BRIDGE DISASTERS IN AMERICA
_The Cause and the Remedy_
BY
GEORGE L. VOSE
AUTHOR OF "MANUAL FOR RAILROAD ENGINEERS AND ENGINEERING
STUDENTS," "LIFE AND WORKS OF GEORGE W. WHISTLER,
CIVIL ENGINEER," ETC.
_"ETERNAL VIGILANCE IS THE PRICE OF LIBERTY"_
BOSTON
LEE AND SHEPARD PUBLISHERS
10 MILK STREET
NEXT OLD SOUTH MEETING HOUSE
1887
NOTE.
The substance of the following pages appeared originally in "The
Railroad Gazette." It was afterwards reproduced in pamphlet form, and
has since been several times delivered as an address to various
bodies, the last occasion being before the Legislature of
Massachusetts, 1887. It is now re-published, with some new matter
added, in the hope that the public attention may be called to a
subject which has so important a bearing upon the public safety.
COPYRIGHT, 1887,
BY LEE AND SHEPARD.
_All rights reserved._
BRIDGE DISASTERS IN AMERICA.
Nearly all of the disasters which occur from the breaking down of
bridges are caused by defects which would be easily detected by an
efficient system of inspection. Not less than forty bridges fall in
the United States every year. No system of public inspection or
control at present existing has been able to detect in advance the
defects in these structures, or to prevent the disasters. After a
defective bridge falls, it is in nearly every case easy to see why it
did so. It would be just about as easy, in most cases, to tell in
advance that such a structure would fall if it ever happened to be
heavily loaded. Hundreds of bridges are to-day standing in this
country simply because they never happen to have received the load
which is at any time liable to come upon them.
A few years ago an iron highway bridge at Dixon, Ill., fell, while a
crowd was upon it, and killed sixty persons. The briefest inspection
of that bridge by any competent engineer would have been sure to
condemn it. A few years later the Ashtabula bridge upon the Lake
Shore Railroad broke down under an express train, and killed over
eighty passengers. The report of the committee of the Ohio
Legislature appointed to investigate that disaster concluded, first,
that the bridge went down under an ordinary load by reason of
defects in its original construction; and, secondly, that the defects
in the original construction of the bridge could have been discovered
at any time after its erection by careful examination. Hardly had the
public recovered from the shock of this terrible disaster when the
Tariffville calamity added its list of dead and wounded to the long
roll already charged to the ignorance and recklessness which
characterize so much of the management of the public works in this
country.
There are many bridges now in use upon our railroads in no way better
than those at Ashtabula and Tariffville, and which await only the
right combination of circumstances to tumble down. There are, by the
laws of chance, just so many persons who are going to be killed on
those bridges. There are hundreds of highway bridges now in daily
use which are in no way safer than the bridge at Dixon was, and which
would certainly be condemned by five minutes of competent and honest
inspection. More than that, many of them have already been condemned
as unfit for public use, but yet are allowed to remain, and invite
the disaster which is sure to come. Can nothing be done to prevent
this reckless and wicked waste of human life? Can we not have some
system of public control of public works which shall secure the
public safety? The answer to this question will be, Not until the
public is a good deal more enlightened upon these matters than it is
now.
It has been very correctly remarked, that, in order to bring a
disaster to the public notice, it must be emphasized by loss of
life. The Ashtabula bridge fell, and killed over eighty persons; and
a storm of indignation swept over the country, from one end to the
other. No language was severe enough to apply to the managers of the
Lake Shore Railroad; but if that very bridge had fallen under a
freight-train, and no one had been injured, the occurrence would have
been dismissed with a paragraph, if, indeed, it had received even
that recognition. In February, 1879, a span one hundred and ten feet
long of an iron bridge on the Chicago and Alton Railroad at
Wilmington fell as a train of empty coal-cars was passing over it,
and three cars were precipitated into the river, a distance of over
thirty feet. No one was injured. Not a word of comment was ever made
in regard to this occurrence. Suppose, that, in place of empty
coal-cars, the train had consisted of loaded passenger-cars, and
that one hundred persons had been killed. We know very well what the
result would have been. Is not the company just as much to blame in
one case as the other? On the night of the 8th of November, 1879, one
span of the large bridge over the Missouri at St. Charles gave way as
a freight-train was crossing it, and seventeen loaded stock-cars fell
a distance of eighty feet into the river. Two brakemen and two
drovers were killed. This bridge, says the only account that appeared
in the papers, did not break apparently, for the whole span "went
down" with the cars upon it. It could hardly make much difference to
the four men who were killed, whether the bridge broke down, or
"went" down. Not a word of comment was ever made in the papers
outside of Missouri in regard to this disaster. Suppose, that, in
place of seventeen stock-cars, half a dozen passenger-cars had fallen
from a height of eighty feet into the river, and that, in place of
killing two brakemen and two drovers, two or three hundred passengers
had been killed. Is not the public just as much concerned in one case
as in the other?
