Transcriber's Notes:
Text between underscores represents _italics_, small capitals have been
transcribed as ALL CAPITALS.
Curly brackets indicate {subscripts}; letters between square brackets
(such as [T] and [U]) represent the shape rahter than the letter itself.
More Transcriber's Notes may be found at the end of this text.
THE
ANATOMY OF BRIDGEWORK
THE
ANATOMY OF BRIDGEWORK
BY
WILLIAM HENRY THORPE
ASSOC. M. INST. C. E.
WITH 103 ILLUSTRATIONS
[Illustration]
London
E. & F. N. SPON, LIMITED, 57 HAYMARKET
New York
SPON & CHAMBERLAIN, 123 LIBERTY STREET
1906
PREFACE
In offering this little book to the reader interested in Bridgework, the
author desires to express his acknowledgments to the proprietors of
“Engineering,” in which journal the papers first appeared, for their
courtesy in facilitating the production in book form.
It may possibly happen that the scanning of these pages will induce
others to observe and collect information extending our knowledge of
this subject--information which, while familiar to maintenance engineers
of experience, has not been so readily available as is desirable.
No theory which fails to stand the test of practical working can
maintain its claims to regard; the study of the behaviour of old work
has, therefore, a high educational value, and tends to the occasional
correction of views which might otherwise be complacently retained.
60 WINSHAM STREET,
CLAPHAM COMMON, LONDON, S.W.
_October_, 1906.
CONTENTS
CHAPTER I.
INTRODUCTION--GIRDER BEARINGS.
PAGE
Pressure distribution--Square and skew bearings--Fixed bearings--
Knuckles--Rollers--Yield of supports 1
CHAPTER II.
MAIN GIRDERS.
_Plate webs_: Improper loading of flanges--Twisting of girders--
Remedial measures--Cracks in webs--Stiffening of webs--[T]
stiffeners 9
_Open webs_: Common faults--Top booms--Buckling of bottom booms--
Counterbracing--Flat members 17
CHAPTER III.
BRIDGE FLOORS.
Liability to defects--Impact--Ends of cross and longitudinal
girders--Awkward riveting--Fixed ends to cross girders--Plated
floor--Liberal depths desirable--Type connections--Effect of “skew”
on floor--Water-tightness--Drainage--Timber floors--Jack arches--
Corrugated sheeting--Ballast--Rail joints--Effect of main girders
on floors 20
CHAPTER IV.
BRACING.
Effect of bracing on girders--Influence of skew on bracing--Flat
bars--Overhead girders--Main girders stiffened from floor--
Stiffening of light girders--Incomplete bracing--Tall piers--Sea
piers 34
CHAPTER V.
RIVETED CONNECTIONS.
Latitude in practice--Laboratory experiments--Care in considering
practical instances--Main girder web rivets--Lattice girders
investigated--Rivets in small girders--Faulty bridge floor--
Stresses in rivets--Cross girder connections--Tension in rivets--
Defective rivets--Loose rivets--Table of actual rivet stresses--
Bearing pressure--Permissible stresses--Proposed table--Immunity of
road bridges from loose rivets--Rivet spacing 45
CHAPTER VI.
HIGH STRESS.
Elastic limit--Care in calculation--Impact--Examples of high stress
--Early examples of high stress in steel girders--Tabulated
examples--General remarks 61
CHAPTER VII.
DEFORMATIONS.
Various kinds--Flexing of girder flanges--Examples--Settlement
deformations--Creeping--Temperature changes--Local distortions--
Imperfect workmanship--Deformation of cast-iron arches 73
CHAPTER VIII.
DEFLECTIONS.
Differences as between new work and old--Influence of booms and web
structure on deflection--Yield of rivets and stiffness of
connections--Working formulæ--Set--Effect of floor system--
Deflection diagrams--Loads quickly applied--“Drop” loads--Flexible
girders--Measuring deflections--New method of observing deflections
--Effect of running load 85
CHAPTER IX.
DECAY AND PAINTING.
