Science and technology

Directions for this passage: Numbered portions are marked [n]…[/n] (underlined on paper, highlighted online). Each numbered portion matches the question with the same number. Paragraphs are numbered for reference.

Listening to the Harrow Creek Bridge

Paragraph 1 The Harrow Creek footbridge has carried walkers across a narrow ravine in the town of Millbeck since 1931. After more than ninety years of rain, frost, and footsteps, town officials wanted to know whether its steel beams and joints were still sound. A traditional inspection would have closed the bridge for several weeks, so the town invited a research team from nearby Ardley Polytechnic to try a newer approach.

Paragraph 2 The team, led by structural engineer Priya Venkataraman, attached twelve matchbox-sized sensors to the underside of the bridge. Each sensor can measure vibrations [1]precise[/1] enough to detect the footsteps of a single jogger. Because the sensors cost less than forty dollars [2]apiece. The team[/2] could afford to cover the bridge far more thoroughly than a few expensive instruments could. Since gusts of wind can [3]effect[/3] the readings, the team also mounted a small weather station on one of the railings.

Paragraph 3 Every ten minutes for eight months, the sensors sent a new set of [4]readings, to a laptop, in Venkataraman’s office[/4]. The team was watching for changes in the bridge’s natural frequency, [5]a kind of measurement[/5]. When a bridge’s beams weaken or its joints loosen, that frequency drops slightly, much as a guitar string sounds lower when it is loosened.

Paragraph 4 In early spring, the three sensors nearest the eastern end of the bridge began to record a small but steady drop in frequency. Because the weather station showed no unusual temperatures or winds, Venkataraman’s team concluded that the drop [6]suggested[/6] a problem with the eastern bearings, the joints that allow the bridge to expand and contract. No one, however, could yet see any damage.

Paragraph 5 Because they knew exactly where to look, inspectors needed only a single afternoon to examine the eastern bearings, and they found two of them badly corroded. Both were replaced over one weekend, and the Harrow Creek footbridge reopened the following Monday. Its twelve sensors are still in place, listening.