Suppose that a bridge now standing is exactly as unsafe as the
Ashtabula bridge was the day before it fell, would it be possible to
awaken public attention enough to have it examined? Probably not.
About two years ago an attempt was made to induce one of the leading
dailies in this country to expose a wretchedly unsafe bridge in New
England. The editor declined, on the ground that the matter was not
of sufficient interest for his readers; but less than a month
afterwards he devoted three columns of his paper to a detailed
account of a bridge disaster in Scotland, and asked why it was that
such things must happen, and if there was no way of determining in
advance whether a bridge was safe, or not?
This editor certainly would not maintain, that, in itself, it was
more important to describe a disaster after it had occurred than to
endeavor to prevent the occurrence; but, as a business man, he knew
perfectly well that his patrons would read an account giving all of
the sickening detail of a terrible catastrophe, while few, if any,
would wade through a dry discussion of the means for protecting the
public from just such disasters. The public is always very indignant
with the effect, but does not care to trouble itself with the cause;
but the effect never will be prevented until the cause is
controlled; and the sooner the public understands that the cause is
in its own hands, to be controlled, or not, as it chooses, the sooner
we shall have a remedy for the fearful disasters which are altogether
too common in the United States.
In a country where government controls all matters on which the
public safety depends, and where no bridge over which the public is
to pass is allowed to be built except after the plans have been
approved by competent authority, where no work can be executed except
under the rigid inspection of the best experts, nor opened to the
public until it has been officially tested and accepted, it makes
little or no difference whether the public is informed, or not, upon
these matters; but in a country like the United States, where any man
may at any time open a shop for the manufacture of bridges, whether
he knows any thing about the business, or not, and is at liberty to
use cheap and insufficient material, and where public officers are
always to be found ready to buy such bridges, simply because the
first cost is low, and to place them in the public ways, it makes a
good deal of difference. There is at present in this country
absolutely no law, no control, no inspection, which can prevent the
building and the use of unsafe bridges; and there never will be until
the people who make the laws see the need of such control.
There is no one thing more important in this matter than that we
should be able to fix precisely the blame in case of disaster upon
some person to whom the proper punishment may be applied. If every
railway director, or town or county officer, knew that he was held
personally accountable for the failure of any bridge in his charge,
we should soon have a decided improvement in these structures. If we
could show that a certain bridge in a large town had been for a long
time old, rotten, worn out, and liable at any moment to tumble down,
and could show in addition, that the public officers having charge of
such a bridge knew this to be the case, and still allowed the public
to pass over it, we can see at once, that, in case of disaster, the
blame would be clearly located, and the action for damages would be
short and decisive. Once let a town have heavy damages to pay, and
let it know at the same time that the town officers are clearly
accountable for the loss, and it is possible that it would be willing
to adopt some system that should prevent the recurrence of such an
outlay.
To see what may be accomplished by an efficient system of public
inspection, it is necessary to know something in regard to the
structures to be inspected. We have now in common use in this
country, both upon our roads and our railroads, bridges made entirely
of iron, bridges of wood and iron combined, and occasionally, though
not often nowadays, a bridge entirely of wood; and these structures
are to be seen of a great variety of patterns, of all sizes, and in
every stage of preservation. Of late so great has been the demand for
bridge-work, that this branch of engineering has become a trade by
itself; and we find immense works fitted up with an endless variety
of the most admirably adapted machine-tools devoted exclusively to
the making of bridges of wood, iron, steel, or all combined. As in
all division of labor, the result of this specialization has been to
improve the quality of the product, to lessen the cost, and to
increase the demand, until many of our large firms reckon the length
of bridging which they have erected by miles instead of feet. As
usual, however, in such cases, unprincipled adventurers are not
wanting, who, taking advantage of a great demand, do not hesitate to
fit up cheap shops, to buy poor material, and to flood the market
with a class of bridges made with a single object in view, viz., to
sell, relying upon the ignorance--or something worse--of public
officials for custom. Not a year passes in which some of these
wretched traps do not tumble down, and cause a greater or less loss
of life, and at the same time, with uninformed people, throw
discredit on the whole modern system of bridge-building. This evil
affects particularly highway bridges. The ordinary county
commissioner or selectman considers himself amply competent to
contract for a bridge of wood or iron, though he may never have given
a single day of thought to the matter before his appointment to
office. The result is, that we see all over the country a great
number of highway bridges which have been sold by dishonest builders
to ignorant officials, and which are on the eve of falling, and await
only an extra large crowd of people, a company of soldiers, a
procession, or something of the sort, to break down.
