In the old days when Rome was supreme a Caesar decreed that a bridge
should be built to carry a military road across a valley, or ordered
that great stone arches should be raised to conduct a stream of water to
a city; and after great toil, and at the cost of the lives of unnumbered
labourers, the work was done--so well done, in fact, that much of it is
still standing, and some is still doing service.
In much the same regal way the managers of a railroad order a steel
bridge flung across a chasm in the midst of a wilderness far from
civilisation, or command that a new structure shall be substituted for
an old one without disturbing traffic; and, lo and behold, it is done in
a surprisingly short time. But the new bridges, in contrast to the old
ones, are as spider webs compared to the overarching branches of a great
tree. The old type, built of solid masonry, is massive, ponderous, while
the new, slender, graceful, is built of steel.
One day a bridge-building company in Pennsylvania received the
specifications giving the dimensions and particulars of a bridge that an
English railway company wished to build in far-off Burma, above a great
gorge more than eight hundred feet deep and about a half-mile wide. From
the meagre description of the conditions and requirements, and from the
measurements furnished by the railroad, the engineers of the American
bridge company created a viaduct. Just as an author creates a story or a
painter a picture, so these engineers built a bridge on paper, except
that the work of the engineers' imagination had to be figured out
mathematically, proved, and reproved. Not only was the soaring structure
created out of bare facts and dry statistics, but the thickness of every
bolt and the strain to be borne by every rod were predetermined
accurately.
And when the plans of the great viaduct were completed the engineers
knew the cost of every part, and felt so sure that the actual bridge in
far-off Burma could be built for the estimated amount, that they put in
a bid for the work that proved to be far below the price asked by
English builders.
And so this company whose works are in Pennsylvania was awarded the
contract for the Gokteik viaduct in Burma, half-way round the world
from the factory.
[Illustration: BUILDING AN AMERICAN BRIDGE IN BURMAH
This structure stretches 820 feet above the bottom of the Gokteik Gorge.
The viaduct was built entirely from above, as shown in this picture.]
In the midst of a wilderness, among an ancient people whose language and
habits were utterly strange to most Americans, in a tropical country
where modern machinery and appliances were practically unknown, a small
band of men from the young republic contracted to build the greatest
viaduct the world had ever seen. All the material, all the tools and
machinery, were to be carried to the opposite side of the earth and
dumped on the edge of the chasm. From the heaps of metal the small band
of American workmen and engineers, aided by the native labourers, were
to build the actual structure, strong and enduring, that was conceived
by the engineers and reduced to working-plans in far-off Pennsylvania.
From ore dug out of the Pennsylvania mountains the steel was made and,
piece by piece, the parts were rolled, riveted, or welded together so
that every section was exactly according to the measurements laid out on
the plan. As each part was finished it was marked to correspond with the
plan and also to show its relation to its neighbour. It was like a
gigantic puzzle. The parts were made to fit each other accurately, so
that when the workmen in Burma came to put them together the tangle of
beams and rods, of trusses and braces should be assembled into a
perfect, orderly structure--each part in its place and each doing its
share of the work.
With men trained to work with ropes and tackle collected from an Indian
seaport, and native riveters gathered from another place, Mr. J.C. Turk,
the engineer in charge, set to work with the American bridgemen and the
constructing engineer to build a bridge out of the pieces of steel that
lay in heaps along the brink of the gorge. First, the traveller, or
derrick, shipped from America in sections, was put together, and its
long arm extended from the end of the tracks on which it ran over the
abyss.
From above the great steel beams were lowered to the masonry foundations
of the first tower and securely bolted to them, and so, piece by piece,
the steel girders were suspended in space and swung this way and that
until each was exactly in its proper position and then riveted
permanently. The great valley resounded with the blows of hammers on
red-hot metal, and the clangour of steel on steel broke the silence of
the tropic wilderness. The towers rose up higher and higher, until the
tops were level with the rim of the valley, and as they were completed
the horizontal girders were built on them, the rails laid, and the
traveller pushed forward until its arm swung over the foundation of the
next tower.
And so over the deep valley the slender structure gradually won its way,
supporting itself on its own web as it crawled along like a spider.
Indeed, so tall were its towers and so slender its steel cords and beams
that from below it appeared as fragile as a spider's web, and the men,
poised on the end of swinging beams or standing on narrow platforms
hundreds of feet in air, looked not unlike the flies caught in the web.
