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GODFREY LOWELL CABOT SCIENCE LIBRARY
of the Harvard College Library
This book is
FRAGILE
and circulates only with permission.
Please handle with care
and consult a staff member
before photocopying.
Thanks for your help in preserving
Harvard's library collections.
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I
1871
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and
REPORT
US A
SUSPENSION BRIDGE
ACROSS THE POTOMAC,
FOR
RAIL ROAD AND COMMON TRAVEL:
ADDRESSED TO THE
MAYOR AND CITY COUNCIL OF GEORGETOWN, D.C.
BY
CHARLES ELLET, Ju.
CIVIL ANCINEER.
(SROOND EDITION.)
#@0
PHILADELPHIA:
JOHN a CLARK, PRINTER, 68 DOOK STREET.
1854.
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Eng 743.54
1866, June 29,
6. W. Hassler
Gift of
Paymaster u.s.r.
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To the Honourable Senate and House of Representatives of the
United States.
The Memorial of the undersigned, Members of a Committee ap-
pointed by the Corporation of Georgetown, to attend to its interests be-
fore Congress, respectfully represents that the people of said town, as
well as of its neighbourhood, still experience serious inconvenience and
injury from the loss of the Suspension Bridge at the Little Falls of the
Potomac. Public opinion concurred to such an extent in favour of the
erection of a Bridge at the point known as the 'Three Sisters," that
the Corporation, during the late recess of Congress, determined to en-
gage CHARLES ELLET, Jr., Esq., to survey that site, not only to ascer-
tain its general suitableness for a Bridge, but, also, the practicability of
connecting roads with it from the North and South, both for National
and local accommodation, at a moderate cost.
After making the survey, Mr. Ellet's opinion was so confirmatory of
the certainty of these results, that the Corporation engaged him to pre-
pare a general outline of the whole improvement, and, also, a neat plan
of the proposed Bridge, and to have it engraved with a descriptive prin-
ted report and an estimate of its cost, attached. This work has been
completed, and is herewith presented to your Honourable Body for
such consideration as you may be pleased to bestow upon it.
Having the fullest confidence in the correctness of Mr. Ellet's
opinion, and that a Bridge can no where else be erected so as to secure
the great objects of promoting the general welfare and public conveni-
ence, without inflicting loss or injury upon any party however remotely
interested in the improvement, we most earnestly and respectfully pray
that Congress may adopt the same, and make an appropriation for its
seasonable erection.
In conclusion, your Memorialists also beg leave, most respectfully, to
renew the expression of the opposition of the People and Authorities of
Georgetown to the erection of any more bridges below their harbour, as
well as to any plans or schemes for other purposes that may invade the
rights to which they are justly and naturally entitled.
H. ADDISON,
A. H. PICKRELL,
WM. H. TENNEY,
H. C. MATTHEWS,
WILLIAM S. NICHOLS, MARIMIS WILLETT,
F. W. RISQUE,
ROBT. OULD,
A. H. DODGE,
JUDSON MITCHELL,
B. J. SEMMES.
GEORGETOWN, D. C., December 22d, 1852.
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REPORT.
To the Mayor and City Councils of Georgetown, D. C.
GENTLEMEN,-I propose to recommend a WIRE
SUSPENSION BRIDGE as the most appropriate plan that
can be adopted for a bridge across the Potomac, whe-
ther it be intended for the accommodation of common
travel or of rail roads.
I am aware of the prejudice which has existed, and
may still exist, against the application of the suspension
bridge for rail road purposes; but as it is only a preju-
dice, I feel sure that it will give way before reflection
and sound argument. It is but a very short time since
a greater prejudice prevailed against the adoption of sus-
pension bridges for common travel; and fifteen years
ago I met with no little difficulty in convincing the
public of their practicability and safety even for that
purpose. It is, indeed, but thirty years since a great
effort was required to satisfy the British Parliament of
the practicability and safety of a common road bridge
of less than 600 feet span, though it was proposed by
the most famous engineer of his day; and nearly all the
practical science and skill of the country was con-
vened to testify for or against the plan.
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But truth and common sense prevailed; for, in the
long run, Truth always prevails. I hesitate not, there-
fore, to propose a rail road suspension bridge for cross-
ing the Potomac. Its practicability and security are
demonstrable; and I regard it as not only the most ap-
propriate, but by far the most beautiful and imposing
structure that can be placed on the site proposed. I
shall not hesitate on account of prejudice. Prejudices
must yield before proof, conviction and expediency.
OF THE SITE OF THE BRIDGE.
The position selected for the construction of this
bridge is a point about half a mile above the George-
town aqueduct. The river is here, at common high
water, 1030 feet wide on the surface. The bed is
generally of rock, and the depth very variable.
At a point about 450 feet from the water's edge, on
the Maryland side, several masses of granite protrude
above the surface of the river in all stages of the water.
These masses, called THE SISTERS, are surrounded by
a sand bar, of which a portion is bare at low tide.
The high water depth of the river between these pro-
truding rocks and the Maryland shore, is about 25 feet;
but between the rocks and the Virginia shore, it in-
creases to 85 feet. This great depth is found at the
distance of 150 feet and 200 feet from the shore, where
the bottom is of rock and extremely rough and irre-
gular.
The great depth of the river, and the want of uni-
formity in the shape of the bottom, would add largely
to the cost of any ordinary stone or iron bridge, re-
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quiring the support of piers. But the excessive depth
offers no additional impediment to the construction of
a suspension bridge.
On the Maryland shore is the Chesapeake and Ohio
Canal, of which the surface is 75 feet wide, and ele-
vated 34 feet above common high tides.
By the side of the canal is a public road, raised a few
feet above the water, and separated from it by a para-
pet wall.
The use both of the canal and road must be protected,
in any plan which may be adopted for the bridge.
