43rd Street Pedestrian Bridge over LSD/Metra/CNRR
CATEGORY: Special Projects
PROJECT FIRM: Benesch
OTHER CONSULTANTS: GSG Consultants, Inc., AAA Enginering, Ltd., R.M. Chin & Associates, Inc., F.H. Paschen, AECOM
OWNER: Chicago Department of Transportation
DESCRIPTION: The project team exemplified ingenuity in their approach to the 43rd Street Pedestrian Bridge project, employing innovative techniques that pushed the boundaries of conventional engineering and construction. One of the project’s most significant challenges was the need to construct inclined arch structures over six incredibly busy railroad tracks and a major highway. With 263 trains passing daily and a constant stream of 100,000 vehicles on the highway, the stakes were high to erect the inclined arch structures over six very active railroad tracks. Further complicating the project, the Metra Electric Line operates on the four westernmost tracks with overhead powered catenary lines which need to be protected. To tackle this complexity, the team worked with the City to carefully construct a temporary bridge to minimize the impacts to the railroads and to eliminate many of the construction constraints. The temporary bridge was constructed to not only support construction loads including temporary supports for erection of the arc hand deck ribs, but also provided a protective shield and safer working environment over the Metra Electric Line. By doing so, the team drastically reduced the need for disruptive track closures and the time-consuming process of de-energizing and re-energizing the Metra Electric Line. This innovative approach not only saved substantial time but also mitigated significant project costs that would have otherwise been incurred due to extended closures and the deployment of railroad flaggers. Furthermore, the team devised a suspended falsework and decking system for the arch over Lake Shore Drive. This system not only ensured the safety of the workers but also minimized traffic disturbances, allowing the project to progress smoothly while minimizing inconvenience to commuters. The erection of the steel arch components was completed with three overnight closures of Lake Shore Drive with traffic detoured around the project site. The innovative design of the 43rd Street Pedestrian Bridge, echoing the aesthetic features of the 41st Street bridge, signifies a significant leap forward for the engineering profession. Not only does this project challenge traditional engineering thinking with its unique elements, such as the twin 30-degree inclined arches on reverse curvature spanning the Metra Electric and CN railroads and Lake Shore Drive (LSD), but it also introduces a novel approach to addressing complex geometric challenges.
The use of hangers on one side to support the 400’ radius curved deck rib pushed the boundaries of conventional design, fabrication and construction techniques. Moreover, the project’s impact extends beyond its technical complexities. The design team, in collaboration with the contractor, demonstrated remarkable adaptability in the face of unprecedented material shortages, particularly with domestically produced stainless steel for hanger assemblies. Their ability to navigate these challenges not only ensured the completion of the main span deck but also averted a potential shutdown. This proactive approach not only showcases innovative problem-solving within the engineering field but also reinforces a positive public image of engineering excellence. By avoiding disruptions visible to the public, the project not only met its technical demands but also maintained a seamless and positive perception of engineering achievements, setting a new standard for future projects and elevating the reputation of the engineering profession. Social, Economic and Sustainable Design Considerations The 43rd Street Pedestrian Bridge was built with a keen focus on social, economic and sustainable considerations, aiming to strengthen the community and promote eco-friendly practices. The bridge was designed to accommodate pedestrians, cyclists, people with disabilities, wheelchairs and strollers, encouraging sustainable living by promoting non-vehicular access to different parts of Chicago via the lakefront pathways. Throughout construction, the project team prioritized social and economic benefits. They minimized the construction site impact by using existing pathways as haul roads and preserving a recently reconstructed path instead of replacing it as originally planned. This approach not only reduced disruption to the landscape but also minimized the need for off-site materials, leading to economic savings. Additionally, the project incorporated sustainable practices by using low-maintenance grasses and plantings, reusing suitable topsoil and on-site excavated embankment, and implementing erosion control measures to protect natural habitats and reduce and collect soil runoff. These efforts not only enhanced the bridge’s sustainability but also contributed positively to the overall well-being of the community and the environment. Complexity The complexity of the 43rd Street Pedestrian Bridge project is visibly apparent in the non-symmetrical, inclined geometric configuration. Challenges arose from unique criteria and site-specific conditions. The bridge’s position relative to the Metra and CN railroad tracks presented extraordinary challenges. The available space for temporary support/protection structures was much narrow between the railroad tracks and pier 3 footing and constrained by the Metra access road and an underground utility corridor. To avoid disruptions, the team ingeniously employed helical pile foundations and surface-level shallow foundations, minimizing disturbances to Metra’s daily operations. Additionally, the project faced complexities in the design of the west approach ramp. Limited space required the use of curves as tight as a 36’ radius, leading to a transition through three different curves as the ramp ascended to the main span. The constrained footprint also required a walk surface with a slope exceeding 5%, necessitating the incorporation of landings. To overcome these challenges, the team implemented innovative solutions, including consistent slope in structural steel design and variable deck inclinations from 8% to 1% at the landings. These creative approaches ensured that deck elevations remained within tolerance during construction, highlighting the project’s requirement for out-of-the-ordinary ingenuity to achieve its goals.
The team’s exceptional performance went beyond mere completion; they exceeded the owner’s needs and expectations at every turn. Despite the challenges posed by a tight two-year schedule and the complications of a global pandemic, the project was successfully completed on March 7, 2023, even with unprecedented material shortages and fabrication shop shutdowns. The initial budget estimate of $31,530,824.27 was met remarkably closely, with the total actual cost amounting to $31,600,000. What truly distinguished this project and team was proactive engagement with the client throughout the entire development process. They not only navigated the complexities of the pandemic to find an economically viable solution that resulted in project success. When faced with the option of an accelerated finish in January, which would have incurred upwards of $1 million in additional costs and extreme overtime conditions, the team chose a more cost-effective approach after discussions with the client. Opening the project in March proved to be a financially prudent decision, aligning with the client’s goals and stakeholders’ expectations while keeping the project very close to its budgeted costs. Success hinged not just on technical expertise but also on fostering seamless collaboration among diverse stakeholders. Coordination efforts with Metra, CN, federal, state, and local authorities were instrumental in the project’s smooth progression, demonstrating their commitment to delivering exceptional results while ensuring economical and cost- effective solutions that exceeded the owner’s requirements. Closing Summary The 43rd Street Pedestrian Bridge stands as a truly one-of-a-kind project. The project team overcame numerous obstacles to create this distinctive structure within remarkably unique circumstances. Challenges arose during material procurement and staging due to limited availability, necessitating the team to employ innovative thinking and creative execution. Moreover, the construction process posed additional complexities, requiring the team to adopt sophisticated strategies uncommon for conventional highway bridges. Despite these hurdles, the project not only established a new connection to the lakefront for another Chicago neighborhood but also managed to deliver an iconic structure while remaining mindful of budget constraints and societal impact.