Examples of rusting of wrought-iron girders--Girder over sea-water
--Rate of rusting--Steelwork--Precautions--Red-lead--Repainting--
Scraping--Girders built into masonry--Cast iron--Effect of sea-
water on cast iron--Examples--Tabulated observations--Percentage of
submersion--Quality of metal 96
CHAPTER X.
EXAMINATION, REPAIR, AND STRENGTHENING OF RIVETED BRIDGES.
Purpose--Methods of examination--Calculations--Stress in old work--
Methods of reducing stress--Repair--Loose rivets--Replacing wasted
flange plates--Adding new to old sections--Principles governing
additions--Example--Strengthening lattice girder bracings--Bracing
between girders--Strengthening floors--Distributing girders 107
CHAPTER XI.
STRENGTHENING OF RIVETED BRIDGES BY CENTRE GIRDERS.
Principal methods in use--Method of calculation--Adjustments--
Connections--Method of execution--Checks--Effect of skew on method
considered--Results of calculation for a typical case--Probable
error--Practical examples--Special case--Method of determining
flexure curves 122
CHAPTER XII.
CAST-IRON BRIDGES.
Limitations of cast iron--Stress examples--Advantages and
disadvantages--Foundry stresses--Examples--Want of ductility of
cast iron--Repairs--Restricted possibilities 141
CHAPTER XIII.
TIMBER BRIDGES.
Perishable nature--Causes of decay--Sag--Lateral bracing--Piles--
Uncertainty respecting decay--Examples--Conditions and practice
favourable to durability--Bracing--Protection--Repair--Piles--Cost 149
CHAPTER XIV.
MASONRY BRIDGES.
Definition--Cause of defects or failure--Spreading of abutments--
Closing in--Example--Stop piers--Example of failure--Strength of
rubble arch--Equilibrium of arches--Effect of vibration on masonry
--Safety centring--Methods of repair--Pointing--Rough dressed
stonework 157
CHAPTER XV.
LIFE OF BRIDGES--RELATIVE MERITS.
Previous history--Causes of limited life--Tabulated examples of
short-lived metallic bridges--Timber and masonry bridges--
Durability--Maintenance charges--First cost--Comparative merits--
Choice of material 165
CHAPTER XVI.
RECONSTRUCTION AND WIDENING OF BRIDGES.
CONCLUSION.
Measuring up--Railway under-bridges--Methods of reconstruction in
common use--Reconstruction of bridges of many openings--Timber
staging--Traffic arrangements--Sunday work--Railway over-bridges--
Widenings--Junction of new and old work--Concluding remarks--Study
of old bridgework 172
INDEX 187
THE
ANATOMY OF BRIDGEWORK.
CHAPTER I.
INTRODUCTION.
No book has, so far as the author is aware, been written upon that
aspect of bridgework to be treated in the following pages. No excuse
need, therefore, be given for adding to the already large amount of
published matter dealing with bridges. Indeed, as it too often happens
that the designing of such constructions, and their after-maintenance,
are in this country entirely separated, it cannot but be useful to give
such results of the behaviour of bridges, whether new or old, as have
come under observation.
In the early days of metallic bridges there was of necessity no
experience available to guide the engineer in his endeavour to avoid
objectionable features in design, and he was, as a result, compelled to
rely upon his own foresight and judgment in any attempt to anticipate
the effects of those influences to which his work might later be
subject. How heavily handicapped he must have been under these
conditions is evident from the mass of information since acquired by the
experimental study of the behaviour of metals under stress, and the
growth of the literature of bridgework during the last forty years. That
many mistakes were made is little occasion for surprise; rather is it a
cause for admiration that some very fine bridges, still in use, were
the product of that time. Much may be learned from the study of defects
and failures, even though they be of such a character that no
experienced designer would now furnish like examples.
Modern instances may, none the less, be found, with faults repeated,
which should long since have disappeared from all bridgework, and are
only to be accounted for by the unnatural divorce of design and
maintenance already referred to. As the reader proceeds, it may appear
that details are occasionally touched upon of a character altogether too
crude and objectionable to need comment; but the consideration of these
cases is none the less interesting, and, so far as the author’s
observation goes, not altogether unnecessary.