On the southern shore, the bridge stands on a base of
uncovered granite; and on both shores the cables will
be anchored in the solid rock.
The site of the bridge has been chosen with a view
to avoid any obstruction to the navigation of the Po-
tomac, and to relieve Georgetown of the impediments
which it is alleged have been formed in the channel of
the river since the construction of the dilapidated Long
Bridge, and the contraction of the water-way; and, at
the same time, to afford a convenient approach for the
rail road trains and common travel crossing the river
on the great northern and southern line.
The chief objection that has been urged against this
location, is the fact that any rail road which may be
projected to unite the road leading from Baltimore to
Washington, with those leading from Alexandria to the
south and south-west, must be carried out of the direct
line in order to cross at this point. The distance is sup-
posed to be about three miles greater than it might be
if the new bridge were placed on the site of the ob-
noxious Long Bridge. I am of opinion, however, judg-
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6
ing from the map of Washington and my personal
knowledge of the localities, that the actual increase
will be less than three miles. Yet, in the absence of
precise information, I shall assume three miles as the
true measure of its value.
This increase of distance might appear to be a sub-
stantial objection to the site proposed. Yet it is, in
my judgment, perfectly clear, that if the object were to
locate the connecting rail road with a view to the least
possible loss of time to the through traveller, this is
precisely the point at which the engineer would wish
to cross the Potomac. A bridge on this site, as already
stated, can be conveniently approached by rail road
lines from both directions. There is no dense popula-
tion on the route of the connecting road to retard the
trains, or to prevent the highest desirable speed.
There is here no long bridge to be traversed at a slow
rate, nor draws to interfere with the passage of trains,
nor streets nor crowds to be provided for. The
through trains may therefore maintain their full head-
way.
The line of the road, crossing at the proposed site
of the bridge, will pass immediately on the western bor-
der of Georgetown; it will then sweep round to the
north, skirting the town in that quarter, and affording
every convenience that can possibly be derived from a
rail road without carrying it actually through the place.
It will cross over Rock creek near the foot of the new
cemetery, and traverse the whole length of Washington,
through the northern wards, from Rock creek to the
Baltimore Rail Road-keeping near enough for the ac-
commodation of the city, yet far enough north of the pre-
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7
sent built up district, to relieve the public of the danger
and nuisance of a through rail road within the city.*
The ordinary time on a good rail road cannot now
be assumed at less than a mile in two minutes, or 30
miles an hour. Much higher speed than this is made
on many English roads, and if less than 30 miles an
hour is now suffered on the main lines of this country,
it will not be long tolerated, after connections are
formed between the finished roads east, and those west
of the Alleghany mountains, and the great flood of
western travel begins to be vented.
To traverse the three miles assumed to be lost by
crossing the Potomac above Georgetown, and passing
north of Washington, will involve an apparent loss of
six minutes in the through time;-in the supposition
that no corresponding loss will be incurred by crossing
at or near the site of the abandoned Long Bridge.
But there will be material losses on any line; and these
losses must be properly noticed in any just compa-
rison.
If we cross at the site of the Long Bridge we shall
meet with the following detentions, which are avoided
on the line proposed.
1. The loss of time required to cross a bridge one
mile longer than the proposed Georgetown Bridge, at a
* I do not wish to be understood to express any opinion adverse to rail roads
in cities along great thoroughfares, such as Broadway, in New York, or Penn-
sylvania Avenue, in Washington, where the roads are properly devised for the
local accommodation of the city. On the contrary, I am of opinion that a
branch from the main line might be carried along Pennsylvania Avenue, under
proper municipal control, with great advantage to the public, and without im-
pediment to the through travel.
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speed of, say eight miles an hour: this loss will be 51
minutes.
2. The loss of time in passing one mile through the
built up part of Washington, at 8 miles an hour; which
would also be 51 minutes.
3. The loss to be encountered in case the draw
should chance to be raised for the passage of a vessel
on the approach of a train; which loss would vary with
circumstances.
It is easy to perceive, without going into a minute
investigation, that the through trips can be made
quicker, on a rail road which crosses the Potomac
where it is narrow, and above the navigation, and which
passes north of Washington, than on any line whatever
leading through the heart of the city.
It is not my purpose, I repeat, to take general
ground against local rail roads in cities, when properly
designed for the accommodation of the cities. Yet I
am clearly of opinion, that, at an early day, the through
trade and travel of this country must be accommodated
by lines which pass around the great cities, and avoid
the obstruction which a dense population offers.
In the case before us, it is my opinion that the best
speed and the best time can be made on a through road
passing west of Georgetown; and, moreover, that the
site selected for the proposed bridge is the only place
where the Potomac can be crossed by a rail road with-
out injury to the navigation, and affording just ground
of complaint, unless the structure be a suspension
bridge of very great height and span and cost.
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9
OF THE PLAN OF THE BRIDGE.
In designing a plan for this bridge, I have concluded
not to make any use of the masses of rock which rise
up from the bed of the Potomac, and form The Sis-
ters. There would be, certainly, a trifling economy in
inserting a pier and dividing the span; but such an ar-
rangement would detract greatly from the beauty and
magnificence of the structure, and form a fixed point to
be exposed to the perpetual assault of the river.
The distance to be bridged, from shore to shore, in
an average stage of the river, is 1000 feet; though at
high water it is 1030 feet; and after careful inquiry
and full consideration, I have decided to span the en-
tire water-way with a single arch.
The length of this arch, from centre to centre of the
supporting towers, is 1000 feet-or ten feet less than
the span of the Wheeling Bridge. It will clear the
entire water-way, with the exception of a few feet of
the beach which is covered by the tides.
The elevation of the flooring is 60 feet above the
high water surface of the river.
