托福考试

【单选题】Listening 9 "EngineeringClass"
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What is this lecture mainly about
A、Reinforced concrete in buildings
B、The stresses caused by earthquakes
C、Earthquake-resistant structures
D、Understanding construction sites
网考网参考答案：C
网考网解析：
[解析] [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 structur 查看试题解析出处>>