BRIDGE FLOORS.
The floors of bridges commonly give more trouble in maintenance, and
their defects are more frequently the cause which renders reconstruction
necessary, apart from reasons not concerning strength, than any other
part of such structures. When it is considered that this portion of a
bridge is first affected by impact of the load which comes upon it, and
is usually light in comparison with the main girders further removed
from the load, and to which the latter is transferred through the more
or less elastic floor, the fact will be readily appreciated by those not
already familiar with it.
The end attachments of cross and longitudinal girders are very liable to
suffer by loosening of rivets, or, more rarely, by breaking of the
angle-irons which commonly make such a connection. A not unusual defect
of old work, which may also sometimes be seen in work quite new, where
the cross-girder depth has from any cause been restricted, is the
extremely cramped position of the rivets securing the ends. There is
small chance of these ever being properly tight, if the act of riveting
is rendered difficult by bad design. This is the more objectionable if
it happens that cross-girder ends abut against opposite sides of the web
of an intermediate main girder, and are secured by the same rivets
passing through. At the best such rivets will not be well placed to
insure good workmanship, and the severe treatment to which they become
subject, as the cross-girders take their load and deflect under it,
will be very apt to loosen them. The author has seen a case of this kind
(see Figs. 11 and 12)--rather extreme, it is true--in which nearly the
whole of the cross-girder end rivets were loose, some nearly worn
through, thus allowing the cross-girders to be carried, not by their
attachments, but by resting upon the main-girder flanges, which in turn,
by repeated twisting, tore the web for a length of 4 feet; there was
also pronounced side flexure of the top booms. The movements generally
on this bridge (of 42-feet span), whether of main or cross-girders, were
very considerable and disturbing. It was removed after about
twenty-three years’ use.
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
There is no necessity, as a rule, for the ends of cross-girders attached
to the same main girder at opposite sides to be placed in line. The
author prefers to arrange them to miss, by which device each connection
is entirely separate, the riveting can be more efficiently executed,
erection is simplified, and the rivets will be more likely to keep
tight. Other special cases of cross-girder ends will be dealt with under
the head “Riveted Connections.”
It is sometimes contended that cross-girders attached at their ends by a
riveted connection should be designed as for fixed ends, in which case
they are usually made of the same flange section throughout, with a view
to satisfy the supposed requirements. But a girder to be rightly
considered as having fixed ends must be secured to something itself
unyielding. With an outer main girder of ordinary construction, and no
overhead bracing, this is so far from being the case as to leave little
occasion for taking the precaution named. As the cross-girders deflect,
the main girders will commonly yield slightly, inclining bodily towards
the cross-girders, if these are attached to the lower part of the main
girders. The force requisite to cant the main girders in this manner is
usually less than that which corresponds to fixing the cross-girder
ends, and is, generally, slight. It is, of course, necessary that this
measure of resistance at the connection should be borne in mind for the
sake of the joint itself, quite apart from any question of fixing.
Possibly, in quite exceptional cases, where very stiff main girders are
braced in such a manner as to prevent canting, it may be proper to
consider the cross-girder ends as fixed, or for those near the bearings
of heavy main girders; but the author has not met with any example where
cross-girders, apart from attachments, appear to have suffered from
neglect of this consideration.
With cross-girders placed on either side of a main girder, and in line,
it may also, for new work, be desirable to regard the ends as fixed, and
to detail them with this in view. It does not, however, appear wise to
carry this assumption to its logical issue, and reduce the flange
section to any appreciable extent on this account. The fixity of the
ends will, in any such case, be imperfect; and when one side only of an
intermediate main girder is loaded, it can have but a moderate effect in
reducing flange stress at the middle of the loaded floor beam.
[Illustration: FIG. 13. FIG. 14. FIG. 15.]
Similar reasons affect the design of longitudinal girder attachments to
cross-girders, which, if intended to support rails, cannot of necessity
be schemed to come other than in line. Where the floor is plated as one
plane surface, there will not usually be any trouble resulting if no
special precautions are used, as the plate itself will insure that the
longitudinals act, in a measure, as continuous beams, relieving the
joints of abnormal stress. If the plating is, however, designed in a
manner which does not present this advantage, or if the floor be of
timber, it is better to decide whether the connections shall be
considered as fixed, and made so; or avowedly flexible, and detailed in
such a manner as to possess a capacity for yielding slightly without
injury. Those connections are most likely to suffer which are neither of
the one character nor the other, offering resistance without the ability
to maintain it. Figs. 13, 14, and 15 give representations of three
“spring joint” methods of insuring yield in a greater or less degree.
