Excellence in Sustainability

"The less material used, the less embodied carbon on a project. The designers removed struts from the truss and created slender, efficient members. The project minimized their transportation impacts by sourcing local steel. They also sourced from producers that had a low GWP per ton." -- Max Puchtel, SE, PE, LEED Green Associate, Director of Government Relations and Sustainability, AISC, 2024 IDEAS² Awards Judge

The Winship Cancer Institute is expanding its footprint in Atlanta with a state-of-the-art new hospital.

The new location, called the Winship Cancer Institute at Emory Midtown, is a 17-story cancer hospital in the heart of Atlanta that brings 450,000 sq. ft of inpatient, outpatient, and research facilities to the Emory University Hospital Midtown campus. It consists of a new cancer care center located directly across the street from Emory’s existing hospital, with the two buildings connected by a new pedestrian bridge.

The two-level bridge--designed by Skidmore, Owings & Merrill (SOM) and constructed by Batson-Cook Construction--spans 184 ft with a main span of 134 ft across Linden Avenue, linking the two buildings without impacting the existing hospital’s drop-off area and architectural design. Its transparent lower level connects public spaces in both buildings, while the upper level provides patient access and is clad in fritted glass to maintain patient privacy. The bridge also carries major mechanical, electrical, and plumbing services across the street to service the new cancer center.

Though the bridge has two levels, the design team determined that the span could be efficiently managed with a single-story truss. The top level of the bridge comprises a structural steel truss at each side, while the lower level is hung from hollow structural section (HSS) hangers at the truss panel points. Besides these efficiency and constructability considerations, this choice also reflected the functional differences between the two levels. The top private level is more enclosed, while the lower public level is as transparent as possible.

The Emory Bridge spans Linden Avenue--a highly trafficked road that functions as an on-ramp to Interstate 85--an exterior courtyard, and a patient drop-off area for the current hospital’s Peachtree Building. Ideally, its design would minimize road closures and bring negligible impact to the hospital, which needed to remain open during construction.

The design team chose structural steel because its bridge could be pre-assembled into the largest possible sections on the ground--before being lifted into place and assembled in just a single weekend. One lane of Linden Avenue was a laydown area to assemble the delivered pieces into two large box trusses that could be erected quickly and safely over the road. Two mobile cranes lift the two box trusses into place in one weekend. The sections were bolted together while suspended in the air, and the cranes remained hooked to the trusses until all back welds were complete.

Construction crews hung the bridge’s lower level from the trusses, allowing Linden Avenue to be reopened while work remained ongoing.

Where exposed to view, the bridge members are architecturally exposed structural steel (AESS), consisting of square HSS sections connected to a milled steel node at the intersections of the truss diagonals. The AESS material’s aesthetic purposes were another factor in choosing steel because they allowed for the most slender profiles possible. The simple yet elegant detailing puts the structural steel’s beauty on full display. The square HSS hangers supporting the bridge’s lower level practically disappear behind the enclosure mullions and give that level the appearance of floating.

One of the major design challenges was the bridge’s need to connect two buildings at different corresponding elevations. The lower level had to reconcile an elevation change of 4 ft 1 in., while the upper level had a change of 1 ft 4 in.

The design team strategically connected the two springing points with straight and slightly inclined walking surfaces to meet the maximum allowable slope. It then investigated several options for forming the geometry of the structural elements to accommodate these elevation constraints. The solution was to keep the top and bottom chords of the truss horizontal to simplify fabrication, and more importantly, to preserve the clarity and purity of the truss geometry.

Additionally, the chosen configuration maintained a consistent node geometry to facilitate fabrication and provided a regular datum for the attachment of the facade modules. The walking surface’s slope was achieved using a slab-on-grade over-shaped geofoam. The bottom level framing, which hangs from the truss, was sloped to follow the walking surface. The lower-level façade units extend down from the bottom of the structure along the bridge to avoid trapezoidal glass elements.

The existing building structure did not have sufficient capacity to support a new bridge and would have required reinforcement. In response, the design team cantilevered the end of the bridge beyond the last support, ensuring that no new loads were imposed on the existing building.

The construction sequence aimed to minimize the time when highly trafficked Linden Avenue was closed by erecting the largest possible sections.

Trusting the Trusses

The bridge’s primary distinctive feature is the upper-level truss that utilizes web members arranged in an innovative geometry, increasing the structure’s efficiency.

SOM used several academic and internally developed structural optimization tools to arrive at an efficient structural form and minimize total material usage. One program, Polytop, starts with a design space of a solid 2D continuum of material and iteratively removes material to arrive at a solid-like approximation of the most efficient structural form. The other program, Ground Structure, utilizes a design space of a densely interconnected grid of linear elements and iteratively removes members to arrive at a sketch-like approximation of the most efficient structural form.

The results were combined with an applied rationalization considering the fabrication and construction of the bridge. The resulting geometry consisted of truss bays with skewed X-bracing that is symmetrically oriented about the mid-span of the bridge.

SOM has successfully designed several high-rises with a similar geometry for vertical bracing, including 800 W. Fulton Market in Chicago. However, the Emory Winship at Midtown Bridge is the first constructed example of a long-span structure with this truss geometry.

The truss diagonals were considered AESS, and the design team worked with aesthetic purposes and facilitating construction in mind. The team decided to maintain a consistent outer dimension for all the truss diagonals within each truss bay, ensuring a visual continuity of elements and avoiding connections with members of varying dimensions.

Similarly, the design team minimized the changes in member dimensions between bays to ensure a seamless transition along the truss. At the asymmetric connection of the truss diagonals, the team used a milled steel node to provide a clean, easily repeatable piece for a consistent connection. The resulting truss consists of W12×96 top and bottom chord members, while the truss diagonals comprise HSS4×4 through HSS8×8 elements.

The design team’s preferred topology optimization tools determined the most structurally efficient form of the truss, ensuring material is used only where required and minimizing the overall embodied carbon footprint of the bridge. The support points of the bridge were also chosen to reduce the impact on existing structures, allowing them to remain in place and minimizing demolition and waste.

Owner: Winship Cancer Institute of Emory University, Atlanta
Owner's representatives: Turner & Townsend Healthcare, Atlanta
General contractor: Batson-Cook Construction, Atlanta
Architects: Skidmore, Owings & Merrill (SOM), New York; May Architecture, Atlanta
Structural engineer: Skidmore, Owings & Merrill (SOM), New York