IELTS Słuchanie – pytania testowe do ćwiczeń, sekcja 2

Marcus: [excited] Hi Hannah.

Hannah: Hey Marcus, are you ready to compare notes for our presentation?

Marcus: Yes I think so. I found some really useful sources describing how the main categories of historic bridge actually work, and how engineers go on to check whether they are still safe today.

Hannah: Me too. So let's start with the categories of bridge themselves. The different categories are based on how the structure carries the load.

Marcus: Right. What about beam bridges?

Hannah: They're like a plank laid across a stream. A beam just sits on its two supports, and the load travels straight down to those supports. Because of that, beam bridges are best for short, straight crossings, once the span gets too long, the beam starts to sag in the middle and you need a different type of bridge.

Marcus: I see.

Hannah: We should probably include arch bridges as well. They're an interesting category.

Marcus: [excited] Yes. I saw a Roman one in Spain once and it was amazing.

Hannah: Yes. What distinguishes them from a beam bridge is the curved shape. The arch forms a curve which transfers the weight outwards to abutments at each end.

Marcus: That makes sense.

Hannah: Exactly. Now we definitely need to include truss bridges. They were a real revolution in nineteenth century railway engineering.

Marcus: Yes. They're built from interlocking triangular sections of iron or steel, and the triangles are what give them their strength, a triangle won't deform under load the way a square will.

Hannah: So you can build a long span and keep it light at the same time.

Marcus: Yes, that's why so many old railway bridges are trusses.

Hannah: Okay. What about suspension bridges?

Marcus: They're very common for very long crossings. The road deck hangs from cables that are slung from tall towers on either side, and the cables themselves are anchored into the ground at each end.

Hannah: Oh, that's the type that crosses big rivers and harbours.

Marcus: Yes. The deck is suspended from the cables, so the towers carry most of the load, which is what allows the spans to be enormous.

Hannah: We should probably include cantilever bridges. They weren't on my list.

Marcus: What are they?

Hannah: They're built so that two arms, called cantilevers, extend out from the anchorages on each shore until they meet in the middle. So instead of one long beam, you've got two stiff arms that are balanced and then joined.

Marcus: Oh, sounds like an interesting type of bridge.

Hannah: So we've talked about how the different categories of bridge work, and in the safety-check part of our presentation we want to demonstrate in a practical way how engineers find out whether an old bridge is still safe to use.

Marcus: Yes. And I thought we could present the information in the form of a flow chart.

Hannah: Great idea. So the procedure begins when an inspection team is assigned a particular bridge to assess.

Marcus: Right. Then before they go out to the site, they go through the paperwork. The team starts by reviewing the bridge's history records, the original drawings, the design calculations, every previous inspection, every repair. Without that background, you've got nothing to compare against.

Hannah: Right.

Marcus: Yes. After that they go out to the site and carry out a visual inspection of the structure, walking the deck, looking under the spans, photographing anything that looks suspicious.

Hannah: That makes sense. And what then?

Marcus: Then stress sensors are attached to record the level of vibration as traffic crosses the bridge. Why vibration? Because every old bridge has a kind of natural rhythm, and if cracks or loose connections develop, that rhythm changes, so vibration tells you a lot about the hidden state of the structure.

Hannah: I see. But something might still be wrong inside the concrete or the steel that doesn't show up in vibration?

Marcus: Yes, exactly. So at this point, cores are extracted from the concrete piers, small cylinders drilled out and taken back to the lab. The samples are then tested for signs of corrosion in the steel reinforcement. Concrete looks fine on the outside, but if the rebar inside is rusting, the bridge is in serious trouble.

Hannah: Right. Just like checking for hidden damage on a car.

Marcus: It's an essential step. Once that's been done, a drone is used to photograph the underside, looking for cracks and small deformations, bulges, twists, any change in shape.

Hannah: Why a drone? Doesn't that miss things?

Marcus: Well, luckily, no. The cameras are very high resolution, and the drone can fly close to surfaces that no inspector could reach safely.

Hannah: And in the next stage of the analysis, all of the data has to be brought together, the history records, the visual notes, the vibration traces, the core results, the drone photos.

Marcus: We'll need to explain that this is where the chief engineer compares everything against the original calculations to decide whether the bridge can still carry today's traffic.

Hannah: Yes. This level of analysis is necessary to make a sound judgement about safety.

Marcus: So once all this has been done, the findings are compiled into a report and submitted to the highway authority. What happens next?

Hannah: The authority can compare them with data from similar studies done on bridges of the same age and type.

Marcus: Fascinating. Let's make a start on our presentation slides, shall we?