Tech company Google and architecture studio Jahn have released visualisations of the planned revamp of the James R Thompson Center, an iconic postmodern-style building in Chicago's Downtown.

The plans for the redesign retain the original rounded form and the 17-storey atrium inside. Its glass curtain wall will change, moving from the slightly darkened patterned glass of the original to a more uniform, sleek look.

Located in the core of Downtown Chicago – known as the Loop – the Thompson Center was designed in the 1980s by local architect Helmut Jahn. It housed offices for the Illinois state government and connections for the Chicago Transit Authority.

Last year, tech company Google purchased the building from the city and plans to make it a centre for its offices and to rent extra space. Google will retain Jahn's practice, named Jahn, for the redesign.

"For the Thompson Center, we're working with the building's original architects at Jahn to help bring the design into the 21st century while maintaining its iconic form," said the company.

It said that the redesign was meant to "honor the postmodern legacy of the building".

However, certain details of the facade are set to change, such as the red and beige panelling of the ground-level collanade. Google's plans see a heightened entry program, with tall collonades that make more clear the ground-level recess in the facade.

The renderings show a second-storey amenities space placed above these columns on the interior of the building.

The developers on the project, The Prime Group and Capri Investment Group, confirmed that the train connections accessible through the atrium will remain under Google's plans, which means that some part of the atrium's ground and subterranean levels will remain open to the public.

Before, these areas held restaurants and services such as postal and licensure offices.

"When it opened, the Thompson Center was intended to be a new kind of gathering space for Chicagoans," said Google.

"[O]ur hope is that this reimagined building becomes something more: a thriving community destination for all Chicagoans in a revitalized Loop neighborhood."

Google has also set ambitious sustainability goals for the structure, aiming for "carbon-free" operation by 2030.

This will be achieved in part through the replacement of the facade with triple-pane glass and by completely replacing the mechanical systems.

The new facade will also allow for less artificial lighting during the day. Google said that the redevelopment will take "several years to complete".

The tech company recently worked to complete its campus in Silicon Valley with Danish studio BIG and UK-based Heatherwick Studio.

Jahn Studio also remains relevant in other ways in the Chicago infrastructural landscape, working towards the completion of a skyscraper in the south of the city, which will be one of the tallest in the area.

Images are courtesy of Google.