BRIDGES
BRIDGES OF TIMBER, THEN STONE, THEN STEEL--THE STARUCCA VIADUCT--THE
FIRST IRON BRIDGE IN THE U. S.--STEEL BRIDGES--ENGINEERING
TRIUMPHS--DIFFERENT TYPES OF RAILROAD BRIDGE--THE DECK SPAN AND THE
TRUSS SPAN--SUSPENSION BRIDGES--CANTILEVER BRIDGES--REACHING THE SOLID
ROCK WITH CAISSONS--THE WORK OF "SAND-HOGS"--THE CANTILEVER OVER THE
PEND OREILLE RIVER--VARIETY OF PROBLEMS IN BRIDGE-BUILDING--POINTS IN
FAVOR OF THE STONE BRIDGE--BRIDGES OVER THE KEYS OF FLORIDA.
When the habitations of man first began to multiply upon the banks of the
water courses, the profession of the bridge-builder was born. The first
bridge was probably a felled tree spanning some modest brook. But from
that first bridge came a magnificent development. Bridge-building became
an art and a science. Men wrought gigantic structures in stone,
long-arched viaducts, with which they defied time. Then for two thousand
years the profession of the bridge-builder stood absolutely still.
With the coming of the iron and steel age it moved forward again. The
development of a fibre of great strength and without the dead weight of
granite gave engineers new possibilities. They began in simple fashion,
and then they developed once again, with marvellous strides. Steel, the
dead thing with a living muscle, could span waterways from which stone
shrank. Steel redrew the maps of nations. Proud rivers at which the paths
of man had halted, were conquered for the first time. Routes of traffic of
every sort were simplified; the railroad made new progress; and economic
saving of millions of dollars was made to this gray old world.
The earliest of the very distinguished list of American bridge-builders
erected great timber structures for the highroads and the post-roads. Some
of them went back many centuries and came to the stone bridge, in many
ways the most wonderful of all the artifices by which man conquers the
obstructive power of a running stream. But the building of stone bridges
took time and money, and time and money were little known factors in a new
land that had begun to expand rapidly.
So at first the railroad followed the course of the highroad and the
post-road, and took the timber bridge unto itself. In some cases it
actually fastened itself upon the highroad bridge, as at Trenton, N. J.,
where a faithful wooden structure built by Theodore Burr in 1803 was
strengthened and widened in 1848 to take the first through railroad route
from New York. It continued its heavy dual work until 1875 when it was
superseded by a steel bridge. A dozen years ago the railroad tracks were
moved from that structure to a magnificent and permanent stone-arch built
near-by. Thus the railroad crossing the Delaware at Trenton has, in this
way, typified step by step every stage of the development of American
bridge-building.
The timber bridges developed the steel truss bridge, the typically
American construction, of to-day. In an earlier day the timber bridges
were the glory of the engineer. Sometimes you see one of these old fellows
remaining, like the long structure that Mr. Walcott built across the
Connecticut River at Springfield, Mass., in 1805, and which still does
good service; but the most of them have passed away. Fire has been their
most persistent enemy. Within the past two years fire destroyed the
staunch toll-bridge at Waterford on the Hudson, just above Troy. The
bridge was a faithful carrier for one hundred and four years. In many ways
it was typical of those first constructions. It consisted of four clear
arch spans--one 154 feet, another 161 feet, the third 176 feet, and the
fourth 180 feet in length. It was built of yellow pine, wonderfully hewn
and fitted, hung upon solid pegs; and save for the renewal of some of the
arch footings, the roof, and the side coverings, it was unchanged through
all the years--even though the heavy trolley-cars of a through interurban
line were finally turned upon it.
