EXAMINATION, REPAIR, AND STRENGTHENING OF RIVETED BRIDGES.
In the preceding chapters defects of various kinds to which riveted
bridgework is liable have been more particularly dealt with; it is now
proposed to consider the examination of such structures, following this
by a reference to methods of repair and strengthening, leaving the
treatment of other classes of bridgework to be developed under their
proper headings, though some of the remarks immediately following will
apply to all.
The exhaustive survey of a bridge is only to be made after considerable
experience in the work, but it may be stated that in looking for defects
it is well to seek where they are least expected, till, with practice,
one knows better where to direct attention. When examining with a view
to pronouncing an opinion upon the fitness of the structure to remain in
place, if in any real doubt, it is wise to give a casting vote against
it; and finally it may be said that upon taking down a bridge condemned
for any one or more defects, it should be examined for worse. This may
seem to be somewhat pessimistic, but is based upon the teachings of
experience.
Preliminary examination of a bridge may reveal such faults or weaknesses
as at once to ensure its condemnation; but if this is not the case, and
there is a reasonable probability that the structure may be given a
fresh lease of life, it will, for the purpose of estimating the
strength, or for possible repairs, commonly be desirable to secure
precise particulars of the existing structure independently of any
drawings that may be in existence, and which will very probably be
incorrect, the finished work, if old, seldom agreeing with the contract
drawings. A final decision may in this case be deferred till after the
measuring up has been completed, the condition of the structure becoming
more familiar in the process.
It is desirable first to ascertain whether the bridge remains in good
form, whether the camber of girders appears to be what might be
expected, or agreeable with existing records, though much reliance must
not be placed upon figured cambers, it being quite common for girders to
leave the bridge yards with the camber something other than that
intended. The deflections under live load will also be observed, and
compared with the calculated result, or checked by judgment. The
calculations upon which strength and deflections are based will, of
course, refer to the actual sections, which are sometimes a little
difficult to ascertain if there has been irregular rusting. In
continuous girders also, levels having been taken, allowance should be
made for effects of settlement, if any; and with arches evidence of
movement of the piers or abutments sought for, with the like object. It
is seldom that the main flanges of girders show signs of weakness,
unless from flexure in the case of long and narrow top members,
insufficiently stiffened; but there may be want of truth from other
causes already dealt with. In plate girders the webs should be most
carefully scanned for possible cracks, particularly where cross-girders
are connected, and along the upper edges of bottom flange angles, if the
floor rest upon the flange. All riveted connections, of course, need
close attention, both for straining effects, where there is a liability
to wracking, and to detect loose rivets. Loose rivets and want of
tightness in other parts of the work may frequently be detected at
sight by a reddish bloom which appears on the neighbouring surfaces,
caused by rust working out and spreading under the effects of weather;
it may be seen round rivet-heads or along the edges of angle-bars, or
other parts where there is movement. Loose rivets, though generally to
be detected also by the hammer, may perhaps in the case of thin-webbed
cross-girders be working in the web-thickness only, possibly to a
considerable extent. This, if not otherwise evident, may sometimes be
detected by simultaneous deflection tests--with rods--at the top and
bottom flanges of a girder, at the same distance from the bearings. Any
difference in the readings may indicate loose web-rivets, or possibly a
tear in the web running parallel to the flange angles.
Bracings between girders are very apt to display a rich harvest of
working rivets. Cross-girders and longitudinals also may have loose
rivets at their connections, and be very badly wasted, with quite
possibly cracks in the webs, or other defects already enlarged upon.
The condition of the road upon the bridge will frequently be an
indication of the state of the floor which carries it; or the existence
of rail-joints which are working badly may very properly lead to a
critical examination of the girder-work immediately below, as this is a
fruitful source of damage in light constructions. Floor-plates, where
these exist, should be scanned for leakages, drainage nozzles, and
guttering, to see that they are free, the attachments of the latter
being often loose and unsatisfactory.
Trough floors may be expected to show loose rivets near the ends, with a
probability of excessive leakage where they abut against the webs of
supporting girders.
Floor plates resting upon abutments or piers, being very liable to
serious decay, require attention, and girder-work entering masonry
should receive close scrutiny, any obstruction to a sufficient
examination being removed so far as is judged sufficient for the
purpose. The structure should, of course, be closely watched during the
passage of live load for any signs of abnormal movement, excessive
vibration, or lurching.
In addition to seeking for these various defects, or others which have
been referred to in these pages at length, it will be well always to be
alive to the possibility of faults to be seen for the first time, or of
which the author has furnished no instance.