For small longitudinals it is, perhaps, sufficient to use end angles
with very broad flanges against the cross-girder web; these to be
riveted in the manner indicated in Fig. 15.
Liberal depth to floor beams is distinctly advantageous where it can be
secured, rendering it easier to design the ends in a suitable manner, by
giving room near mid-depth of the attachment to get in the necessary
number of rivets; or where the ends are rigidly attached direct to
vertical members of an open-work truss, the greater depth is effective
in reducing the inclination of the end from the vertical, with a
correspondingly reduced cant of the main girders and flexure of the
vertical member, with smaller consequent secondary stresses. In any case
deep girders will contribute to stiffness of the floor itself,
favourable in railway bridges to the maintenance of permanent-way in
good order.
[Illustration: FIGS. 16, 17, 18.]
A point in connection with skew-bridge floors occasionally overlooked is
the combined effect of the skew, and main girder camber, in throwing the
floor structure out of truth, if no regard has been paid to this. The
result is bad cross-girder or other connections; or, in the case of
bearers running over the tops of main girders, a necessity for special
packings to bring all fair (Fig. 17). The author has in such cases,
where cross-girders are used, set the main girder beds at suitable
levels, in order that the cross-bearers may all be horizontal (see Figs.
16 and 18). This may not always be permissible; but, however the
difficulty may be met, it should be dealt with as part of the design.
For small angles of skew only may it be neglected.
Rivets attaching cross-girder angles to the web will occasionally
loosen, probably due in most cases to bad work, together with some
circumstance of aggravation, as in the case of a bridge floor consisting
of girders spaced 3 feet 6 inches apart, with short timber bearers
between, carrying rails. In many girders the top row of rivets, of
ordinary pitch and size, had loosened, allowing the web, about 1/4 inch
thick, a movement of 1/8 inch vertically. The rails being very close
down upon the cross-girder tops, though not intended to touch, had at
some time probably done so, and by “hammering” produced the result
described.
Plated floors are often found which are objectionable on account of
their inability to hold water, arising sometimes from bad work, as often
from wide spacing of rivets. With rivets arranged to be easily got at,
and pitched not more than 3 inches apart, a tight floor may be expected;
but it is still necessary to drain the floor by a sufficient number of
holes, provided with nozzles projecting below the underside of the
plate, and sufficiently long to deliver direct into gutters, where these
are necessary. Drain-holes should not be less frequent than one to every
50 square feet of floor, if flat, and may advantageously be more so.
Gutters should slope well, and care be taken to insure practicable
joints and good methods of attachment--a matter too often left to take
care of itself, with considerable after-annoyance as a result.
The use of asphalt, or asphalt concrete, to render a plated floor
water-tight is hardly to be relied upon for railway bridges, though no
doubt effective for those carrying roads. It is extremely difficult to
insure that it shall stand the jarring and disturbance to which it may
be subject, and under which it will commonly break up, and make matters
worse by holding moisture, and delaying the natural drying of the floor.
In bulk, as in troughs, it may be useful, but in thin coverings on
plates it cannot be depended upon.
Floors having plated tops are sometimes finished over abutments or piers
in a manner which is not satisfactory, either as regards the carrying of
loads or accessibility for painting. If the plates are carried on to a
dwarf wall with the intention that the free margin of the plate shall
rest upon it, there will be a difficulty in securing this in an
efficient manner. Commonly such a wall is built up after the girder work
is in place, making it difficult to insure that the wall really supports
the plate, the result being that this may have to carry itself as best
it can. In any case, severe corrosion will occur on the underside, and
the plate rust through much before the rest of the floor; the masonry
also will usually be disturbed.
It appears preferable to form the end of the floor with a vertical
skirting-plate having an angle or angles along the lower edge. This may
come down to a dwarf wall, but preferably not to touch it, the skirting
being designed to act as a carrying girder. A convenient arrangement is
shown in Fig. 19, which may be used either for a square or skew bridge.
It will be seen that the plate-girders have no end-plates, the skirting
referred to being carried continuously along the floor edge, and
attached to each girder-web, the whole of the more important parts being
open to the painter.
[Illustration: FIG. 19.]
