RECONSTRUCTION AND WIDENING--CONCLUSION.
The need for the reconstruction of bridges, arising from various causes
which have been treated in the preceding chapters, original weakness or
faults in design, decay or defects, may also be caused by such
extraneous considerations as the growth of loads, widening of the
openings spanned, or improvement of the headway.
In any case, a precise survey or measuring up of the structure and its
immediate surroundings is required, in the execution of which the
greatest care is desirable, and with respect to which it may be well to
give a few hints.
The surveying chain, when used, should be tested, the measure of
accuracy required rendering this imperative in a degree peculiar to work
of this class. Linen tapes should also be compared with a reliable steel
tape, and used only where sufficiently accurate for the particular
purpose. A careful and observant man may do very good work with a linen
tape, making just that allowance in the sag of the tape which corrects
for the inevitable stretch; but there is still some uncertainty involved
in its use, and the author prefers to rely upon a steel tape,
notwithstanding the inconvenience commonly experienced from its
intractable nature and liability to damage.
Instruments used must also be in the best adjustment; as errors, which
in ordinary field work may not be of great importance, are inadmissible
in bridge work.
It is not necessary here to enter upon the methods of small survey
work, but it may be desirable to point out that abutment walls should be
plumbed for verticality; girders, which are liable to be leaning,
defined in position by reference to their bearings; and generally that
it should never be taken for granted that there is truth in old work, or
that this may be assumed as to line or level.
In cases where disputes with any local authority as to headway are
likely to arise, it is prudent to supplement the information as to level
of soffits by rods cut to length in strict agreement with the clear
height, before removing the old superstructure.
It is apparent that in cases where the superstructure is already
condemned, the detail measurements may be confined to that part of the
structure which is to remain, securing only such information as to the
work superseded which may be required in arranging for the new work.
In taking particulars of skew bridges, needless as the warning may seem,
it is yet necessary to remark that there may be right or left-hand skews
which will not reverse. The author has known a disregard of this to make
serious trouble in two instances.
Dealing first with reconstruction of the superstructure of railway
under-bridges, these, if small, may not give much trouble, though the
demand for greater strength will, perhaps, involve some difficulty in
working to the limiting construction depth--i.e., the distance from the
top of rail to soffit of bridge--particularly as many old bridges have a
very niggardly allowance in this respect. It may be, and quite commonly
is, necessary to raise the rails a small amount, or, if headway is not
restricted, to lower the soffit. Clearances between the running gauge
and girder-work may also be difficult to secure, more liberal allowances
being now required than formerly. Complications in the character of the
permanent way, so frequently found upon old bridges, should, of course,
be got rid of, if possible; but the endeavour may introduce further
difficulties. Regard must throughout be had to the methods to be adopted
in removing old work and in erecting the new. Perhaps the simplest case
to deal with is that where girders lie parallel to, and under the rails,
with a timber floor upon which the permanent way is carried, as sections
of the road involving pairs of girders may be readily removed, and
replaced by the new girder-work (see Fig. 93). If the deck be of trough
flooring or old rails, the matter may not be so simple, as regard must
then be had to the position of joints in the existing floor, and the new
work be schemed with respect to the number and office of girders which
may be got in at any one breaking of the road. A slight slewing of rails
may sometimes be resorted to on occasion, where this has the effect of
releasing some part of the work not otherwise to be dealt with.
[Illustration: FIGS. 93 and 94.]
Bridges having main girders, with timber or trough flooring resting upon
the bottom flanges, or suspended by bolts, will, if carrying many roads,
cause some little difficulty, as the dismantling of any one span
involves the disturbance of others; where, however, many lines are
concerned, it may be feasible to put one or more temporarily out of use,
preserving the continuity of traffic over those which remain, but
refraining from any diversion of the more important roads.
Somewhat similar troubles occur where main girders with cross-girders at
the lower flanges are found, particularly if the cross-girders are
arranged in line, the ends abutting on each side of the same main girder
webs. It is seldom, however, that this construction is used in bridges
of small span carrying many roads; but where it does occur, it may
necessitate the use of timbering below, to carry the ends of
cross-girders when freed from their supporting main girders. (See Fig.