About the same time, the once-famed Permanent Bridge across the Schuylkill
River at Philadelphia was built. It had two arches of 150 feet each and
one of 195 feet. In its day it was regarded as nothing less than a
triumph. A very old publication says:
"The plan was furnished by Mr. Timothy Palmer, of Newburyport, Mass.,
a self-taught architect. He brought with him five workmen from New
England. They at once evinced superior intelligence and adroitness in
a business which was found to be a peculiar art, acquired by habits
not promptly gained by even good workmen in other branches of framing
in wood.... The frame is a masterly piece of workmanship, combining in
its principles that of king-post and braces or trusses with those of a
stone arch."
In after years, the Permanent Bridge was also entrusted with the carrying
of a railroad. It has, however, disappeared these many years.
The early railroad builders did not neglect the possibilities of the stone
bridge. Two notable early examples of this form of construction still
remain--the Starrucca Viaduct upon the Erie Railroad, near Susquehanna,
Pa., and an even earlier structure, the stone-arch bridge across the
Patapsco River at Relay, Md., which B. H. Latrobe, the most distinguished
of all American railroad engineers, built for the Baltimore & Ohio
Railroad, in 1833-35. The Thomas Viaduct, as it has been known for
three-quarters of a century, was the first stone-arch bridge ever built to
carry railroad traffic. It was erected in a day when the railroad was just
graduating from the use of teams of horses as motive-power. In this day,
when locomotives have begun to reach practical limits of size and weight,
that viaduct is still in use as an integral part of the main line of the
Baltimore & Ohio. It is built on a curve, and consists of 8 spans of stone
arches, 67 feet 6 inches, centre to centre of piers, which, together with
the abutments at each end, make the total length of the structure 612
feet. It is in as good condition to-day as upon the day it was built.
When the Erie Railroad was being constructed across the Southern Tier
counties of New York in 1848, its course was halted near the point where
the rails first reached the beautiful valley of the Susquehanna. A
side-valley, a quarter of a mile in width, stretched itself squarely
across the railroad's path. There was no way it could be avoided, and it
could be crossed only at a high level. For a time the projectors of the
Erie considered making a solid fill, but the tremendous cost of such an
embankment was prohibitive. While they were at their wits' ends, James P.
Kirkwood, a shrewd Scotchman, who had been working as a civil engineer
upon the Boston & Albany, appeared. Kirkwood spanned the valley with the
Starucca Viaduct, one of the most beautiful bridges ever built in America.
He opened quarries close at hand and by indefatigable energy built his
stone bridge in a single summer. It has been in use ever since. The
increasing weight of its burdens has never been of consequence to it, and
to-day it remains an important link in a busy trunk-line railroad. It is
1,200 feet in length and consists of 18 arches of 50 feet clear span
apiece.
But stone bridges even then cost money, and so the timber structure still
remained the most available. Many men can still remember the tunnels, into
whose darkness the railroad cars plunged every time they crossed a stream
of any importance whatsoever. They have nearly all gone. The wooden bridge
was ill suited to the ravages of weather and of fire--ravages that were
quickened by the railroad, rather than hindered. A substitute material
was demanded. It was found--in iron.
The first iron bridge in the United States is believed to be the one
erected by Trumbull in 1840 over the Erie Canal at Frankfort, N. Y. Record
is also held of one of these bridges being built for the North Adams
branch of the Boston & Albany Railroad, in 1846. About a year later,
Nathaniel Rider began to build iron bridges for the New York & Harlem, the
Erie, and some others of the early railroads. His bridges--of the truss
type, of course, that type having been worked out in the timber bridges of
the land--were each composed of cast-iron top-chords and post, the
remaining part of the structure being fabricated of wrought-iron. The
members were bolted together. Still, the failure of a Rider bridge upon
the Erie in 1850, followed closely by the failure of a similar structure
over the River Dee, in England, influenced officials of that railroad to a
conclusion that iron bridges were unpractical, and to order them to be
removed and replaced by wooden structures. For a time it looked as if the
iron bridge were doomed. That was a dark day for the bridge engineers. A
contemporary account says:
"The first impulse to the general adoption of iron for railroad
bridges was given by Benjamin H. Latrobe, chief engineer of the
Baltimore & Ohio Railroad. When the extension of this road from
Cumberland to Wheeling was begun, he decided to use this material in
all the new bridges. Mr. Latrobe had previously much experience in the
construction of wooden bridges in which iron was extensively used; he
had also designed and used the fish-bellied girder constructed of cast
and wrought-iron."
