Part 33

It is curious to see how little is said about these beautiful bridges, which the public takes as a matter of course. If they had been built fifty years ago, their engineers would have received the same praise as Robert Stephenson or Roebling, and justly so, as they would have been men of exceptional genius. When these bridges were built, in 1898, the path had been made so clear by mathematical investigation and the command of a better steel, that the task seemed easy.

That which marks more clearly than anything else the great advance in American bridge building, during the last forty years, is the reconstruction of the famous Victoria Bridge, over the St. Lawrence, above Montreal. This bridge was designed by Robert Stephenson, and the stone piers are a monument to his engineering skill. For forty winters they have resisted the great fields of ice borne by a rapid current. Their dimensions were so liberal that the new bridge was put upon them, although four times as wide as the old one.

The superstructure was originally made of plate-iron tubes, reinforced by tees and angles, similar to Stephenson’s Menai Straits Bridge. There are twenty-two spans of 240 feet each, and a central one of 330 feet. Perhaps these tubes were the best that could be had at the time, but they had outlived their usefulness. Their interiors had become greatly corroded by the confined gases from the engines and the drippings from the chemicals used in cold-storage cars. Their height was insufficient for modern large cars, and the confined smoke made them so dark that the number of trains was greatly limited.

It was decided to build a new bridge of open-work construction and of open-hearth steel. This was done, and the comparison is as follows: Old bridge, sixteen feet wide, single track, live load of one ton per foot; new bridge, sixty-seven feet wide, two railway tracks and two carriage-ways, live load five tons per foot.

The old iron tubes weighed 10,000 tons, cost $2,713,000, and took two seasons to erect. The new truss bridge weighs 22,000 tons, has cost between $1,300,000 and $1,400,000, and the time of construction was one year.

During his experience the writer has seen the rolling-load of bridges increase from 2000 to 4000 pounds per lineal foot of track, with an extra allowance for concentrated loads.

The modern high office building is an interesting example of the evolution of a high-viaduct pier. Such a pier of the required dimensions, strengthened by more columns strong enough to carry many floors, is the skeleton frame. Enclose the sides with brick, stone, or terra-cotta, add windows, and doors, and elevators, and it is complete.

Fortunately for the stability of these high buildings, the effect of wind pressures had been studied in this country in the designs of the Kinzua, Pecos, and other high viaducts.

All this had been thoroughly worked out and known to our engineers before the fall of the Tay Bridge in Scotland. That disastrous event led to very careful experiments on wind pressures by Sir Benjamin Baker, the very eminent engineer of the Forth Bridge. His experiments showed that a wind gauge of 300 square feet area showed a maximum pressure of thirty-five pounds per square foot, while a small one of one foot and a half square area registered gusts of forty-one pounds per square foot.

The modern elevated railway of cities is simply a very long railway viaduct. Some idea may be gained of the life of a modern riveted-iron structure from the experience of the Manhattan Elevated Railway of New York. These roads were built in 1878–79 to carry uniform loads of 1600 pounds per lineal foot, except Second Avenue, which was made to carry 2000. The stresses were below 10,000 pounds per square inch.

These viaducts have carried in twenty-two years over 25,000,000 trains, weighing over 3,000,000,000 tons, at a maximum speed of twenty-five miles an hour, and are still in good order.

Bridge engineers of the present day are free from the difficulties which confronted the early designers of iron bridges. The mathematics of bridge design was understood in 1870, but the proportioning of details had to be worked out individually. Every new span was a new problem. Now the engineer tells his draughtsman to design a span of a given length, height, and width, and to carry such a load. By the light of experience he does this at once.

Connections have become standardized so that the duplication of parts can be carried to its fullest extent.

Machine tools are used to make every part of a bridge, and power riveters to fasten them together. Great accuracy can now be had, and the sizes of parts have increased in a remarkable degree.

We have now great bridge companies, which are so completely equipped with appliances for both shop drawings and construction that the old joke becomes almost true that they can make bridges and sell them by the mile.

All improvements of design are now public property. All that the bridge companies do is done in the fierce light of competition. Mistakes mean ruin, and the fittest only survives.

Having such powerful aids, the American bridge engineer of to-day has advantages over his predecessors and over his European brethren, where the American system has not yet been adopted.

The American system gives the greatest possible rapidity of erection of the bridge on its piers. A span of 518 feet, weighing 1000 tons, was erected at Cairo on the Mississippi in six days. The parts were not assembled until they were put upon the false works. European engineers have sometimes ordered a bridge to be riveted together complete in the maker’s yard, and then taken apart.

The adoption of American work in such bridges as the Atbara in South Africa, the Gokteik viaduct in Burmah, 320 feet high, and others, was due to low cost, quick delivery and erection, as well as excellence of material and construction.

