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 This
picture reminds us that reliability begins with foundations.
This building was built near a beach, and the ground has proved
inadequate; perhaps a hurricane contributed. Bridge-building has generally been a safe discipline,
but no activity can progress without a few disasters, unless we are prepared never to tackle the unknown.
The Tay railway bridge was designed
with a badly underestimated resistance to high winds, and there were
instances of faulty materials and inadequate responses to detected
problems.
Several suspension bridges have
succumbed to induced oscillation. In the year 2000, the Millennium
bridge across the river Thames was closed after only two days of use because of oscillation.
One way to reduce the probability of
trouble from unknown causes is to make tests on scale models.
Computer programs can be used to simulate many structures. In both
cases the results will of course be no more accurate than the
faithfulness of the simulation.
Accidents and problems during
construction
Some early box-bridge spans fell down
during construction, a period during which many structures are more
vulnerable than when complete. These bridges were designed as
continuous hollow beams, with diaphragms at intervals. This is a
strong and light design. But during construction, the growing
beams were cantilevers, supported only at one end. Until they were
joined to their partners, the parts near the piers had to withstand
bending moments that would not occur after completion. In addition
to this, there was also the weight of cranes and other equipment, of
necessity at the tip of the cantilever.
The Milford Haven bridge in west Wales
and the West Gate bridge over the river Yarra near Melbourne, Australia,
collapsed during construction, with loss of life.
The incomplete spans of cable-stayed
bridges, or the towers of incomplete suspension bridges are obvious
candidates for wind-induced oscillation.
Moving and lifting spans or parts of
spans are particularly dangerous operations for bridge builders.
Large and heavy objects in motion have huge momentum if they move
quickly. Lifting them puts them in positions where they are not
yet secured in their final places. Wind, tide and river flow are
among the factors that can cause trouble at these times.
Operations underground and under water
are particularly dangerous, and great care is taken to minimise risk to
safety.
Problems during use
Scouring by rivers is a powerfully
destructive force, which can destroy the most solid masonry. Piers
from early times have been shaped to reduce resistance to the flow of
water, and have often been surrounded by a pavement of stone.
Their foundations have to go so far down that scouring cannot remove a
dangerous amount of material.
Maintenance is a huge task on big
structures – "painting the Forth bridge" is in the UK a
well-known metaphor. Ideally a structure would be designed so as
to require no attention once complete. In practice this is an
impossible ideal. At the very least, any structure needs
inspection, however well it was built. On any large structure you
can see inspection hatches for access to the inside. The
collapse of the Tacoma Narrows bridge is well known. Spare a
thought for the designer. Books do not record a general outcry
against the design, which was presumably accepted as an example of
progress in design economy. It is the designer’s bad luck that he took
his design too far for the current state of knowledge. But other suspension
bridges had been damaged or destroyed by oscillations. History is
not bunk. Download
Accidents during use Some
events are outside the control of the designers and constructors.
Some earthquakes are so powerful that no structure can withstand
them. Ships have been known to collide with bridges, bringing
spans crashing into the river. This happened to the Sharpness
railway bridge over the river Severn. It has never been
replaced. The Sunshine Skyway bridge in Florida was struck by a
ship in 1986, bringing down a span. A
tragic consequence of building a big bridge at a dramatic location is
the attraction to suicidal people. The Clifton bridge is well
equipped with cameras and signs about the Samaritans, but it has proved
necessary to add curved fences to make climbing over the side very
difficult. People sometimes jump off quite small bridges over
roads or railways. People
have occasionally decided to fly aeroplanes under them. The Tower
bridge in London has been the subject of such behaviour. A Vampire
was flown under Clifton bridge, but the pilot misjudged the difficult
manoeuvre needed to escape from the curving gorge. He didn’t make
it. Had he flown slower he might have succeeded. This type
of manoeuvre is extremely dangerous because it cannot be practised – the pilot
can only study maps and models. Maintenance Even
if nothing apparently goes wrong, without maintenance, most structures
will degenerate. Painting the Forth bridge is a well-known
metaphor. On the whole, it is rare for a bridge to be left to rot,
unless it is disused. Here are pictures that show how plants can
seize the smallest opportunity to grow where they are not wanted.