94.)
[Illustration: FIG. 95.]
If it is proposed to use new main and cross-girders, it is desirable to
arrange these in the manner already recommended, the cross-girders not
in line; this has peculiar advantages in reconstruction work, as the
bolting up and riveting of the cross-girder ends is not hampered by
other cross-girder attachments, leaving each piece of floor complete in
itself. Twin main girders are occasionally used with the same object,
and present the advantage of simplicity in erection and independence of
one span from those adjoining (see Fig. 95); but the method is wasteful
of space, and involves a somewhat greater total weight in the main
girders.
The foregoing observations apply more generally to small single-span
bridges, the operations on which may be effected without any material
disturbance of traffic arrangements; though this can seldom be wholly
avoided, it should be confined, where practicable, to a few hours on a
Sunday.
The reconstruction of bridges over 70-feet span may have to be dealt
with under more elaborate arrangements, if carrying two lines only,
possibly with single-line working for a period more or less protracted;
or it may be necessary, having regard to the weight of main girders to
be removed, to carry the whole structure upon temporary staging,
supporting the road independently, cutting up and removing the old work,
and later putting the new work in place, either by detailed erection in
its ultimate position, or by erection at one side and drawing across.
The latter method is, however, commonly reserved for cases in which no
special staging is used under the old structure.
Bridges of a number of openings are usually dealt with by securing full
possession of one road at a time, which for double-line bridges
necessitates single-line working. It is commonly out of the question,
even with moderate spans, to deal with some of these only at a time, and
so avoid continuous possession of one road, for a lengthened period; and
it can only, as a rule, be managed where the ends of the new main
girders do not in any way interfere with those of the old, and where it
is not necessary to reset bed-stones, or make other alterations in the
bearings which necessitate the complete clearance of the pier-tops. In
exceptional cases it may be found possible to arrange for the complete
removal of a small number of moderate spans on a Sunday, and the putting
in place of the new work, as in the case of small single spans.
Spans erected to one side of the final position, to be later travelled
across, are commonly mounted upon gantry staging, and up to 50 tons
weight may rest directly upon rails well greased. The power adopted to
move the span is usually that of screw or hydraulic jacks, or
occasionally engine haulage, special tackle being in that case necessary
to apply the engine power in the right direction. If the time is
limited, or weight considerable, a more elaborate arrangement by which
the load is supported upon wheels, may be necessary, with a view to
reducing the resistance to a manageable amount. All work which it is
possible to do before shifting into place, including the permanent way,
where this is of a special character, should be executed in advance,
leaving only the rail connections to be made good when the span is in
position.
Where timber staging is used to carry the permanent way before
dismantling an old structure, it is convenient to begin by placing stout
balks of timber under the sleepers from end to end of the bridge, or
directly under the rails if space is limited; the staging is then
arranged to give support to the running timbers.
Metallic under-bridges of ample headway, perhaps over coal-workings
(since settled down), or for some less sufficient reason made of metal,
may be cheaply replaced by brick arches built below the old
superstructure, the springings of the arch being checked into the face
of the existing abutments. With stout walls, careful work and good
material will make this an efficient and durable job.
It being a primary condition of reconstruction work to interfere but
little with ordinary traffic arrangements, single-line working is
avoided wherever practicable; as this, always objectionable, may
necessitate the erection of special signals and signal apparatus,
besides the temporary remodelling of the roads, and in this country may
involve also a Board of Trade inspection--altogether a troublesome and
expensive business.