Not many years ago, a new highway bridge of iron was to be made over
one of the principal rivers in New England. The county commissioners
desired a well-known engineer, especially noted as a bridge-builder,
to superintend the work, in order to see that it was properly
executed. The engineer, after inspection of the plans, told the
commissioners plainly that the design was defective, and would not
make a safe bridge; and that, unless it was materially changed, he
would have nothing to do with it. The bridge, however, was a cheap
one, and, as such, commended itself to the commissioners, who
proceeded to have it erected according to the original plan; and
these same commissioners now point to that bridge, which has not yet
fallen, but which is liable to do so at any time, as a complete
vindication of their judgment, so called, as opposed to that of the
engineer who had spent his life in building bridges.
An impression exists in the minds of many persons, that it is purely
a matter of opinion whether a bridge is safe, or not. In very many
cases, however,--perhaps in most,--it is not at all a matter of
opinion, but a matter of fact and of arithmetic. The whole question
always comes to this: Is the material in this bridge of good quality?
Is there enough of it? Is it correctly disposed, and properly put
together? With given dimensions, and knowing the load to be carried,
it is a matter of the very simplest computation to fix the size of
each member. We know what one square inch of iron will hold, and we
know, also, the total number of pounds to be sustained; and it is no
matter of opinion, but one of simple division, how many times one
will go into the other.
But it may be asked, Can the precise load which is coming upon any
structure be exactly fixed? are not the circumstances under which
bridges are loaded very different? Bridges in different localities
are certainly subjected to very different loads, and under very
different conditions; but the proper loads to be provided for have
been fixed by the best authority for all cases within narrow enough
limits for all practical purposes. Few persons are aware of the
weight of a closely packed crowd of people. Mr. Stoney of Dublin, one
of the best authorities, packed 30 persons upon an area of a little
less than 30 square feet; and at another time he placed 58 persons
upon an area of 57 square feet, the resulting load in the two cases
being very nearly 150 pounds to the square foot. "Such cramming,"
says Mr. Stoney, "could scarcely occur in practice, except in
portions of a strongly excited crowd; but I have no doubt that it
does occasionally so occur." "In my own practice," he continues, "I
adopt 100 pounds per square foot as the standard working-load
distributed uniformly over the whole surface of a public bridge, and
140 pounds per square foot for certain portions of the structure,
such, for example, as the foot-paths of a bridge crossing a navigable
river in a city, which are liable to be severely tried by an excited
crowd during a boat-race, or some similar occasion." Tredgold and
Rankine estimate the weight of a dense crowd at 120 pounds per square
foot. Mr. Brunel used 100 pounds in his calculations for the
Hungerford Suspension Bridge. Mr. Drewry, an old but excellent
authority, observes that any body of men marching in step at from 3
to 3-1/2 miles an hour will strain a bridge at least as much as
double the same weight at rest; and he adds, "In prudence, not more
than one-sixth the number of infantry that would fill a bridge should
be permitted to march over it in step." Mr. Roebling says, in
speaking of the Niagara Falls Suspension Bridge, "In my opinion, a
heavy train, running at a speed of 20 miles an hour, does less injury
to the structure than is caused by 20 heavy cattle under full trot.
Public processions marching to the sound of music, or bodies of
soldiers keeping regular step, will produce a still more injurious
effect."
Evidently a difference should be made in determining the load for
London Bridge and the load for a highway bridge upon a New-England
country road in a thinly settled district. A bridge that is strong
enough is just as good and just as safe as one that is ten times
stronger, and even better; for in a large bridge, if we make it too
strong, we make it at the same time too heavy. The weight of the
structure itself has to be sustained, and this part of the load is a
perpetual drag on the material.
In 1875 the American Society of Civil Engineers, in view of the
repeated bridge disasters in this country, appointed a committee to
report upon The Means of Averting Bridge Accidents. We might expect,
when a society composed of some hundreds of our best engineers
selects an expert committee of half a dozen men, that the best
authority would be pretty well represented; and such was eminently
the case. It would be impossible to have combined a greater amount of
acknowledged talent, both theoretical and practical, with a wider and
more valuable experience than this committee possessed. The first
point taken up in the report is the determination of the loads for
which both railroad and highway bridges should be proportioned. In
regard to highway bridges, a majority of the committee reported that
for such structures the standard loads should not be less than as
shown in the following table:--
+-------------------+----------+----------+----------+
| | POUNDS PER SQUARE FOOT. |
| SPAN. +----------+----------+----------+
| | CLASS A. | CLASS B. | CLASS C. |
+-------------------+----------+----------+----------+
| 60 feet and less | 100 | 100 | 70 |
| 60 to 100 feet | 90 | 75 | 60 |
| 100 to 200 feet | 75 | 60 | 50 |
| 200 to 400 feet | 60 | 50 | 40 |
+-------------------+----------+----------+----------+
Class A includes city and suburban bridges, and those over large
rivers, where great concentration of weight is possible. Class B
denotes highway bridges in manufacturing districts having
well-ballasted roads. Class C refers to ordinary country-road
bridges, where travel is less frequent and lighter. A minority of the
committee modified the table above by making the loads a little
larger. The whole committee agreed in making the load per square foot
less as the span is greater, which is, of course, correct. It would
seem eminently proper to make a difference between a bridge which
carries the continuous and heavy traffic of a large city, and one
which is subjected only to the comparatively light and infrequent
traffic of a country road. At the same time it should not be
forgotten, that, in a large part of the United States, a bridge may
be loaded by ten, fifteen, or even twenty pounds per square foot by
snow and ice alone, and that the very bridges which from their
location we should be apt to make the lightest, are those which would
be most likely to be neglected, and not relieved from a heavy
accumulation of snow. In view of the above, and remembering that a
moving load produces a much greater strain upon a bridge than one
which is at rest, we may be sure, that, as the committee above
referred to recommend, the loads should not be less than those given
in the table. We can easily see that in special cases they should be
more.