Under the influence of the really great Latrobe, an iron span of 124 feet
was built in 1852 at Harpers Ferry. In that same year, the B. & O. built
its Monongahela River Bridge, a really pretentious structure of 3 spans of
205 feet each, and the first really great iron railroad bridge in all the
land. The path was set. The conquest of iron over wood as a bridge
material was merely a problem of good engineering. The iron bridge quickly
came into its own. The Pennsylvania Railroad began building cast-iron
bridges of from 65 to 110 feet span at its Altoona shops for the many
creeks and runs along the western end of its line. The other railroads
were following in rapid order. Squire Whipple, Bollman, Pratt--all the
others who could design and build iron bridges--were kept more than busy
by the work that poured in upon them.
And in the day when the iron bridge was coming into its own, Sir Henry
Bessemer, over in England, was bringing the steel age into existence,
first making toy cannon models for the lasting joy of Napoleon III, and
then making a whole world see that steel--that dead thing with the living
muscle--was no longer to be limited for use in tools and cutting surface.
Steel was to become the very right-hand of man. And so steel came to the
bridge-builders, at first only in the most important wearing points such
as pins and rivets, finally to be the whole fabric of the modern bridge.
The transition was gradual. The early engineers began using less and less
of cast-iron and more and more of wrought, until they had practically
eliminated cast-iron as a bridge material. Then there came a quick change;
there was another dark day for the railroad bridge engineers of America.
In 1876--that very year when the land was so joyously celebrating its
Centennial--a passenger train went crashing through a defective bridge at
Ashtabula, Ohio. There was a great property loss--thousands and thousands
of dollars, and a loss of lives that could never be expressed in dollars.
An outraged land asked the bridge-builders if they really knew their
business.
Out of that Ashtabula wreck came the scientific testing of bridges and
bridge materials, and the abolition of the rule-of-thumb in the cheaper
sorts of construction. Out of that miserable wreckage came also the use of
steel in the railroad bridge. Steel had found itself; and how the steel
bridges began to spring up across the land! They spanned the Ohio, and
they spanned the Mississippi, and they spanned the Missouri; a great
structure threw itself over the deep gorge of the Kentucky River. When the
day came that fire destroyed the famous wooden viaduct of the Erie over
the Genesee River at Portage, N. Y. (you must remember the pictures of
that tremendous structure in the early geographies), steel took its place.
All this while the bridge engineer attempted more and more. He built over
the deep gorge of the Niagara. He conquered the St. Lawrence in and about
Montreal. He laughed at the mighty Hudson and flung a dizzy steel trestle
over its bosom at Poughkeepsie. He built at Cairo, at Thebes, and at
Memphis, on the Mississippi, and again and again and still again at St.
Louis. The East River no longer halted him or compelled him to resort to
the alternative of the very expensive types of suspension bridge. He has
finally thrown a great cantilever over it, from Manhattan to Long Island.
The steel bridge has come into its own.
* * * * *
Let us study for a moment the construction of the different types of
railroad bridge. For the tiny creeks--the little things that are mad
torrents in spring, and run stark-dry in midsummer--where they cannot be
poured through a pipe or a concrete moulded culvert, the simplest of
bridge forms will suffice. And the simplest of bridge forms consists of
two wooden beams laid from abutment to abutment and holding the ties and
rails of the track-structure. As the first development of that simplest
idea comes the substitution of steel for wood, giving, as we have already
seen, protection against fire and a far greater strength. The steel beam
has greater strength than a wooden beam of the same outside dimension and
yet in its design it effects for itself a great saving of material, by
cutting out superfluous parts and becoming the structural standard of
to-day, the I beam. When the I beam becomes too large to be made in a
single pouring or a single rolling, it may be constructed of steel plates
and angles firmly riveted together, and thus still remains the possibility
of the simplest form of bridge. That single span may be further increased,
or the bridge developed into a succession of increased spans by the
substitution of the lattice-work girder, effecting further saving in
weight without material loss of strength for the solid-plate girder. The
track may be laid atop of such girders or--to save clearance in overhead
crossing--swung between them at their bases.
