TIMBER BRIDGES.
Timber bridges, though probably the most ancient in type, are yet the
least durable in any particular instance. The perishable nature of the
material when used for exposed construction renders it peculiarly liable
to develop defects which quickly put a limit to the life of the
structure. In addition to decay in the body of the main members--which
may perhaps be long delayed, so that a simple beam bridge may last for
many years--there is in more complex designs decay at connections and
joints, which proves very detrimental to the integrity of the whole.
Water running upon the surface of a member gravitates to its lower end,
and, if there be a joint or other connection, settles there, to be
productive of lasting mischief. From this cause, together with a very
common deficiency of bearing surface relative to the forces to be met,
the joints soon develop some movement; working of the structure
commences under passing loads, its final destruction being then a
question of time only. Each joint is, in fact, in timber bridge
construction a source of serious weakness to a degree which has no
parallel in well-designed metallic bridges.
Wrought-iron straps to confine the ends of raking members, or for other
uses, are liable to crush into the wood, and bolts are apt to enlarge
the hole through which they pass. Wood keys, where these are introduced
to prevent one timber from sliding upon another, are also prone to
develop cracks in the main members, and fibre crippling from excess of
stress. All these defects are, however, in timber-work more easily
defined than efficiently remedied, as it is barely practicable for any
but the harder woods to ensure, for heavy loads, a sufficiency of
bearing surfaces.
The most readily detected evidence of deterioration in timber bridges is
the sag of its bearing members, or trusses, for the simple reason that
if there is no local trouble at the joints, there will probably be no
appreciable drop at the centre of the span. The existence of such a
depression may, however, be caused in rare instances by the spread of
the supporting piers or abutments, particularly in the case of beams
trussed by end diagonal rakers and having no tie.
Bridges formed of deep trusses, with the road upon the top, are
sometimes found to be wanting in lateral bracing, the result of which is
that the main trusses go out of line, leaning considerably one way or
the other, being checked only by such rigidity as the joints and
floor-beam attachments may have, with possibly some assistance from the
end connections of the span.
The decay of piles where entering the ground or water is, of course, a
fruitful source of trouble, as also is the sinking of piles, where these
are insufficient in number, or have not been well driven in the first
place.
A vital difficulty with timber structures generally is the uncertainty
that will commonly exist as to how far decay extends in those cases
where it has started. Timber does not necessarily show upon its surface
the evidences of internal rotting. Memel timber may, indeed, be
sometimes found to have become thoroughly unreliable, yet showing no
sign of this upon its painted surface. By sounding the wood with a
hammer, or by probing, its condition may commonly be ascertained. In
cases of doubt, an auger-hole will make it clear as to whether the
interior be good or otherwise, as to the particular parts tested; but
only as to those parts, leaving it a matter of guesswork as to the
remainder.
[Illustration: FIG. 85.]
A railway bridge having many of the defects which have been indicated
may be quoted as an example. This structure crossed a canal, supported
upon piles, some of which were in water, others carrying land spans. The
canal span consisted of four trusses, one under each rail, or nearly so,
framed in the manner shown in Fig. 85, precise details not, however,
being now available. The trusses, apart from deflection under live load,
sagged considerably--in one instance, 4-1/2 inches; one inside truss was
also leaning towards the centre line of the bridge as much as 3 inches.
One raker, or diagonal strut, was rotted half through its thickness, and
many other timbers were badly decayed. The end connections and joints
were also in a bad condition. The vertical tie-bolts of the main trusses
were all slack. The piles generally, many of which were badly decayed,
had sunk and inclined towards one end of the bridge about 4 inches in 7
feet of height, the ground being soft and unreliable.
Movement under a passenger train crawling over the bridge was very
appreciable, but not startling. There had been introduced, from time to
time, additional timbers and iron ties, with the object of rendering the
spans more reliable, but leaving it somewhat difficult to determine the
function of the several members. The bridge was, of course,
reconstructed.
[Illustration: FIG. 86.]
[Illustration: FIG. 87.]
[Illustration: FIG. 88.]
An instance may here be cited showing how badly distorted a timber
structure may become without actually falling. The bridge referred to
consisted of three spans of 29 feet, each span having two trusses,
between which ran a colliery tramroad, 1-foot 6-inch gauge; the corves
running upon this, at 4 feet 6 inch centres, weighed, when full, about
10 cwt. each. The trusses were badly out of shape, the centre span
having sagged 5-1/2 inches, with one truss of the same span nearly 10
inches out of line at the centre. This little bridge, of which some
details are shown in Figs. 86, 87, and 88, had been in use about twenty
years.
