The Largest Monument - Gateway Arch

The Gateway Arch, also known as the Gateway to the West, is an integral part of the Jefferson National Expansion Memorial and the iconic image of St. Louis, Missouri. It was designed by Finnish-American architect Eero Saarinen and structural engineer Hannskarl Bandel in 1947. It stands 630 feet (192 m) tall, and is 630 feet (192 m) wide at its base, making it the tallest monument in the United States. Construction of the arch started on February 12, 1963 and was completed on October 28, 1965. The monument opened to the public on July 10, 1967.
The cross-sections of its legs are equilateral triangles, narrowing from 54 feet (16 m) per side at the base to 17 feet (5.2 m) at the top. Each wall consists of a stainless steel skin covering a sandwich of two carbon steel walls with reinforced concrete in the middle from ground level to 300 feet (91 m), with carbon steel and rebar from 300 feet (91 m) to the peak. The interior of the Arch is hollow and contains a unique transport system leading to an observation deck at the top. The interior of the Arch also contains two emergency stairwells of 1076 steps each, in the event of a need to evacuate the Arch or if a problem develops with the tram system.

The base of each leg at ground level had an engineering tolerance of one sixty-fourth of an inch or the two legs would not meet at the top.

During construction, both legs were built up simultaneously. When the time came to connect both legs together at the apex, thermal expansion of the sunward facing south leg prevented it from aligning precisely with the north leg. This alignment problem was solved when the St. Louis Fire Department sprayed the south leg with water from firehoses until it had cooled to the point where it aligned with the north leg.
It is the tallest habitable structure in Missouri, 7 feet higher than the 623 foot spire of One Kansas City Place in Kansas City, and 37 feet higher than the roof of Metropolitan Square in St. Louis, Missouri.

Observation area

Near the top of the arch, the rider exits the compartment and climbs a slight grade to enter the arched observation area. Thirty-two small windows (16 per side) measuring 7 by 27 inches (180 mm × 690 mm) allow views across the Mississippi River and southern Illinois with its prominent Mississippian culture mounds to the east at Cahokia Mounds, and the City of Saint Louis and St. Louis County to the west beyond the city. On a clear day, one can see up to thirty miles (48 km).

Source: wikipedia.org