The limit in this form of bridge is generally in a 65-foot or a 100-foot
span. It is not practical to build the girders up outside of a shop; and
the 65-foot length represents the two flat-cars that must be used to
transport any one of them to the bridge location. Some railroads have used
three cars for the hauling of a single girder, and so increased these
spans to 100 feet; but as a rule, over 65 feet, and the truss, the most
common form of railroad bridge in this country, comes into use.
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.
You have expressed your interest in the sand-hog, given vent to a desire
to go down into their underworld. You wonder what three pressures is going
to feel like. Permission is given and a physician begins examining you.
You cannot go into the caisson unless you are sound of heart and stout of
body. This is no joking matter. The sand-hogs' rules read like the
training instructions for a college football team. No drink, regular
hours, simple diet, the donning of heavy clothes after they leave the
pressure, constant reëxamination--these rules are inflexible when the
caissons go to far depths. By their observance the difficult foundation
construction of this new bridge has been kept free from accident--there
have been few cases of the "bends" brought to the specially constructed
hospital in the bottom of the cavity.
The "bends" sounds complicated, and is, in reality, almost the simplest of
human ailments in its diagnosis. A "bubble" of high pressure air works its
way into the human structure while a man is in the caisson. When he comes
out into the normal atmosphere the bubble is caught and remains. If it is
caught near any vital organ that bubble is apt to spell death. Generally
the bubbles are caught in the joints--frequently the elbow or the
knee--where they cause excruciating pain. Then the specially constructed
hospital crowded on the narrow platform formed by the top of the pier,
comes into full play. Its sick room is incased in an air-tight cylinder.
The man suffering from the "bends," together with physicians and nurses,
is put under a pressure that gradually increases until it reaches that of
the caisson. After that it is a comparatively simple matter to relieve the
bubble and bring the air in the hospital back to a normal pressure.
The path is clear for us to go down into the caisson. A party of
sand-hogs, hot and exhausted after forty minutes of work within, come out
of the little manhole at the top of the air-lock. We step through the
little manhole and into a tiny steel bucket that rests within the air-lock
there at the top of the shaft. A word of command--farewell to the bright
blue sky overhead--the black manhole cover is replaced. It is suddenly
very dark. A single faint incandescent gives a dim glow in the tiny
place.
[Illustration: CONCRETE AFFORDS WONDERFUL OPPORTUNITIES FOR THE
BRIDGE-BUILDERS]
[Illustration: THE LACKAWANNA IS BUILDING THE LARGEST CONCRETE BRIDGE IN
THE WORLD ACROSS THE DELAWARE RIVER AT SLATEFORD, PA.]
[Illustration: THE BRIDGE-BUILDER LAYS OUT AN ASSEMBLYING-YARD FOR
GATHERING TOGETHER THE DIFFERENT PARTS OF HIS NEW CONSTRUCTION]
[Illustration: THE NEW BRANDYWINE VIADUCT OF THE BALTIMORE & OHIO, AT
WILMINGTON, DEL.]
You are not thinking of that. They are putting the pressure on. You can
feel it. Your eardrums feel as if they would break; they vibrate. You must
show your distress.
"Pinch your nose and swallow hard," says the man who stands beside you in
the bucket.
He stands so close to you that you can fairly feel the pulsation of his
heart, but his voice sounds miles away. You swallow hard, the hardest you
have ever swallowed, and you pinch your nose. You feel better. The
far-away voice speaks again in your ear. "Three atmospheres," is all it
says. The caisson shaft is no place for extended conversation. You descend
in an express elevator car; in that bucket you just drop. You have all the
eerie sensations that a Coney Island "novelty ride" might give you. There
is a row of dim incandescents all the way down the smooth side of the
shaft, and when you look you forget that this is vertical traction and
think of an uptown subway tube as you see it recede from the rear of an
express. A final manhole, the gate at the foot of the shaft and you stop
abruptly. It seems as if you had almost bumped against the under side of
China.
