41st and 43rd Street Bridges City of Chicago As the ordinary directness of line in town-streets, with its resultant regularity of plan would suggest eagerness to press forward without looking to the right hand or the left… we should recommend the general adoption, in the design of your roads, of gracefully-curved lines, generous spaces, and the absence of sharp corners, the idea being to suggest leisure, contemplativeness, and happy tranquility.” –Frederick Law Olmsted These bridges use innovative double-curved arch monotruss structures, to form a large, graceful S-curves. Their leisurely curves echo those of the lakefront park’s walkways, extending the park west over Lake Shore Drive. The bridges curve horizontally and vertically, creating an urban promenade that is both graceful and dynamic. The double curvature is visually dynamic from all approaches. Its slender, minimal detailing provides excellent views to and from all areas of the bridge: There are no hidden areas. The double curvature provides greater structural dynamism incorporating the simple and unifying structure of its supporting arches. Slender and elegant arches provide the structure for a curving deck that widens at its midspans and projects outwards to create grand balconies. Continuous lighting parallel to the surface of the deck highlights its curves and provides even, low-glare illumination for the entire bridge. The bridges glide gently to rest at either side of their spans, with ramps supported on tapering rusticated prairie-stone. The structure, built and galvanized entirely in the factory, is plated in zinc to prevent corrosion. The main arches are off-white, with a very light gray- green cast to help blend them with the landscape. Continuous stainless-steel shrouds and railings highlight the bridge decks. Natural beige prairie stone, used at the intermediate structural support and the beneath the bridge ramps as they taper into the landscape, help the bridges blend with and extend the park environment.
S t r u c t u r e The lean, muscular, dynamic form of these bridges is in keeping with Chicago’s tradition structural expressiveness. Form, function, and structure are one. The double horizontal and vertical curvature adds inherent structural rigidity and wind resistance. The structural detailing, which uses large steel tube and plate sections, is kept simple and clean. The concrete deck is slightly elevated from the steel structure. This highlights the bridge structure and simplifies maintenance by eliminating hidden cavities and nesting places.
The main sections of the bridges will be factory pre-fabricated. They will then be delivered to the site and joined together, using scaffolding at the medians. This will minimize construction downtime, both for Lake Shore Drive and the railroad.
Lighting Indirect lighting adjacent to the bridge decks highlights the curves of the bridges and provides even, low-glare illumination. This same light source is used to cast light onto the large steel-plate fins supporting the main arches. These highlight the structure and provide visual drama at night. Energy-efficient color-corrected luminaries are housed in the continuous stainless-steel shrouds at the base of the guardrails. These are protected from vandalism by high-impact plastic lenses, but are readily accessible for maintenance. Cordogan Clark & Associates is the Design Architect for these bridges, and is teamed on them with AECOM for engineering and construction. (Photography: James Steinkamp, copyright 2019)