On the Maryland shore, a stone arch of 85 feet span
will bring the bridge to the tow-path of the Chesapeake
and Ohio Canal. Another arch of 85 feet, will clear
the Canal and its tow-path. A third arch, of 35 feet,
will clear the public road, and reach the high ground
north of the Canal.
* Since writing this, I have made an estimate of the cost of a rail road
bridge of equal strength and rigidity, in two spans, with a pier. The cost of
such a bridge will be $240,000.
B
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On the opposite, or Virginia shore, a stone arch of
85 feet brings the bridge to the rocky slope of the hill.
These stone arches are all elliptical, and rise 15 feet
above the skew-backs.
The breadth of the flooring will be 32 feet in the
clear, between the parapets of the foot-ways.
Ten feet in the centre of the flooring is appropriated
to a railway track; 81 feet on each side thereof, to car-
riage-ways; and 21 feet outside of the carriage-ways,
to foot-ways.
The foot-ways are carried around, outside the tow-
ers, and protected by a projecting parapet, as shown in
the annexed engravings.
This bridge, thus arranged, will be supported by 16
wire cables, each of which will be composed of 1350
strands of No. 10 iron wire-or wire which weighs one-
twentieth of a pound per lineal foot.
The diameter of each of the 16 cables will be about
six inches. Their length will be 1400 feet. The de-
flection of the cables, below their respective points of
suspension, will be 65 feet.
On the south side of the river, or Virginia shore, the
road will curve to the east, after leaving the bridge, and
follow the slope of the hill.
On the north side, it will rise with a grade not ex-
ceeding 50 feet per mile, and pass through a depres-
sion in the hill forming the heights of Georgetown, by
a short tunnel, or a cut about 65 feet deep at the sum-
mit.
No difficulty whatever is presented to the continua-
tion of the rail road, from this site, on either side of
the river.
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On the Maryland side, a branch track, half a mile
long, turning off north of the canal road, will bring the
accommodation cars to a point on Bridge Street nearly
opposite the aqueduct and convenient for the local bu-
siness of Georgetown.
OF THE FLOORING.
In the plan of the flooring herewith presented, ar-
rangements are made for a railway track. It may hap-
pen, however, that for want of legislation, the bridge
will be built before the rail road connexion is formed at
Washington. In that case the framed girders which
separate the track from the carriage-ways, and the stone
flagging with which the track is covered, and four of the
cables, may be left out, until the railway needs accom-
modation. The first cost of the bridge would thus be
reduced about $50,000.
In the plan, as prepared and submitted, the railway
is separated from the carriage-ways by heavy timber
girders, 31 feet high. Under the flooring, and corres-
ponding with these upper girders, are two suspend-
ed girders, 18 inches deep. These upper and lower
girders are firmly drawn together by iron bolts passing
through them both, and through the heavy joists of the
flooring. The intention of these girders is at the same
time to give weight to the structure, to protect the
common travel from the trains, and also to distribute
the weight of the trains along the flooring, and break
the vibrations which are experienced in lighter and
looser structures. The vertical girders are sustained
laterally by knees bolted to them and to the heavy
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joists beneath. Between these girders is the railway
track, supported on transverse sills, in the usual mode
-the space not occupied by the sills being covered with
a stone flagging.
The flooring of the carriage-ways is composed of
two courses of 21 inches plank, laid longitudinally with
the bridge. The upper course is of oak. The lower
course of pine.
The foot-ways are raised 8 inches above the car-
riage-ways, and covered with a single course of 21 in-
ches pine plank.
Outside of the foot-ways is a lattice parapet, well
bolted to the timbers below, so as to assist somewhat
in stiffening the structure and in distributing the tran-
sitory loans along the flooring.
The details of the platform are exhibited in the an-
nexed engraving, where the parts are drawn to a con-
venient scale.
The total volume of timber in each lineal foot of the
structure, is 471 cubic feet. Of this volume we shall
have of
White pine, 40 cubic feet, at 35 lbs.
-
-
1400 lbs.
White oak, 7½
"
at 60 lbs.
-
-
450 "
Total weight of timber in the bridge,
1850 lbs.
There will be no difficulty in substituting iron in
place of timber, for the greater part of this platform;
but as the timber in such a framing will last for many
years, and may be easily renewed, I have not deemed
it worth while to incur the additional expense which
the use of iron would involve.
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:- the railway
in usual mode
sered with
connsed of
realty with
The cower
above
'attice persupe
...'
is excipt SO:
at
di- muting
an-
exhi
'.l are
1 con-
1400 lbs.
150 19
1850 lhe
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ELEVATION OF FLOORING.
SECTION OF FLOORING.
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SCALE 5 FEET TO THE INCH.
Engraved by JB Neagle
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13
There will be, in the flooring, in addition to the tim-
ber, a considerable weight of iron bolts and stone flag-
ging. The total weight of the platform, including these
items, will be as follows :-
Weight of Flooring.
Weight of timber, per lineal foot, as above,
-
1850 lbs.
Weight of iron in bolts, rails, and suspenders,
-
140 "
Weight of stone paving,
-
230 "
Total weight of flooring,
2220 lbs.
OF THE TRANSITORY LOADS ON THE BRIDGE.
The transitory loads which will come on the bridge
are not likely to exceed 150 tons at any one time; but
in estimating the possible loads, with a view to deter-
mine the strength of the cables, it is proper, and, in-
deed, due to the public, to assume an extreme case. I
shall therefore make the computation of the needful
strength, and the cost of the bridge, in the hypothesis
that the railway track may be occupied by engines and
trains of cars from one abutment to the other; and
that, at the same time, both the carriage-ways may be
covered from one end of the bridge to the other with
loaded teams. The cables must then be so propor-
tioned that they will bear thrice the weight of the
bridge, and thrice the tension produced by this load
upon it, and still possess a surplus of force.
The weight of this load would be as follows :-
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Maximum Transitory Load.
