Passage: Spicer & Oppenheim agreed to lease 14 percent of the space, but for more than a year, as Black Monday and other factors adversely affected the Lower Manhattan real estate market, Silverstein was unable to find tenants for the remaining space. By April 1988, he had lowered the rent and made other concessions.In November 1988, Salomon Brothers withdrew from plans to build a large new complex at Columbus Circle in Midtown, and agreed to a 20-year lease for the top 19 floors of 7 World Trade Center. The building was extensively renovated in 1989 to accommodate the needs of Salomon Brothers. This led to the alternative naming of the building as the Salomon Brothers building. Most of three existing floors were removed as tenants continued to occupy other floors, and more than 350 tons (U.S.) of steel were added to construct three double-height trading floors. Nine diesel generators were installed on the 5th floor as part of a backup power station. "Essentially, Salomon is constructing a building within a building – and it's an occupied building, which complicates the situation", said a district manager of Silverstein Properties. The unusual task was possible, said Larry Silverstein, because it was designed to allow for "entire portions of floors to be removed without affecting the building's structural integrity, on the assumption that someone might need double-height floors."At the time of the September 11, 2001, attacks, Salomon Smith Barney was by far the largest tenant in 7 World Trade Center, occupying 1,202,900 sq ft (111,750 m2) (64 percent of the building) which included floors 28–45. Other major tenants included ITT Hartford Insurance Group (122,590 sq ft/11,400 m2), American Express Bank International (106,117 sq ft/9,900 m2), Standard Chartered Bank (111,398 sq ft/10,350 m2), and the Securities and Exchange Commission (106,117 sq ft/9,850 m2). Smaller tenants included the Internal Revenue Service Regional Council (90,430 sq ft/8,400 m2) and the United States Secret Service (85,343 sq ft/7,900 m2). The...

Question: How much steel was added to the structure?
350 tons