Any bridgework which is accompanied by breaking or blocking the road can
only be undertaken by arrangement with the traffic department, after
notice duly given and published in the periodical record of such
matters; it is generally fixed for a Sunday. Preparatory to this, it is
necessary to make all ready by getting as much done beforehand as is
possible. Wherever practicable and prudent, the whole work is released
from its surroundings, masonry cut away, rivets cut out and replaced by
good bolts, nuts removed from holding down bolts, or the bolts cut
through, etc. Particular care should be exercised to ascertain what
remains to be done immediately prior to removal. It is necessary further
to arrange for trucks to be in readiness to receive old material, and
others containing new girder work to be conveniently stationed, having
been loaded up to come right end foremost; engine power, cranes, empty
and loaded trucks, being all marshalled and so placed as to be available
in proper order, and as wanted. There must be no mistake as to what
roads will be fouled by swinging the crane with its load, or as to the
reach of the crane in effecting its work.
The whole operation to be conducted on any Sunday should be well within
the resources of the men and plant engaged in it, or so managed that it
is a matter of no serious importance if the whole cannot be completed as
originally desired.
Possession of the roads to be blocked having been secured between
certain hours, if some part only of the work to be carried out has been
completed as the time grows short, any attempt to execute the remainder
may result in checking trains until such time as the line may be
reported clear--a contingency to be avoided--though the temptation to
save another Sunday’s work by delay of a few minutes to some one train
may be considerable.
In scheming any reconstruction, it may be insisted that at least one
feasible method of carrying out the work must be secured, though it is
the author’s experience that frequently some other method than that
contemplated is in the end adopted, when, some months later, the final
arrangements for fixing are made. The tendency of a zealous erector is
commonly to take full advantage of any facilities offered, with a view
to a moderate amount of work being done at any one time, and to achieve
as much more as he can himself secure by scheming, or a liberal use of
labour; all Sunday work, with attendance of engines and cranes, being of
necessity expensive.
Railway over-bridges do not commonly present any particular
difficulties. The spans to be dealt with are usually small, and the
weights to be lifted moderate. The height above rails may, however, be
above the lift of any crane; and, for the purpose of raising main
girders, a derrick may become necessary, the rearing and guying of which
may block many roads during the time it is in use. The girders of larger
spans, too unmanageable to be lifted whole, may be erected upon staging;
to secure the requisite headway it may be necessary to build the girders
at a level above that at which they will finally be, lowering them into
position when self-supporting, and after the removal of the staging.
The widening of railway under-bridges is, as a rule, a matter of no
special difficulty, but some remarks may be of use. Widenings should be
planned with a regard to later reconstruction of the original bridge, if
that is at all likely to be necessary, and with the object that, when
complete, the whole should be a consistent piece of work.
It may, indeed, happen that widening of a bridge may involve the
remodelling or reconstruction of the old work, to enable the new roads
to be laid down as desired; this is more likely to be necessary where
there exist main girders not competent to take any additional load, and
to duplicate which would sacrifice space between the new and old roads;
or it may be unavoidable because of slewing of the old rails, as part of
a general rearrangement.
[Illustration: FIG. 96.]
Dealing with widenings simply, there is often some little trouble in
contriving a connection between the new and the old work, as this may
have to be made under, or close to, the sleeper ends of the existing
roads. It is desirable to arrange this part so that no drilling of old
work for rivets or bolts shall be necessary, there being, in fact, no
strict connection. By judicious scheming, this may be effected, whilst
securing freedom from leakage of water at the joint. (See Figs. 96 and
97.) If tying of the new and old structure is desired, this can usually
be done quite simply, well below the floor at some more accessible
level.
[Illustration: FIGS. 97 and 98.]
The strict jointing-up of trough flooring, new to old, at right angles
to the troughs, cannot be contemplated, but may be dealt with by
treating each part independently, the ends being near together,
separated by the space of an inch or so. Each trough end being closed up
by a diaphragm or oak block to prevent ballast dropping through, the top
of the space may be covered by a loose strip, secured to prevent it
shifting, the bottom provided with a gutter of liberal dimensions to
take away leakage, as it is practically impossible to make this
arrangement “drop dry” under the conditions common in executing work of
this kind (see Fig. 98).
[Illustration: FIGS. 99 and 100.]