There is another point in regard to loading a highway bridge, which
is to be considered. It often happens that a very heavy load is
carried over such bridges upon a single truck, thus throwing a heavy
and concentrated load upon each point as it passes. Mr. Stoney states
that a wagon with a crank-shaft of the British ship "Hercules,"
weighing about forty-five tons, was refused a passage over
Westminster iron bridge, for fear of damage to the structure, and had
to be carried over Waterloo bridge, which is of stone; and he says
that in many cases large boilers, heavy forgings, or castings reach
as high as twelve tons upon a single wheel. The report of the
American Society of Civil Engineers, above referred to, advises that
the floor system be strong enough to carry the following loads upon
four wheels: Class A, 24 tons; Class B, 16 tons; Class C, 8 tons;
though it is stated that these do not include the extraordinary loads
sometimes taken over highways. "This provision for local loads,"
says Mr. Boller, one of the committee, "may seem extreme; but the jar
and jolt of heavy, spring-less loads come directly on all parts of
the flooring at successive intervals, and admonish us that any errors
should be on the safe side."
To pass now to railroad bridges, we find here a very heavy load
coming upon the structure in a sudden, and often very violent,
manner. Experiment and observation both indicate that a rapidly
moving load produces an effect equal to double the same load at rest.
This effect is seen much more upon short bridges, where the moving
load is large in proportion to the weight of the bridge, than upon
long spans, where the weight of the bridge itself is considerable.
The actual load upon a short bridge is also more per foot than upon
a long one, because the locomotive, which is much heavier than an
equal length of cars, may cover the whole of a short span, but only a
part of a longer one. The largest engines in use upon our railroads
weigh from 75,000 to 80,000 pounds on a wheel-base of not over twelve
feet in length, or 2,800 pounds per foot for the whole length of the
engine, and from 20,000 to 24,000 pounds on a single pair of wheels.
The heaviest coal-trains will weigh nearly a ton to the foot,
ordinary freight-trains from 1,600 to 1,800 pounds, and
passenger-trains from 1,000 to 1,200 pounds per foot. Any bridge is
liable to be traversed by two heavy freight-engines followed by a
load of three-quarters of a ton to the foot; so that if we proportion
a bridge to carry 3,000 pounds per foot for the total engine length,
and one ton per foot for the rest of the bridge, bearing in mind
that any one point may be called upon to sustain 24,000 pounds, and
regarding the increase of strain upon short spans due to high speeds,
we have the following loads for different spans exclusive of the
weight of the bridge:--
+---------+-----------------+
| SPAN. | LBS. PER FOOT. |
+---------+-----------------+
| 12 | 7,000 |
+---------+-----------------+
| 15 | 6,000 |
+---------+-----------------+
| 20 | 4,800 |
+---------+-----------------+
| 25 | 4,000 |
+---------+-----------------+
| 30 | 3,600 |
+---------+-----------------+
| 40 | 3,200 |
+---------+-----------------+
| 50 | 3,000 |
+---------+-----------------+
| 100 | 2,800 |
+---------+-----------------+
| 200 | 2,600 |
+---------+-----------------+
| 300 | 2,500 |
+---------+-----------------+
| 400 | 2,450 |
+---------+-----------------+
| 500 | 2,400 |
+---------+-----------------+
The above does not vary essentially from the English practice, and is
substantially the same as given by the committee of the American
Society of Civil Engineers.
The load which any bridge will be required to carry being determined,
and the general plan and dimensions fixed, the several strains upon
the different members follow by a simple process of arithmetic,
leaving to be determined the actual dimensions of the various parts,
a matter which depends upon the power of different kinds of material
to resist different strains. This brings us to the exceedingly
important subject of the nature and strength of materials.
It has been said that we know what one square inch of iron will hold.