Trough floors consisting of one or other of the forms of pressed or
rolled section present the objection that it is almost impracticable to
arrange an efficient connection at the ends, if they abut against main
girders, and but little connection is, as a rule, attempted, and
sometimes none. The result is that the load from these troughs is
delivered in an objectionable manner, and the ends being open or
imperfectly closed, water and dirt escape on to the flange, or other
ledge, which supports them. A description of pressed floor which
promises to overcome this objection, and provide a ready means of
attachment to the webs of plate-girders, or of booms having vertical
plate-webs, has within the last few years been introduced. This has the
ends shaped in such a manner as to close them and provide a flat surface
of sufficient area for connection by rivets. Each hollow is separately
drained by holes with nozzles. Whether this type of trough will develop
faults of its own, due to over-straining of the metal in the act of
pressing, remains to be seen; but as it appears possible to produce the
desired form without any material thinning or thickening of the metal,
the contention that no severe usage accompanies the process appears to
be reasonable.
That form of troughing in which the top and bottom portions are
separately formed, and connected by a horizontal seam of rivets at
mid-depth, is found in use upon railway bridges to be very liable to
loosening of those rivets near the ends; less surprising, perhaps,
because the sloping sides are usually thin.
It is a distinctly difficult matter to join two or more lengths of any
trough flooring having sloping sides, in a workmanlike manner; the fit
of covers is apt to be imperfect, and some rivets, being difficult of
access, are likely to be but indifferently tight, so that if the joint
occurs where it will be more than lightly stressed, trouble will
probably follow. A bad place for such joints is immediately over girders
supporting the troughs, as there the stress will be most severe, any
leakage come directly upon the girder, and remedial measures be more
difficult to carry out.
Timber floors of the best timber, close jointed, are more durable than
might be supposed. The disadvantage is a difficulty in ascertaining the
precise condition of the timber after many years’ use. The author has
seen timbers, 9 inches by 9 inches, forming in one length a close floor,
carried by three girders, and supporting two lines of way, which, when
taken out, could as to a considerable part be kicked to pieces with the
foot; whilst in another case, with the same arrangement of girders and
close-timbered floor, the wood, after being in place for thirty-two
years, was, when taken out, found to be perfectly sound, with the
exception of a very few bad places of no great extent. In this instance,
however, it is known that the floor--pitch-pine--was put in by a
contractor who prided himself upon the quality of the timber that he
used; the floor being also covered with tar concrete, which had in this
instance so well performed its office as to keep the timber quite dry on
the top.
Jack arches between girders make an excellent floor for road bridges,
though heavy; and for small bridges may be used to carry rails, if the
girders are designed to be stiff under load. The apprehension that
brickwork or concrete will separate from the girder-work, or become
broken up under even moderate vibration, does not seem to be well
founded, if the deflection is small and the brickwork or concrete good.
The use of corrugated sheeting as a means of rendering the underside of
a bridge drop dry cannot be too strongly deprecated. If it must be
adopted, the arrangement should be such as to permit ready removal for
inspection and painting. It is evident that by boxing up the floor
structure, rust is favoured, and serious defects may be developed, not
to be discovered till the sheeting is removed, or something happens.
[Illustration: FIG. 20.]
A case may be instanced in which it was found, on taking down sheeting
of this description, that the floor girders, previously hidden, were
badly wasted in the webs. One of these girders had cracked, as shown in
Fig. 20, and others were in a condition only less bad.
In any floor carrying ballast or macadam, if means are not adopted to
keep the road material from the structure of the floor, or from the main
girders, corrosion may be serious in its effects. Cinder ballast is,
perhaps, the worst in this respect, in its action upon steel or
ironwork, being distinctly more damaging than any other kind commonly
used.
Rail-joints upon bridge floors are to be avoided where practicable by
the use of rails as long as can be obtained; if the bridge is small
enough, crossing it in one length. At each joint there is likely to be
hammering and working extremely detrimental to floor members and
connections; indeed, it may happen that loose rivets will be found in
the neighbourhood of such joints, and nowhere else on the bridge. Where
rail-joints cannot be avoided, their position should, if there be any
choice, be judiciously selected, and the plate-layers taught to close
the joints and jam the fish-bolts.
[Illustration: FIG. 21.]
As rail-joints upon a bridge may injuriously affect the floor, so also
will a weak floor be very trying to the rails. A remarkable instance of
this has come under the writer’s notice, where a bridge (Fig. 21) of
three 33-feet spans, having outer and centre main girders, with
cross-girders spaced 3 feet apart, resting upon the girder flanges, but
not attached, and carrying two roads, had the permanent-way in a very
bad state. The rails proper, with supplementary angle-plates, rested
direct upon the cross-girders, which were decidedly light, and the whole
floor had much “life” in it, the ill-effect of which was shown in
thirteen breaks in the angle-plates, in each case near their ends,
generally at holes.