Where trough flooring, new and old, has to be made good parallel to the
troughs, the difficulty of making a direct connection is less marked,
and it is not unusual to introduce a strip cover simply; but if
accessible, the work is still troublesome, as there is commonly a want
of strict alignment and truth as to level, between the new and the old
troughs. It is preferable to arrange for junctions of a more convenient
type, as in Figs. 99 and 100.
When widening masonry arch bridges by girder-work, it is desirable to
insure that any girders parallel to the masonry face shall be
sufficiently far removed from it to enable painting to be executed. The
space remaining between the girder and the arch may then be bridged by
floor-plates, or an extension of the timber floor if that is adopted.
In effecting a junction such as this, the author has used the
arrangement shown in Fig. 101, the advantage being that the piece of
connecting-floor is sufficiently wide, and also sufficiently flexible,
to allow the girder-work freedom to deflect without doing harm. The load
carried by the width of floor is, as to one part, delivered well on to
the old masonry, in preference to being imposed near to the face. If it
should for any reason be imperative to place the girder close to the
arch face, it is preferable to scheme the floor so that there shall be
no actual contact, the new floor in that case slightly overhanging the
masonry, as in Fig. 102, or dealt with as in Fig. 103, if depth is
restricted.
The widening of masonry arch bridges by masonry, calls for no other
remark than that the new work should be free from the old; though it may
be advisable, when the widening is narrow, to tie the new work to the
old in such a way as to permit independent settlement.
If the widening is exceptionally narrow, there may be no choice but to
bond the new and old work together, and in the best manner, with the
object of minimising the risk of separation.
[Illustration: FIGS. 101 and 102.]
[Illustration: FIG. 103.]
The above matters relative to widenings, though apparently trifling, may
by neglect cause much trouble and expense in maintenance. They
principally concern small bridges, the extension of larger structures
coming rather in the category of independent works.
CONCLUSION.
In bringing these chapters, dealing largely with questions affecting
maintenance, to a close, it may be well to draw attention to the fact
that economy in design (apart from improper reduction of sections) goes
hand-in-hand with economy of upkeep. Given good material, that which
favours low first cost, simplicity of detail, fewness of parts, absence
of smithing, the use of rolled sections, and good depth to girders,
favours also small expenditure in maintenance. The less complex the
design, the easier will it be to keep the structure in order; the less
the number of parts, the fewer will be the connections. Freedom from
smithing eliminates liability to failure at cranks, or other work which
has been subject to fire. It is apparent also that the free use of
rolled instead of built-up sections, reduces the liability to trouble
from bad riveting, or from good riveting overstressed. A liberal depth
to all girders, by reducing deflections, limits the inclination of the
ends and gives the connections a better chance of remaining intact.
Lastly, with work of this character, the labour of scraping and painting
is simplified and cheapened.
The author wishes to reiterate the statement made in the opening
paragraphs of this book, that all instances of decrepitude, failure, or
peculiar behaviour cited, have been under his direct observation. The
fact is insisted upon simply that the reader may appreciate that the
information is at first hand.
It has not been thought necessary, nor was it considered desirable, to
indicate the locality of each case referred to; but it may be said that
the matter of these chapters has been accumulating during many years,
and relates to structures under the control of many different bodies.
The study of old bridges is strongly recommended, particularly with
respect to stress and strain, which in structures new or old, occur
possibly as may be expected--certainly as they must. Consideration of
existing work may thus be a useful check upon the fanciful requirements
of some methods of design. There is a recent tendency, for instance, in
English practice to over-stiffen the webs of plate-girders, such that if
the theory upon which the results are based were true, many old bridges
carrying their loads with no sign of distress, should have failed long
ago. Excess in riveting is a common extravagance, to which the same
criticism may in a less degree apply. Considerable impact allowances for
girders of large span may also be referred to as an application of
empiric theory not justified by experience, which, as in all cases where
such considerations fight with facts, should be modified or rejected.
