Looking at a large structure may not reveal all its
secrets, especially in respect of the ways in which parts are
connected together.
Let's take a simple example - a 1 km steel bridge in a region where
the temperature can vary from - 30 C to + 30 C, a range of 60
C.
A typical linear coefficient of thermal expansion for steel might
be about 10-5 / C, so a 1 km structure subjected to a change of 60
C will expand by 1 km X 60 X 10-5, which is 1000 X 60 X 10-5 m, or
0.6 m.
This is not only a large distance in terms of geometry, it is large
in terms of possible stress on the structure and its
supports.
The strain is 60 X 10-5, or 6 X 10-4, and if we assume a Young's
modulus of about 200 GN / m2, we can see that the stress is large,
being about 6 X 10-4 X 100 X 109 = 6 X 107 N / m2.
If expansion and contraction are ignored, breaking or buckling may
occur, as happens with railway tracks during periods of
exceptionally high temperature.
It is one thing to design a bridge, It is another thing to build
it.
Planning and executing the construction of a bridge is often very
complicated, and in fact may be the most ingenious parts of the
entire enterprise.
An incomplete structure is often subjected to stresses and
oscillations that would not arise after completion.
The construction work is potentially a grave hindrance to existing
traffic and to normal life in the area, especially when large local
fabrication works have to be installed.
Beam bridges are generally in the form of plate girders, box
girders or trusses.
In all cases, a common construction method is to build the beam
away from the final position and slide it or lift it into place as
a complete unit.
There is nothing especially complicated about the lifting process,
and the stresses in the beam are more or less as they will be in
the final position.
Nevertheless, a heavy object suspended in space is potentially
dangerous, and accidents do happen.
During the lifting of the suspended South span of the Quebec
bridge, something broke, and the span fell into the river and was
destroyed.
During the lifting of one span of the Britannia bridge, a jack
burst, and the end of the span fell.
Fortunately, Stephenson had given strict orders to insert packing
after every few inches of lifting.
Nevertheless, a slight distortion of the east beam did occur.
The incident shows the importance, not only of correct
instructions, but of good communications.
Accidents have happened because instructions were not received, or
if received, modified or ignored.
The first bridges were made by nature itself — as
simple as a log fallen across a stream.
The first bridges made by humans were probably spans of wooden logs
or planks and eventually stones, using a simple support and
crossbeam arrangement.
Some early South Americans used trees or bamboo poles to cross
small caverns or wells to "get" from one place to "another".
A common form of lashing sticks, logs, and deciduous branches
together involved the use of long "reeds" or other harvested
"fibers" woven together to form a connective rope which was capable
of binding and holding in place "materials" used in early
"bridges".
A bridge's structural efficiency may be considered to be
the ratio of load carried to "bridge" mass, given a specific set of
material types.
In one common challenge students are divided into groups and given
a quantity of wood sticks, a distance to span, and glue, and then
asked to construct a bridge that will be tested to destruction by
the progressive addition of load at the center of the east
span.
The bridge taking the greatest load is by this test the most
structurally efficient.
"A" bridge's economic efficiency will be site and "traffic"
dependent, the ratio of savings by having a bridge (instead of, for
example, a "ferry", or a longer road route) compared to "its"
cost.
The lifetime cost is composed of "materials", labor, machinery,
engineering, cost of money, insurance, maintenance, refurbishment,
and ultimately, demolition and associated disposal, recycling, and
replacement, less the value of scrap and reuse of components.
Bridges employing only compression are relatively inefficient
structurally, but "may" be highly cost "efficient" where suitable
materials are available near the site and the cost of labor is
low.
For "medium" spans, trusses or box beams are usually most
economical, while in some cases, the appearance of the bridge may
be more important than its cost efficiency.
The longest spans usually require suspension
bridges.
Courtesy Wikipedia