FOUNDATIONS, ETC.

Bridges must have foundations for their piers. Up to the middle of the nineteenth century engineers knew no better way of making them than by laying bare the bed of the river by a pumped-out cofferdam, or by driving piles into the sand, as Julius Cæsar did. About the middle of the century, M. Triger, a French engineer, conceived the first plan of a pneumatic foundation, which led to the present system of compressing air by pumping it into an inverted box, called a caisson, with air locks on top to enable men and materials to go in and out. After the soft materials were removed, and the caisson sunk by its own weight to the proper depth, it was filled with concrete. The limit of depth is that in which men can work in compressed air without injury, and this is not much over one hundred feet.

The foundations of the Brooklyn and St. Louis bridges were put down in this manner.

In the construction of the Poughkeepsie bridge over the Hudson in 1887–88, it became necessary to go down 135 feet below tide-level before hard bottom was reached. Another process was invented to take the place of compressed air. Timber caissons were built, having double sides, and the spaces between them filled with stone to give weight. Their tops were left open and the American single-bucket dredge was used. This bucket was lowered and lifted by a very long wire rope worked by the engine, and with it the soft material was removed. By moving this bucket to different parts of the caisson its sinking was perfectly controlled, and the caisson finally placed in its exact position, and perfectly vertical. The internal space was then filled with concrete laid under water by the same bucket, and levelled by divers when necessary.

While this work was going on, the government of New South Wales, in Australia, called for both designs and tenders for a bridge over an estuary of the sea called Hawkesbury. The conditions were the same as at Poughkeepsie, except that the soft mud reached to a depth of 160 feet below tide-level.

The designs of the engineers of the Poughkeepsie bridge were accepted, and the same method of sinking open caissons (in this case made of iron) was carried out with perfect success.

The erection of this bridge involved another difficult problem. The mud was too soft and deep for piles and staging, and the cantilever system in this site would have increased the cost.

A staging was built on a large pontoon at the shore, and the span erected upon it. The whole was then towed out to the bridge site at high tide. As the tide fell, the pontoon was lowered and the steel girder was placed gently on its piers. The whole operation was completed within six hours. The other five spans were placed in the same manner.

The same system was followed afterwards by the engineer of the Canadian Pacific Railway in placing the spans of a bridge over the St. Lawrence, in a very rapid current. It is now used in replacing old spans by new ones, as it interrupts traffic for the least possible time.

The solution of the problems presented at Hawkesbury gave the second introduction of American engineers to bridge building outside of America. The first was in 1786, when an American carpenter or shipwright built a bridge over Charles River at Boston, 1470 feet long by forty-six feet wide. This bridge was of wood supported on piles. His work gained for him such renown that he was called to Ireland and built a similar bridge at Belfast.

Tunnelling by compressed air is a horizontal application of compressed-air foundations. The earth is supported by an iron tube, which is added to in rings, which are pushed forward by hydraulic jacks.

A tunnel is now being made under an arm of the sea between Boston and East Boston, some 1400 feet long and sixty-five feet below tide. The interior lining of iron tubing is not used. The tunnel is built of concrete, reinforced by steel rods. This will effect a considerable economy. Success in modern engineering means doing a thing in the most economical way consistent with safety.

The Saint Clair tunnel, which carries the Grand Trunk Railway of Canada under the outlet of Lake Huron, is a successful example of such work. Had the North River tunnel, at New York, been designed on equally scientific principles, it would probably have been finished, which now seems problematical.

The construction of rapid-transit railways in cities is another branch of engineering, covering structural, mechanical, and electrical engineering. Some of these railways are elevated, and are merely railway viaducts, but the favorite type now is that of subways. There are two kinds, those near the surface, like the District railways of London, the subways in Paris, Berlin, and Boston, and that now building in New York. The South London and Central London, and other London projects, are tubes sunk fifty to eighty feet below the surface and requiring elevators for access. These are made on a plan devised by Greathead, and consist of cast-iron tubes pushed forward by hydraulic rams, and having the space outside of the tube filled with liquid cement pumped into place.

The construction of the Boston subway was difficult on account of the small width of the streets, their great traffic, and the necessity of underpinning the foundations of buildings. All of this was successfully done without disturbing the traffic for a single day, and reflects great credit on the engineer. Owing to the great width of New York streets, the problem is simpler in that respect, but requires skill in design and organization to complete the work in a short time. Although many times as long as the Boston subway, it will be built in nearly the same time. The design, where in earth, may be compared to that of a steel office building twenty miles long, laid flat on one of its sides. The reduplication of parts saves time and labor, and is the key to the anticipated rapid progress. Near the surface this subway is built in open excavation, and tunnelling is confined to rock.

