York and Brooklyn, that only a simple statement of some of
its characteristic features will be given. Its clear span is
1,595½ feet. With its approaches its length is 3,455 feet. The
clear waterway is 135 feet high. The towers rise 272 feet above
high water and extend on the New York side down to rock 78 feet
below. The four suspension cables are of steel wire and support
six parallel steel trusses, thus providing two carriage ways, two
lines of railway, and one elevated footway. The cables are carried
to bearing anchorages in New York and in Brooklyn. The cars on the
bridge are propelled by cables, and the amount of travel is now
so great as to demand some radical changes in the methods for its
accommodation, which a few years ago were supposed to be ample.
Except under special circumstances of location or length of span,
the truss bridge is a more economical and suitable structure for
railway traffic than a suspension bridge.
The advance from the wood truss to the modern steel structure has
been through a number of stages. Excellent bridges were built
in combinations of wood and iron, and are still advocated where
wood is inexpensive. Then came the use of cast iron for those
portions of the truss subject only to compressive strains, wrought
iron being used for all members liable to tension. Many bridges
of notable spans were built in this way and are still in use.
The form of this combination truss varied with the designs of
different engineers, and the spans extended to over three hundred
feet. The forms bore the names of the designers, and the Fink, the
Bollman, the Pratt, the Whipple, the Post, the Warren, and others
had each their advocates. The substitution of wrought for cast
iron followed, and until quite recently trusses built entirely of
wrought iron have been used for all structures of great span. The
latest step has been made in the use of steel, at first for special
members of a truss and latterly for the whole structure. The art
of railway bridge building has thus, in a comparatively few years,
passed through its age of wood, and then of iron, and now rests in
the application of steel in all its parts.
Two distinct ways of connecting the different parts of a structure
are in common use, riveting and pin connections.
In riveted connections the various parts of the bridge are fastened
at all junctions by overlapping the plates of iron or steel and
inserting rivets into holes punched through all the plates to be
connected. The rivets are so spaced as to insure the best result
as to strength. The pieces of metal are brought together, either
in the shop or at the structure during erection, and the rivets,
which are round pieces of metal with a head formed on one end,
are heated and inserted from one side, being made long enough to
project sufficiently to give the proper amount of metal for forming
the other head. This is done while the rivet is still hot, either
by hammering or by the application of a riveting machine, operated
by steam or hydraulic pressure. Ingenious portable machines are
now manufactured which are hung from the structure during erection
and connected by flexible hose with the steam power, by the use of
which the rivet heads can be formed in place with great celerity.
The connections of plates by rivets of proper dimensions and
properly spaced give great strength and stiffness to such joints.
In pin connections the members of a structure are assembled at
points of junction and a large iron or steel pin inserted in
a pin-hole running through all the members. This pin is made
of such diameter as to withstand and properly transmit all the
strains brought upon it. Joints made with such pin connections
have flexibility, and the strains and stresses can be calculated
with great precision. Eye-bars are forged pieces of iron or steel,
generally flat, and enlarged at the ends so as to give a proper
amount of metal around the pin-hole or eye, formed in those ends.
Structures connected by pins at their principal junctions have, of
course, many parts in which riveting must be used.
The elements which are distinctively American in our railway
bridges are the concentration of material in few members and
the use of eye-bars and pin connections in place of riveted
connections. The riveted methods are, however, largely used in
connection with the American forms of truss construction.
[Illustration: Truss Bridge of the Northern Pacific Railway over
the Missouri River at Bismarck, Dak.--Testing the central span.]
An excellent example of an American railway truss bridge is shown
on the opposite page. This structure spans the Missouri River at
its crossing by the Northern Pacific Railroad. It has three through
spans of 400 feet each and two deck spans of 113 feet each. The
bottom chords of the long spans are 50 feet above high water,
which at this place is 1,636 feet above the level of the sea. The
foundations of the masonry piers were pneumatic caissons. The
trusses of the through spans, 400 feet long, are 50 feet deep and
22 feet between centres. They are divided into 16 panels of 25 feet
each. The truss is of the double system Whipple type, with inclined
end posts. The bridge is proportioned to carry a train weighing
2,000 pounds per lineal foot, preceded by two locomotives weighing
150,000 pounds in a length of 50 feet. The pins connecting the
members of the main truss are 5 inches in diameter.
