| Before the frame can go up |
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| A skyscraper must be able to swing |
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| The Burj Khalifa is a multi-use building |
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| As soon as steel became an important building material |
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| In the early 20th century |
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| New York's World Trade Centre |
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| In the past few decades most of the highest buildings |
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| The invention of the elevator made it easier |
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| Service areas belong |
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| Air conditioning and heating systems in skyscrapers |
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