2 locomotive engines, each 18 tons,
-
-
36 tons.
2 tenders, each 9 tons,
-
-
-
-
18
"
40 freight cars, loaded, each 10 tons,
-
-
400 "
100 carts, loaded, on the carriage-ways,
-
-
90 "
100 horses, say
-
-
-
-
-
-
56 "
Total transitory load,
600 tons.
This load will be distributed over a space of 1000 feet,
making the increment of weight on each unit of length,
or one foot of the platform, 1200 pounds.
This, it will be conceded, is an extreme supposition;
for it is not at all probable that we shall ever see a
bridge across the Potomac occupied at the same time,
from end to end with loaded cars, on the central track,
and with 100 loaded carts and 100 horses, on the side
tracks. But to make this assumption, as the basis of
calculation, is certainly to err on the side of safety
and it is therefore proper, even though it may seem to
border on extravagance.
OF THE CABLES.
We have now all the elements for ascertaining the
proper strength of the cables, or the ability of the ca-
bles proposed, to sustain the weight of the bridge, and
the loads which may come upon it. And it is fortu-
nate that, on this point, there need be nothing left for
conjecture or speculation. The dimensions of the ca-
bles, or the quantity of material of which they must be
composed, may be determined, where the weight of the
bridge and its load are prescribed, with the most per-
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fect certainty, by a calculation as reliable as anything
in the circle of mechanics.
The total weight of the bridge, including that of the
cables, is as follows :-
Total Weight to be supported.
Weight of the flooring, per lineal foot,
-
-
2220 lbs.
Weight of 16 cables, each composed of 1350 strands
of wire weighing 210 lb. per foot,
-
-
-
1080
"
Total permanent load, per lineal foot,
-
-
3300
"
Add transitory loads,
-
"
-
1200
"
Total, or maximum load to be provided for, per
lineal foot of flooring,
-
-
-
-
4500 lbs.
To ascertain the effect which will be produced upon
the cables by this load, we will observe that the great-
est strain is at the points of support, where its value is
equal to the resultant of the horizontal tension, and
the vertical or proper weight of the bridge and load.
To calculate the tension at this point, we must first
deduce the value of the horizontal strain from the ab-
solute weight to be supported.
The vertical weight on each foot of the flooring,
when the bridge is loaded, is, as above, 4500 pounds.
The length of the flooring is 966 feet. The total ver-
tical weight, then, is
4500 X 966 = 4,347,000 pounds,
or 2173 tons-the one-half of which, or 1086.5 tons, is
supported at each bearing point of the cables.
But the vertical weight is to the horizontal tension,
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as twice the sagitta is to half the span of the curve, or
as
2 X 65 : 500.*
The horizontal tension will then be
130 500 X 1086.5 tons = 4179 tons.
The resultant of these two forces will be
(1086.52 + 41792)1/2 = 4318 tons.
Or, the greatest strain that can come upon any part
of the cables, when the bridge is loaded with heavy
teams and rail road cars, as above described, will be
4318 tons.
Iron wire of the size known as No. 10-or of such
a diameter that a strand 20 feet in length will weigh
one pound-if of good quality and free from flaws, will
sustain a weight of about 1400 pounds. If we load
strands of wire capable of bearing 1400 pounds with a
weight not exceeding 400 pounds, we may regard any
work supported by such strands as perfectly secure.
Now, I propose SO to adjust the proportions of this
bridge, that when there is a load of 600 tons on the
flooring, and when the tension in the cables, all told, is
4318 tons, each strand of wire, found by experiment
capable of bearing 1400 lbs., shall not suffer a strain
exceeding 400 pounds.
The total strain is 4318 tons, or 8,636,000 pounds.
The number of strands required to support this strain,
with the degree of security implied by the condition, is
8636000 400 = 21,590 strands.
* For the usual formulas for these calculations, see Note A.
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The bridge will be upheld by 16 cables; and there
will therefore be 1350 strands in each cable.
The total weight of the cables, per lineal foot, will
be, from these data, at 20 of a pound per lineal foot,
21590 20 = 1079 lbs. per foot.
The total weight of wire in the 16 cables, each of
which will be 1400 feet long, will be
1079.5 lbs. X 1400 feet = 1,511,300 lbs.
or 7557 tons.
The absolute strength of all the cables will be
21,590 strands X 1400 lbs.
=
30,226,000 lbs.
The maximum strain is, as above shown, 8,636,000 lbs.
Or, the absolute strength of the cables is to the greatest
absolute strain which can come upon them, as 31 to 1.
Now when it is considered that the weight which is
supposed to be on the bridge, in this calculation, very
far exceeds any load that is ever likely to be collected
on the flooring; and that the cables are so adjusted, in
dimensions and in the mode of suspension, that they
ought, if sound, to be capable of sustaining the weight of
more than three such bridges and three such loads, before
breaking, it will not be doubted that, on the score of
strength, every reasonable precaution is taken, and the
fullest allowance made.
The question might indeed, be raised, as to whether
it is really practicable so to manufacture these ca-
bles, that the strands of wire of which they are com-
posed may each be depended on to fulfil its share of
the duty. On this score there is, however, no practical
C
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difficulty. Every strand does in fact do its duty, in
the cable, as completely as if it acted separately.
Neither can there be any doubt of the soundness of the
material; for every strand of wire used in the cables is
necessarily subjected to a test, in the last drawing to
which it must be subjected in its manufacture, much
exceeding the greatest strain that can come upon it in
the bridge.
The processes of protecting the material against ox-
idation, so as to preserve its original strength, have
been tested by long experience; and there is now no
reason to doubt that the wire cables of a suspension
bridge, properly varnished, and protected externally
by an occasional coat of paint, will be of almost in-
definite durability.
