听写

This is the first of a series of lectures on historic engineering structures. Today, we’re looking at the Clifton Suspension Bridge in Bristol, which we hope to visit later this term, and I’d like to begin with a (1) about the bridge’s history and about bridge building in general.

Now, people have been building bridges since prehistoric times. Over the centuries, bridge design has evolved using a variety of (2), but the objective has always been the same: to get to the other side.

One of the most basic types of bridge is the arch, and there’s evidence from the Middle East that people knew how to construct arches using (3) as early as 3200 BC. The stone arch had the advantage of being quite simple to build, and it remained the main type of bridge design from Roman times until the early 1700s.

Another type of bridge with a (4) is the suspension bridge, where the road is suspended from cables hanging between towers. The first suspension bridges were simple affairs, made of rope and wood, and the earliest (5) were constructed around AD 550 in China, but rope has limited strength, and it only became possible to build longer bridges when iron became available. The first major iron suspension bridge in Europe, completed in 1826, was the Menai Strait Bridge in Wales.

The story of Clifton Suspension Bridge in Bristol began just (6), in 1829. At that time, the city authorities wanted to build a bridge over the River Avon. In order to choose the best design, they organized a (7), and the winner, announced in 1831, was an engineer by the name of Isambard Kingdom Brunel. Work began the same year, but was almost immediately interrupted when serious riots broke out in the city. As a result, investors (8), and work stopped until 1836. The two supporting piers had been completed by 1843, but unfortunately, at this point the money ran out, and work on the bridge came to a halt for a second time. Then, in 1851, all the ironwork for the bridge was sold off in order to pay back the creditors, and the project seemed to have (9). However, in 1860 there was a stroke of luck when a suspension bridge in London was demolished. That bridge had chains which were almost the same as the ones designed for Clifton, and these chains were available to buy. Events moved quickly after that. Money was (10), and work went ahead again in 1862. The bridge was finally completed amid great celebration two years later, in 1864.

We’ll be examining some of the design features in (11) in the second half of this talk. But just as a footnote to this section, it’s worth looking ahead to the future, and a couple of proposals for “super bridges” linking not only river banks or even countries, but (12).

One proposal is for a bridge between Alaska and Siberia, which would be six lanes wide and 80 kilometers long. The water of the Bering Sea beneath is only about 50 meters deep, but the biggest challenge is the (13) of the location. This would restrict construction to five months a year and also close the road during winter.

There’s a different obstacle facing a second proposal, a bridge linking Europe and Africa across the Straits of Gibraltar, and that’s the (14). Although it’s only 28 kilometers across, the water is as deep as 1,500 meters in places. In such deep water, a bridge may not be able to support its own weight, so engineers are considering using (15) which have never been attempted before.

A third seaway that engineers hope to cross in the near future is the Straits of Messina, between the island of Sicily and mainland Italy. Unlike the other two proposals, the Messina Bridge only involves one (16), and the distance is relatively short at two and a half kilometers, so there’s a good chance it will be built.