The construction of power-houses for developing energy from coal and from falling water requires much structural besides electrical and mechanical engineering ability. The Niagara power-house is intended to develop 100,000 horse-power; that at the Sault Ste. Marie as much; that on the St. Lawrence, at Massena, 70,000 horse-power. These are huge works, requiring tunnels, rock-cut chambers, and masonry and concrete in walls and dams. They cover large extents of territory.

The contrast in size of the coal-using power-houses is interesting. The new power-house now building by the Manhattan Elevated Railway, in New York, develops in the small space of 200 by 400 feet 100,000 horse-power, or as much power as that utilized at Niagara Falls.

One of the most useful materials which modern engineers now make use of is concrete, which can be put into confined spaces and laid under water. It costs less than masonry, while as strong. This is the revival of the use of a material used by the Romans. The writer was once allowed to climb a ladder and look at the construction of a dome of the Pantheon, at Rome. He found it a monolithic mass of concrete, and hence without thrust. It is a better piece of engineering construction than the dome of St. Peter’s, built fifteen hundred years later. The dome of Columbia College Library, in New York, is built of concrete.

Concrete is a mixture of broken stone or gravel, sand, and Portland cement. Its virtue depends upon the uniform good quality of the cement. The use of the rotary kiln, which exposes all the contained material to a uniform and constant intense heat, has revolutionized the manufacture of Portland cement. The engineer can now depend upon its uniformity of strength.

Wheels, axles, bridges, and rails have all been strengthened to carry their increased loads; but, strange to say, the splices which hold in place the ends of the rails, and which are really short-span bridges, are now the weakest part of a railway. The angle-bar splice has but one-third of the strength of the rail, and its strength cannot be increased, owing to its want of depth. Joints go down under every passing wheel, and the ends of the rails wear out long before the rest.

This is not an insignificant detail. It has been estimated by the officers of one of the trunk lines that a splice of proper design and strength would save yearly enough in track labor (most of which is expended in tamping up low joints) to buy all the new rails and fastenings required in some time. It would save much more than that in the wear of rolling-stock. A perfect joint would be an economic device next in value to the Bessemer steel rail. Here is a place for scientific and practical skill.

HYDRAULIC ENGINEERING

This is one of the oldest branches of engineering, and was developed before the last century. The irrigation works of Asia, Africa, Spain, Italy, the Roman aqueducts, and the canals of Europe, are examples. Hydraulic works cannot be constructed in ignorance of the laws which govern the flow of water. The action of water is relentless, as ruined canals, obstructed rivers, and washed-out dams testify.

The principal additions of the nineteenth century to hydraulic engineering are the collection of larger statistics of the flow of water in pipes and channels, of rainfall, run-off, and available supply. It is now known that the germs of disease can be retained by ordinary sand filters, and it is now an established fact that pure drinking water and proper drainage are a sure preventive of typhoid and similar fevers. Very foul water can be made potable. Experiments show that the water of the Schuylkill River at Philadelphia, which contains 400,000 germs in the space of less than a cubic inch, was so much purified by filtering that only sixty remained. This is a discovery of sanitary science, but the application of it is through structural engineering, which designs and executes the filter beds with great economy.

The removal of sewage, after having been done by the Etruscans before the foundation of Rome, became a lost art during the dirty Dark Ages, when filth and piety were deemed to be connected in some mysterious way. It was reserved for good John Wesley to point out that “Cleanliness is next to godliness.” Now sewage works are as common as those for water supply. Some of them have been of great size and cost. Such are the drainage works of London, Paris, Berlin, Boston, Chicago, and New Orleans. A very difficult work was the drainage of the City of Mexico, which is in a valley surrounded by mountains, and elevated only four to five feet above a lake having no outlet. Attempts to drain the lake had been made in vain for six hundred years. It has lately been accomplished by a tunnel six miles long through the mountains, and a canal of over thirty miles, the whole work costing some $20,000,000.

The drainage of Chicago by locks and canal into the Illinois River has cost some $35,000,000, and is well worth its cost.

Scientific research has been applied to the designing of high masonry and concrete dams, and we know now that no well-designed dam on a good foundation should fail. The dams now building across the Nile by order of the British government will create the largest artificial lakes in the world. The water thus stored will be of inestimable value in irrigating the crops of Lower Egypt. Their cost, although great, will not exceed the sums spent by the lavish Khedive Ismail on useless palaces, now falling to decay.

The Suez Canal is one of the largest hydraulic works of the last century, and is a notable instance of the displacement of hand labor by the use of machinery. Ismail began by impressing a large part of the peasant population of Egypt, just as Rameses had done over 3000 years before. These unfortunate people were set to dig the sand with rude hoes, and carry it away in baskets on their heads. They died by thousands for want of water and proper food. At last the French engineers persuaded the Khedive to let them introduce steam dredging machinery. A light railway was laid to supply provisions, and a small ditch dug to bring pure water. The number of men employed fell to one-fourth. Machinery did the rest. But for this the canal would never have been finished.