This bridge is a characteristic illustration of the latest type
of American methods. The extreme simplicity of its lines of
construction, the direct transfer of the strains arising from
loads, through the members, to and from the points where those
strains are concentrated in the pin connections at the ends of each
member, are apparent even to the untechnical eye. The apparent
lightness of construction arising from the concentration of the
material in so small a number of members, and the necessarily great
height of the truss, give a grace and elegance to the structure,
and suggest bold and fine development of the theories of mechanics.
[Illustration: Curved Viaduct, Georgetown, Col.; the Union Pacific
crossing its own Line.]
An interesting viaduct is shown in the above illustration, where
the railway crosses its own line on a curved truss.
The truss bridges which have been mentioned as types of the modern
railway bridge are erected by the use of false works of timber,
placed generally upon piling or other suitable foundation, between
the piers or abutments, and made of sufficient strength to carry
each span of the permanent structure until it is completed and all
its parts connected, or, as is technically said, until the span
is swung. Then the false works are removed and the span is left
without intermediate support. But there are places where it would
be impossible or exceedingly expensive to erect any false works. A
structure over a valley of great depth, or over a river with very
rapid current, are instances of such a situation.
A suspension bridge would solve the problem, but in many cases not
satisfactorily. The method adopted by Colonel C. Shaler Smith at
the Kentucky River Bridge [p. 55] shows ingenuity and boldness
worthy of special remark. The Cincinnati Southern Railroad had
here to cross a cañon 1,200 feet wide and 275 feet deep. The
river is subject to freshets every two months, with a range of 55
feet and a known rise of 40 feet in a single night. Twenty years
before, the towers for a suspension bridge had been erected at
this point. The design adopted for the railroad bridge was based
upon the cantilever principle. The structure has three spans of
375 feet each, carrying a railway track at a height of 276 feet
above the bed of the river. At the time of its construction this
was the highest railway bridge in the world, and it is still the
highest structure of the kind with spans of over 60 feet in length.
The bridge is supported by the bluffs at its ends and by two
intermediate iron piers resting upon bases of stone masonry. Each
iron pier is 177 feet high, and consists of four legs, having a
base of 71½ × 28 feet, and terminating at its top in a turned pin
12 inches in diameter under each of the two trusses. Each iron pier
is a structure complete in itself, with provision for expansion and
contraction in each direction through double roller beds interposed
between it and the masonry, and is braced to withstand a gale of
wind that would blow a loaded freight-train bodily from the bridge.
The trusses were commenced by anchoring them back to the old
towers, and were then built out as cantilevers from each bluff to
a distance of one-half the length of the side spans, and at this
point rested upon temporary wooden supports. Thence they were again
extended as cantilevers until the side spans were completed and
rested upon the iron piers. This cantilever principle is simply the
balancing of a portion of the structure on one side of a support by
the portion on the opposite side of the same support. Similarly the
halves of the middle span were built out from the piers, meeting
with exactness in mid-air. The temporary support used first at
the centre of one side span and then at the other, was the only
scaffolding used in erecting the structure, none whatever being
used for the middle span.
When the junction was made at the centre of the middle span, the
trusses were continuous from bluff to bluff, and, had they been
left in this condition, would have been subjected to constantly
varying strains resulting from the rise and fall of the iron piers
due to thermal changes. This liability was obviated by cutting the
bottom chords of the side spans and converting them into sliding
joints at points 75 feet distant from the iron piers. This done,
the bridge consists of a continuous girder 525 feet long, covering
the middle span of 375 feet, and projecting as cantilevers for 75
feet beyond each pier, each cantilever supporting one end of a
300-foot span, which completes the distance to the bluff on each
side.
