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ON THIS DAY: March 15, 1967, U.S. Steel Tower groundbreaking

PITTSBURGH — The tallest building in Pittsburgh was once the tallest building in the country, excluding New York and Chicago, a title it no longer owns. It does however, somewhat interestingly, still have the title of the tallest building in the world with a completely flat roof.

On March 15, 1967, the company held a groundbreaking ceremony for the new tower. The president of U.S. Steel, Leslie B. Worthington, called it the “most significant day in the history of two great partners in progress—the city of Pittsburgh and U.S. Steel."

Steel for the building was fabricated at U.S. Steel’s American Bridge Division Plant in Ambridge and at the former U.S. Steel Homestead Works.

It was a new type of steel at the time, called Cor-ten, which is designed to resist corrosion by preemptively forming a coating of dark brown oxidation, which gives the tower its color.

Unlike other buildings that have their structure clad and hidden behind glass, stone or other decorative panels, the tower’s skeleton was intentionally left proudly bare to showcase its local steel. The building is effectively modular. Each floor has about an acre of office space and every third floor is attached to the exterior columns.

The sparse structure did require inventive ways of hiding protective systems. The 18 main columns are fireproofed, hollow and filled with a water/antifreeze/rust inhibitor mixture. It was the first building to use this technique.

After years of construction, the triangular 64-story, 840-foot-tall building was dedicated on Sept. 30, 1971. At the time, it boasted of a rooftop heliport that has not been used since 1992.

In 1988, the building’s name was changed to the USX Tower to match the rebranding of its namesake company, but this was reversed in 2002 when U.S. Steel reverted back to its original name.

Since 2007, UPMC has made the tower its corporate headquarters.

UPMC installed large signs on the building, each letter measuring about 20 feet high.