"This is it," says the far-away voice.
A timbered room, not larger than a parlor in a city flat and not near so
high. A close and murky place, filled with a little company of
men--shadowy humans of a real underworld there under the dull electric
glow.
"They're finding the footing for the shaft," says the voice. "We're on
rock at last at 94 feet."
* * * * *
When the footings are finished and the caisson's edges have ceased to cut
its path straight downward, that timbered construction will rest here far
below the city for long ages. The sand-hogs will come out of their working
chamber for the last time--it will be poured full of concrete, more solid
than rock itself. The air pressure will be withdrawn--there is no longer
mud or shifting sand for it to withhold. Then, section by section, the
steel lining of the caisson shaft will be withdrawn, while concrete,
tramped into place, makes the shaft a hidden monolith 100 feet or so in
length. Upon the tops of all these monoliths a close grillage of steel
beams will be laid; upon that grillage will be riveted the steel plates
and columns of the bridge tower. The great structure is to have sure
footing; these giant feet bind and clasp themselves throughout the years
against the mighty river that has been conquered and humbled by the work
of man.
* * * * *
"You should have been down in one of the boxes when they had to burn
torches, before they got the electric light," says one of the bridge
engineers. "I worked in one of those that we left under a stone tower of
the Brooklyn Bridge. Now we're almost in clover. They even cool and dry
the compressed air before we breathe it."
An order goes aloft over an electric wire, the engineer who sits smoking
his pipe on the sun-baked platform of the traveller derrick pulls a lever,
and we go slipping up the shaft toward fresh air and freedom only a little
less rapidly than we descended it. We do not reach it too quickly. There
is a long wait in the air-lock after the lower manhole has closed, while
the pressure is being reduced. You begin to worry and you ask your guide
as to the delay. Nothing wrong?
He smiles at your timorous question and explains. It would be dangerous to
come out from the caisson pressure quickly. He does not want to have to
send you to that air-tight hospital with a bad case of the "bends."
"How long in the air-lock?" you ask.
"Fifty minutes," he answers.
Then he explains in more detail. You have been under a pressure of 50
pounds to the square inch--that's your three atmospheres, and under the
rules you must spend fifty minutes in the tiny air-lock. Up to a pressure
of 36 pounds you must spend two minutes there for every three pounds of
pressure. When you get above that "law of 36" it is a minute to the pound.
When that manhole cover overhead finally slides open you feel blinded by
the light, even though the sun is hidden behind a passing cloud. The
air-lock tender reaches down with his arms and gives you a lift up onto
his narrow perch.
"Want to be a sand-hog?" he smiles.
"Not yet a while," you answer, in all truth. "Not until every other job is
gone."
* * * * *
You are standing aloft, balancing yourself upon tiny planks at the
steadily advancing end of the bridge, as it forces itself over a stream of
formidable width. Overhead, a gigantic, ungainly traveller, equipped with
steel derricks at every corner, is advancing foot by foot as the bridge
advances foot by foot. Underneath, through the thin network of planks, of
girder and of supporting false work, you can see the surface of the river
a full hundred feet below. A steamboat is passing directly beneath you.
From your perch she looks like a great yellow bird. Those fine black
specks upon her back are the humans who are gathered upon her upper deck.
Whistles call and the derricks groan as they swing the thousands of
bridge-members, that are flying together at the beck of the engineer, into
their final resting-places. There is the deafening racket of the riveters,
here and there and everywhere. There are crude railroad tracks upon the
temporary flooring of the bridge deck, and the calls of the dummy
locomotives add to the racket. The railroad tracks lead to the shore, to
temporary yards where the bridge materials are assembled as fast as they
come from the shops in a city three hundred miles distant.
