In early days the building of a bridge was a matter of great
ceremony, and it was consecrated to protect it from evil spirits. Its
construction was controlled by priests, as the title of the Pope of
Rome, “Pontifex Maximus,” indicates.
Railways changed all this. Instead of the picturesque stone bridge,
whose long line of low arches harmonized with the landscape, there came
the straight girder or high truss, ugly indeed, but quickly built, and
costing much less.
Bridge construction has made greater progress in the United States than
abroad. The heavy trains that we have described called for stronger
bridges. The large American rolling-stock is not used in England, and
but little on the continent of Europe, as the width of tunnels and
other obstacles will not allow of it. It is said that there is an
average of one bridge for every three miles of railway in the United
States, making 63,000 bridges, most of which have been replaced by new
and stronger ones during the last twenty years.
This demand has brought into existence many bridge-building companies,
some of whom make the whole bridge, from the ore to the finished
product.
Before the advent of railways, highway bridges in America were made
of wood, and called trusses. Few of them existed before railways.
The large rivers and estuaries were crossed in horse-boats, a trip
more dangerous than an Atlantic voyage now is. A few smaller rivers
had wooden truss bridges. Although originally invented by Leonardo
da Vinci, in the sixteenth century, they were reinvented by American
carpenters. Some of Burr’s bridges are still standing after more
than one hundred years’ use. This shows what wood can do when not
overstrained and protected from weather and fire.
The coming of railways required a stronger type of bridge to carry
concentrated loads, and the Howe truss, with vertical iron rods, was
invented, capable of 150-foot spans.
About 1868 iron bridges began to take the place of wooden bridges.
Die-forged eyebars and pin connections allowed of longer panels and
longer spans. One of the first long-span bridges was a single-track
railway bridge of 400-foot span over the Ohio at Cincinnati, which was
considered to be a great achievement in 1870.
The Kinzua viaduct, 310 feet high and over half a mile long, belongs to
this era. It is the type of the numerous high viaducts now so common.
About 1885 a new material was given to engineers, having greater
strength and tenacity than iron, and commercially available from its
low cost. This is basic steel. After many experiments, the proper
proportions of carbon, phosphorus, sulphur, and manganese were
ascertained, and uniformity resulted. The open-hearth process is now
generally used. This new chemical metal, for such it is, is fifty per
cent. stronger than iron, and can be tied in a knot when cold.
The effect of improved devices and the use of steel is shown by the
weights of the 400-foot Ohio River iron bridge, built in 1870, and a
bridge at the same place, built in 1886.
The bridge of 1870 was of iron, had panels twelve feet long, and its
height was forty-five feet, and span 400 feet.
The bridge of 1886 was of steel, had panels thirty feet long, and its
height was eighty feet. Its span was 550 feet. The weights of the two
were nearly alike.
The cantilever design, which is a revival of a very ancient type,
came into use. The great Forth Bridge, in Scotland, 1600-foot span,
is of this style, as are the 500-foot spans at Poughkeepsie, and now
a new one is being designed to cross the St. Lawrence near Quebec, of
1800-foot span.
This is probably near the economic limit of cantilever construction,
but the suspension bridge can be extended much farther, as it carries
no dead weight of compression members.
The Niagara Suspension Bridge, of 810-foot span, built by Roebling, in
1852, and the Brooklyn Bridge, of 1600 feet, built by Roebling and his
son, twenty years after, marked a wonderful advance in bridge design.
Thirty years later, when a new bridge of 1600 feet was wanted to
cross another part of the East River at New York, the same lines of
construction were followed, and they will be followed in the 2700-foot
span, designed to cross the North River some time in the present
century. The only radical advance is the use of a better steel than
could be had in earlier days.
Steel-arched bridges are now scientifically designed. Such are the new
Niagara Bridge, of 840-foot span, and the Alexandra Bridge at Paris.
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.