Once they have taken root, they can generate more soil, and they can
generate pressure that widens the cracks. This allows more water
in, increasing the risk of damage by the expansion caused by freezing. 
Plants,
of course, are welcome in the right place. Many shops, pubs and
other buildings have hanging baskets and window boxes, providing colour
during the summer. Some towns even have hanging baskets along the
centre line of major roads. Dürer, in The Nativity, Paumgartner
altar, shows some substantial plants growing from a ruined arch.
    Plants
are sometimes welcome in a wall, even planted deliberately. That
they are wanted in the wall does not, of course, reduce their eventual
effects on the wall. But the owners of a wall that could last for
hundreds of years are not likely to worry about a possible reduction of
its life by a very small amount.
During
the summer of 2000, the
central reservation of the Barnwood bypass in Gloucester was alive with
colour because someone had had the idea of sowing thousands of wild-flower
seeds. In Birmingham, central fences have been decorated with
flower baskets, and in many towns, spring flowering bulbs give pleasure
to thousands of people.
Domestic
gardeners too, often grow plants on the tops and outsides of their
walls, where others can enjoy them. In the background
of this picture is Gloucester cathedral. Unlike the Forth
railway bridge, which is continually painted to keep it from rusting,
cathedrals have to cope with wind and weather without surface
coatings. The Cotswold limestone, used extensively in this
cathedral, and in places like Bath and many Cotswold towns and villages, is vulnerable to acidic impurities in
the air. The next pictures illustrate examples of erosion on the
outside of Gloucester cathedral.
Erosion by wind-borne dust adds to the damage. Masonry at
the top of the cathedral tower became so dangerous that it had to be replaced.
The outside of the cathedral is continually being checked and
repaired. Many buildings in Bath were almost black until a vigorous
cleaning program restored them to the normal pale colour. The
Passing of Time
 Many
a structure that looked so good in the artist’s impression, depicting the
scene on a sunny spring day, with imaginary trees in leaf, later looks
dowdy, as a result of fading or peeling paint, wrinkled or cracked
surfaces, corrosion, creep, dirt, erosion, graffiti, and other forms of entropy.  On
the other hand, the designer of this sculpture must have known that it
would rust. The use of copper on roofs to form an attractive green
patina was once quite common, but is now quite rare.
These
pictures show the effects of time on a multi-arch railway bridge. The
two
left hand pictures shows a crack from top to bottom, where the retaining wall
of the embankment meets the bridge. The two right hand pictures show a
number of small stalactites, formed by dripping rainwater.
In a limestone cave, we marvel at
the size of the stalactites and the curtains of limestone. We
forget the sheer arithmetic of time. If a drop of water runs down
every ten seconds, that’s three million drops in a year. Three
thousand million drops in a millennium. And what is a millennium
in geological time? Not much. Even on this bridge, dry for most days of the
year, less than a hundred years old, we see measurable progress.
The other two pictures show the effects of water running down the sides
of the arch.
From
Gloucestershire to Yorkshire and beyond, stone walls delineate the
landscape. Most have stood for hundreds of years, but as their
utility has declined, so has the care invested in them. Cotswold
limestone in particular, soft and susceptible to cracks, frost,
colonization by plants, damage by people and animals, is suffering
badly. This wall is typical of many that remain. As soon as
any part starts to collapse, it is used as a way through by animals, whose
continual passage hastens the process of decay. The positions of
many other walls
are marked by little more than elongated ridges in the ground.
Intermediate
stages can form useful habitats for snakes, lizards, insects, mammals
and plants. The picture at left shows an adder basking on the
wreckage of an ancient Cotswold wall. The adders find refuge and
prey within these walls.
The
end is nigh . . . . 
http://www.ketchum.org/bridgecollapse.html
Bridge
pro – many useful links including disasters
Ribblehead
Viaduct refurbishment
http://www.iti.nwu.edu/clear/bridge/bri_dis.html
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Mitan suspension bridge collapse bridge
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