III.

From data furnished by Mr. D. J. Whittemore, chief engineer of the Chicago, Milwaukee, and St. Paul system (which had a total length of 5,688 miles on January 1, 1888), the length of open bridges on these lines was 115⁹¹/₁₀₀ miles, and of culverts covered over with embankment, 39²/₁₀ miles. "Everything," says Mr. Whittemore, "not covered with earth, except cattle guards, be the span 10 or 400 feet, is called a bridge. Everything covered with earth is called a culvert. Wherever we are far removed from suitable quarries, we build a wooden culvert in preference to a pile bridge, if we can get six inches of filling over it. These culverts are built of roughly squared logs, and are large enough to draw an iron pipe through them of sufficient diameter to take care of the water. We do this because we believe that we lessen the liability to accident, and that the culvert can be maintained after decay has begun, much longer than a piled bridge with stringers to carry the track. Had we good quarries along our line, stone would be cheaper. Many thousands of dollars have been spent by this company in building masonry that after twenty to twenty-five years shows such signs of disintegration that we confine masonry work now only to stone that we can procure from certain quarries known to be good."

Bergen Tunnels, Hoboken, N. J.

Mr. Whittemore is an engineer of great experience, skill, and judgment, and there is food for much reflection in these words of his: First—that it is better to use temporary wooden structures, to be afterward renewed in good stone, rather than to build of the stone of the locality, unless first-class. Second—that a structure covered with earth is much safer than an open bridge; which, if short and apparently insignificant, may be, through neglect, a most serious point of danger, as was shown in the dreadful accident of 1887 on the Toledo, Peoria, and Western road in Illinois, where one hundred and fifty persons were killed and wounded, and by the equally avoidable accident on the Florida and Savannah line, in March, 1888. Had these little trestles been changed to culverts covered with earth, many valuable lives would not have been lost.

Bergen Tunnels, Hoboken, N. J.

Beginning a Tunnel.

It was safely estimated that there were, in 1888, 208,749 bridges of all kinds, amounting in length to 3,213 miles, in the United States.[4]

The wooden bridge and the wooden trestle are purely American products, although they were invented by Leonardo da Vinci in the sixteenth century. From the above statistics it will be seen how much our American railways owe to them, for without them over 150,000 miles could never have been built.

The art of building wooden truss-bridges was developed by Burr & Wernwag, two Pennsylvania carpenters, some of whose works are still in use after eighty years of faithful duty ([p. 28]). A bridge built by Wernwag across the Delaware in 1803 was used as a highway bridge for forty-five years, was then strengthened and used as a railway bridge for twenty-seven years more, and was finally superseded by the present iron bridge in 1875.

These old bridge-builders were very particular about the quality of their timber, and never put any into a bridge less than two years old. But when we began to build railways, everything was done in a hurry, and nobody could wait for seasoned timber. This led to the invention of the Howe truss, by the engineer of that name, which had the advantage of being adjustable with screws and nuts, so that the shrinkage could be taken up, and which had its parts connected in such a way that they were able to bear the heavy concentrated weight of locomotives without crushing. This bridge was used on all railways, new and old, from 1840 to about 1870. Had it been free from liability to decay and burn up, we should probably not be building iron and steel bridges now, except for long spans of over 200 feet; and as the table opposite shows, the largest number of our spans are less than 100 feet long.

The Howe truss forms an excellent bridge, and is still used in the West on new roads, with the intention of substituting iron trusses after the roads are opened.

After 1870, the weights both of locomotives and other rolling stock began to be increased very rapidly. This, together with the development of the manufacture of iron, and especially the invention of rolled beams and of eye-bars, gave a great impetus to the construction of iron bridges. At first cast-iron was used for the compression members, but the development of the rolling-mill soon enabled us to make all parts of rolled iron sections at no greater cost, and rolled iron, being a less uncertain material, has replaced cast-iron entirely. Iron bridges came in direct competition with the less costly Howe truss, and during the first decade of their construction every attempt was made to build them with as few pounds of iron as would meet the strains.

Old Burr Wooden Bridge.

S. Whipple, C.E., published a book in 1847 which was the first attempt ever made to solve the mathematical questions upon which the due proportioning of iron truss-bridges depends. This work bore fruit, and a race of bridge designers sprang up. The first iron bridges were modelled after their wooden predecessors, with high trusses and short panels. Riveted connections were avoided, and every part was so designed that it might be quickly and easily erected upon staging or false works, placed in the river. This was very necessary, for our rivers are subject to sudden freshets, and if we had adopted the English system of riveting together all the connections, the long time required before the bridge became self-sustaining would have been a serious element of danger.

Following the practice of wooden bridge building, iron bridges were contracted for by the foot, and not by the pound as is now the custom. To this accidental circumstance is greatly due the development of the American iron bridge. The engineer representing the railway company fixed the lengths of spans, and other general dimensions, and also the loads to be carried and the maximum strains to be allowed. The contracting engineer was left perfectly free to design his bridge, and he strained every nerve to find the form of truss and the arrangement of its parts that should give the required strength with the least number of pounds weight per foot, so that he could beat his competitors. When the different plans were handed in, an expert examined them and rejected those whose parts were too small to meet the strains. Of those found to be correctly proportioned, the lowest bid took the work.

By the rule of the survival of the fittest all badly designed forms of trusses disappeared and only two remained: one the original truss designed by Mr. Whipple, and the other, the well-known triangular, or "Warren" girder, so called after its English inventor.