For, remember that while the sand-hogs were burrowing under the surface of
the river to find footholds for this monster, other men were burrowing
into the hillsides to find the precious ore for the welding of his
muscles. A hundred thousand picks must have fought in his behalf, furnaces
blazed for miles before the crude ore became the finished, perfect steel.
Of the forging and the rolling of the steel a whole book might be written.
It is enough now to say that of the 50,000,000 pounds of steel, every
pound was made on honor. The railroad had its inspectors everywhere, but
the rolling-mill men held to their formulas for perfect steel, and perfect
steel was the result. A slight flaw in the metal, and possibly at some
unexpected day, a great catastrophe. The safety of human life was upon the
men who forged the steel, and they forged honor into every great girder,
into every rod and bolt and plate. This conqueror of the river was a
warrior built in honor.
The safety of human life depends upon the men who build this bridge. Study
carefully the face of this man who stands beside you, the man who evolved
this bridge as a season's work of his restless mind. His face is the face
of a man who has high regard for human safety; that factor creeps to the
fore as he talks to you. He is telling of the method of constructing the
upper works of a bridge of this size.
"We're getting ahead all the time," he laughs, "and we're moving rather
forward in our construction methods. In an older day we did this work with
derricks of a rather simple sort, operated them by small portable steam
engines. You can't handle bridge-members--units that are only held down by
the clearances of tunnels and the transporting powers of the
railroads--that way to-day. We've nearly half a million dollars tied up
here in constructing-appliances. These steel-boom derricks, travellers,
and steel-wire hoists, the compressing engines for handling the riveters,
cost big money.
"Our method? That's a simple enough affair as a rule. We set up this
spindly tower on rails, that we call the 'traveller' and it moves
backwards and forwards over the trusses and the timber falsework that we
build before the steel really begins to be set up. When the steel--the
trusses--is up and riveted, then away with the falsework. Our bridge
stands by itself. You can put up a 500-foot span in no time at all by
using the falsework."
You make bold to ask what the engineer does when the river is too deep to
admit of falsework. He is quick to answer.
"We generally fall back on a cantilever," he says, without hesitation.
Then he begins to tell you about one of the latest of American
problems--the new bridge of the Idaho & Washington Northern Railroad, just
now being built over the Pend Oreille River, Washington. They could span
that narrow cleft only on the cantilever principle, and when they began to
balance their cantilever, there was not enough room for the back arm. But
the engineers only chewed off fresh cigars and began forcing their great
span out mid-air. They made the balance by placing 600 tons of steel rails
on the back-arm. For every foot the span reached out anew over a so-called
"bottomless" they added a few more rails. You can generally trust an
engineer in such a time as that.
Look closely now upon the workmen who are fabricating this giant bridge.
Look closely upon them. They are different from those whom we saw toiling
in the caissons below. Scandinavians may and do toil as sand-hogs at the
bottom of the stream; Lithuanians may mine the ore, and Hungarians roll it
into steel; Americans build upon their toil and erect this bridge. These
builders speak no unfamiliar tongue. They are the product of Ohio, the
Middle West, the South, the Pacific Coast, New England; they rise
immeasurably superior to every other class of labor employed upon the
work. Some of them have been sailors, and their talk has the savor of the
sea. All of them are men, clear-headed, cool-headed, true-headed men.
If you come upon them at the noon-hour, sprawled along the narrow ledge of
a single plank you may be impressed by two things--their Americanism and
their cosmopolitanism. The first of these is writ upon each man as you
look at him; the second is evident in talk with him. This big fellow must
have been a sheriff out in Montana, and he must have been a sheriff for
bad men to dodge; his neighbor is talking about his last job, a sky-high
cantilever down in Peru. The two side-partners over by the tool-box are
just back from India. American bridge-building talent encircles the world.
Here is a boss who got his first training down on the Nile; his assistant
has done some mighty big work on the Trans-Siberian.