The cables in this bridge will be anchored, on both
sides, in the granite rock. The cost of anchorage will
therefore be very small, and the security of the fasten-
ings undoubted.
OF THE TOWERS.
The towers of this bridge are represented in the an-
nexed engraving. They are each 128 feet high, mea-
sured from the surface of the river, at high water, to
the bearing point of the cables. To the summit of the
parapets, the height is 135 feet. The summits of the
towers are 75 feet above the flooring of the bridge.
Each of the towers is 35 feet thick, and 64 feet
wide, at the level of the water; and 10 feet thick, and
521 feet wide, at the height of the saddles. They are
pierced, at the level of the roadway, with pointed
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SCALE 30 FEET TO THE INCH
PLAN
A
I
NOTIVASTS 3018
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FRONT ELEVATION OF TOWER.
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E SE "TION THROUGH A A H H
SCALE 30FEET TO THE INCH
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arches 27 feet wide and 54 feet high, for the passage of
the railway and carriage tracks. The foot-ways are
carried around, on the outside of the towers, and sup-
ported by a projecting cornice with an embattled para-
pet.
Each tower and abutment contains 4650 cubic yards
of masonry.
The piers of the elliptical arches, which span the
canal and the space between the northern tower and
the canal, are 10 feet thick.
The total amount of masonry in the abutments, tow-
ers, piers, arches and spandrils, is 16,550 cubic yards.
Of this volume 7100 cubic yards are in the abutments,
2200 cubic yards in the towers, and 7250 cubic yards
in the arches, piers, wing-walls, and spandrils.
ESTIMATED COST OF THE BRIDGE.
We have now all the material needed for determining
the cost of this bridge. And in making this estimate,
as in arranging the plan of the structure, I have not
deemed it at all expedient or proper to practice a close
economy. It would seem to be more appropriate in
designing a structure to be erected within sight of the
Capitol, for the conveyance both of the rail road and
common travel across the Potomac, on the great north-
ern and southern thoroughfare, rather to seek to make
the edifice a monument of strength and stability worthy
of the site and its purposes. I shall therefore submit a
liberal estimate and assume that a work of the first
order is to be erected.
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ESTIMATE.
1,520,000 pounds of wire, including the cost of manu-
facturing, raising and adjusting the cables, at 10 cts.
$152,000
40,000 pounds No. 12 wire, in the suspenders, at 12 cts.
4,800
190,000 pounds bar iron in the anchorage, at 7 cts.
-
13,300
31,000 pounds bar iron in bolts, for the flooring, para-
pets, &c., at 7 cts.
-
-
-
-
-
-
2,170
50,000 pounds cast iron, for saddles, rollers, roller plates,
&c., at 4 cts.
-
-
-
-
-
-
-
2,000
47,500 cubic feet of timber in the flooring and parapets,
including framing and raising, at 30 cts.
-
-
14,250
600 cubic yards rock excavation in the fastening cham-
bers, at $7.00,
-
-
-
4,200
2200 cubic yards masonry in the towers, at $10.00
-
22,000
7100 cubic yards masonry in the abutments, at $4.00,
28,400
7250 cubic yards masonry in the arches, piers, and
wing-walls, at $7.00,
:-
-
-
-
-
-
50,750
500 cubic yards masonry in the fastening chambers, or
anchorage, at $8.00,
-
-
-
-
-
4,000
Total cost,
-
-
$297,870
Or, in round numbers, I estimate the entire cost of
the bridge at $300,000.
If the structure were in all respects the same, ex-
cepting only that the railway track were left out, but
preparation made for its introduction; and four of the
cables, which would then be unnecessary, were tempo-
rarily dispensed with, the cost of the bridge would
be reduced to $250,000. No change of plan would be
required to build it in this way. It would merely in-
volve the omission of the stone flagging, the upper and
lower girders, and four of the cables; all of which
might be supplied whenever they would be required for
the use of a connecting link of railway. The anchor-
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age would, however, need to be prepared for these ad-
ditional cables in the progress of the work.
Still, I think it scarcely probable that the construc-
tion of the rail road track can be delayed, beyond the
completion of the bridge. Indeed, if the erection of
this bridge be authorized by Congress, I hesitate not to
say that the railway to connect the northern and south-
ern lines would be promptly undertaken as a private
enterprise. All that now stands in the way, and pre-
vents this connecting link from offering inducements
sufficient to justify parties in undertaking the work,
is the want of a charter for the road, and the need of
this bridge across the Potomac.
OF THE STIFFNESS OF SUSPENSION BRIDGES.
Considering the suspension bridge in its simplest
form, as without any rigidity or stiffness of its own,
and resisting displacement solely by its weight, or vis
inertia, it is easy to calculate the utmost movements
which will be produced by the application of given
forces. A very simple rule, deduced, however, from a
very profound and elaborate investigation, will suffice
for such computations, when we know the weight of a
bridge, the span and deflection of the curve formed
by the cables, and the weight of any extraneous load
brought upon the centre of the flooring. This rule,
when no allowance is made for the proper resistance
of the flooring itself, is as follows :-
Multiply the sagitta, or deflection of the curve of the
cables, in feet, by the weight in tons placed in the cen-
tre of the flooring, and divide the product by twice the
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weight of the bridge, in tons : the result will be the
depression, or bending, which that weight will produce
in the centre of the bridge, expressed in feet-provided
there be no stiffness in the flooring. (See Note B.)
Let us now suppose that a looomotive engine of 15
tons weight were brought on the centre of the flooring
of the proposed bridge. In this case the weight of the
bridge is 1594 tons; and the sagitta, or deflection of
the cables, is 65 feet.
The rule will then give
2 65 X X 1594 15 = 3066 of a foot.