Like the question of loads above examined, this is a matter which has
been settled, at any rate within very narrow limits, by the
experience of many years of both European and American engineers. A
bar of the best wrought-iron an inch square will not break under a
tensile strain of less than sixty thousand pounds. Only a small part
of this, however, is to be used in practice. A bar or beam may be
loaded with a greater weight applied as a permanent or dead-load than
would be safe as a rolling or moving weight. A load may be brought
upon any material in an easy and gradual manner, so as not to damage
it; while the same load could not be suddenly and violently applied
without injury. The margin for safety should be greater with a
material liable to contain hidden defects, than with one which is not
so; and it should be greater with any member of a bridge which is
subjected to several different kinds of strain, than for one which
has to resist only a single form of strain. Respect, also, should be
had to the frequency with which any part is subjected to strain from
a moving load, as this will influence its power of endurance. The
rule in structures having so important an office to perform as
railroad or highway bridges, should be, in all cases, absolute safety
under all conditions.
The British Board of Trade fixes the greatest strain that shall come
upon the material in a wrought-iron bridge, from the combined weight
of the bridge and load, at 5 tons per square inch of the net section
of the metal. The French practice allows 3-8/10 tons per square inch
of the cross section of the metal, which, considering the amount
taken out by rivet-holes, is substantially the same as the English
allowance. The report of the American Society of Civil Engineers,
above referred to, recommends 10,000 pounds per inch as the maximum
for wrought-iron in tension in railroad bridges. For highway bridges
a unit strain of 15,000 pounds per square inch is often allowed. A
very common clause in a specification is that the _factor of safety_
shall be four, five, or six, as the case may be, meaning by this that
the actual load shall not exceed one-fourth, one-fifth, or one-sixth
part of the breaking-load. It is a little unfortunate that this term,
factor of safety, has found its way into use just as it has; for it
by no means indicates what is intended, or what it is supposed to.
The true margin for safety is not the difference between the
working-strain and the breaking-strain, but between the
working-strain and that strain which will in any way unfit the
material for use. Now, any material is unfitted for use when it is
so far distorted by overstraining that it cannot recover, or,
technically speaking, when its elastic limit has been exceeded. The
elastic limit of the best grades of iron is just about half the
breaking-weight, or from 25,000 to 30,000 pounds per inch. A poor
iron will often show a very fair breaking-strength, but, at the same
time, will have a very low elastic limit, and be entirely unfit for
use in a bridge. A piece of iron of very inferior quality will often
sustain a greater load before breaking than a piece of the best and
toughest material, for the reason that a tough but ductile iron will
stretch before giving way, thus reducing the area of section, while a
hard but poor iron will keep nearly its full size until it breaks. A
tough and ductile iron should bend double, when cold, without
showing any signs of fracture, and should stretch fifteen per cent of
its length before breaking; but much of the iron used in bridges,
although it may hold 40,000 or 50,000 pounds per inch before failing,
will not bend over 90 degrees without cracking, and has an elastic
limit as low as 18,000 pounds. It is thus full as important to
specify that an iron should have a high elastic limit as that it
should have a high breaking-weight. A specification which allowed no
material to be strained by more than 10,000 pounds per inch, and no
iron to be used with a less elastic limit than 25,000 pounds, would,
at the same time, agree with the standard requirement, both in
England and in the United States, and would also secure a good
quality of iron.
Two documents published some time since illustrate the preceding
remarks. The first is the account of the tests of the iron taken from
the Tariffville bridge after its failure, and the second is the
specification for bridges on the Cincinnati Southern Railroad. The
Tariffville bridge, though nominally a wooden one, like most
structures of the kind relied entirely upon iron rods to keep the
wood-work together. Although the rods were too small, and seriously
defective in manufacture, the bridge ought not to have fallen from
that cause. The ultimate strength of the iron was not what it should
have been, but yet it was not low enough to explain the disaster; but
when we look at the _quality_ of the iron, we have the cause of the
fall. The rods taken from the bridge show an ultimate tensile
strength of 47,560 pounds per inch, but an elastic limit of only
19,000 pounds; while the strain which was at any time liable to come
on them was 22,000 pounds per inch, or 3,000 pounds more than the
elastic limit. The fracture of the tested rods, which, it is stated,
broke with a single blow of the hammer very much in the manner of
cast-iron, showed a very inferior quality of metal. The rods broke in
the bridge exactly where we should look for the failure; viz., in the
screw at the end. No ordinary inspection would have detected this
weakness. No inspection _did_ detect it, but a proper specification
faithfully carried out would have prevented the disaster.