These are the men who are building the bridge. In a little time there will
be no advancing ends, finding their path from pier-top to pier-top. There
will be, instead, a long and slender path for the railroad; the bridgemen
will have done their work well; a great river will have once again been
conquered.
* * * * *
The bridge problem is always different, it constantly has the fascination
of variety. That variety will come into play at unexpected turns. Once,
down in a deep Colorado cañon, whose walls rose precipitously for a
thousand-odd feet, and which was all but filled by a deep and rapid river,
the engineers of the Rio Grande & Western found absolutely no ledge
whatsoever upon which they might rest their rails. They puzzled upon the
problem for a little while, and then they swung a girder bridge parallel
with the river. The bridge was supported by braced girders, that fastened
their feet in the walls of the cañon, hardly wider there than a narrow
city house. The railroad has been running over that construction for more
than thirty years; it is one of the scenic wonders of the land, and a
triumph for the engineer that built it. In constructing the expensive West
Shore Railroad up the Hudson River, similar difficulties were experienced
south of West Point, and truss bridges were built parallel with the steep
river banks to carry the tracks from ledge to ledge. It is not an unusual
matter for the construction engineer to spend a quarter of a million
dollars to span some deep, waterless gully in the mountains, which could
not be filled for more than twice that sum.
Many times, in these days of increasing weight of equipment, it becomes
necessary to replace a bridge, without interrupting the traffic. The
construction engineer never fails to meet the problem. Years ago, he took
Roebling's famous suspension bridge at Niagara Falls, removed the stone
towers and replaced them with towers of steel, without delaying a single
train; and a little later he took that bridge itself, and substituted a
heavy cantilever for it, while all the time a heavy traffic poured itself
over the structure. The rebuilder of bridges works like the original
builder--with plentiful falsework. He timbers in and around his structure,
and then step by step and with exceeding caution removes the old and
substitutes the new. An old girder is taken out between trains; before
another train of cars shall roll over the structure a new one is ready,
temporarily bolted until the riveters can make it fast. It sounds
complicated, but it is remarkably simple, under the careful plans of a
patient engineer, who has that infinite thing that we call genius.
Sometimes a bold engineer strikes out into a new method, quicker and less
expensive than these piecemeal efforts. Of such was the job at
Steubenville, O., where a 205-foot double-track span was erected on heavy
falsework alongside the old bridge. In a carefully chosen interval between
a service of frequent trains, both the old and the new spans--together
weighing 1,300 tons--were fastened together and drawn sideways a distance
of twenty-five feet in one minute and forty seconds. The new span was then
in place, and the old one--ready to be dismantled--stood on falsework at
the side. The entire job had been accomplished in an interval of seventeen
minutes between trains.
That is not unusual. The floating method is sometimes adopted with
remarkable success--especially in the case of draw-bridge spans. There the
problem complicates itself exceedingly, for both the water and the land
highways must be kept open for traffic; yet it is a matter of record that
the Pennsylvania Railroad, operating a fearfully heavy suburban service in
and out of Jersey City, recently substituted one draw for another on its
Hackensack River Bridge without delaying a single train.
* * * * *
But even in this high noon of the day of steel, the stone bridge holds its
own. The big chiefs of railroad construction look upon it with favor.
Higher priced than a steel bridge of equal capacity it requires initial
outlay. But forever after, it represents a saving--a saving chiefly in
that very important figure, maintenance. A steel bridge requires constant
attention and constant expense. A stone bridge requires little of either;
and therein lies its strength in its old age. Engineers point to such
structures as the Thomas Viaduct down at Relay, or to the wonderful stone
bridges that have stood through the centuries in older lands; they bear in
mind the constant battle that a steel bridge must make against the ravages
of weather and against the sinister thefts of corrosion, and ofttimes they
rule in favor of the oldest type of sizable bridge.