Or, a locomotive engine weighing 15 tons, placed in
the centre of the arch of this bridge, would produce a
depression in the flooring, of three-tenths of a foot, in
the case assumed-viz: that the flooring itself is
without strength or stiffness. Of course, if the framing
offer any resistance, or if the bridge cannot rise at the
haunches of the inverted arch, the depression must be
less. But it would be superfluous to contend that a
depression of three-tenths of a foot, or, indeed, of a foot,
in an elastic bridge of 1000 feet span, is utterly insig-
nificant, and would be entirely harmless.
The Wheeling Bridge, of which the span is 1010
feet, rises or falls frequently 8 or 10 inches, in the
course of a few hours, from mere changes of tempera-
ture. It is, in truth, often depressed eight inches by
the passage of a loaded six-horse team along the floor-
ing; and there are frequently several of these teams
upon it at the same time. Two loaded six-horse teams
meeting on the arch of that bridge, will depress the
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flooring 15 or 16 inches; yet a person crossing at the
same time would be unconscious of any such flexure in
the arch. The movement is, in fact, perfectly harmless.
A depression of 16 inches in a bridge of a thousand feet
span produces the same angular motion, or the same
strain on the material of the flooring, as a depression
of inch in an arch of 100 feet span.
To produce a depression of 16 inches-such as often
occurs in the Wheeling Bridge-in the structure before
us, would require a weight of 66 tons to be concen-
trated in the middle of the arch. But in the practical
use of a bridge, no such load can be concentrated on any
point of its flooring. The rail road train is distributed
over many feet, and the depression is consequently
greatly reduced, both because of this distribution, and
because of the stiffness of the bridge itself.
Yet 16 inches, an amount of bending often expe-
rienced in the Wheeling Bridge, is really of no conse-
quence. It is not much more than sometimes occurs
in the hulls of the Western steamboats, from improper
loading, without causing the seams to open or pro-
ducing leakage.
The proposed bridge over the Potomac will possess
more than four times the resistance to displacement that
is offered by the Wheeling Bridge, for the reason that
it is more than four times as heavy; and it will present a
still greater resistance, for the further reason, that the
cables are drawn taughter; and, finally, for the ad-
ditional reason, that the flooring is made purposely of
great strength, so that the heavy weights which come
upon it, instead of pressing on a given point, may be
distributed to the right and left along the track. Yet
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the Wheeling Bridge, even as it is-built for common
travel-is fully competent to bear the weight of an or-
dinary rail-way passenger train, without injury, or dan-
ger to its security.
It might seem that the vibrations of a suspension
bridge would be materially increased by the speed of the
train. Under certain circumstances this is so. Yet ex-
periment shows that the extent of the depression pro-
duced by a given weight is less when the team which
produces it is moving across the flooring than when
resting quietly upon it. In other words, that if a team
be brought upon the flooring of a bridge, and the de-
pression thereby produced be carefully measured, while
the team is at rest; and then measured again when the
same team is moved briskly across the flooring, the de-
pression will be perceptibly less in the second than in
the first instance.
I have made no particular computation, in this re-
port, of the effect of the winds in causing vibration;
because we have abundant experience to show that the
bridge will be safe against the strongest hurricanes.
A suspension bridge resists the action of the wind by
its own weight; and we know that bridges of one-
fifth the weight of this-such as the Freibourg
Bridge-have resisted the heaviest blows and some-
times tornadoes. * The most violent hurricane would
assail this bridge without danger to its safety.
*
The cables are inclined towards the axis of the bridge, so as to serve also
as lateral stays. But still it is the weight of the structure that makes this late-
ral support effective.
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OF THE EFFECTS OF VARIATIONS OF THE TEMPERATURE.
Suspension bridges, like all other bridges, and struc-
tures supported by iron, are subject to perpetual eleva-
tions or depressions consequent on changes of tempe-
rature. As the temperature of the air rises, the cables
undergo a corresponding dilatation, and the flooring is
consequently depressed. The value of this depression
may be very closely calculated by the following simple
rule :-
Divide three-eighths of the span of the cables by the
sagitta of the curve, and multiply the quotient by the
dilatation of half the length of the cables. The result
will be the depression in feet. (See Note C.)
For an application of this rule to the present plan-
where the span is 1000 feet, the sagitta 65 feet, and
half the length of the cables 700 feet-we must first
ascertain the dilatation of the cables due to a change
of one degree of temperature.
The average result of experiments on the dilatation
of iron is 150000 for each degree of Fahrenheit. The
dilatation of half the length of the cables, or 700 feet,
will therefore be
150000 = 1500 of a foot
for each degree.
Consequently, by applying the preceding rule, we
shall have
3 x 1000 8 x 65 X = 260 of a foot,
for the sinking of the platform of the bridge, conse-
quent on each degree of elevation of the temperature.
D
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The extreme variation at Georgetown occasionally ap-
proaches 100° of Fahrenheit: and the extreme change
in the height of the flooring might, consequently,
amount to
70 = 2½ feet;
or the bridge might possibly be two feet and seven-
tenths higher in the extreme cold weather of winter,
than in midsummer.
This might seem to be a dangerous movement. But
no practical inconvenience whatever is found to result
from it. The bridge rises and falls, but no eye disco-
vers the movement of two or three feet distributed
along a platform 1000 feet in length. The same move-
ments take place also in the rigid frames of cast iron
bridges, some of which must rise and fall not less than
six or eight inches in the variations of the year. The
iron houses in which we live undergo similar changes.
They are taller at midday than at midnight, and taller
in the summer than in the winter. Iron ships are
equally liable to be warped. The hull of an iron
steamer at anchor, struck on its broadside by the
morning sun, must turn its bow and stern to the west;
and in the evening, when the sun shines on the oppo-
site side, the boat is necessarily warped in the opposite
direction. The elasticity of the material compensates
for these movements, and no injury results.