Having formed a reliable opinion as to the state of the bridge, this, if
satisfactory, may leave to be determined only the question of strength
relative to the loads carried. It is apparent that stress limits
suitable for a new structure, which has all its life before it, of
purpose moderate to cover possible deteriorations, the growth of loads,
and other adverse influences, may to avoid immediate reconstruction,
reasonably be permitted of a higher value for a further term of years in
the case of a structure which it is known has for a considerable period
behaved well, and remains in good condition. What this higher value may
be will be greatly influenced by the circumstances of each case, and,
being largely a matter of judgment, may be expected to vary with
different engineers. Experience shows, however, that the nominal unit
stress in an old bridge may be a very considerable amount in excess of
that allowed for new work, without, of necessity, showing any ill
effects; and the author is of opinion that for old bridges in good
condition it is quite prudent to allow an excess of 33 per cent. beyond
that permissible for a new design. If the structure is too weak to
satisfy this modified condition, it may be possible to bring it within
the stress limit by a reduction of ballast or other removable dead
weight. If this expedient does not promise to be satisfactory, or the
bridge shows actual signs of weakness, or palpable defects, it will be
necessary to deal with the question of repair, strengthening, or
reconstruction.
The repair of built up bridgework resolves itself largely into a matter
of replacing loose rivets by cutting these out, rhymering the holes, if
desirable, and again riveting. It will often be sufficient to do this
with no particular precautions as to bolting up temporarily; the rivets
having been loose, may very well be spared for a time. In re-riveting
cross-girder connections it may, however, be imperative to remove all
the rivets, bolting up securely as this is done, in order to make a
tight job, taking out each bolt in turn as required, and again filling
the holes; or it may be well in a bad case first to remove all loose
rivets, substituting good bolts, in order that work which has gone out
of shape owing to defective rivets may first be brought true.
Cross-girder webs, cracked vertically or nearly so, are commonly
repaired with splice-plates on either side; but in doing this it is
undesirable to add plates of excessive thickness relative to the
web--probably poor--as by an abrupt change of web section it appears not
unlikely a fresh break may be favoured.
[Illustration: FIG. 60.]
[Illustration: FIG. 61.]
[Illustration: FIG. 62.]
[Illustration: FIG. 63.]
Replacing wasted flange-plates, or adding new plates to those which
exist, is occasionally resorted to in the case of main girders, the
flanges of which are sufficiently accessible, but the operation is
difficult, takes some little time, and should only be attempted under
the constant supervision of a thoroughly capable man. When done, if the
girder has not been relieved of load by staging, the stress under full
load will be unequally distributed between the old and the new section,
the old always taking more by the amount of the dead-load stress
previously carried. The method which the author has seen applied to
lattice girders of about 80 feet span, having good angle-bars in the
flanges, with a shallow vertical web for attachment of diagonals,
consisted in first cutting out the old flange rivets, and substituting
bolts well screwed up, till all the rivets necessary had been removed.
The new plate length having been prepared, was, on a Sunday, during a
few hours’ cessation of traffic, marked off, the temporary bolts being
removed for the purpose, and then replaced. After the plate had been
drilled, on a later Sunday, it was finally put into position, bolted up,
and riveted at leisure; cover-plates make additional trouble, but are
dealt with on the same principle. The method as shown in Fig. 60 is,
however, barely practicable for so many plates. It is preferable, if it
is proposed to add section, to do this with as little interference as
possible with existing rivets of importance. This may be accomplished,
if the existing plates are not too wasted at their edges, by riveting on
new strips or angle-bars (see Figs. 61 to 63). Occasionally the strength
of a girder is increased by the addition to the top or bottom boom of
material in such a form as sensibly to increase the depth, and thus,
while adding increased section to one boom, to reduce the stress in
each, though to dissimilar amounts. By this device also the relief is
effective only as regards the live-load stress; under dead load only the
new material does no work, provided, of course, that no relief staging
was used during the alterations. For girders carrying any considerable
proportion of dead load the method is very inefficient, though for
others, in which the live load is relatively large, the result should be
more satisfactory.
As this question of adding new section to old is of much importance in
dealing with repairs and strengthening operations, a few general remarks
upon the subject will be pertinent. The difficulty in such work commonly
is to cause the new to render any considerable assistance to the old in
those cases which occur in practice. If a bar be imagined under
longitudinal stress varying between 0 and a maximum, then, if the area
of the piece be increased at the time when it takes no stress, its
capacity for resisting the maximum amount will be increased, and for
added material of similar elasticity the unit stress proportionately
reduced. If, however, the load on the bar does not vary, the mere
addition of metal will not relieve the original section in any degree.