[Illustration: The Niagara Cantilever Bridge in Progress.]
A most interesting example of cantilever construction is the
railway bridge built several years ago at Niagara, only a few rods
from the suspension bridge and a short distance below the great
falls. It is shown in the illustrations above and on page 91. The
floor of the bridge is 239 feet above the surface of the water,
which at that point has a velocity in the centre of 16½ miles per
hour and forms constant whirlpools and eddies near the shores.
The total length of the structure is 910 feet, and the clear span
over the river between the towers is 470 feet. The shore arms of
the cantilever, that is to say, those portions of the structure
which extend from the top of the bank to the top of the tower built
from the foot of the bank, are firmly anchored at their shore
ends to a pier built upon the solid rock. These shore-arms were
constructed on wooden false works, and serve as balancing weights
to the other or river arms of the lever, which project out over
the stream. These river-arms were built by the addition of metal,
piece by piece, the weight being always more than balanced by
the shore-arms. The separate members of the river-arms were run
out on the top of the completed part and then lowered from the
end by an overhanging travelling derrick, and fastened in place
by men working upon a platform suspended below. This work was
continued, piece by piece, until the river-arm of each cantilever
was complete, and the structure was then finished by connecting
these river-arms by a short truss suspended from them directly
over the centre of the stream. This whole structure was built in
eight months, and is an example both of a bold engineering work
and of the facility with which a pin-connected structure can be
erected. The materials are steel and iron. The prosecution of this
work by men suspended on a platform, hung by ropes from a skeleton
structure projecting, without apparent support, over the rushing
Niagara torrent, was always an interesting and really thrilling
spectacle.
[Illustration: The Niagara Cantilever Bridge Completed.]
The Lachine Bridge recently built over the St. Lawrence near
Montreal, illustrated below, has certain peculiar features. It has
a total length of 3,514 feet. The two channel spans are each 408
feet in length and are through spans. The others are deck spans.
Through spans are those where the train passes between the side
trusses. Deck spans are those where the train passes over the top
of the structure. These two channel spans and the two spans next
them form cantilevers, and the channel spans were built out from
the central pier and from the adjacent flanking spans without the
use of false works in either channel. A novel method of passing
from the deck to the through spans has been used, by curving the
top and bottom chords of the channel spans to connect with the
chords of the flanking spans. The material is steel.
[Illustration: The Lachine Bridge, on the Canadian Pacific Railway,
near Montreal, Canada.]
This structure, light, airy, and graceful, forms a strong contrast
to the dark, heavy tube of the Victoria Bridge just below.
The enormous cantilever Forth Bridge, with its two spans of 1,710
feet each, is in steady progress of construction and will when
completed mark a long step in advance in the science of bridge
construction.
Of entirely different design and principle from all these trusses
are the beautiful steel arches of the St. Louis Bridge [p. 95], the
great work of that remarkable genius, James B. Eads. This structure
spans the Mississippi at St. Louis. Difficult problems were
presented in the study of the design for a permanent bridge at that
point. The river is subject to great changes. The variation between
extreme low and high water has been over 41 feet. The current
runs from 2¾ to 8½ miles per hour. It holds always much matter
in suspension, but the amount so held varies greatly with the
velocity. The very bed of the river is really in constant motion.
Examination by Captain Eads in a diving-bell showed that there was
a moving current of sand at the bottom, of at least three feet in
depth. At low water, the velocity of the stream is small and the
bottom rises. When the velocity increases, a "scour" results and
the river-bed is deepened, sometimes with amazing rapidity. In
winter the river is closed by huge cakes of ice from the north,
which freeze together and form great fields of ice.