The towers, however, were designed to sustain a heavy train and
locomotive and to withstand the terrific wind of the monsoon. The
pressure of such a wind on a 320-foot tower is tremendous. The bridge
was completed within the specified time and bore without flinching all
the severe tests to which it was put. Heavy trains--much heavier than
would ordinarily be run over the viaduct--steamed slowly across the
great steel trestle while the railroad engineers examined with utmost
care every section that would be likely to show weakness. But the
designers had planned well, the steel-workers had done their full duty,
and the American bridgemen had seen to it that every rivet was properly
headed and every bolt screwed tight--and no fault could be found.
The bridge engineer's work is very diversified, since no two bridges are
alike. At one time he might be ordered to span a stream in the midst of
a populous country where every aid is at hand, and his next commission
might be the building of a difficult bridge in a foreign wilderness far
beyond the edge of civilisation.
Bridge-building is really divided into four parts, and each part
requires a different kind of knowledge and experience.
First, the designer has to have the imagination to see the bridge as it
will be when it is completed, and then he must be able to lay it out on
paper section by section, estimating the size of the parts necessary for
the stress they will have to bear, the weight of the load they will have
to carry, the effect of the wind, the contraction and expansion of cold
and heat, and vibration; all these things must be thought of and
considered in planning every part and determining the size of each. Also
he must know what kind of material to use that is best fitted to stand
each strain, whether to use steel that is rigid or that which is so
flexible that it can be tied in a knot. On the designer depends the
price asked for the work, and so it is his business to invent, for each
bridge is a separate problem in invention, a bridge that will carry the
required weight with the least expenditure of material and labour and at
the same time be strong enough to carry very much greater loads than it
is ever likely to be called upon to sustain. The designer is often the
constructor as well, and he is always a man of great practical
experience. He has in his time stepped out on a foot-wide girder over a
rushing stream, directing his men, and he has floundered in the mud of a
river bottom in a caisson far below the surface of the stream, while the
compressed air kept the ooze from flowing in and drowning him and his
workmen.
The second operation of making the pieces that go into the structure is
simply the following out of the clearly drawn plans furnished by the
designing engineers. Different grades of steel and iron are moulded or
forged into shape and riveted together, each part being made the exact
size and shape required, even the position of the holes through which
the bolts or rivets are to go that are to secure it to the neighbouring
section being marked on the plan.
The foundations for bridges are not always put down by the builders of
the bridge proper; that is a work by itself and requires special
experience. On the strength and permanency of the foundation depends the
life of the bridge. While the foundries and steel mills are making the
metal-work the foundations are being laid. If the bridge is to cross a
valley, or carry the roadway on the level across a depression, the
placing of the foundations is a simple matter of digging or blasting out
a big hole and laying courses of masonry; but if a pier is to be built
in water, or the land on which the towers are to stand is unstable, then
the problem is much more difficult.
For bridges like those that connect New York and Brooklyn, the towers of
which rest on bed-rock below the river's bottom, caissons are sunk and
the massive masonry is built upon them. If you take a glass and sink it
in water, bottom up, carefully, so that the air will not escape, it will
be noticed that the water enters the glass but a little way: the air
prevents the water from filling the glass. The caisson works on the same
principle, except that the air in the great boxlike chamber is highly
compressed by powerful pumps and keeps the water and river ooze out
altogether.
The caissons of the third bridge across the East River were as big as a
good-sized house--about one hundred feet long and eighty feet wide. It
took five large tugs more than two days to get one of them in its proper
place. Anchored in its exact position, it was slowly sunk by building
the masonry of the tower upon it, and when the lower edges of the great
box rested on the bottom of the river men were sent down through an
air-lock which worked a good deal like the lock of a canal. The men, two
or three at a time, entered a small round chamber built of steel which
was fitted with two air-tight doors at the top and bottom; when they
were inside the air-lock, the upper door was closed and clamped tight,
just as the gates leading from the lower level of a canal are closed
after the boat is in the lock; then very gradually the air in the
compartment is compressed by an air-compressor until the pressure in the
air-lock is the same as that in the caisson chamber, when the lower door
opened and allowed the men to enter the great dim room. Imagine a room
eighty by one hundred feet, low and criss-crossed by massive timber
braces, resting on the black, slimy mud of the river bottom; electric
lights shine dimly, showing the half-naked workmen toiling with
tremendous energy by reason of the extra quantity of oxygen in the
compressed air. The workmen dug the earth and mud from under the
iron-shod edges of the caisson, and the weight of the masonry being
continually added to above sunk the great box lower and lower. From time
to time the earth was mixed with water and sucked to the surface by a
great pump. With hundreds of tons of masonry above, and the watery mud
of the river on all sides far below the keels of the vessels that passed
to and fro all about, the men worked under a pressure that was two or
three times as great as the fifteen pounds to the square inch that every
one is accustomed to above ground. If the pressure relaxed for a moment
the lives of the men would be snuffed out instantly--drowned by the
inrushing waters; if the excavation was not even all around, the balance
of the top-heavy structure would be lost, the men killed, and the work
destroyed entirely. But so carefully is this sort of work done that such
an accident rarely occurs, and the caissons are sunk till they rest on
bed-rock or permanent, solid ground, far below the scouring effect of
currents and tides. Then the air-chamber is filled with concrete and
left to support the great towers that pierce the sky above the waters.