Most of the instances cited are of bridges, or parts of bridges, of
quite small dimensions; but it is these which most commonly give
trouble, both because the effects of impact are in such cases most
severely felt, and possibly because the smaller class of bridges is very
generally designed by men of less experience, than large and imposing
structures.
The particulars given relate in all cases to bridges of wrought iron,
unless otherwise described.
An endeavour has been made to secure some kind of order in dealing with
the subject, but it has been found difficult to avoid a somewhat
disjointed treatment, inseparable, perhaps, from the nature of the
matter. Finally, the reader may be assured that every case quoted has
come under the writer’s personal notice.
GIRDER BEARINGS.
In girder-work generally, and more particularly in plate-girders,
considerable latitude obtains in the amount of bearing allowed. Clearly,
the surface over which the pressure is distributed should be
sufficiently ample to avoid overloading and possible crushing or
fracture of bedstones where these exist; but if no knuckles are
introduced, this is an extremely difficult matter to insure. A long
bearing may deliver the load at the extreme end of the surface on which
it rests, or, more probably, near the face.
If the girder is made with truly level bearings, and the beds set level,
it will certainly, when under load, throw an extreme pressure upon that
part of the bearing surface immediately under the forward edge of the
bearing-plate. These considerations probably account for bedstones
frequently cracking, in addition to which possibility there is the
disadvantage that the designer does not know where the girder will rest,
and cannot truly define the span. The variation of flange-stress due to
this cause may, in a girder of ordinary proportions, having bearings
equal in length to the girder’s depth, be as much as 15 per cent. above
or below that intended.
If great care be taken in setting beds, in the first instance, to dip
toward the centre of the span an amount depending upon the anticipated
girder deflection, it may be possible to insure that when under full
load the girder bearing shall rest equally upon its seat; but this is
evidently a difficult condition to obtain practically, is good only for
one degree of loading, and may at any time be nullified by a disturbance
of the supports, as, for instance, the very common occurrence of a
slight leaning forward of abutment walls.
Double or treble thicknesses of hair-felt are sometimes placed beneath
girder bearings, with the object of securing a better distribution of
pressure, no doubt with advantage; but this practice, though it may be
quite satisfactory as applied to girders carrying an unchangeable load,
hardly meets the case for loads which are variable. Notwithstanding the
faulty nature of the plain bearing ordinarily used for girders of
moderate span, its extreme simplicity commends it to most engineers. It
must be admitted that no serious inconvenience need be anticipated in
the majority of cases, particularly if the bearings are limited in
length, do not approach nearer than 3 inches to the face of bedstones,
and are furnished with hair-felt or similar packing.
[Illustration: FIG. 1.]
Whether with long or short bearings, the forward edge should be at right
angles to the girder’s length. In skew bridges it is sometimes seen that
this edge follows the angle of skew. The effect on the girder is to
twist it, as will be clear from a little consideration. In evidence of
this the case may be quoted of a lattice girder of 95 feet effective
span and 7 feet deep, which, resting on a skew abutment right up to the
masonry face at a rather bad angle (about 15 degrees), was, after twenty
years, found canted over at the top to the extent of 4 inches, with the
further result of springing a joint in the top flange at about the
middle of the girder, causing some rivets to loosen. The bedstone was
also very badly broken at the face, and had to be replaced in the course
of repairs (Fig. 1). This girder had, in addition to the canting from
the upright position at its end, and the distortion of the top flange, a
curvature in the same direction, though less in amount, at the
bottom--an effect very common in the main girders of skew bridges, and
possibly accounted for in part by a tendency of the girder end to creep
along the abutment away from the point at which it bears hardest, under
frequent applications and removals of the live load, and accompanying
deflections.