Look now at an extract from the specification for bridges upon the
Cincinnati Southern Railway:--
"All parts of the bridges and trestleworks must be proportioned to
sustain the passage of the following rolling-load at a speed of not
less than 30 miles an hour: viz., two locomotives coupled, each
weighing 36 tons on the drivers in a space of 12 feet, the total
weight of each engine and tender loaded being 66 tons in a space of
50 feet, and followed by loaded cars weighing 20 tons each in a space
of 22 feet. An addition of 25 per cent will be made to the strains
produced by the rolling-load considered as static in all parts which
are liable to be thrown suddenly under strain by the passage of a
rapidly moving load. A similar addition of 50 per cent will be made
to the strain on suspension links and riveted connections of
stringers with floor-beams, and floor-beams with trusses. The
iron-work shall be so proportioned that the weight of the structure,
together with the above specified rolling-load, shall in no part
cause a tensile strain of more than 10,000 pounds per square inch of
sectional area. Iron used under tensile strain shall be tough,
ductile, of uniform quality, and capable of sustaining not less than
50,000 pounds per square inch of sectional area without fracture, and
25,000 pounds per square inch without taking a permanent set. The
reduction of area at the breaking-point shall average 25 per cent,
and the elongation 15 per cent. When cold, the iron must bend,
without sign of fracture, from 90 to 180 degrees."
A specification like this, properly carried out, would put an
absolute stop to the building of such structures as the Tariffville
Bridge, and would prevent a very large part of the catastrophes which
so often shock the community, and shake the public faith in iron
bridges. Reference has been made above to the proper loads to be
placed upon wrought-iron when under a tensile strain. Similar loads
have been determined for other materials, and for other kinds of
strain.
The preceding remarks in regard to the loads for which bridges should
be designed, and the safe weight to be put upon the material, are
given to show how far the safety of a bridge is a matter of fact, and
how far a matter of opinion. It will be seen that the limits within
which we are at liberty to vary, are quite narrow, so that
bridge-building may correctly be called a science; and there is no
excuse for the person who guesses, either at the load which a bridge
should be designed to bear, or at the size of the different members
of the structure. Still less can we excuse the man who not only
guesses, but who, in order to build cheaply, persistently guesses on
the wrong side.
It will, of course, be understood, when it is said that
bridge-building may be called a science, that it can only be so when
in the hands of an engineer whose judgment has been matured by wide
experience, and who understands that no mechanical philosophy can be
applied to practice which is not subject to the contingencies of
workmanship. There are many bridges which will stand the test of
figures very well, and which are nevertheless very poor structures.
The general plan of a bridge may be good, the computations all right,
and yet it may break down under the first train that passes over it.
There are many practical considerations that cannot be, at any rate
have not yet been, reduced to figures. It is not enough that the
strains upon each member of a bridge should be correctly estimated,
and fall within the safe limits: the different members of the bridge
must be so connected, and the mechanical details such, as to insure,
under all conditions, the assumed action of the several parts. In
fine, while we can say that a bridge that does not stand the test of
arithmetic is a bad bridge, we cannot always say that a structure
which does stand such a test is a good one.
We often hear it argued that a bridge must be safe, since it has been
submitted to a heavy load, and did not break down. Such a test means
absolutely nothing. It does not even show that the bridge will bear
the same load again, much less does it show that it has the proper
margin for safety. It simply shows that it did not break down at that
time. Every rotten, worn-out, and defective bridge that ever fell has
been submitted to exactly that test. More than this, it has
repeatedly happened that a heavy train has passed over a bridge in
apparent safety, while a much lighter one passing directly afterwards
has gone through. In almost all such cases, the structure has been
weak and defective; and finally some heavy load passes over, and
cripples the bridge, so that the next load produces a disaster. For
the test of a bridge to be in any way satisfactory, we must know just
what effect such test has had upon the structure. We do not find this
out by simply standing near, and noting that the bridge did not
break down. We must satisfy ourselves beyond all question that no
part has been overstrained.
A short time ago the builders of a wretchedly cheap and unsafe
highway bridge, in order to quiet a fear which had arisen that the
structure was not altogether sound, tested a span 122 feet long with
a load of 58,000 pounds; and inasmuch as the bridge did not break
down under this load, which was less than a quarter part of what it
was warranted to carry safely, the county commissioners considered
the result eminently satisfactory, and remarked that the test was
made merely to satisfy the public that the bridge was abundantly safe
for all practical uses. The public would, no doubt, have been
satisfied that the Ashtabula bridge was abundantly safe for all
practical uses had it stood on that bridge in the morning and seen a
heavy freight-train go over it, and yet that very bridge broke down
directly afterwards under a passenger-train.
Now, according to the common notion, that was a good bridge in the
morning, and a very bad bridge, or rather, no bridge at all, in the
evening. The question for the public is, When did it cease to be a
good bridge, and begin to be a bad one? A test like the one referred
to above can do no more than illustrate the ignorance or lack of
honesty of those who make it, or those who are satisfied with it.