It was decided to be necessary that the foundations should
go to rock, and they were so built. The general plan of the
superstructure, with all its details, was elaborated gradually and
carefully, and the result is a real feat of engineering. There are
three steel arches, the centre one having a span of 520 feet and
each side arch a span of 502 feet. Each span has four parallel
arches or ribs, and each arch is composed of two cylindrical steel
tubes, 18 inches in exterior diameter, one acting as the upper
and the other as the lower chord of the arch. The tubes are in
sections, each about twelve feet long, and connected by screw
joints. The thickness of the steel forming the tubes runs from
1-3/16 to 2-1/8 inches. These upper and lower tubes are parallel
and are 12 feet apart, connected by a single system of diagonal
bracing. The double tracks of the railroad run through the bridge
adjacent to the side arches at the elevation of the highest point
of the lower tube. The carriage road and footpaths extend the full
width of the bridge and are carried, by braced vertical posts, at
an elevation of twenty-three feet above the railroad. The clear
headway is 55 feet above ordinary high water. The approaches on
each side are masonry viaducts, and the railway connects with the
City Station by a tunnel nearly a mile in length. The illustration
shows vividly the method of erection of these great tubular ribs.
They were built out from each side of a pier, the weight on one
side acting as a counterpoise for the construction on the other
side of the pier. They were thus gradually and systematically
projected over the river, without support from below, till they met
at the middle of the span, when the last central connecting tube
was put in place by an ingenious mechanical arrangement, and the
arch became self-supporting.
The double arch steel viaduct recently built over the Harlem Valley
in the city of New York [p. 97] has a marked difference from the
St. Louis arches in the method of construction of the ribs. These
are made up of immense voussoirs of plate steel, forming sections
somewhat analogous to the ring stones of a masonry arch. These
sections are built up in the form of great I beams, the top and
bottom of the I being made by a number of parallel steel plates
connected by angle pieces with the upright web, which is a single
piece of steel. The vertical height of the I is 13 feet. The span
of each of these arches is 510 feet. There are six such parallel
ribs in each span, connected with each other by bracing. These
great ribs rest upon steel pins of 18 inches diameter, placed at
the springing of the arch. The arches rise from massive masonry
piers, which extend up to the level of the floor of the bridge.
This floor is supported by vertical posts from the arches and is
a little above the highest point of the rib. It is 152 feet above
the surface of the river--having an elevation fifty feet greater
than the well-known High Bridge, which spans the same valley
within a quarter of a mile. The approaches to these steel arches
on each side are granite viaducts carried over a series of stone
arches. The whole structure forms a notable example of engineering
construction. It was finished within two years from the beginning
of work upon its foundations, the energy of its builders being
worthy of special commendation.
* * * * *
[Illustration: The St. Louis Bridge during Construction.]
[Illustration: The 510-feet Span Steel Arches of the New Harlem
River Bridge, New York, during construction.]
In providing for the rapid transit of passengers in great
cities the two types of construction successfully adopted are
represented by the New York Elevated and the London Underground
railways. The New York Elevated is a continuous metal viaduct,
supported on columns varying in height so as to secure easy grades.
The details of construction differ greatly at various parts of the
elevated lines, those more recently built being able to carry much
heavier trains than the earlier portions. The roads have been very
successful in providing the facilities for transit so absolutely
necessary in New York. The citizens of that city are alive to the
present necessity of adding very soon to those facilities, and it
is now only a question of the best method to be adopted to secure
the largest results in a permanent manner.
The London Underground road has also been very successful. Its
construction was a formidable undertaking. Its tunnels are not
only under streets but under heavy buildings. Its daily traffic is
enormous. The difficult question in its management is, as in all
long tunnels, that of ventilation, but modern science will surely
solve that, as it does so many other problems connected with the
active life of man.
[Illustration: London Underground Railway Station.]
* * * * *
Many broad questions of general policy, and innumerable matters of
detail are involved in the development of railway engineering. In
the determination, for instance, of the location, the relations
of cost and construction to future business, the possibilities
of extensions and connections, the best points for settlements
and industrial enterprises, the merits and defects of alternative
routes must be weighed and decided.
Where structures are to be built, the amount and delicacy of detail
requisite in their design and execution can hardly be described.
