Diarmuid Kelleher
Thornton Tomasetti, Chicago

Fifty years ago, in 1969, Sears, Roebuck & Co set in motion plans for a consolidated office headquarters that would result in one of the most iconic buildings in the world. This year, the new podium at its base will open to the public, marking the most significant upgrade to the building since its inception. This milestone serves as an ideal study of the many advances in the field of façade engineering, and is a prime example of the sustainable practice of existing building modernization.

The original building was a feat of structural engineering and building to greater heights through Fazlur Kahn’s innovative use of bundled tubes. However, clad in its distinctive dark skin of captured mullions and monolithic glass, it was the very definition of rudimentary façade engineering characteristic if its time. Today, the new podium is clad in a high-performance custom unitized curtain wall. The structural ingenuity of the original is echoed in the new feature entryways that consist of self-supporting AESS glass winter gardens and an undulating glazed grid shell.

The reconceived podium invites the public in and offers green space to the city through a newly landscaped roof. In addition, the tenants gain the respectfully modernized architectural experience and amenities of a new high-rise. The sustainable practices also extended to the façade design via advanced thermal modelling of the façade assemblies, daylight analyses of the interior public spaces for occupant comfort, and reflection analysis of the new skylights to mitigate unwanted glare.

The Willis Tower has transformed from its original signature magnificence, to a modern architectural technological achievement though the use of innovate façade systems and design approaches.