Civil Engineering Reference
In-Depth Information
time. When the car accelerates, it delivers a force on the bridge in a direc-
tion opposite the acceleration. Another way to put this is that the dynamic
load changes in magnitude or intensity over the time that it is on the
bridge. When the driver hits the brake, screeching to a halt, he imposes a
more intense load than if he had slowed gradually. In normal traffic, these
combined vehicular loads are asynchronous and intermittent, but on some
occasions, as when a tractor-trailer jackknifes and the cars behind it simul-
taneously hit their brakes, the combined load jolts the bridge.
Another and extreme kind of dynamic force is a one-shot blow known
as
impact load
. This may be an out-of-control truck that smashes into a
column. It could also be a shake from an earthquake or a block of floating
ice hitting a pier. Such matters are among the bridge engineer's greatest
concerns, but we get to them in due course in chapter 6. In the meantime,
let us just keep in mind the general lesson that the structure must bear the
dead loads plus the various stationary and dynamic live loads to which it
will be exposed.
STRESSES AND STRAINS
To begin even to assess the effects of a load on a structural member, engi-
neers use a measure of what is defined as
stress.
Stress refers simply to load
applied per unit area of the structural member. The reason for concern about
stress should be clear. Imagine 100,000-pound weights placed on each of two
upright cylinders of the same material. One has a diameter of 3.5 inches
and the other of 7 inches. The 100,000-pound weight has much more of
an effect on the thinner cylinder, but let us explain why. It is because a
horizontal slice through the thinner one has an area of 9.6 square inches
and through the thicker one of about 38 square inches. The thicker one has
double the diameter, but four times as much cross-sectional area.
So as to not have to say “thousands of pounds,” American structural
engineers take recourse to a unit used by no one else: the kilo-pound, refer-
ring to 1000 pounds, and known for short as a
kip
. It is a hybrid between
metric and the English customary system (to be contrasted to the traditional
ton of 2000 pounds and the metric ton of 1000 kilos). In this parlance,
an applied load of 1000 pounds per square inch is known 1 kip per square
inch, and abbreviated 1ksi.
Recall that stress is defined as applied force per unit area. So the 100
kip applied to the thinner cylinder (about 10 square inches) exerts stress of
10 ksi, but on the thicker cylinder (let us round it off to 40 square inches),
about 2.5 ksi. The stress from the load is four times greater on the thinner
than the thicker cylinder (figure 3.1).