OF THE FITNESS OF SUSPENSION BRIDGES FOR RAIL ROAD
PURPOSES.
There is still an undefined prejudice against the ap-
plication of suspension bridges to rail road purposes;
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yet it is doubtful whether any engineer will under-
take now to deny the practicability of constructing
a safe and sufficient bridge for the passage of locomo-
tives and their trains, upon this plan. Indeed, the
denial itself would involve an absurdity apparent, on
reflection, to the intelligence of every practical mind.
We know that a single strand of wire stretched be-
tween two fixed points, at a given distance, and in a
given manner, will bear safely a given weight. We
know that two strands, stretched in the same manner,
will bear twice that weight with equal safety; and that
a million of strands, under like circumstances, will bear
a million times that same weight. If the single strand
will only bear with safety a hundred pounds, a million
strands will bear with equal safety, a million times 100
pounds, or fifty thousand tons. There is nothing
therefore to limit the capacity of the bridge to sustain,
but the limit which we set to the amount of material
to be used. We can make a suspension bridge of
wire with equal certainty whether it be intended to
bear the weight of a horse and carriage, or to bear the
weight of a hundred locomotives. The problem is pre-
cisely the same; its solution, in either case, equally
certain. The principle is fixed, the amount of material
and cost are the only variable elements.
But, while it is admitted on all hands that such
bridges may be made strong enough to bear any given
weight, there is yet often a lurking suspicion that they
may not be steady enough for the use of heavy engines
and their trains. But this is, in fact, precisely the
same error as the other, and equally clear to demon-
stration that it is SO.
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If a single strand of wire be stretched between two
fixed points, it matters not what may be the dimensions
of that strand, it is evident that a weight may be
chosen SO small that when drawn or rolled along it, no
appreciable, or at least no hurtful vibration, will be pro-
duced thereby.
If then, a single strand is unmoved, or not hurtfully
moved, by a given weight, ever so small, it is again
clear that a million of such strands will bear a million
of such weights, with equal steadiness. There is ob-
viously no limit to the stability or inertia that may be
given to a suspension bridge-for the very chains
which support it may be made heavy enough to be prac-
tically proof against oscillations in a vertical direction.
It is, in fact, the weight of a suspension bridge that
gives it stability. The heavier it is, the more difficult
it is to displace.
A light bridge may be shaken by the shock of light
bodies; while a heavy bridge, under like circum-
stances, can only be equally moved by bodies heavier
in proportion as its own weight is heavier.
This, it is true, is mere general reasoning leading to
results that are apparent to every one. But I treat the
subject in this mode for the reader who will not con-
sent to the more precise deductions of mechanics.
In short, to render a suspension bridge steady, we
must properly proportion, not only the strength, but
the weight of the bridge, to the weight of the moving
burthens which it is to support. If we adopt the pro-
portions which experience shows to be barely sufficient
for a foot bridge, in a structure intended to bear the
weight of farm wagons, the bridge will certainly
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shake, and appear to be dangerous. And, on the other
hand, if we adopt the proportions which would be safe
and sufficient for farm wagons, in a bridge designed
to sustain droves of fatted cattle, or such loaded six-
horse teams as are used on the National road, it will
certainly be unsteady and may seem to be unsafe. And
so, if we adopt proportions fitted only for six-horse
teams or droves of heavy oxen, in a bridge intended
to support locomotives and heavy trains of freight cars,
it also will oscillate and bend unduly beneath its bur-
then. These facts, though they ought to be perfectly
apparent, have not been considered by the opponents
of railway suspension bridges; who draw their hasty
conclusions from experience obtained upon light bridges
suitable for common travel, and misapplied to rail road
purposes.
The suspension bridge across the Tyne, on the
Stockton and Darlington Rail Road, was a signal fail-
ure, because it was a common road bridge, of a very light
pattern, subjected to the heavy usage of a coal railway.
I have seen that bridge, and speak with personal know-
ledge on this point. It should never have been applied
to any such duty; or rather, it should have been made
five or six times as heavy as it was.
The same inexperience, however, has led to the fail-
ure not only of a single suspension bridge, but of
nearly all the first rail road bridges, of every descrip-
tion, on the great thoroughfares of this country.
The fact that all-I believe all-the first wooden
bridges failed on the Reading road, and on the Balti-
more and Ohio and other early lines, as the weight of the
locomotive engines was increased, is notorious. But it
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was not concluded from that fact that wooden arches, or
wooden trusses, were inadequate to rail road service.
It was merely concluded that heavy locomotives and
freight cars required heavier and stronger framings for
their support than were needed to bear the weight of
light engines or common teams. These early bridges
were therefore reconstructed-often on the same plan
-but they were made stronger.
In all works of art we must proportion the structure
to the duties which it has to perform.
A foot bridge erected by the writer, for a temporary
purpose, 800 feet long, and only three feet wide, and
suspended more than 200 feet in the air, quivered un-
der the weight of a man, and oscillated greatly under
the movement of a crowd. But that bridge weighed
only twenty-five pounds for each lineal foot of the
flooring. Yet the Wheeling Bridge, of more than a
thousand feet span, is steady enough under trains of
heavy wagons, and does not vibrate injuriously beneath
droves of the heaviest bullocks. The reason is, sim-
ply, that the Wheeling Bridge, instead of weighing only
25 pounds per lineal foot, weighs more than 800 pounds
per foot; and is, consequently for that reason alone,
more than thirty times steadier, or capable of bearing
loads more than thirty times as great as the foot bridge
at Niagara, with no greater vibration. But the weight
of the bridge now proposed will be four times as great
as that of the Wheeling Bridge; its cables will be four
times as strong, and its flooring will be framed with a
view to the distribution of the pressure of the trains.
The day seems to be nearly gone when a practical
or scientific truth, susceptible of direct demonstration,
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can be resisted on the authority of a popular prejudice.