To take a third case, of the maximum being twice the minimum load, it
will be necessary, in order to lower the maximum unit stress by 25 per
cent., to double the original section of the bar if, as supposed, the
extra metal has been added to the piece when under the smaller load, so
that the new section is only effective in assisting to carry the
remainder of the load at such times as it may be imposed. The
relationship stands thus:--
Live load New area
---------------- × -------------- = relief.
Live + dead load New + old area
These statements will be true under the conditions named, within the
elastic limit of the material; but some advantage would be derived in
the second case, and a more marked benefit in the third, if the load
assumed to be a maximum were exceeded, or if the composite bar were
tested to destruction; as, however, these effects would be outside the
limiting conditions imposed, it must be a matter of judgment as to how
far this reserve of strength may be considered of value.
If, instead of simply adding section to the bar, some part of the
constant load is put upon the new section by the manner of attachment,
the combination will, of course, be more effective.
To apply these considerations and illustrate the way in which the two
methods of adding flange section work out when reduced to figures, the
case will be supposed of a girder 6 feet deep, carrying a load of which
one-third is dead and two-thirds live. To the flanges of this girder are
added plates equal to 50 per cent. of the original areas, in order to
reduce the stress of 7 tons per square inch to which the girder before
strengthening is liable, the depth remaining substantially unaltered.
With dead load only the original section would be stressed to 2·3 tons
per square inch, the new section being then unstressed. Under full load
the new and old material take 3·1 tons per square inch additional,
making the modified stress on the original section 5·4 tons per square
inch, as against 7 tons; or a reduction of 22 per cent. This compares
with 33 per cent., the relief due to 50 per cent. increase of flange
area under ordinary conditions of stress distribution.
Let the second method of strengthening the girder now be considered,
using, for purposes of comparison, the same total amount of new material
to increase the girder depth by an addition to the top flange. This
section will be equal to the area of one flange, which, though it may
be applied in many different ways, giving a greater or a less increase
to the depth, would probably be used in some such manner as that shown
in Fig. 64, increasing the effective depth for live-load stress by
nearly 10 inches.
[Illustration: FIG. 64.]
The added material will, as in the previous case, leave the dead-load
stress unaltered, or 2·3 tons per square inch. The stress in the bottom
flange due to live load will, however, now be 4·1 tons per square inch,
making a total stress of 6·4 tons per square inch, against 7 tons--the
original stress. The reduction here is 8 per cent. only, as compared
with 12 per cent., the relief due, under ordinary conditions, to an
increase of effective depth from 6 feet to 6 feet 10 inches, and by the
use of additional material, equal, as before, to one-half of the total
flange areas before the alteration.
The effect on the top flange need not be here gone into in detail, but
it may be said that, owing to the increase of gross section and of
depth, the ultimate stresses of both the new and old material are
greatly less than as given for the bottom flange.
Girders strengthened by the first of these two methods would, it is
probable, if tested to destruction, give results more nearly in accord
with the actual percentage increase of flange section, plastic
deformation of the metal, before failure, tending to reduce the
differences of stress on the new and old material of the sections.
[Illustration: FIGS. 65 and 66.]
Web members of lattice girders may, if weak, sometimes be dealt with by
the introduction of supplementary bars, parallel to and between the old
members, or by the addition of strips or angles to the existing
diagonals. The treatment will be largely influenced by the nature of the
old detail, which may lend itself to some one arrangement much better
than to any other.
End riveting of web members may, if it has become loose, be dealt with
by simply rhymering the holes a size larger, and re-riveting in the best
manner, if the stresses are not excessive; or it may be necessary to
devise some additional attachments by which new rivets are brought into
use (see Figs. 65 and 66). The effective relief due to supplementary
rivets will be influenced by similar considerations to those governing
increase of section.
[Illustration: FIG. 67.]
[Illustration: FIG. 68.]