[Illustration: FIG. 89.]
A third case which may be named is that of a road bridge, about 12 feet
wide, crossing by thirteen spans a shallow river liable to floods. The
construction was of a simple character, as indicated in Fig. 89, and
consisted of piles supporting trussed beams, which had sagged in some
instances over 2-1/2 inches. The bridge had, some years previous to the
author’s inspection, been heavily repaired, many new strut and
stretching pieces having been introduced, the piles also being
reinforced or renewed. Five years before, a traction engine, said to
weigh 5 tons, had passed across the bridge in safety; but the author
noticed that a coal wagon, which, with the horse, weighed about 50 cwt.,
when walked slowly over set up much movement. This bridge had been in
use nearly thirty years, and was very much out of line from end to end.
Though timber bridges cannot at the best be considered durable, yet, by
attention to certain points in design and construction, their length of
life may be materially enhanced. Every cut across the grain may be
considered an element of weakness by exposing the material to quicker
decay, for which reason the number of ends, or of joints, should be
reduced to a minimum. An additional reason for reducing the number of
joints or other connections is the liability of these to develop
movement, as already stated, the yield of any one joint, being the cause
of movement in others, which might, but for this, have remained close.
These considerations lead to the conclusion that fewness of parts is, in
timber construction, as in structural work generally, an excellent
principle to observe. Mortising, elaborate scarf joints, recessing, or
any cutting into the timber which is not essential, should be avoided,
the simplest forms of connection being preferable, if at all suitable.
If a step or butt surface is wanted for any member, it is commonly
better to provide this by a cleat or other added piece, rather than by
cutting into the timber butted against.
A complicated joint formed in the body of main timbers can only be
renewed by renewal of the timber itself, whereas by the method indicated
the joint is readily tightened, or re-made, without involving the main
member. Bearing surfaces should be ample, straps of liberal dimensions,
and bolts large (with good washers), both for the sake of bearing
surface in the holes, and reduction of any liability to bend under
cross-stress. In trusses of the form shown in Figs. 85 and 86, it is
desirable to introduce diagonal members in the middle bay, even though
it may appear that the stiffness of the main beams is sufficient to
render this unnecessary as a matter of strength, as without these there
is apt to be, under rolling load, a slight distortion, leading to
working of the joints and free entry of moisture. Lateral bracings
should also, for much the same reasons, be introduced, even though they
may not appear necessary in the new structure, with joints all close and
effective.
Projecting ends of timbers should be carried out well beyond the
requirement of strength or bearing, in order to ensure a liberal margin
for that decay in the end fibres which commonly develops. Timbers
resting upon abutments, or running into confined spaces, should be
arranged for free ventilation and ready drying. Occasionally joints at
the lower ends of timbers are protected by lead or zinc flashings to
prevent water running into them, a method which should have some
protective value. Whatever measures may be adopted, whether in the
design or execution of timber bridge-work, will, however, be but little
effective, if the timber itself is not good of its kind, and well
seasoned.
Creosoting to be useful should be thorough and something more than skin
deep. The timber itself should be well dried before treatment.
The repair of timber bridges very largely consists in the renewal of
decaying timbers, where this is practicable, or in adding supplementary
pieces where the old cannot conveniently be displaced. Joints may be
tightened up by hard-wood wedges, properly secured to prevent slacking
back, all bolts being also screwed up tight, perhaps some additional
being introduced.
Piles standing in water, which have decayed, may be strengthened by
driving other piles between the old, or on either side, but not of
necessity opposite to them, and by means of waling timbers bolted to the
old piles, put in a position to take load, either by the walings resting
upon their tops, or being bolted to them. Piles decayed where entering
solid ground may generally be strengthened by bolting on supplementary
timbers to reach well above and below the decayed part, or by cutting
out the bad length, introducing a new piece, and fishing the butt-joints
in a proper manner. But all remedial measures have generally to be
considered with reference to cost, as compared with the probable
increase of life of the structure. With a bridge in an advanced state of
decrepitude, such repairs may prove anything but economical, and at the
best defer reconstruction but a very moderate length of time.