Civil Engineering Reference
In-Depth Information
THREE
UNDERSTANDING STRESSES AND STRAINS
WHAT THE STRUCTURE MUST DO
Let us recall that the purpose of the bridge structure is to stand up to the
forces that would drag it down. To engineers, these forces are
loads,
and
the structure's capacity to withstand them is
resistance.
The engineer's fun-
damental job is to assure that loads imposed on the bridge do not exceed
its capacity to withstand (to resist)
the loads. The critical single lesson in
bridge engineering, the indispensable idea, the one never to be forgotten,
is that resistance should equal or exceed load. So a good place to start
thinking about a future bridge is with an estimate of the loads it will have
to carry.
To begin with, there is what is known as
dead load:
the structure's
own mass along with those things permanently affixed to it. In almost all
bridges, the greatest mass to be borne is that of the dead load. It may not
be obvious that, for the vast majority of bridges, the paved deck on which
one travels is actually not an integral part of the structure, but is rather
carried on it—it is an additional item of dead load. Other dead loads are
railings, traffic signs, traffic signals, and light poles.
Then there is
live load
, which is in turn divided into
stationary load
and
dynamic load
, the latter also known as
time-varying load
. The former consists
of masses temporarily resting on the bridge—these might include cars and
trucks stuck in a traffic jam or waiting at a tollbooth, heavy equipment
(critical during construction and maintenance), people, vehicles' contents,
and ice buildup.
Moving vehicles exemplify dynamic loads. When a car moves along
the bridge deck, it bounces or vibrates, slightly jarring the structure each
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