St Louis' Gateway Arch has undergone a redevelopment, with architecture studio Cooper Robertson expanding and renovating the museum beneath the record-holding structure.

Sitting in the shadow of the Eero Saarinen-designed arch – the tallest structure of its kind in the world – a total of 4,200sq m (45,000sq ft) of new museum area has been added to the west of Saarinen’s underground original, which has itself been renovated and reconfigured to create new exhibition galleries, public education facilities and visitor services.

A circular stainless steel and glass entrance by James Carpenter Design Associates refers to the Arch in its materiality and form, and is precisely inserted into the topography of the National Register-listed site. Visitors to enter the building through the landscape rather than descending underground.

Formerly known as The Museum of Westward Expansion, the new Gateway Arch Museum tells the story of American history in a modern, expanded and inclusive way, with exhibits by Haley Sharpe Design on indigenous American people as well as the historic pioneers and European settlers of the American West.

Landscape architects Michael Van Valkenburgh Associates have created a new park from the original by Dan Kiley, which now crosses the adjacent highway and connects the site to the city centre of St. Louis.

According to the architects, the new project was conceived to restore the link between the city, the museum and the arch.

"Our design makes the museum part of Downtown and you move in a linear way through exhibits which explain the meaning behind it," said Scott Newman, a partner at Cooper Robertson.

"The last part explains how Saarinen built it, and then you exit at the Arch and take the tram to the top. We have connected the museum to the landmark itself, both architecturally and experientially."

Saarinen’s 630ft (192m) monument was completed in 1965 and opened two years later. It has become one of the most visited tourist attractions in the world, with over four million visitors travelling to the observation platform at the top of the structure each year.