Final pressures upon foundations must be ascertained and provided
for. Accurate calculations of strains and stresses, involving
the application of difficult processes and mechanical theories,
must be made. The adjustment of every part must be secured with
reference to its future duty. Strength and safety must be assured
and economy not forgotten. Every contingency must, if possible, be
anticipated, while the emergencies which arise during every great
construction demand constant watchfulness and prompt and accurate
decision.
The financial success of the largest enterprises rests upon
such practical application of theory and experience. Even more
weighty still is the fact that the safety of thousands of human
lives depends daily upon the permanency and stability of railway
structures. Such are some of the deep responsibilities which are
involved in the active work of the Civil Engineer.
FOOTNOTE:
[8] Reference is made to the substitution of locks in the Panama
Canal for the original project of a canal at the sea-level.
AMERICAN LOCOMOTIVES AND CARS.
BY M. N. FORNEY.
The Baltimore and Ohio Railroad in 1830--Evolution of the Car
from the Conestoga Wagon--Horatio Allen's Trial Trip--The
First Locomotive used in the United States--Peter Cooper's
Race with a Gray Horse--The "De Witt Clinton," "Planet," and
other Early Types of Locomotives--Equalizing Levers--How Steam
is Made and Controlled--The Boiler, Cylinder, Injector, and
Valve Gear--Regulation of the Capacity of a Locomotive to
Draw--Increase in the Number of Driving Wheels--Modern Types of
Locomotives--Variation in the Rate of Speed--The Appliances by
which an Engine is Governed--Round-houses and Shops--Development
of American Cars--An Illustration from Peter Parley--The Survival
of Stage Coach Bodies--Adoption of the Rectangular Shape--The
Origin of Eight-wheeled Cars--Improvement in Car Coupling--A
Uniform Type Recommended--The Making of Wheels--Relative Merits
of Cast and Wrought Iron, and Steel--The Allen Paper Wheel--Types
of Cars, with Size, Weight, and Price--The Car-Builder's
Dictionary--Statistical.
Among the readers of this volume there will be some who have
reached the summit of the "divide" which separates the spring
and summer of life from its autumn and winter, and whose first
information about railroads was received from Peter Parley's "First
Book of History," which was used as a schoolbook forty or fifty
years ago. In his chapter on Maryland, he says:
But the most curious thing at Baltimore is the railroad. I must
tell you that there is a great trade between Baltimore and the
States west of the Alleghany Mountains. The western people buy a
great many goods at Baltimore, and send in return a great deal of
western produce. There is, therefore, a vast deal of travelling
back and forth, and hundreds of teams are constantly occupied in
transporting goods and produce to and from market.[9]
Now, in order to carry on all this business more easily, the
people are building what is called a railroad. This consists of
iron bars laid along the ground, and made fast, so that carriages
with small wheels may run along upon them with facility. In this
way, one horse will be able to draw as much as ten horses on a
common road. A part of this railroad is already done, and if you
choose to take a ride upon it, you can do so. You will mount a
car something like a stage, and then you will be drawn along by
two horses, at the rate of twelve miles an hour.
[Illustration: Fig. 1.--Conestoga Wagon and Team. (From a recent
photograph.)]
The picture reproduced below (Fig. 2) of a car drawn by horses
was given with the above description of the Baltimore & Ohio
Railroad. The mutilated copy of the book from which the engraving
and extract were copied does not give the date when it was written
or published. It was probably some time between the years 1830 and
1835. That the car shown in the engraving was evolved from the
Conestoga wagon is obvious from the illustrations.
[Illustration: Fig. 2.--Baltimore & Ohio Railroad, 1830-35.]
This engraving and description, made for children, more than
fifty years ago, will give some idea of the state of the art of
railroading at that time; and it is a remarkable fact that the
present wonderful development and the improvements in railroads and
their equipments in this country have been made during the lives of
persons still living.
[Illustration: Fig. 3.--Boston & Worcester Railroad, 1835.]