[Illustration: THE SPIDER-WEB-LIKE VIADUCT ACROSS CANON DIABLO
The slender steel structure supporting a loaded train that stretches
along its entire length.]
The pneumatic tube, which is practically a steel caisson on a small
scale operated in the same way, is often used for small towers, and many
of the steel sky-scrapers of the cities are built on foundations of this
sort when the ground is unstable.
Foundations of wooden and iron piles, driven deep in the ground below
the river bottom, are perhaps the most common in use. The piles are
sawed off below the surface of the water and a platform built upon them,
which in turn serves as the foundation for the masonry.
The great Eads Bridge, which was built across the Mississippi at St.
Louis, is supported by towers the foundations of which are sunk 107 feet
below the ordinary level of the water; at this depth the men working in
the caissons were subjected to a pressure of nearly fifty pounds to the
square inch, almost equal to that used to run some steam-engines.
The bridge across the Hudson at Poughkeepsie was built on a crib or
caisson open at the top and sunk by means of a dredge operated from
above taking out the material from the inside. The wonder of this is
hard to realise unless it is remembered that the steel hands of the
dredge were worked entirely from above, and the steel rope sinews
reached down below the surface more than one hundred feet sometimes;
yet so cleverly was the work managed that the excavation was perfect all
around, and the crib sank absolutely straight and square.
It is the fourth department of bridge-building that requires the
greatest amount not only of knowledge but of resourcefulness. In the
final process of erection conditions are likely to arise that were not
considered when the plans were drawn.
The chief engineer in charge of the erection of a bridge far from
civilisation is a little king, for it is necessary for him to have the
power of an absolute monarch over his army of workmen, which is often
composed of many different races.
With so many thousand tons of steel and stone dumped on the ground at
the bridge site, with a small force of expert workmen and a greater
number of unskilled labourers, in spite of bad weather, floods, or
fearful heat, the constructing engineer is expected to finish the work
within the specified time, and yet it must withstand the most exacting
tests.
In the heart of Africa, five hundred miles from the coast and the source
of supplies, an American engineer, aided by twenty-one American
bridgemen, built twenty-seven viaducts from 128 to 888 feet long within
a year.
The work was done in half the time and at half the cost demanded by the
English bidders. Mr. Lueder, the chief engineer, tells, in his account
of the work, of shooting lions from the car windows of the temporary
railroad, and of seeing ostriches try to keep pace with the locomotive,
but he said little of his difficulties with unskilled workmen, foreign
customs, and almost unspeakable languages. The bridge engineer the world
over is a man who accomplishes things, and who, furthermore, talks
little of his achievements.
Though the work of the bridge builders within easy reach of the steel
mills and large cities is less unusual, it is none the less adventurous.
In 1897, a steel arch bridge was completed that was built around the old
suspension bridge spanning the Niagara River over the Whirlpool Rapids.
The old suspension bridge had been in continuous service since 1855 and
had outlived its usefulness. It was decided to build a new one on the
same spot, and yet the traffic in the meantime must not be disturbed in
the least. It would seem that this was impossible, but the engineers
intrusted with the work undertook it with perfect confidence. To any one
who has seen the rushing, roaring, foaming waters of unknown depth that
race so fast from the spray-veiled falls that they are heaped up in the
middle, the mere thought of men handling huge girders of steel above the
torrent, and of standing on frail swinging platforms two hundred or more
feet above the rapids, causes chills to run down the spine; yet the work
was undertaken without the slightest doubt of its successful fulfilment.
It was manifestly impossible to support the new structure from below,
and the old bridge was carrying about all it could stand, so it was
necessary to build the new arch, without support from underneath, over
the foaming water of the Niagara rapids two hundred feet below. Steel
towers were built on either side of the gorge, and on them was laid the
platform of the bridge from the towers nearest to the water around and
under the old structure. The upper works were carried to the solid
ground on a level with the rim of the gorge and there securely anchored
with steel rods and chains held in masonry. Then from either side the
arch was built plate by plate from above, the heavy sheets of steel
being handled from a traveller or derrick that was pushed out farther
and farther over the stream as fast as the upper platform was completed.