INDEX
Abutments, leaning, 82, 173
-- movements of, 158
-- settlement of, 78
Adjustment of centre girders, 128
-- of distributing girders, 120
Angular distortions, 88
Arches, equilibrium of, 162
-- repair of, 163
Arrangement of cross girders, 21, 175
Asphalt, 26
Ballast, 29
Bearing pressure on rivets, 47, 51, 57
Bearings, skew, 4
Bottom booms, end bays, 18
Bracing, additional, 117
-- effects of, 34
-- flat bars, 37
-- incomplete, 41
-- sea piers, 42
Bridge floors, 20
-- repairs, 107
-- surveys, 107
Bridges, life of, 165
Breaks in [T] bars, 16
Buckling of webs, 16
Camber, 24, 80
Cast-iron arches, 80, 145
-- -- bridges, 141
-- -- columns, 7,144
-- -- girders, 141
-- -- in sea water, 101
Centre girders, 122
Cinder ballast, 29
Cold-blast iron, 141
Cooling stresses in cast iron, 145
Construction depth, 173
Corrugated sheeting, 29
Cost of centre girders, 135
-- of maintenance, 168
Counterbracing, 19
Cracked bedstones, 3
-- columns, 7
-- web plates, 13, 14, 15, 50
Cross girder arrangement, 21
-- girders, fixed ends, 22, 118
-- -- rusted, 29, 97
-- -- weak, 30, 66
Decay and painting, 96
-- of floor plates, 109
-- of timber, 28, 150
Deflection, 85
-- due to booms and web, 86
-- exceptional cases, 88
-- in new and old work, 85
-- working formulæ, 87
Deformations, 73
Depth of girders, 23, 89, 184
Diagonal ties, 19, 42
Distortion due to temperature changes, 79, 84
Distributing girders, 120
Drainage holes, 25
“Drop” loads, 89
Dwarf walls under floors, 26
Early steel girders, 68
Economy, 184
Effect of earth slips, 157
-- of floor on deflection, 88
-- -- -- on stresses, 23, 30
-- of high stress, 86
-- of permanent way on stresses, 18
--of skew on bridge floors, 25
-- -- -- on centre girders, 131
-- of transverse bracings, 34
-- of vibration on masonry, 162
-- of wave action on sea piers, 42
End bays, bottom booms, 18
Equilibrium of masonry arches, 162
Examination of bridges, 107
Examples of cast-iron bridges overstressed, 141
-- of high stress, 70
-- of life of bridges, 167
-- of rivet stress, 56
-- of strengthening, 114
-- -- -- by centre girders, 131, 134
Excessive bearing pressure, 51
Faulty workmanship, 80
Fixed ends to cross girders, 22, 118
Flange stresses, 63, 66, 67
Flanges, side loaded, 9, 73
Flat bar bracing, 37
Flexing of girders, 9, 74, 76
Flexure curves, 138
Fractured bedstones, 3
-- rails, 30
-- webs, 13, 14, 15, 50
Fractures in cast iron, 7, 145
Girder bearings, 2
Girders on columns, 7
-- on masonry, 8
Girderwork in masonry, 101
Headway, 173
High stress, 61
-- in cast iron, 141
-- in rivets, 47, 52
Holes for drainage, 25
Impact, 20, 62
Inclination of girder ends, 23, 53
Incomplete bracing, 41
Initial set, 88
Interference with traffic, 177
Jack arches, 29, 101
Joints in rails, 29, 109
-- in trough floors, 28, 180
Junction between metallic and masonry bridges, 183
-- -- new and old masonry bridges, 182
-- -- new and old metallic bridges, 180
Lattice girder stresses, 47-66
Liberal depth to girders, 23, 89, 184
Life of bridges, 165
Limit of elasticity, 61
Linen tapes, 172
Longitudinal floor girders, 23
Loose rivets, 21, 25, 51, 53, 56, 109
Main girders, 9-17
Masonry bridges, 157
-- enduring character of, 161
Memel timber, 150
Methods of calculation, 46, 61
-- of observing deflection, 90
-- of setting out deflection curves, 138
Movements of abutments, 158
-- of cast-iron bridge, 82
