"Engineering Class" Why does the professor say this:

[Narrator] Listen to part of a lecture in an engineering class.
[Professor]
    Because every earthquake presents us with unique conditions, it’s difficult to anticipate the stresses that
will ultimately affect the structures we design and build. So our challenge is to try to design a building  Q46
that will be as... safe as possible... for all types of earthquakes. Besides that, during the past decade,
the expectations for earthquake-resistant structures have changed. Whereas in the past, it was considered
adequate for a building not to collapse during an earthquake, now insurance companies and...
and even clients... they’re demanding buildings that will be able to maintain their structural integrity
through an earthquake and, uh... remain sound... after the earthquake subsides.
    So, in recent years were developed several techniques for building more earthquake-resistant  Q46
structures: For relatively small buildings, all we have to do, really, is bolt the buildings to their foundations
and, uh... provide some support walls. Remember these walls are referred to as shear walls in
your textbook. They’re made of reinforced concrete, and by that I mean concrete with steel rods embedded
in it. This not only strengthens the structure but... but it also diminishes the forces that tend to
shake a building during a quake. And in addition to the shear walls that surround a building, shear walls
can be situated in the center of a building around an elevator shaft or a stairwell. This is really an excellent
reinforcement. It’s commonly known as a shear core, and it contains reinforced concrete, too.
    Okay. Let’s talk about walls. Walls can also be reinforced, using a technique called cross-bracing  Q47
Imagine steel beams that cross diagonally from the ceiling to the floor... and this happens on each
story in a building. So before the walls are finished, you can see a vertical row of steel x’s on the structure.
And this cross-bracing tends to make a building very rigid, and consequently, very strong.
    But besides steel reinforcements, engineers have also devised base isolators, which are positioned
below the building, and their purpose is to absorb the shock of the sideways shaking that can undermine
a building and cause it to collapse. Most of the base isolators that are currently being used are made of  Q48
alternating layers of steel and synthetic rubber. The steel is for strength, but, uh... the rubber absorbs
shock waves. In higher buildings, a... a moat... of flexible materials allows the building to sway during
seismic activity. Or... or large rubber cylinders support all of the corners of the building, and in
between each floor, and they allow the building to sway during an earthquake. So, you can see that
these alternatives are quite different from cross-bracing or shear walls.
    So the combination of reinforced structures and flexible materials has been proven to reduce earth
quake damage. But even these engineering techniques are insufficient if the building has been constructed
on filled ground. Soil used in fill dirt can lose its bearing strength when subjected to the shock  Q49
waves of an earthquake, and the buildings constructed on it can literally disappear into the Earth. So, in
areas where earthquakes are known to occur, it’s important to understand the terrain, and you have to
be sure that the ground is either solid or it’s been adequately prepared.
    Okay, let’s assume that we do everything right... we choose and prepare the construction site and
we design a building with plenty of reinforcements and flexible materials. With cross-bracing, we probably  Q50
have a building with the strength to holdup under earthquakes, even those of relatively high magnitudes.
And building, what about the occupants? Well, the structure may be
strong, but the furniture will probably be overturned or shifted during the earthquake and that could
result in major injuries for the people inside. So, now that we’ve made progress in solving the problem
of how to preserve the buildings, uh... one of the more recent areas of research is how to better protect
the occupants during an earthquake.
    One interesting possibility is to design buildings that house a series of pistons, and these pistons  Q51
are filled with fluid and controlled by sensors in a computer. So... by analyzing signals from the sensors,
the computer should be able to determine the magnitude of an earthquake in progress... and
when it does that, it can trigger electromagnets in the pistons to increase or decrease the... the rigidity
of the shock absorbers... built into the structure. If the earthquake is minor, then the building can be
programmed to sway gently, and the people and everything else inside get a safe ride. But during high-magnitude
earthquakes, the shock absorbers can freeze the building to prevent it from shaking at all. So
the beauty of the concept is that the computer sensors work very quickly, reacting within one one-thousandth
of a second, and they can run on battery power since the electrical system usually fails during an
earthquake. Will the concept work? Well, the National Science Foundation is supporting more research  Q51
into the potential of pistons, and the results so far are promising.