The St. Louis Gateway Arch is a national monument to westward expansion in the United States. Located on the banks of the Mississippi River in St. Louis, Missouri, the 630-foot tall stainless steel arch weighs 43,000 tons.
Construction of the arch began on February 12, 1963, and was finished on October 28, 1965 at a cost of roughly $13 million. The two bases are equilateral triangles (triangles with three, 54 foot-long sides). At the top of the arch, the triangle is only 17 feet long on each side. Visitors enter the arch from an underground visitors center and can travel to the observation deck at the top in a 40-passenger tram that runs along the inside.
The arch has 60-foot deep foundations and is built to withstand high winds and earthquakes. The structure sways about one inch in a 20 mph wind; it is designed to sway up to 18 inches in 150 mile per hour winds.