This tendency to travel may be aggravated in bridges carrying a
ballasted road, in which there may be a considerable thickness of
ballast near the bearings, by the compacting and spreading of this
material taking effect upon the girder end, tending to push it outwards,
being tied only by a few light cross-girders badly placed for useful
effect. The movement may be prevented in new work for moderate angles of
skew by carrying the end cross-girders well back, and securing them in
some efficient manner; or by the introduction of a diagonal tie
following the skew face, and attached to cross and main girder flanges
(Fig. 2)--a method which may be applied to existing work also.
[Illustration: FIG. 2.]
For such a case as that cited it is imperative that ballast pressure at
the girder end should be altogether eliminated.
The fixing of girder ends by bolts--a practice at one time usual--hardly
calls for remark, as it is now seldom resorted to unless for special
reasons; but it may be well to point out the weakening effect of holes
for any purpose in bedstones. Bed-plates commonly need no fixing; the
weight carried keeps them in position, or if, in the case of very light
girders upon separate plates, it is considered well to secure these from
shifting, it may best be done by letting the plate in bodily a small
amount, or by means of a very shallow feather sunk into a chase.
[Illustration: FIG. 3.]
As an improvement upon the plain bearing usually adopted, it is an easy
matter so to design girder-ends as to deliver the load by a narrow strip
of bearing-plate carried across the bottom flange, distributing the
pressure upon the stone, if there be one, by means of a simple
rectangular plate of sufficient stoutness (Fig. 3). An imperfect knuckle
will by this means result, with freedom to slide, and the girder span be
defined within narrow limits. A true knuckle is, of course, the best
means of securing imposition of the load always in the same place; but
this by itself is not sufficient where the girder is of a length to make
temperature and stress variations important, in which case rollers, or
freedom to slide, become necessary. Bridges exist in which
roller-bearings have been adopted without the knuckle, or its
equivalent, but this is wholly indefensible, as it is obvious that the
forward roller will in all probability take the whole load, and cannot
be expected to keep its shape and roll freely under this mal-treatment.
It is sometimes asserted that rollers are never effective after some
years’ use; that they become clogged with dirt, and refuse to perform
their office.
There is no reason why rollers should not be boxed in to exclude dirt by
a casing easily removed, some attention being given to them, and any
possible accumulation of dirt removed each time the bridge is painted.
To test the behaviour of rollers under somewhat unfavourable conditions
for their proper action--that of the bearings of main roof trusses of
crescent form, 190 feet span--the author, some thirty years since, took
occasion to make the necessary observations, and found evidence of a
moderate roller movement, though there was in this case no direct
horizontal member to communicate motion. With girders resting upon
columns, particularly if of cast iron, a roller and knuckle arrangement
is most desirable for any but very small spans, as, if not adopted, the
result will be a canting of the columns from side to side--a very small
amount, it is true, but sufficient to throw the load upon the extreme
edges of the base, though the knuckle alone will relieve the top of this
danger. The author at one time took the trouble to examine, so far as it
could be done superficially and without opening out the ground to make a
complete inspection possible, a number of bridges crossing streets, in
which girders rested upon and were secured to cast-iron columns standing
in the line of kerb; and he found cracks, either at the top or bottom,
in about one of every four columns.
When girders passing over columns are not continuous, it may be
difficult to find room for a double roller and knuckle arrangement; but
this inconvenience may be overcome by carrying one girder-end wholly
across the column-top, and securing the next girder-end to it in a
manner which a little care and ingenuity will render satisfactory, one
free bearing then serving to carry the load from both girders.
Though the wisdom of using rollers is apparent in spans exceeding some
moderate length, say 80 feet--as to which engineers do not seem quite
decided--and varying with the conditions, it need not be overlooked that
in some cases masonry will be sufficiently accommodating to render them
unnecessary; piers, if sufficiently tall and slender, will yield a small
amount without injury, and though shorter, if resting upon a bottom not
absolutely rigid, will rock and give the necessary relief; but it is
obvious, if the resistance to movement is sufficiently great, and the
girder cannot slide or roll on its bearings, bedstones will probably
loosen, as, indeed, frequently happens.