The Nichols Bridgeway at the Art Institute of Chicago

Abstract

The Nichols Bridgeway is a 620’ long steel pedestrian bridge that spans from the centre of Chicago’s Millennium Park, over a city street, and onto the top floor of the New Modern Wing of the Art Institute. Both the bridge and Art Institute are designed by renowned architect Renzo Piano. The structural engineer for the bridge superstructure is Ove Arup and Partners Ltd (Arup), and for the substructure Wiss, Janney, Elstner Associates (WJE). Conceived by Piano as a fine blade, the bridge is extremely slender in elevation. The structural depth varies from 2’-2” to 4’-6”. For the primary span of 200ft the span-to-depth ratio is 44. The superstructure comprises a one-of-a-kind lightweight steel box-girder. The bottom flange of the girder is a 10-ft wide curved plate that is exposed to public view, making the structural and architectural design inseparable. The south end of the bridge is supported by the new Art Institute structure, using cantilever beams that support the deck so avoiding the need for columns immediately below. The bridge piers to the north are founded upon the roof of the existing Millennium Park parking garage and rail yard structure that was completed several years before the bridge project was envisioned. The existing structure was designed to support several feet of soil, but not the sizeable forces imparted by the new bridge. To accommodate the bridge loads, existing soil surcharge was strategically removed or replaced with lightweight fill, and some garage roof framing members were strengthened by constructing supplemental concrete members atop the garage roof. The bridge crosses several expansion joints in the existing structure, requiring the bridge structure to accommodate thermal movement of the underlying structural framing. The performance of the bridge is defined by footfall induced vertical and lateral vibration criteria, which were key factors in the design and steered the development of the concept. Providing high lateral stiffness together with relatively low mass ensures that the frequencies of the natural modes preclude resonant lateral response. Vertical vibration response is limited by balancing the mass and stiffness in the relevant modes of vibration, eliminating the requirement for added damping.