In the summer of 1978, a recently completed tower in New York looked like a bold engineering bet.
Citicorp Center, now Citigroup Center, stood on four huge columns at the middle of each side, not in the corners. Its 59 stories sat raised over the street.
The building's structural engineer, William LeMessurier, had packed unusual choices into the design.
At first glance, the structure held.
Then the math flipped.
The bolted joints were too weak. By his estimate, a storm expected once every 16 years could bring the tower down.
He warned the owners and took on the fix.
A crew welded steel plates at night over each of more than 200 bolted joints.
Six weeks later?
Only half the work was done.
Hurricane Ella sat off Cape Hatteras and headed for New York. The city was hours from emergency evacuation.
Ella turned east and veered out to sea.
After the last weld, by his estimate, the tower could withstand an event expected once every 700 years.
And that span?
That is almost 44 times rarer than the storm that could have brought it down.
The repair passed overhead, with no noise at all.
A tower ready to fall stayed standing on one new calculation.
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