Civil Engineering Reference
In-Depth Information
Roads hug the earth's surface. Pipelines and tunnels burrow under-
neath it. But bridges soar through the air, without ever really leaving the
ground. On beholders they make a distinct impression. Unlike buildings,
which are more numerous but clad with outer surfaces that usually keep the
underlying structure hidden, bridges reveal the structural principles that keep
them aloft. They are the most visible expressions of engineering as art, or of
architecture as science. Some are gateways to regions, symbols for entire cit-
ies, and world-renowned monuments in their own right. Some bridges, like
some violin concertos, have magnificence that cannot be expressed in words.
While many kinds of human contrivances mar the natural landscape,
bridges—even ones that are not particularly famous—are likely to comple-
ment it. They provide sequentially shifting panoramas for those crossing
them, dramatic objects for those observing them from a shore or embank-
ment, and framed horizons for those looking through or past them. Bridges
as structural art are to be appreciated in their own right, but also as environ-
mental art: pieces of artifice that enhance awareness not just of the artwork
itself but also of the hills, chasms, torrents, skylines, or forests among which
they are situated.
Before they can be art, they are economic infrastructure. They are
essential because we move around on the earth and the earth's surface is,
fortunately, not a flat and solid expanse. It has gullies, rivers, valleys, hills,
swamps, crags, coves, and cliffs that must be crossed if we're to get about.
Since we build roads and railways, it is often also wise to make them leap
over each other instead of intersecting.
To accomplish that crossing by which it becomes an economic asset,
the bridge must first be designed and built as a physical structure—which
now needs definition.
WHAT IS A BRIDGE?
In movies when a galloping cavalry reaches a river, the riders inevitably
coax the horses to swim across, just their heads above water, even if their
mounts are in full armor. This way of crossing the river works, we suspect,
only in the movies. Moses developed the method of getting the waters to
part, a procedure that is no longer recommended since too many regula-
tory approvals would be needed. A ferry may be pleasant, if the waves are
not too choppy and the wait at the dock not too long. In a pinch, and in
the absence of a ferry or rowboat, a brisk swim might do; a catapult is best
declined, even in desperation.
A bridge differs from the other ways of getting across in that it is a
fixed structure that affords passage across; but, as a tunnel does the same
by a rather different route, we have to add that the bridge reaches across
