a curve, and consists of 8 spans of stone arches, 67 feet 6 inches, centre to centre of piers, which, together with the abutments at each end, make the total length of the structure 612 feet. It is in as good condition to-day as upon the day it was built.

When the Erie Railroad was being constructed across the Southern Tier counties of New York in 1848, its course was halted near the point where the rails first reached the beautiful valley of the Susquehanna. A side-valley, a quarter of a mile in width, stretched itself squarely across the railroad’s path. There was no way it could be avoided, and it could be crossed only at a high level. For a time the projectors of the Erie considered making a solid fill, but the tremendous cost of such an embankment was prohibitive. While they were at their wits’ ends, James P. Kirkwood, a shrewd Scotchman, who had been working as a civil engineer upon the Boston & Albany, appeared. Kirkwood spanned the valley with the Starucca Viaduct, one of the most beautiful bridges ever built in America. He opened quarries close at hand and by indefatigable energy built his stone bridge in a single summer. It has been in use ever since. The increasing weight of its burdens has never been of consequence to it, and to-day it remains an important link in a busy trunk-line railroad. It is 1,200 feet in length and consists of 18 arches of 50 feet clear span apiece.

But stone bridges even then cost money, and so the timber structure still remained the most available. Many men can still remember the tunnels, into whose darkness the railroad cars plunged every time they crossed a stream of any importance whatsoever. They have nearly all gone. The wooden bridge was ill suited to the ravages of weather and of fire—ravages that were quickened by the railroad, rather than hindered. A substitute material was demanded. It was found—in iron.

The first iron bridge in the United States is believed to be the one erected by Trumbull in 1840 over the Erie Canal at Frankfort, N. Y. Record is also held of one of these bridges being built for the North Adams branch of the Boston & Albany Railroad, in 1846. About a year later, Nathaniel Rider began to build iron bridges for the New York & Harlem, the Erie, and some others of the early railroads. His bridges—of the truss type, of course, that type having been worked out in the timber bridges of the land—were each composed of cast-iron top-chords and post, the remaining part of the structure being fabricated of wrought-iron. The members were bolted together. Still, the failure of a Rider bridge upon the Erie in 1850, followed closely by the failure of a similar structure over the River Dee, in England, influenced officials of that railroad to a conclusion that iron bridges were unpractical, and to order them to be removed and replaced by wooden structures. For a time it looked as if the iron bridge were doomed. That was a dark day for the bridge engineers. A contemporary account says:

Under the influence of the really great Latrobe, an iron span of 124 feet was built in 1852 at Harpers Ferry. In that same year, the B. & O. built its Monongahela River Bridge, a really pretentious structure of 3 spans of 205 feet each, and the first really great iron railroad bridge in all the land. The path was set. The conquest of iron over wood as a bridge material was merely a problem of good engineering. The iron bridge quickly came into its own. The Pennsylvania Railroad began building cast-iron bridges of from 65 to 110 feet span at its Altoona shops for the many creeks and runs along the western end of its line. The other railroads were following in rapid order. Squire Whipple, Bollman, Pratt—all the others who could design and build iron bridges—were kept more than busy by the work that poured in upon them.

And in the day when the iron bridge was coming into its own, Sir Henry Bessemer, over in England, was bringing the steel age into existence, first making toy cannon models for the lasting joy of Napoleon III, and then making a whole world see that steel—that dead thing with the living muscle—was no longer to be limited for use in tools and cutting surface. Steel was to become the very right-hand of man. And so steel came to the bridge-builders, at first only in the most important wearing points such as pins and rivets, finally to be the whole fabric of the modern bridge. The transition was gradual. The early engineers began using less and less of cast-iron and more and more of wrought, until they had practically eliminated cast-iron as a bridge material. Then there came a quick change; there was another dark day for the railroad bridge engineers of America. In 1876—that very year when the land was so joyously celebrating its Centennial—a passenger train went crashing through a defective bridge at Ashtabula, Ohio. There was a great property loss—thousands and thousands of dollars, and a loss of lives that could never be expressed in dollars. An outraged land asked the bridge-builders if they really knew their business.

Out of that Ashtabula wreck came the scientific testing of bridges and bridge materials, and the abolition of the rule-of-thumb in the cheaper sorts of construction. Out of that miserable wreckage came also the use of steel in the railroad bridge. Steel had found itself; and how the steel bridges began to spring up across the land! They spanned the Ohio, and they spanned the Mississippi, and they spanned the Missouri; a great structure threw itself over the deep gorge of the Kentucky River. When the day came that fire destroyed the famous wooden viaduct of the Erie over the Genesee River at Portage, N. Y. (you must remember the pictures of that tremendous structure in the early geographies), steel took its place.

All this while the bridge engineer attempted more and more. He built over the deep gorge of the Niagara. He conquered the St. Lawrence in and about Montreal. He laughed at the mighty Hudson and flung a dizzy steel trestle over its bosom at Poughkeepsie. He built at Cairo, at Thebes, and at Memphis, on the Mississippi, and again and again and still again at St. Louis. The East River no longer halted him or compelled him to resort to the alternative of the very expensive types of suspension bridge. He has finally thrown a great cantilever over it, from Manhattan to Long Island. The steel bridge has come into its own.

