Civil Engineering Reference
In-Depth Information
by spanning a gap. By definition, then, a bridge is a structure that affords
passage at a height across a gap. Let us now take the three pieces of the
definition and consider them each, though in reverse order: the gap to be
spanned, that which will make passage across it, and the structure that will
support the passers' weight.
For the
gap
that the bridge crosses, a river most readily comes to mind,
but it could just as well be a channel, lake, estuary, or the like. Or it may
be a chasm, canyon, mining pit, ice crevice, or space between buildings.
All these taken together still form a minority of the gaps that bridges cross.
Many of the rest are the spaces between the raised sides of a roadway or
railway. The curved ramp that raises or lowers traffic at highway interchanges
is a bridge, too. So is the elevated highway, sometimes known as a viaduct,
which spans the gap as it traverses a row of piers, sometimes casting its
shadow over another highway running below.
That to which the bridge affords passage
—well, it is people, vehicles, and
the goods they carry, perhaps with livestock tagging along. Some bridges
are solely for pedestrians and bicycles; a large number are for railways. In
present-day America, that to which the bridge gives passage is overwhelm-
ingly automobile traffic. Unless we specify otherwise, when we say “bridge”
in this topic, we mean one primarily meant to carry motorized road vehicles,
though it may carry pedestrians and trains in addition.
The things that cross have weight and momentum. To afford them
passage, the bridge must consist of an assembly of parts—a
structure
—that
supports the forces acting on it. The structure must carry its own weight,
stand up to the loads vehicles impart to it, and resist the forces of winds
and waves and of the occasional errant barge that hits a pier. Those who
would like to be informed about bridges should be able to understand the
basics: the thinking by which engineers decide which kind of structure will
safely carry the loads imposed on it.
THE BRIDGE DECISION
Even in a road transportation system as large as America's, we have far
more bridges than most would guess, some 600,000 in fact. Every 500 or
so Americans owns a bridge, or better put, each American owns a share in
the nation's vast bridge portfolio. And that means many decisions have to
be made about bridges, whether to build them, upgrade them, or close and
replace them. At many places in America, every few years, citizens and their
representatives, along with expert advisers, have to make such decisions.
We should pause, however, to consider whether it might be better
to burrow underground to the other side than to span the gap above. It is
rarely a good idea. Only in exceptional cases is a tunnel the right choice,