Such a test might come within a dozen pounds of breaking the bridge
down, and no one be the wiser. The entire absurdity of such testing
has recently been illustrated in the most decided manner. The very
same company that built the bridge above referred to, made also
another one on exactly the same plan, and of almost precisely the
same size, and tested it when done by placing almost exactly the same
load upon it. The bridge did not break down; and the county
commissioners, for whom the work was done, were satisfied that it was
"abundantly safe for all practical uses," accepted it, paid for it;
and in less than ten years it broke down under a single team and a
little snow, weighing in all not over one-tenth part of the load the
bridge was warranted to carry, and not over one-half the load with
which it had been previously tested. If this bridge had been "tested"
by five minutes of honest arithmetic, it would have been promptly
condemned the very day it was finished.
In view of the preceding, what shall we say of a bridge company that
deliberately builds a bridge in the middle of a large town, where it
will be subjected to heavy teaming, and, owing to its peculiar
location, to heavy crowds, and warrants to the town that it shall
safely hold a ton per running-foot, when the very simplest
computation shows beyond chance of dispute that such a load will
strain the iron to 40,000 pounds per square inch? We are to say,
either that such a company is so ignorant that it does not know the
difference between a good bridge and a bad one, or else so wicked as
to knowingly subject the public to a wretchedly unsafe bridge. The
case referred to is not an imaginary one, but existed recently in the
main street of a large New-England town. The joints in that bridge,
which could safely hold but 20,000 pounds, were required to hold
60,000 pounds under the load which the builders had warranted the
bridge to carry safely. The case was so bad, that, after a lengthy
controversy, the town officers had a thorough expert examination of
the bridge, which promptly condemned it as in imminent danger of
falling, and as having a factor of safety of only 1-15/100, which is
practically no factor at all. Notwithstanding all this, and in the
face of the report, the president of the bridge company came out with
a letter in the papers, in which he pronounced the bridge "perfectly
safe." Thus we actually have the president of a bridge company in
this country stating openly that a factor of safety of 1-15/100 makes
a bridge perfectly safe, or, in other words, that a bridge can safely
bear the load that will break it down, for he very wisely made not
the slightest attempt to disprove any of the conclusions of the
commission; and this company has built hundreds of highway bridges
all over the United States, and is building them to-day wherever it
can find town or county officers ignorant enough to buy them.
It might be supposed, that, under the above condemnation, the
authorities controlling the bridge would have taken some steps to
prevent the coming disaster. They did, however, nothing of the kind,
but allowed the public to travel over it for more than a year, at the
most fearful risk, until public indignation became so strong that a
special town-meeting was called, and a committee appointed to remove
the old bridge, and to build a new one.
One of the worst cases of utterly dishonest bridge-building that we
have had of late years in Massachusetts, was that of the iron highway
bridge across the Merrimac River at Groveland, a few miles below
Haverhill, one span of which broke down in January, 1881. This bridge
was built in 1871-1872, and consisted of 6 spans, each about 125 feet
long. The whole cost of the structure was $80,000, and the contract
price for the iron-work was $28,000. The company which made that
bridge, agreed in their contract to give the county a structure that
should carry safely 3,000 pounds per running-foot besides its own
weight; but they built a bridge, which, if they knew enough to
compute its strength at all, they knew perfectly well could not
safely carry over one-quarter part of that load. In fact, the weight
of the bridge alone is more than it ever ought to have borne. The
company warranted each span of that bridge to carry safely a net or
moving load of 165 tons, and it broke down under a single team and a
small amount of snow. The company warranted that bridge to carry
safely a load which would strain the iron to 50,000 pounds per inch,
when it knew perfectly well that 15,000 pounds per inch was the most
that could safely be borne.
There are several concerns in the United States which make a
specialty of highway bridges, and which, taking advantage of the
ignorance of public officials, are flooding the country with bridges
no better than that at Groveland. On an average, at least twenty of
these miserable traps tumble down every year, and nothing is done to
bring the guilty parties to punishment. Dishonest builders cheat
ignorant officials, and the public suffers the damage and pays the
bills. Is human life worth enough to pay for having these structures
inspected, and, if found unsafe, strengthened or removed? Can we do
any thing to prevent towns and counties from being imposed upon by
dishonest builders? We certainly can, if those who control these
matters care enough about it to do it. There are two ways of buying a
bridge,--a good way and a bad one; and these two ways are so plain
that no one can misunderstand. To buy a bad bridge, just as soon as
your town or county votes money for a new bridge, certain agents--and
they are as numerous as the agents for sewing-machines or
lightning-rods--will call on, or write to, the town or county
officers, and will offer to build any thing under heavens you want of
any size, shape, or material, and for almost any price. They will
produce testimonials from all the town and county officers in the
country for the excellence of their bridges, and would not hesitate
to give reference, even, for their moral character, if you should ask
it. If they find that you don't know any thing about bridges, they
will, to save you the trouble, furnish a printed specification; which
document will commit you to pay the money, but will not commit the
bridge company to any thing at all. When the bridge is put up, you
never will know whether the iron is good or bad, nor whether the
dimensions and proportions are such as to be safe or not. You will
know that you have paid your money away, but you never will know what
you have got for it until some day when your bridge gets a crowd upon
it, and breaks down, and you have the damage to pay. This mode of
buying a bridge is very common. To buy a good bridge, first determine
precisely what you want; and if you don't know any thing in regard to
bridge-building yourself, employ an engineer who does, to make a
specification stating exactly what you want, and what you mean to
have. Then advertise for bridge-builders to send in plans and
proposals. Let the contractors understand that all plans and
computations are to be submitted to your engineer, that all materials
and workmanship will be submitted to your inspectors, and that the
whole structure is to be made subject to the supervision of a
competent engineer, and accepted by him for you. You will find at
once, that, under such conditions, all travelling agents and builders
of cheap bridges will avoid you as a thief does the light of day. You
will have genuine proposals from responsible companies, and their
bids should be submitted to your engineer. When you have made your
choice, let the contract be written by your lawyer, and have the
plans and specifications attached. Employ a competent engineer to
inspect the work as it goes on; and when it is done, you will have a
bridge which will be warranted absolutely sound by the best
authority. This mode of buying a bridge is very uncommon.
