Earthquakes & Bridges

1
Have you ever wondered how earthquakes affect
bridges? I must admit that I had not thought
about that subject until quite recently.
2
In the last few years in the North Texas area, we
have been experiencing a significant increase in
the frequency and magnitude of earthquakes.
3
There are many who say that this is the result of
the exploration for underground gas reserves,
which has become widespread in this area.
4
No matter the root cause, the subject of how
earthquakes affect bridges is now on the minds of
many.
5
When evaluating the affects of earthquakes the
most important factors to take into account are
the magnitude of the earthquake, the distance
from the epicenter of the earthquake and the
geologic conditions at the site of the earthquake
itself.
6
When it comes to how earthquakes can affect
bridges, the main concern is the damage that can
be caused by the ground motion that the
earthquake creates.
7
Going deeper into this subject, one must take a
look at the local geology and type of soil that
the bridge has been built upon.
8
When soft soil lies on top of hard bedrock the
ground motion is amplified, this is known as
ground-motion amplification.
9
This type of amplification can result in much
greater damage, even from bridges that are quit a
distance from the epicenter of the earthquake
itself.
10
In fact, soft sandy soils can actually be
liquefied by strong ground motion in process
called liquefaction.
11
When liquefaction occurs, the foundation of
structures, like bridges, can be completely
compromised.
12
The general rule of thumb is that earthquakes
below a magnitude of 4.0 do not cause
infrastructure damage.
13
Since 2013 there have been over fifty reported
earthquakes in a two mile area alone just west of
the Dallas city limit.
14
None of these earthquakes have been recorded to
be above 4.0., But there have been earthquakes in
the North Texas area in the past that have been
above 4.0.
15
The largest recorded earthquake in the State of
Texas occurred in 1931, when a magnitude 5.8
earthquake was recorded in far West Texas near
the town of Valentine.
16
Highway engineers have devised standards in an
attempt to earthquake-proof bridges that have
been built since the early 1990's.
17
Most bridges that we drive on daily were built
before these higher standards were implemented
however.
18
Today, the subject of how earthquakes affect
bridges is something that all of us must pay
attention to avoid a potential catastrophe.