It appears probable that severe stresses may be thrown upon the parts of
a floor, whether placed at the level of the bottom booms or of the top,
by changes of length in the booms due to stress. The author has,
unfortunately, no direct evidence to offer in reference to this, tending
either for or against the contention. If an unplated floor of cross and
longitudinal girders of usual arrangement be at the bottom boom of a
large bridge, as the boom lengthens with the imposition of load upon the
bridge, all the cross-girders from the centre towards the abutments will
be curved horizontally, the middle portion being restrained by the
longitudinals from moving bodily with the ends. Each cross-girder except
that at the centre, if there be one, will thus present a figure in plan,
concave towards the abutment to which it is nearest. This will be
accompanied by stressing of the connections, and a transfer to the
longitudinals of as much of the tensile stress properly belonging to the
booms as the stiffness of the cross-girders may communicate.
This in itself will hardly be considerable, and will be the less on
account of a slight yielding which may be expected at the end
connections of each longitudinal; but the effect upon the cross-girders
by horizontal bending will be much marked. If the case be supposed of a
200-feet span in steel at ordinary loads and stresses, carrying one line
of way, with cross-girders 20 feet apart, and having no floor-plates, it
may be ascertained, neglecting for the moment any slight yielding of the
longitudinal girder connections, that upon the bridge taking its full
live load there will be the following approximate results: Movement at
each end of the end cross-girders of 3/10 inch, equivalent to a force of
7-1/2 tons, tending to bend them horizontally, and a mean stress on the
outer edges of the girders, 12 inches wide, of 8 tons per square inch
due to flexure, which, compounded with the ordinary flange stresses,
will seem to give rather alarming results. There will also be a
longitudinal stress in the rail-girders, at centre part of bridge, of
3/4 ton per square inch. Normal elongation of the longitudinal girder
bottom flanges, and compression of the top, modifies the figures
unfavourably as to the cross-girder top flange. Yielding of the
connections named before has been neglected in arriving at these
stresses. If they are sufficiently accommodating to give freely, to a
mean extent, as between the top and bottom of each joint, of 1/29 inch,
these results will disappear. It is evident, however, that we cannot
rely upon good work yielding without the existence of considerable
forces to cause it. In the issue it is justifiable to apprehend that the
flexing and stressing of the cross-girders will be considerable.
The most favourable case has been taken; if now it is assumed that the
floor has continuous plating, the results would seem to be much more
astonishing. It will appear on this supposition that the boom stresses,
instead of being taken wholly by the booms, are about equally divided
between these and the floor structure, each cross-girder connection
communicating its share of boom stress to the floor, which for the end
cross-girders will approach 40 tons at each connection--considerably
more than the vertical reaction under normal loads.
Palpably, these conclusions must be greatly modified by the yield of
longitudinal girder ends, and slip of the floor rivets in transverse
seams. If these rivets be 3-1/2 inch pitch and 3/4 inch in diameter, the
stress at each, as estimated, would be sufficient to induce shear of
about 6 tons per square inch--more than enough to cause “slip.” After
making this allowance, it is still evident there must be very serious
forces at work about the ends of cross-girders under the conditions
supposed, probably not less than one half the amounts named, as with
this reduction the floor rivets should not yield, given reasonably good
work. It is to be observed that the effect of live load only has been
introduced, on the presumption that the longitudinals and floor-plating
have not been riveted up till the main girders have been allowed to
carry the major part of the dead load; but even this cannot always be
conceded. The deduction appears to be that the floor and cross-girder
connections should be studied with special reference to these possible
effects, either with the object of rendering the communication of these
forces harmless, or making the floor so that it shall take little or no
stress from the main booms, by arranging joints across the floor
specially designed to yield, the ends of longitudinals being schemed
with the same object. Where there is no plating, the case is, perhaps,
sufficiently provided for by making the cross-girders narrow, and the
longitudinal girder connections flexible, or by putting these girders
upon the top of the cross-girders, when stretching of the bottom flanges
of the rail-bearers under load may be expected, within a little, to keep
pace with the lengthening of the main booms.
It would appear that light pressed troughs running across the
longitudinals would, by yielding in every section, also furnish relief,
as compared with the rigidity of flat plates.
By placing the floor at a level corresponding to the neutral axis of the
main girders, the communication of stress to the floor may be avoided;
but it seldom happens that there is so free a choice as to floor height
relative to the girders. This solution is, therefore, of limited
application.
It is obvious that somewhat similar effects must obtain to those
considered in detail, when the floor structure lies at the level of top
booms, but with forces of compression from the booms to deal with,
instead of tension.