The Ashtabula bridge, it is stated in the report of the committee of
the Ohio Legislature appointed to investigate that disaster, had
factors,--we can hardly call them factors of safety,--in some parts
as low as 1-6/10 and 1-2/10, such factors referring to the
breaking-weight; and even these factors were obtained by assuming the
load as at rest, and making no allowance for the jar and shock from a
railroad train in motion. Well may the commissioners say, as they do
at the end of their report, "The bridge was liable to go down at any
time during the last ten years under the loads that might at any time
be brought upon it in the ordinary course of the company's business,
and it is most remarkable that it did not sooner occur."
One point always brought forward when an iron bridge breaks down, is
the supposed deterioration of iron under repeated straining; and we
are gravely told that after a while all iron loses its fibre, and
becomes crystalline. This is one of the "mysteries" which some
persons conjure up at tolerably regular intervals to cover their
ignorance. It is perfectly well known by engineers the world over,
that with good iron properly used, nothing of the kind ever takes
place. This matter used to be a favorite bone of contention among
engineers, but it has long since been laid upon the shelf. No
engineer at the present day ever thinks of it. We have only to allow
the proper margin for safety, as our first-class builders all do, and
this antiquated objection at once vanishes. The examples of the long
duration of iron in large bridges are numerous and conclusive. The
Niagara-Falls railroad suspension bridge was carefully inspected
after twenty-five years of continued use under frequent and heavy
trains, and not only was it impossible to detect by the severest
tests any defect in the wire of the cables, but a piece of it, being
thrown upon the floor, curled up, showing the old "kink" which the
iron had when it was first made, and wound on the reel. The Menai
suspension bridge, in which 1,000 tons of iron have hung suspended
across an opening of 600 feet for sixty years, shows no depreciation
that the most rigid inspection could detect. Iron rods, recently
taken from an old bridge in this country, have been carefully tested
after sixty years of use, and found to have lost nothing, either of
the original breaking-strength, or of the original elasticity.
The question is frequently asked, Does not extreme cold weaken iron
bridges? To this, it may be replied, that no iron bridge, made by a
reliable company, has ever shown the slightest indication of any
thing of the kind, though they have been used for many years in
Russia, Norway, Sweden, and Canada, and nothing that we know in
regard to iron gives us any reason to suppose that any thing of the
kind ever will happen. But here, again, every thing turns upon the
quality of the iron. Iron containing phosphorus is "cold-short," or
brittle when cold, and will break quicker under repeated and sudden
shocks in cold weather than when it is warm. With good iron, properly
used, we need have no fear on this point. The securing such iron is a
matter to which the utmost attention is paid by our first-class
bridge-building firms, but it is a matter to which no attention is
paid by the builders of cheap bridges. We might suppose that a
person, in putting an insufficient amount of iron into a bridge,
would be careful to get the best quality; but exactly the reverse
seems to be the case, on the ground, perhaps, that the less of a bad
thing we have, the better.
Many persons, in building wooden bridges, take no pains to get iron
rods which are suitable for such work, but purchase what is easiest
to be had in the market, and in many cases never find that the iron
was bad until a bridge tumbles down. There are, without the slightest
question, hundreds of bridges now in use in this country, which, as
far as mere proportions and dimensions go, would appear to be
entirely safe, but which, on account of the quality of the iron with
which they are made, are entirely unsafe; and there always will be,
as long as public officials purchase iron which they know nothing
about, to put into bridges. When a bridge is finished, the ordinary
examinations never detect the quality of the iron; so that the wise
remarks of many inspectors, or the opinions of those in charge of
these structures, as to the exact condition of a bridge, are of
little or no value.