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← Why doesn’t the Leaning Tower of Pisa fall over? - Alex Gendler

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Showing Revision 1 created 12/01/2019 by lauren mcalpine .

  1. In 1990, the Italian government
    enlisted top engineers
  2. to stabilize Pisa’s famous Leaning Tower.
  3. There’d been many attempts to right the
    tower during its 800 year history,
  4. but this team’s computer models revealed
    the urgency of their situation.
  5. They projected the tower would topple if
    it reached an angle of 5.44 degrees—
  6. and it was currently leaning at 5.5.
  7. No one knew how the tower was still
    standing, but the crisis was clear:
  8. they had to solve a problem that
    stumped centuries of engineers,
  9. and they needed to do it fast.
  10. To understand their situation,

  11. it’s helpful to understand why the
    tower tilted in the first place.
  12. In the 12th century, the wealthy
    maritime republic of Pisa
  13. set about turning its cathedral square
    into a magnificent landmark.
  14. Workers embellished and enlarged
    the existing church,
  15. and added a massive domed
    baptistry to the plaza.
  16. In 1173, construction began on a
    free-standing campanile, or bell tower.
  17. The engineers and architects of the
    time were masters of their craft.

  18. But for all their engineering knowledge,
  19. they knew far less about the
    ground they stood on.
  20. Pisa’s name comes from a
    Greek word for “marshy land,"
  21. which perfectly describes the clay, mud,
    and wet sand below the city’s surface.
  22. Ancient Romans counteracted similar
    conditions with massive stone pillars
  23. called piles which rest on
    Earth’s stable bedrock.
  24. However, the tower’s architects believed
    a three-meter foundation would suffice
  25. for their relatively short structure.
  26. Unfortunately for them,
    less than five years later,
  27. the tower’s southern side
    was already underground.
  28. Such a shifting foundation would
    normally have been a fatal flaw.

  29. If workers added more weight,
  30. the pressure from upper stories
    would sink the structure
  31. and fatally increase the lean.
  32. But construction halted at the
    fourth story for nearly a century
  33. as Pisa descended into prolonged warfare.
  34. This long pause allowed
    the soil to settle,
  35. and when construction
    began again in 1272,
  36. the foundation was on
    slightly more stable footing.
  37. Under the direction of
    architect Giovanni di Simone,

  38. workers compensated for
    the tower’s minor tilt
  39. by making the next few floors taller
    on the southern side.
  40. But the weight of the extra masonry
    made that side sink even deeper.
  41. By the time they completed the seventh
    floor and bell chamber,
  42. the angle of the tilt was 1.6 degrees.
  43. For centuries, engineers tried numerous
    strategies to address the lean.

  44. In 1838, they dug a walkway around the
    base to examine the sunken foundation.
  45. But removing the supporting sand only
    worsened the tilt.
  46. In 1935, the Italian Corps of Engineers
    injected mortar to strengthen the base.
  47. However, the mortar wasn’t evenly
    distributed throughout the foundation,
  48. resulting in another sudden drop.
  49. All these failed attempts, along with
    the ever-sinking foundation,

  50. moved the tower closer to
    its tipping point.
  51. And without definitive knowledge
    of the soil composition,
  52. engineers couldn’t pinpoint
    the tower’s fatal angle
  53. or devise a way to stop its fall.
  54. In the years following WWII,

  55. researchers developed tests to
    identify those missing variables.
  56. And in the 1970’s, engineers calculated
    the curved tower’s center of gravity.
  57. With this data and new
    computing technology,
  58. engineers could model how stiff
    the soil was, the tower’s trajectory,
  59. and the exact amount of excavation
    needed for the tower to remain standing.
  60. In 1992, the team drilled diagonal tunnels

  61. to remove 38 cubic meters of soil
    from under the tower’s north end.
  62. Then, they temporarily counterbalanced
    the structure with 600 tons of lead ingots
  63. before anchoring the base
    with steel cables.
  64. More than six centuries
    after its construction,
  65. the tower was finally straightened…
    to a tilt of about four degrees.
  66. No one wanted the tower to fall,
  67. but they also didn’t want to lose the
    landmark’s most famous feature.
  68. Today the tower stands at
    55– or 56– meters tall,

  69. and it should remain stable
    for at least 300 years
  70. as a monument to the
    beauty of imperfection.