-- of piers, 42, 83, 159
-- of rollers, 7
-- of wrought-iron bridges, 4, 21, 38, 73
New members to old work, 116
Oiling steelwork, 99
Old drawings unreliable, 108
-- rivets, 21, 51, 54, 55
Open webs, 17
Overhead bracing, 39
-- girders, 118
Painting, 98
Parapets, 79
Permissible stress in old work, 110
Piers, movements of, 42, 83, 159
-- out of plumb, 159
Piles, decay of, 102, 150
Pitch pine, 28
Plasticity, 61
Plate webs, 9
Plated floors, 23, 25, 30
Pointing masonry, 164
Proposed rivet stresses, 58
Quickly applied loads, 89
Rail joints, 29, 109
Rails, breaks in, 30
Reaction of cross girder with centre support, 127
Red-lead, 98, 100
Relative merits of bridges, 169
Relief by centre girders, 122
Repainting, 100
Repair of bridges, 107, 147, 155, 163
-- of timber piles, 156
Replacing flange plates, 110
-- rivets, 111
Resistance of cast iron to rust, 101
Riveted connections, 45, 86
Rivets in cramped positions, 20
-- in cross girder ends, 21, 49, 53, 54
-- in road bridges, 60
-- in webs of main girders, 46
-- spacing of, 60, 80
-- stresses in, 56
Rocking of piers, 8, 83, 159
Roller bearings, 7
Rubble masonry, 161-164
Running load and deflection, 95
Rusting, instances of, 29, 96
-- of steelwork, 99
-- over sea-water, 98
Sag in timber bridges, 150
-- of tapes, 172
Scour under piers, 161
Sea piers, 42, 102
Setting bedstones, 3, 129, 176
Settlements, 76, 157
Skew bearings, 4
-- bridges, right and left, 173
-- -- floors, 25
Skirting plate, 26
Slope of girder ends, 92
Softening of cast-iron in sea-water, 101
Spacing of rivets, 60, 79
Spread of abutments, 157
Spring joints, 23
Steel trough girders, 68
-- troughing, 70
Stiffening girders from floor, 12, 40
-- to webs, 16, 38, 185
Stop piers, 158
Strength of light top booms, 40
Strengthening of bridges, 107, 122
-- bridge floors, 118
-- cross girders, 123
Stress in plated floors, 31
Study of old bridges, 185
Tall piers, 42, 159
Timber bridges, 149
-- floors, 27
-- staging, 177
Top booms, 18
Traffic during reconstruction, 177
Transverse bracing, 34, 117
Trough floors, 27, 180
-- girders, 50, 74
[T] stiffeners, breaks in, 16
Twin girders, 15, 75
Twisting of girders, 11, 68, 73
-- -- -- corrected, 12
Types of reconstruction, 174
[U]-shaped booms, 18
Uncomplicated stress, 62
Uniform pressure on bearings, 6
Value of E in deflection formulæ, 87
Vibration, 162
Wasted webs, 14, 29, 96
Water, scour of, 161
Web buckling, 16
-- plates, cracked, 13, 14, 15, 50
-- rivets, 46, 50
-- stiffening, 16, 38, 185
Widening masonry bridges, 182
-- metallic bridges, 179
Wide spaced rivets, 79
Wind pressure, 41, 43
Yielding of piers, 8, 83, 159
LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S.E.
Transcriber's Notes:
The text of the original work (including inconsistent spelling,
hyphenation, formatting etc.) has been retained, except as mentioned
below.
The slight differences between the Table of Contents and the text have
not been changed.
Changes made to the text:
Some punctuation errors and obvious typographical errors have been
corrected silently.
Several illustrations have been moved to where they are described in the
text.
Page 123, formula (2): the original shows an unclear superscript after
the first L. As described in the following line of the text, this has
been changed to L{_l_}.
End of Project Gutenberg's The Anatomy of Bridgework, by William Henry Thorpe