That which can be demonstrated will now be believed.
In the earlier periods of practical science, it was
enough to doubt, in order to condemn, a new proposi-
tion. Information was spread slowly, and confidence
was won by a painful struggle. Long after the loco-
motive was invented, its power to propel itself for-
ward was doubted, because it was supposed that the
wheels would slip on the rails. To guard against this
imaginary difficulty the engine was provided with
claws, and other rude appendages, equally unnecessary
and awkward, to enable it to push or drag itself along.
But even then it was perfectly demonstrable, without
a single item of additional information, that no such
difficulty existed.
The difficulty which Brindley encountered when, 90
years ago, he proposed to construct a stone aqueduct
across the Irwell, at an elevation of 39 feet above the
stream, is matter of history. The project was consi-
dered wild and extravagant." Consultation was sug-
gested. A gentleman of official eminence was accord-
dingly called, who, says the historian of the work, being
conducted to the place where it was intended that the
aqueduct should be made, ridiculed the project, and re-
marked contemptuously, 'that he had often heard of
castles in the air, but never was shown before where
any of them were to be erected." But the aqueduct
was built for all that; and there are engineers enough
now, both in Europe and America, who would take
such an aqueduct as that on a carriage, and launch it
across the Potomac.
The power to navigate the ocean by steam, was de-
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nied for years after the world was in possession of all
the art and means for its accomplishment, in the un-
founded assumption that the vessel could not carry
fuel enough for the voyage. Nay, we are told, and it
seems to be matter of history, that for 30 years after
iron rails for rail roads were first attempted, they were
laid aside, and their fitness denied, on the ground that
they would not bear the weight of the heavy coal cars
in use. Thirty years, it seems, passed by before it
was perceived that by making the cars lighter or the
rails stronger, the application might be made.
But in this age it would be strange indeed, if after
we know that suspension bridges have been made
strong enough to bear footmen; and increased from
that limit until they were strong enough to support
teams; then trains of artillery, and droves of cattle,
and columns of troops-it would be strange indeed,
with this experience, if we could not comprehend
that by making the same sort of bridges heavier and
stronger, we could make them bear safely the weight
of a locomotive, or if need were, of a ship of the line.
Certainly no such doubt will be felt. Twenty years
ago the first question asked of every new project-
canal or rail road-was, Is it practicable? But who
asks now whether a rail road is practicable anywhere?
The writer of this Report is at this time construct-
ing a rail road across the Blue Ridge, in Virginia, to
be traversed by locomotives, on which the grades are
295 feet per mile; yet no one will question its suc-
cess. *
*
February, 1854. This work will be in use in the course of a few weeks.
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I have no fear that the practicability of this bridge
will be doubted. Twenty years ago, when I proposed
a suspension bridge across the Potomac, for common
travel, in place of that long causeway called " a bridge,"
from which you have so long suffered, and of which
you have so often and so loudly complained, the prac-
ticability of the plan was a subject of doubt. * But
those doubts have been set at rest by the Fairmount,
the Niagara, and the Wheeling bridges, as they ought
to have been by numerous works then existing in Eu-
rope. To prove that these bridges might be made
heavier and stronger, and hence capable of bearing
greater weights, is a difficulty of a secondary order.
I have no fear of the result of the investigation.
The tribunal before which you appear is just that from
which you may look for an enlightened decision. An
American Congress will not deny the practicability of
a work demonstrably safe.
If the appropriation is refused, it will be on other
grounds.
Your obd't servant,
CHARLES ELLET, JR.
Civil Engineer.
Philadelphia, December 15, 1852.
*
For this plan, see Congressional Documents for 1832.
E
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NOTES.
NOTE A.
The formula for calculating the tension in the cables at the points of
suspension, is
Where T is the tension;
h, half the distance between the points of suspension in feet;
f, the sagitta of the curve in feet; and
p, the weight of each lineal foot of the suspended portion of the
bridge; or bridge and load.
The horizontal tension is
Q-2ha
NOTE B.
The formula for calculating approximately the depression of the
flooring which will be produced by a given weight placed in the cen-
tre, is
in which f' is the depression in feet, To the weight producing it, and f,
p, and h, as above.
NOTE C.
The formula for calculating the depression of the flooring produced
by any elongation of the cables, consequent on additional tension, or
changes of temperature, is
in which c' is the elongation of half the length of the cables.
The example in the text must be regarded as an extreme case.
Two feet is probably the greatest range that will ever be experienced.
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NOTE D.
The actual length of the Long Bridge is within a fraction of one
mile, or about four-fifths of a mile greater than that of the proposed
Georgetown Bridge. If the flooring were elevated, as it ought to be,
to protect the navigation adequately, its length would be somewhat in-
creased: and I have therefore assumed one mile for the difference of
the lengths of the two bridges in estimating the loss of time.
If the calculation on page 7 had been made for a speed of eight
miles an hour, and a difference in the lengths of the bridges of four-
fifths of a mile, that loss would have been 42 minutes.
For a speed of six miles an hour, and a difference of four-fifths of a
mile, it would have been 62 minutes.
By crossing the Potomac at the Observatory hill, there would be a
small saving of distance. But this suggestion, if we have respect for
the navigation, also involves the necessity of a very long, high, and
costly bridge; besides the loss of time due to the diminished speed in
traversing the City of Washington more than two miles, where, for a
considerable portion of the distance, it is already densely built up.
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THE BORROWER WILL BE CHARGED
THE COST OF OVERDUE NOTIFICATION
IF THIS BOOK IS NOT RETURNED TO
THE LIBRARY ON OR BEFORE THE LAST
DATE STAMPED BELOW.
BOOK DUE: WID
JUN 30 1980
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Eng 748.54
Report on a suspension bridge acros
Cabot Science
002666316
3 2044 091 915 116