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 reexamination--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.
[Illustration: CONCRETE AFFORDS WONDERFUL OPPORTUNITIES FOR THE BRIDGE-BUILDERS]
[Illustration: THE LACKAWANNA IS BUILDING THE LARGEST CONCRETE BRIDGE IN THE WORLD ACROSS THE DELAWARE RIVER AT SLATEFORD, PA.]
[Illustration: THE BRIDGE-BUILDER LAYS OUT AN ASSEMBLYING-YARD FOR GATHERING TOGETHER THE DIFFERENT PARTS OF HIS NEW CONSTRUCTION]
[Illustration: THE NEW BRANDYWINE VIADUCT OF THE BALTIMORE & OHIO, AT WILMINGTON, DEL.]
You are not thinking of that. They are putting the pressure on. You can feel it. Your eardrums feel as if they would break; they vibrate. You must show your distress.
"Pinch your nose and swallow hard," says the man who stands beside you in the bucket.
He stands so close to you that you can fairly feel the pulsation of his heart, but his voice sounds miles away. You swallow hard, the hardest you have ever swallowed, and you pinch your nose. You feel better. The far-away voice speaks again in your ear. "Three atmospheres," is all it says. The caisson shaft is no place for extended conversation. You descend in an express elevator car; in that bucket you just drop. You have all the eerie sensations that a Coney Island "novelty ride" might give you. There is a row of dim incandescents all the way down the smooth side of the shaft, and when you look you forget that this is vertical traction and think of an uptown subway tube as you see it recede from the rear of an express. A final manhole, the gate at the foot of the shaft and you stop abruptly. It seems as if you had almost bumped against the under side of China.
"This is it," says the far-away voice.
A timbered room, not larger than a parlor in a city flat and not near so high. A close and murky place, filled with a little company of men--shadowy humans of a real underworld there under the dull electric glow.
"They're finding the footing for the shaft," says the voice. "We're on rock at last at 94 feet."
When the footings are finished and the caisson's edges have ceased to cut its path straight downward, that timbered construction will rest here far below the city for long ages. The sand-hogs will come out of their working chamber for the last time--it will be poured full of concrete, more solid than rock itself. The air pressure will be withdrawn--there is no longer mud or shifting sand for it to withhold. Then, section by section, the steel lining of the caisson shaft will be withdrawn, while concrete, tramped into place, makes the shaft a hidden monolith 100 feet or so in length. Upon the tops of all these monoliths a close grillage of steel beams will be laid; upon that grillage will be riveted the steel plates and columns of the bridge tower. The great structure is to have sure footing; these giant feet bind and clasp themselves throughout the years against the mighty river that has been conquered and humbled by the work of man.
"You should have been down in one of the boxes when they had to burn torches, before they got the electric light," says one of the bridge engineers. "I worked in one of those that we left under a stone tower of the Brooklyn Bridge. Now we're almost in clover. They even cool and dry the compressed air before we breathe it."
An order goes aloft over an electric wire, the engineer who sits smoking his pipe on the sun-baked platform of the traveller derrick pulls a lever, and we go slipping up the shaft toward fresh air and freedom only a little less rapidly than we descended it. We do not reach it too quickly. There is a long wait in the air-lock after the lower manhole has closed, while the pressure is being reduced. You begin to worry and you ask your guide as to the delay. Nothing wrong?
He smiles at your timorous question and explains. It would be dangerous to come out from the caisson pressure quickly. He does not want to have to send you to that air-tight hospital with a bad case of the "bends."
"How long in the air-lock?" you ask.
"Fifty minutes," he answers.
Then he explains in more detail. You have been under a pressure of 50 pounds to the square inch--that's your three atmospheres, and under the rules you must spend fifty minutes in the tiny air-lock. Up to a pressure of 36 pounds you must spend two minutes there for every three pounds of pressure. When you get above that "law of 36" it is a minute to the pound.
When that manhole cover overhead finally slides open you feel blinded by the light, even though the sun is hidden behind a passing cloud. The air-lock tender reaches down with his arms and gives you a lift up onto his narrow perch.
"Want to be a sand-hog?" he smiles.
"Not yet a while," you answer, in all truth. "Not until every other job is gone."
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. A steamboat is passing directly beneath you.
From your perch she looks like a great yellow bird. Those fine black specks upon her back are the humans who are gathered upon her upper deck.
Whistles call and the derricks groan as they swing the thousands of bridge-members, that are flying together at the beck of the engineer, into their final resting-places. There is the deafening racket of the riveters, here and there and everywhere. There are crude railroad tracks upon the temporary flooring of the bridge deck, and the calls of the dummy locomotives add to the racket. The railroad tracks lead to the shore, to temporary yards where the bridge materials are assembled as fast as they come from the shops in a city three hundred miles distant.
For, remember that while the sand-hogs were burrowing under the surface of the river to find footholds for this monster, other men were burrowing into the hillsides to find the precious ore for the welding of his muscles. A hundred thousand picks must have fought in his behalf, furnaces blazed for miles before the crude ore became the finished, perfect steel.
Of the forging and the rolling of the steel a whole book might be written.
It is enough now to say that of the 50,000,000 pounds of steel, every pound was made on honor. The railroad had its inspectors everywhere, but the rolling-mill men held to their formulas for perfect steel, and perfect steel was the result. A slight flaw in the metal, and possibly at some unexpected day, a great catastrophe. The safety of human life was upon the men who forged the steel, and they forged honor into every great girder, into every rod and bolt and plate. This conqueror of the river was a warrior built in honor.
The safety of human life depends upon the men who build this bridge. Study carefully the face of this man who stands beside you, the man who evolved this bridge as a season's work of his restless mind. His face is the face of a man who has high regard for human safety; that factor creeps to the fore as he talks to you. He is telling of the method of constructing the upper works of a bridge of this size.
"We're getting ahead all the time," he laughs, "and we're moving rather forward in our construction methods. In an older day we did this work with derricks of a rather simple sort, operated them by small portable steam engines. You can't handle bridge-members--units that are only held down by the clearances of tunnels and the transporting powers of the railroads--that way to-day. We've nearly half a million dollars tied up here in constructing-appliances. These steel-boom derricks, travellers, and steel-wire hoists, the compressing engines for handling the riveters, cost big money.