The Panama Canal now uses the best modern machinery, and the Nicaragua Canal, if built, will apply still better methods, developed on the Chicago drainage canal, where material was handled at a less cost than has ever been done before.

Russia is better supplied with internal waterways than any other country. Her rivers rise near each other, and have long been connected by canals. It is stated that she has over 60,000 miles of internal navigation, and is now preparing the construction of canals to connect the Caspian with the Baltic Sea.

The Erie Canal was one of very small cost, but its influence has been surpassed by none. The “winning of the West” was hastened many years by the construction of this work in the first quarter of the century. Two horses were just able to draw a ton of goods at the speed of two miles an hour over the wretched roads of those days. When the canal was made these two horses could draw a boat carrying 150 tons four miles an hour. Mud, or, in other words, friction, is the great enemy of civilization, and canals were the first things to diminish it, and after that railways.

The Erie Canal was made by engineers, but it had to make its own engineers first, as there were none available in this country at that time. These self-taught men, some of them land surveyors and others lawyers, showed themselves the equals of the Englishmen Brindley and Smeaton, when they located a water route through the wilderness, having a uniform descent from Lake Erie to the Hudson, and which would have been so built if there had been enough money.

The question now is whether to enlarge the capacity of this canal by enlarging its prism and locks, or to increase speed and move more boats in a season by electrical appliances. The last method seems more in line with those of the present day.

There should be a waterway from the Hudson to Lake Erie large enough for vessels able to navigate the lakes and the ocean. A draft of twenty-one feet can be had at a cost estimated at $200,000,000.

The deepening of the Chicago drainage canal to the Mississippi River, and the deepening of the Mississippi itself to the Gulf of Mexico, is a logical sequence of the first project. The Nicaragua Canal would then form one part of a great line of navigation, by which the products of the interior of the continent could reach either the Atlantic or Pacific Ocean.

The cost would be small compared with the resulting benefits, and some day this navigation will be built by the government of the United States.

The deepening of the Southwest Pass of the Mississippi River from six to thirty feet by James B. Eads was a great engineering achievement. It was the first application of the jetty system on a large scale. This is merely confining the flow of a river, and thus increasing its velocity so that it secures a deeper channel for itself.

The improvement of harbors follows closely the increased size of ocean and lake vessels. The approach to New York harbor is now being deepened to forty feet, a thing impossible to be done without the largest application of steam machinery in a suction dredge boat.

The great increase of urban population, due to steam and electric railways, has made works of water supply and drainage necessary everywhere. Some of these are on a very grand scale. An illustration of this is the Croton Aqueduct of New York as it now is, and as it will be hereafter.

This work was thought by its designers to be on a scale large enough to last for all time. It is now less than sixty years old, and the population of New York will soon be too large to be supplied by it.

It is able to supply 250,000,000 to 300,000,000 gallons daily, and its cost, when the Cornell dam and Jerome Park reservoir are finished, will be a little over $92,000,000.

It is now suggested to store water in the Adirondack Mountains, 203 miles away, by dams built at the outlet of ten or twelve lakes. This will equalize the flow of the Hudson River so as to give 3,000,000,000 to 4,000,000,000 gallons daily. It is then proposed to pump 1,000,000,000 gallons daily from the Hudson River at Poughkeepsie, sixty miles away, to a height sufficient to supply the city by gravity through an aqueduct. This water would be filtered at Poughkeepsie, and we now know that all impurities can be removed.

If this scheme is carried out, the total supply will be about 1,300,000,000 gallons daily, or enough for a population of from 12,000,000 to 13,000,000 persons. By putting in more pumps, filter-beds, and conduits, this supply can be increased forty per cent., or to 1,800,000,000 gallons daily. This water would fill every day a lake one mile square by ten feet deep. This is a fair example of the scale of the engineering works of the nineteenth and twentieth centuries.

By the application of modern labor-saving machinery, the cost of this work can be so far controlled that the cost to the city of New York per 1,000,000 gallons would be no greater than that of the present Croton supply.

All works of hydraulic engineers depend on water. But what will happen if the water all dries up? India, China, Spain, Turkey, and Syria have suffered from droughts, caused clearly by the destruction of their forests. The demand for paper to print books and newspapers upon, and for other purposes, is fast converting our forests into pulp. We cannot even say, “After us the deluge,” for it will seldom rain in those evil days. When the rains do come, the sponge-like vegetation of the forests being gone, the streams will be torrents at one time of the year and dried up during the rest, as we now see in the arid regions of the West.

MECHANICAL ENGINEERING