Let us study for a moment the construction of the different types of railroad bridge. For the tiny creeks—the little things that are mad torrents in spring, and run stark-dry in midsummer—where they cannot be poured through a pipe or a concrete moulded culvert, the simplest of bridge forms will suffice. And the simplest of bridge forms consists of two wooden beams laid from abutment to abutment and holding the ties and rails of the track-structure. As the first development of that simplest idea comes the substitution of steel for wood, giving, as we have already seen, protection against fire and a far greater strength. The steel beam has greater strength than a wooden beam of the same outside dimension and yet in its design it effects for itself a great saving of material, by cutting out superfluous parts and becoming the structural standard of to-day, the I beam. When the I beam becomes too large to be made in a single pouring or a single rolling, it may be constructed of steel plates and angles firmly riveted together, and thus still remains the possibility of the simplest form of bridge. That single span may be further increased, or the bridge developed into a succession of increased spans by the substitution of the lattice-work girder, effecting further saving in weight without material loss of strength for the solid-plate girder. The track may be laid atop of such girders or—to save clearance in overhead crossing—swung between them at their bases.

The limit in this form of bridge is generally in a 65-foot or a 100-foot span. It is not practical to build the girders up outside of a shop; and the 65-foot length represents the two flat-cars that must be used to transport any one of them to the bridge location. Some railroads have used three cars for the hauling of a single girder, and so increased these spans to 100 feet; but as a rule, over 65 feet, and the truss, the most common form of railroad bridge in this country, comes into use.

The truss is a distinct evolution from those old timber bridges of which we have already spoken. Burr and Latrobe and Bollman and Howe and Squire Whipple—those distinguished engineers of other days—have evolved it, step by step. It is, in one sense, no more than an enlarged form of lattice girder, the work of the different designers having been to accomplish at all times, a maximum of strength with a minimum of weight. It is built of members that stand pulling-strain, and those that stand pressure-strain; and these are respectively known as tension and as compression members. In them rests the real strength of the truss. But in addition to the structure are the bracing-rods, generally placed as diagonals and built to sustain the structure against both lateral and wind-strains. The members that form the trusses are stoutly riveted together; the rapid rat-a-tap-tap of the riveter is no longer a novelty in any corner of the land. Sometimes certain of the important bearing-points are connected by steel pins instead of rivets—another survival of the old days of the timber bridge.

As a rule, the railroad is carried through the truss—and this is known as the through span. Sometimes it is carried upon the top of the structure, and then the truss becomes known as a deck span. A long bridge may effectively combine both of these types of span. The splendid new double-track truss bridge recently built by the Baltimore & Ohio Railroad over the Susquehanna River between Havre-de-Grace and Aiken, Md., to replace a single-track bridge in the same location, is a splendid example of the best type of such structures. At the point of crossing, the river is divided into channels by Watson Island; the width of the west channel being approximately 2,600 feet and that of the east channel being approximately 1,400 feet. The distance across the low-lying island is 2,000 feet—making the length of the entire bridge about 6,000 feet. The bridge, as originally constructed when the line from Baltimore to Philadelphia was built, in 1886, had a steel trestle over Watson Island. In building the new structure, this viaduct was eliminated in favor of a bridge structure of 90-foot girder spans, placed upon concrete piers. Additional piers were placed in the west channel, shortening the deck spans from 480 to 240 feet; the through span over the main channel was kept at the original length—520 feet. In the east channel, the span lengths remained unchanged, with a single slight exception. The changes in the span lengths involved new masonry, and all piers were sunk to solid rock, those in the west channel being carried by caissons to a depth of more than seventy feet beneath low-water. The total amount of new masonry and concrete approximated 62,000 cubic yards. The long span-lengths of the deck span over the east channel and the through span over the navigable portion of the west channel—each 520 feet in length—occasioned heavy construction. The deck span, for instance, weighed 12,000 pounds to each foot of bridge. The total weight of this very long bridge reaches the enormous figure of 32,000,000 pounds. And yet, even the untechnical observe the extreme simplicity of its lines of construction, and feel that the engineer, A. W. Thompson, has done his work well. The construction of the giant took two years and a half. During that time, the trains of the B. & O. were diverted to the closely adjacent Pennsylvania, so that the bridge-builders might continue with a minimum of delay.

The truss span reaches its limitations at a little over 500 feet in length—we have just seen how the Susquehanna structure had its spans cut in halves in the non-navigable portions of the river. The spans of two great railroad bridges over the Ohio at Cincinnati reached 519 and 550 feet, but they were built in a day when the weights of locomotives and of train-loads had not yet begun to rise. Nowadays the shorter span is the safer and by far the best. The engineer builds plenty of midstream piers, looking out only for a decent width for any navigable channels.