Old structures are very frequently deficient in bracing, which may, in
such cases, be advantageously introduced; or girders individually weak
may be rendered collectively efficient by suitable bracing. In
considering the advisability of this, however, the case should be viewed
with regard to the possible effects of such members, as already dealt
with in the chapter relating to these questions. There it has been
pointed out that bracing between a system of parallel girders may have
the effect, under live load, of increasing the stress on the outer
girders due to twisting of the structure as a whole, though the inner
girders will, except for full loading of the whole bridge, be advantaged
as to stress values, and in any event bettered by being held up to their
work. The effect upon the outer girders may be met by increasing their
strength, if this appears to be necessary. In all such alterations the
detail should be schemed with special care to ensure simplicity in
execution, smith’s work being rigorously avoided. A good arrangement for
supplementary bracing between plate-girders, which gives little trouble
in carrying out, is shown in Fig. 67; or where the stiffeners of such
girders are in line across the bridge, the detail given in Fig. 68 may
involve less expenditure. Difficulties may be experienced in riveting,
unless great care is taken in the positioning of rivets. Fitting-bolts
are only to be relied upon as such, if they really justify the name;
they are, though easy to specify, by no means easy to secure under the
conditions of practical work. Weak cross-girders may make
alterations--in some cases considerable--necessary, to rectify the
defect of strength. The removal of old girders to make room for new is
seldom resorted to, unless the existing detail renders this a simple
operation; but it is not unusual to introduce new girders between the
old in cases where there is no plated floor to make the work difficult.
By this method there is, of course, an increase of appreciable amount in
the dead load carried by the main girders, which would in many instances
be objectionable. With deep and heavy main girders, having plate webs,
cross-girders may be strengthened by improving the end connections by
suitable gussets, and attachment to good vertical stiffeners, the fixity
of the ends thus aimed at being assured by overhead struts or girders,
from one main girder to its fellow, at intervals apart well considered
with reference to the horizontal strength of the top flanges, the whole
thus making a closed frame, as shown in Fig. 69. The method appears
feasible, but it should be stated that the author has not known it to be
applied in its entirety as a means of strengthening an old floor.
[Illustration: FIG. 69.]
A simple and very common device consists in substituting for the
ordinary cross-sleeper road, where this exists, stout timber
longitudinals under the rails, which have, where the cross-girders do
not exceed 5 feet centres, a marked distributive effect, tending to
reduce the maximum load upon any individual girder. With a similar
object, trough girders containing longitudinal timbers are sometimes
adopted where the depth available is not enough to enable sufficiently
stiff timbers to be used alone. In either case the object sought is the
same--to modify the effect of the heavier wheel loads upon isolated
cross-girders. When the spacing is so close as 4 feet, the beneficial
result of this treatment is considerable, but at 8 feet centres it can
have but a moderate effect where timbers alone are used.
Occasionally, for long cross-girders, a distributing girder is placed,
with the same intent, in the 6 feet way, its function being limited to
this use only if the depth and strength are sufficiently small to serve
this object alone, as distinct from the case in which it becomes a
carrying girder transferring load to the abutments. As a distributor
simply, the girder has to equalise the bending moments amongst the
cross-girders, to effect which it will be evident that these moments
having been ascertained for the several cross-girders previous to
alteration, for a position of the wheel loads such that the heaviest
comes upon a centre cross-girder, the mean of these moments will, when
compared with that for each girder, show the difference to be induced as
a result of introducing the distributor. These differences of moment
render necessary at the centre of the cross-girders reactions upwards or
downwards, as the case may be, of amounts competent to induce moments
below the inner rails equal to these differences.
It is these reactions which must be provided by the distributing girder
at a moderate stress, and without flexure of such an amount as sensibly
to modify the reactions. The greatest section necessary at any one point
may then be adopted for the girder throughout. The result will commonly
work out to a moderate section, but there will be no harm in a little
excess in a case of this kind, the total cost being but little affected
by some small addition to the weight, where labour upon the site is so
considerable an item as in work of this description. The ends of the
distributing girder should be carried on to the abutments or piers to
ensure adequate relief of the end cross-girders. It will be found
desirable in arranging for distributing girders to ascertain at an early
stage, by boning or by levelling, the condition of the cross-girders as
to uniformity of heights, as this may affect the length most suitable
for separate sections. Between the underside of the distributor and the
cross-girder tops there will commonly be spaces of varying amounts,
which should be filled by packings to fit, rather than by pulling the
work together by force, introducing undesirable stresses of uncertain
amount.
In the earlier remarks upon the strengthening of bridgework by the use
of new material, it has been assumed that the modulus of elasticity of
the new metal is similar to that of the old; it may, however, as in
cases where wrought-iron work is reinforced by additions in steel, be
necessary to take the difference of elastic properties into account,
with which object the new section should be multiplied by a quantity
(greater or less than unity) inversely proportional to the higher or
lower modulus of the new material, that is to say, by
E of old material
-----------------
E of new material