It was built as a commemoration to the Lewis and Clark Expedition commissoned by Thomas Jefferson. The intent was to start at St Louis and go westward looking for a northwest passage across the uncharted part of the Louisiana Purchase and explore and map it.
St Louis has since been known as the Gateway to the West, and the Arch is part of the Gateway Expansion Memorial.
It's outside is constructed of 2" stainless steel. The arch was assembled with both legs going at the same time. It was constructed by adding pre-built triangular sections.
The Arch is the same width as height. It forms two equalateral triangles back to back. In 150mph winds the top of the arch will only sway 9" either way.