Ground was broken for the US Steel Building in March of 1967. The United States Steel Corporation, one of the largest steel producers in the world, wanted to make a statement about the strength and beauty of steel. The original concept was to design the tallest building in the country, with an outer facing completely made of Cor-Ten Steel. It was to be a huge advertisement for the steel industry, which was centered in Pittsburgh.

U.S. Steel deliberately placed the massive steel columns on the exterior of the building to showcase a new product called Cor-ten steel, which resists the corrosive effects of rain, snow, ice, fog, and other meteorological conditions by forming a coating of dark brown oxidation over the metal. This inhibits deeper penetration and doesn't need painting and costly rust-prevention maintenance over the years. The Cor-Ten steel for the building was made at the U.S. Steel Homestead Works.

  
Construction work on the foundation for the U.S. Steel Building in 1968.

  
Foundation work for the U.S. Steel Building in 1968 (left), and the building rising in 1970.

  
The Steel Building in 1969 (left) and an ironworker standing on scaffolding near the top in 1970.
Below is the original U.S. Steel Building, now Three Mellon Center

The building was completed in 1970 and became headquarters of U.S. Steel Corporation. The towering 64-story skyscraper did not quite eclipse the tallest buildings in the country, but in 1970 it was the third tallest in the land, behind the Empire State Building and the Sears Tower. The steel tower rose to a height of 841 feet, well above 600 foot Mount Washington, and is visible from the South Hills. The building does, however, have the largest roof in the world on a structure of its size, an expanse of nearly one acre.

The 64-story U.S. Steel, under construction in 1969, rises above the City of Pittsburgh

The tower was the first structure in the nation to use liquid-filled, fireproofed columns as a safety feature. The eighteen support columns are filled with a mixture of water and anti-freeze. In case of fire, the liquid solution would dissipate its heat by convection allowing the columns to resist flames for four hours. It was the best that the steel industry could produce, remaking the city skyline and towering over the urban landscape.

From 1971 to 2001, there was a fine dining restaurant on the upper floor of the building called the "Top of the Triangle." The bird's eye view of the surrounding city from the restaurant was spectacular. In the mid-1980s, the city's subway was built underneath Grant Street, with one of the stops called Steel Plaza. The "T" station lies underneath the nearby Mellon Tower, with a connecting tunnel to the USX Tower.

  
The tunnel connecting the USX Tower to the Steel Plaza Subway station.

Originally called the U.S. Steel Tower, the name was changed in 1988 to the USX Tower. It remained that way until 2002, when the name changed to U.S. Steel Tower. In 2007, the University of Pittsburgh Medical Center (UPMC) leased several floors and also the right to mount signs reading "UPMC" to the top of each of the three sides. These were attached in 2008 and since then the tower is often refered to as the UPMC Building.

The current US Steel Building is the third downtown building used in the history of the USX Corporation. In 1898 the Carnegie Building was considered "steel headquarters." The building was razed to build the Kaufman's Annex. In 1952, a 40-story structure, to be shared by the U.S. Steel Corporation and the Mellon Bank and Trust Company, was constructed adjacent to present-day Mellon Square. This is considered the first official US Steel Building, now called Three Mellon Center, part of Mellon's downtown campus.

  
The USX Tower (left) and the previous U.S. Steel Building.

Wikipedia: U.S. Steel Tower.

Wikipedia: 525 William Penn Place (Original Steel Building).