It speaks well for the skill and honesty of American bridge engineers that many of their old bridges are still in use, designed for loads of 2,500 pounds per lineal foot, and now daily carrying loads of 4,000 pounds and over per foot. Sometimes the floor has been replaced by a stronger one, but the trusses still remain and do good service. The writer may be permitted to point to the bridge over the Mississippi River at Quincy, Ill., built in 1869, as an example. Most bridge-accidents can be traced to derailed trains striking the trusses and knocking them down. Engineers (both those specially connected with bridge works, and those in charge of railways) know much better now what is wanted, and the managers of railways are willing to pay for the best article. The introduction of mild steel is a great step in advance. This material has an ultimate strength, in the finished piece, of 63,000 to 65,000 pounds per square inch, or forty per cent. more than iron, and it is tough enough to be tied in a knot, or punched into the shape of a bowl, while cold. With this material it is as easy to construct spans of 500 feet as it was spans of 250 feet in iron.

Bridges are now designed to carry much heavier loads than formerly. The best practice adopts riveted connections except at the junction of the chord-bars and the main diagonals, where pins and eyes are still very properly used. Plate girders below the track are preferred up to 60 or 70 feet long, then riveted lattice up to 125 feet. The wind strains also are now provided for with a considerable excess of material, amounting in very long spans to nearly as much as the strains due to gravity. Observing the rule that no bridge can be stronger than its weakest part, a vast deal of care and skill has been applied in perfecting the connections of the parts of a truss, and many valuable experiments have been made which have greatly enlarged our knowledge of this difficult subject. The introduction of riveting by the power of steam or compressed air is another very great improvement.[5]

Kinzua Viaduct;
Erie Railway.

Valleys and ravines are now crossed by viaducts of iron and steel, of which the Kinzua viaduct, illustrated here, is an example. A branch line from the Erie, connecting that system with valuable coal-fields, strikes the valley of the Kinzua, a small creek, about 15 miles southwest of Bradford, Pa. At the point suitable for crossing, this ravine is about half a mile wide and over 300 feet deep. At first it was proposed to run down and cross the creek at a low level by some of the devices heretofore illustrated in this article. But finally the engineering firm of Clarke, Reeves & Co. agreed to build the viaduct, shown above, for a much less sum than any other method of crossing would have cost. This viaduct was built in four months. It is 305 feet high and about 2,400 feet long. The skeleton piers were first erected by means of their own posts, and afterward the girders were placed by means of a travelling scaffold on the top, projecting over about 80 feet. No staging of any kind was used, nor even ladders, as the men climbed up the diagonal rods of the piers, as a cat will run up a tree.

The Manhattan Elevated Railway, about 34 miles long, is nothing but a long viaduct, and is as strong and durable as iron viaducts on railways usually are, while from the slower speed of its trains it is much safer.

Kinzua Viaduct; Erie Railway.

Kinzua Viaduct.

It may not be out of place for the writer to state here what, in his belief, is the next series of steps to be taken to insure safety in travelling over our bridges: Replace, wherever possible, all temporary trestles by wood or stone culverts covered with earth. Where this cannot be done, build strong iron or steel bridges and viaducts with as short spans as possible and having no trusses above the track where it can possibly be helped. Cover these and all new bridges with a solid deck of rolled-steel corrugated plates, coated with asphalt to prevent rusting. Place on this broken stone ballast, and bed the ties in it as in the ordinary form of road-bed.

By this means the usual shock felt in passing from the elastic embankment to the comparatively solid bridge will be done away. Has a crack formed in a wheel or axle, this shock generally develops it into a break, the car or engine is derailed, and if it strikes the truss the bridge is wrecked. The cost of this proposed safety floor is insignificant, compared with the security resulting from it.

The improvements in the processes of putting in the foundations of bridges have been as great as those above water. All have shortened greatly the time necessary, and have made the results more certain. The American system may briefly be described as an abandonment of the old engineering device of coffer-dams, by which the bed of the river is enclosed by a water-tight fence and the water pumped out. For this we substitute driving piles and sawing them off under water; or sinking cribs down to a hard bottom through the water. In both cases we sink the masonry, built in a great water-tight box (called a caisson) with a thick bottom of solid timber, until it finally rests on the heads of the piles sawn to a level, or on the top of a crib which is filled with stone, dumped out of a barge. Sometimes it is filled with concrete lowered through the water by special apparatus.[6]

Another process, developed within the last twenty years, is to sink cribs through soft or unreliable material to a harder stratum by compressed air. This is an improvement on the old diving-bell. The air, forced into the bell-shaped cavity, expels the water and allows the men to work and remove the material, which is taken up by a device called an air-lock. The crib slowly sinks, carrying the masonry on its top.

By this means the foundations of the Brooklyn bridge and of the St. Louis bridge were sunk a little over 100 feet below water. A recent invention is that of a German engineer, Herr Poetsch, who freezes the sand by inserting tubes filled with a freezing mixture, and then excavates it as if it were solid rock.

The process of sinking open cribs through the water by weighting them and dredging out the material was followed at the new bridge recently built over the Hudson at Poughkeepsie, where the cribs were sunk 130 feet below water, and at the bridge building over the Hawkesbury River, in Australia. The Hawkesbury piers are sunk to a depth of 175 feet below water, and are the deepest foundations yet put in. The writer (who derives his knowledge from being one of the designing and executive engineers of both these bridges) sees no difficulty in putting down foundations by this process of open dredging to even much greater depths. The compressed-air process is limited to about 110 feet in depth.