The great mass of metal on both sides of the Niagara hung over the
stream, and was only held from toppling over by the rods and chains
solidly anchored on shore. Gradually the two ends of the uncompleted
arch approached each other, the amount of work on each part being
exactly equal, until but a small space was left between. The work was so
carefully planned and exactly executed that the two completed halves of
the arch did not meet, but when all was in readiness the chains on each
side, bearing as they did the weight of more than 1,000,000 pounds, were
lengthened just enough, and the two ends came together, clasping hands
over the great gorge. Soon the tracks were laid, and the new bridge took
up the work of the old, and then, piece by piece, the old suspension
bridge, the first of its kind, was demolished and taken away.
Over the Niagara gorge also was built one of the first cantilever
bridges ever constructed. To uphold it, two towers were built close to
the water's edge on either side, and then from the towers to the shores,
on a level with the upper plateau, the steel fabric, composed of slender
rods and beams braced to stand the great weight it would have to carry,
was built on false work and secured to solid anchorages on shore. Then
on this, over tracks laid for the purpose, a crane was run (the same
process being carried out on both sides of the river simultaneously),
and so the span was built over the water 239 feet above the seething
stream, the shore ends balancing the outer sections until the two arms
met and were joined exactly in the middle. This bridge required but
eight months to build, and was finished in 1883. From the car windows
hardly any part of the slender structure can be seen, and the train
seems to be held over the foaming torrent by some invisible support, yet
hundreds of trains have passed over it, the winds of many storms have
torn at its members, heat and cold have tried by expansion and
contraction to rend it apart, yet the bridge is as strong as ever.
Sometimes bridges are built a span or section at a time and placed on
great barges, raised to just their proper height, and floated down to
the piers and there secured.
A railroad bridge across the Schuylkill at Philadelphia was judged
inadequate for the work it had to do, and it was deemed necessary to
replace it with a new one. The towers it rested upon, therefore, were
widened, and another, stronger bridge was built alongside, the new one
put upon rollers as was the old, and then between trains the old
structure was pushed to one side, still resting on the widened piers,
and the new bridge was pushed into its place, the whole operation
occupying less than three minutes. The new replaced the old between the
passing of trains that run at four or five-minute intervals. The Eads
Bridge, which crosses the Mississippi at St. Louis, was built on a novel
plan. Its deep foundations have already been mentioned. The great
"Father of Waters" is notoriously fickle; its channel is continually
changing, the current is swift, and the frequent floods fill up and
scour out new channels constantly. It was necessary, therefore, in order
to span the great stream, to place as few towers as possible and build
entirely from above or from the towers themselves. It was a bold idea,
and many predicted its failure, but Captain Eads, the great engineer,
had the courage of his convictions and carried out his plans
successfully. From each tower a steel arch was started on each side,
built of steel tubes braced securely; the building on each side of every
tower was carried on simultaneously, one side of every arch balancing
the weight on the other side. Each section was like a gigantic seesaw,
the tower acting as the centre support; the ends, of course, not
swinging up and down. Gradually the two sections of every arch
approached each other until they met over the turbid water and were
permanently connected. With the completion of the three arches, built
entirely from the piers supporting them, the great stream was spanned.
The Eads Bridge was practically a double series of cantilevers balancing
on the towers. Three arches were built, the longest being 520 feet long
and the two shorter ones 502 feet each.
Every situation that confronts the bridge builder requires different
handling; at one time he may be called upon to construct a bridge
alongside of a narrow, rocky cleft over a rushing stream like the Royal
Gorge, Colorado, where the track is hung from two great beams stretched
across the chasm, or he may be required to design and construct a
viaduct like that gossamer structure three hundred and five feet high
and nearly a half-mile long across the Kinzua Creek, in Pennsylvania.
Problems which have nothing to do with mechanics often try his courage
and tax his resources, and many difficulties though apparently trivial,
develop into serious troubles. The caste of the different native gangs
who worked on the twenty-seven viaducts built in Central Africa is a
case in point: each group belonging to the same caste had to be
provided with its own quarters, cooking utensils, and camp furniture,
and dire were the consequences of a mix-up during one of the frequent
moves made by the whole party.
[Illustration: BEGINNING AN AMERICAN BRIDGE IN MID-AFRICA]
And so the work of a bridge builder, whether it is creating out of a
mere jumble of facts and figures a giant structure, the shaping of
glowing metal to exact measurements, the delving in the slime under
water for firm foundations, or the throwing of webs of steel across
yawning chasms or over roaring streams, is never monotonous, is often
adventurous, and in many, many instances is a great civilising
influence.