And when because of peculiarities of location he cannot place his pier midstream, then it is time for him to get out his pencils and begin his drawings all over again. He can perhaps build a suspension bridge—a clear span of 1,500 feet will be as nothing to it,—but suspension bridges take a long time to build and are fearfully expensive in the building. It is more than likely, then, that he will turn to the cantilever. In the cantilever, two giant trusses are cunningly balanced upon string supporting towers. They are constructed by being built out from the towers, evenly, so that the balance of weight may never be lost for a single hour. The two projecting arms are finally caught together in mid-air and over the very centre of the span—caught and made fast by the riveters. The result is a bridge of surpassing strength and fairly low cost, a real triumph for the bridge engineer.

The first of these cantilever bridges built in the United States was of iron. It was designed and constructed by C. Shaler Smith across the deep gorge of the Kentucky River in 1876-77. Mr. Smith also built the second cantilever, the Minnehaha, across the Mississippi, at St. Paul, Minn., in 1879-80. The third and fourth were the Niagara and the Frazer River bridges built in the early eighties. In their trail came many others—one of the most notable among them being the great Poughkeepsie Bridge.

We are going to see something of the construction of one of these great railroad bridges. Let us begin at the beginning, and see the men, as they work upon the foundations of abutments and of piers—many times hundreds of feet under the waters of the very stream that they will eventually conquer. For months this important work of getting a good foothold for the monster will go forth almost unseen by the workaday world—by the aid of the great timber footings, which the engineer calls his caissons. These caissons (they are really nothing more or less than great wooden boxes), are slowly sunk into the sand or soft rock under the tremendous weight of the many courses of masonry. They sink to solid rock—or something that closely approximates solid rock.

We are going down into one of the caissons that form the foothold of a single great pier of a modern railroad bridge; we are going to stand for a very few minutes under air-pressure with the “sand-hogs”—men whom we first came to know when we studied the boring of a tunnel. Air pressure spells danger. It takes a good nerve to work high up on the exposed steel frame of some growing bridge, but the bridge-builders have air and sunlight in which to pursue their hazardous work. The sand-hog has neither. He toils in a box down in the depths of the unknown, working with pick and shovel under artificial light and under a pressure that becomes all but intolerable. The knowledge that the most precious and vital of all man’s needs—fresh air—is controlled by another, and through delicate and intricate mechanism, cannot add to his peace of mind.

No wonder, then, that it is the highest paid of all merely manual work. The sand-hog working 50 feet below datum is paid $3.50 for an eight-hour day. But 50 feet is but the beginning to these human worms, who burrow deep into the earth. Below it they first begin to divide their day into two working periods. The air begins to count, and men with steel muscled arms must rest. As they approach 80 feet below datum—the engineers’ phrase for sea level,—they are working two periods each day of one hour and a half apiece, while their daily pay has risen to $4. There is your rough arithmetical law of sand-hogs. As your caisson goes down so does the length of your working-day decrease; inversely, their air pressures and the pay of the men increase. The cost? The cost leaps forward in geometrical progression. It is the owner’s turn to groan this time.

One hundred feet is the limit. At 100 feet the air pressure is more than 50 pounds to the square inch—three additional atmospheres—and the limit of human endurance is reached. The men work two shifts of forty minutes each as a daily portion and the law steps in to say that they must rest four hours between the shifts. They are paid $4.50 for that day’s work—which means something more than $4 an hour for the time that they are actually at work in the caisson.

You have expressed your interest in the sand-hog, given vent to a desire to go down into their underworld. You wonder what three pressures is going to feel like. Permission is given and a physician begins examining you. You cannot go into the caisson unless you are sound of heart and stout of body. This is no joking matter. The sand-hogs’ rules read like the training instructions for a college football team. No drink, regular hours, simple diet, the donning of heavy clothes after they leave the pressure, constant reËxamination—these rules are inflexible when the caissons go to far depths. By their observance the difficult foundation construction of this new bridge has been kept free from accident—there have been few cases of the “bends” brought to the specially constructed hospital in the bottom of the cavity.

The “bends” sounds complicated, and is, in reality, almost the simplest of human ailments in its diagnosis. A “bubble” of high pressure air works its way into the human structure while a man is in the caisson. When he comes out into the normal atmosphere the bubble is caught and remains. If it is caught near any vital organ that bubble is apt to spell death. Generally the bubbles are caught in the joints—frequently the elbow or the knee—where they cause excruciating pain. Then the specially constructed hospital crowded on the narrow platform formed by the top of the pier, comes into full play. Its sick room is incased in an air-tight cylinder. The man suffering from the “bends,” together with physicians and nurses, is put under a pressure that gradually increases until it reaches that of the caisson. After that it is a comparatively simple matter to relieve the bubble and bring the air in the hospital back to a normal pressure.

The path is clear for us to go down into the caisson. A party of sand-hogs, hot and exhausted after forty minutes of work within, come out of the little manhole at the top of the air-lock. We step through the little manhole and into a tiny steel bucket that rests within the air-lock there at the top of the shaft. A word of command—farewell to the bright blue sky overhead—the black manhole cover is replaced. It is suddenly very dark. A single faint incandescent gives a dim glow in the tiny place.