Two things are all-important in the choice between the steel bridge and
the arch bridge of stone or concrete. The first is the accessibility of
the quarries. If they are not very near the solid bridge will cost four
times that of one of steel and the average American railroad is not able
to spend money in that fashion, even in the hopes of future economies in
maintenance. If the quarries are close at hand, as they were years ago
when Kirkwood built the Starucca Viaduct for the Erie, the cost of a
masonry bridge will hardly exceed that of steel trusses, and the concrete
structure may cost a little less. Then there comes into play the second
consideration. The stone or concrete bridge has tremendous weight, no
ordinary foundation work will serve it. If the river bed and banks be of
sand or poor earth, the engineer had best give up his hopes of the Roman
form of structure. He can build steel towers and trusses on piles of
caissons--hardly solid stone piers and abutments and aides.
All these things considered, the stone bridge is still more than holding
its own in modern railroad construction. The Boston & Albany Railroad
began building these splendidly permanent structures along its lines
through the Berkshires more than twenty years ago. More recently both the
Pennsylvania and the Baltimore & Ohio have been looking with favor upon
this type of bridge. The Baltimore & Ohio has just finished building its
massive Brandywine Viaduct, near Wilmington, a splendid double-track
structure, 764 feet in length, and composed of two 80-foot, two 90-foot,
and three 100-foot arches.
The three great stone bridges that the Pennsylvania has built upon its
main line are all four-tracked. Two splendid examples of these span the
Raritan River at New Brunswick, and the Delaware at Trenton, New Jersey.
The third, spanning the Susquehanna at Rockville, Pa., just north of
Harrisburg, is the largest stone bridge in the world. It is over a mile in
length, and is composed of 48 arches; 220,000 tons of masonry was employed
in its construction.
Concrete viaducts were first employed in interurban electric railroad
construction, and latterly they have been brought more to the service of
the steam railroad. A splendid example of this very new form of
construction exists in the extension of the Florida East Coast Railroad
over the keys and shallow waters of Southern Florida, for seventy-five
miles between Homestead and Key West. A considerable portion of the line
is over the sea.
The Florida keys are like a series of stepping-stones, leading into the
ocean from the tip of the peninsula to Key West. They lie in the form of a
curve, the channels separating the islands varying from a few hundred feet
to several miles in width. Nearly thirty of these islands were used in the
construction of the new railroad. More than fifty miles of rock and
earthen embankment have been built where the intervening waters are
shallow, but where the water is deeper and the openings are exposed to
storms by breaks in the outer reef, concrete arch viaducts have been used.
These viaducts consist of 50-foot reinforced concrete arch spans and
piers, with here and there a 60-foot span.
There are four of these arch viaducts aggregating 5.78 miles in length.
The longest is between Long Key and Grassy Key, 2.7 miles, and is called
the Long Key Viaduct; across Knight's Key Channel, 7,300 feet; across
Moser's Channel, 7,800 feet, and across Bahia Honda Channel, 4,950 feet.
The material of these islands is coralline limestone. In many places the
embankment for the roadway is 8 or 9 feet in height, and the roadbed is
ballasted with the same material. The result is one of the finest and
safest railway roadbeds in the world.
Across the Delaware River at Slateford, Pa., the Delaware, Lackawanna &
Western Railroad is building the largest concrete bridge in the world, a
few feet longer than the great structure by which the Illinois Central
crosses the Big Muddy River and just 100 feet longer than the Connecticut
Avenue Bridge, at Washington, D. C. The Lackawanna's bridge is 1,450 feet
long, with five arches of 150-foot span, and a number of shorter arches.
The track is carried at an elevation of 75 feet above highwater; and to
find living-rock as a solid foundation for a structure of so great a
weight, the abutments and piers were carried about 61 feet below the
surface of the ground.
* * * * *
With the bridge-builder at his elbow, the railroad constructing engineer
hesitates at no river, no arm of the sea, no deep valley, no wild ravine,
no cleft in the mountain-side. He calls to his aid the magic of the men
who have made this branch of American practical science famous: a feathery
trestle appears, as if by magic. Across its narrow edge the steel rails
follow their resistless path.