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.