Nestled in the northeast region of the Financial District North and bounded by Clay, Montgomery, Sansome, and Washington streets, stands an architectural sculpture, the brainchild of architect William Pereira. The Transamerica Pyramid was designed to be part architectural masterpiece and part environmental practicality: The unique pyramid shape was chosen to allow natural light and fresh air to filter down to the streets below. Excavation to a depth of 52 feet commenced in December 1969, with the first steel placed in November 1970. Components of the base include approximately 16,000 cubic yards of concrete, encasing more than 300 miles of steel reinforcement rods. The depth of the concrete mat foundation is nine feet, the result of a 24-hour continuous concrete pour. The building's base is a four-level, multi-use, including lobby, restaurant, and garage. The maximum number of parking spaces in the building's underground garage is 280. The Pyramid lobby features the work of many artists through a rotating art exhibition. The building has 48 floors, with the fifth floor being the largest, measuring 145 feet per side and containing 21,025 square feet of space. The smallest floor is the 48th, measuring only 45 feet per side and containing 2,025 square feet of space. The edifice has a whopping total space of 530,000 square feet. Its total height is 853 feet, including the “spire.” The spire is the upper 212 feet, and is covered with vertically louvered aluminum panels. The lower portion of the spire houses mechanical equipment. The building has 18 elevators, with two reaching the top floor. With 3,678 windows, it requires a month to wash them. The "wings" that start at the 29th floor are necessary near the top of the pyramid to support elevators on the east side, and a stairwell and smoke tower on the west side. In keeping with its artistic concept, the exterior material is white pre-cast quartz aggregate. The first occupancy occurred during the summer of 1972. In 1969, and throughout its construction, the boarding signage surrounding the emerging site boldly boasted, “A San Francisco landmark since 1972.” On October 17, 1989, the magnitude 7.1 Loma Prieta earthquake struck the Santa Cruz Mountains in central California. Sixty miles away, in downtown San Francisco, the occupants of the Transamerica Pyramid were unnerved as the 49-story office building shook for more than a minute. U.S. Geological Survey (USGS) instruments, installed years earlier, showed that the top floor swayed more than 1 foot from side to side. However, no one was seriously injured, and the Transamerica Pyramid was not damaged. The survival of the Transamerica Pyramid can be attributed to the building's careful structural engineering, designed to withstand tremors in the seismically active Bay Area. Although the Transamerica Pyramid is no longer the headquarters of Transamerica Corporation, the company retains a small presence as a tenant and still uses the building's image as its registered trademark logo. Instead, it is an excellent office tower and home to more than 50 high profile firms employing about 1,500 people.