Innovative Pedestrian Bridges Connect University of Colorado Campus to Neighborhoods and Nature
January, 2021
With the opening of the 19th Street & 23rd Street pedestrian bridges in Boulder, the University of Colorado (CU) has not only created better ADA compliant connectivity between the campus and the surrounding community but now has an iconic landmark that will help define the campus’s identity for decades to come. These two innovative pedestrian bridges, designed by Otak Colorado’s structural team, in coordination with CU Facilities Staff and Design Review Board, are the culmination of engineering ingenuity and practical problem solving incorporating lessons learned as the first of the two bridges (23rd Street) was being constructed.
CU’s main Boulder campus is separated from areas containing student housing and residential neighborhoods by Boulder Creek, the Boulder Creek Trail, a riparian corridor, and a 30 to 60-foot-tall elevation differential. Otak was retained by CU to design two bridges at 23rd and 19th Streets, with a primary goal to provide ADA compliant, user-friendly connections, between the main campus and areas north of the creek. The design team was also charged with the need to integrate bridge design with the surrounding natural environment and provide a level of aesthetics commensurate with CU’s flagship university.
What the university envisioned was a “bridge through the canopy,” weaving pedestrians through trees and connecting them to the natural beauty and wildlife in the area, while efficiently getting them to where they need to go. These bridges replace steep pathways that required pedestrians to literally huff it up a lung-pumping 20% grade, which was difficult to maintain and dangerous during winter conditions.
The overall project involved the design and construction of the 23rd Street Bridge, completed in 2018, and the 19th Street Bridge, completed in 2020. Project manager, Dan Beltzer, P.E., in Otak’s Colorado office, oversaw the design of the two bridges, guiding the project from inception through successful completion. David Graff, P.E., served as the project engineer and oversaw the bridges’ construction. The two bridges offer practical solutions for pedestrian traffic, while also preserving the natural environment and wildlife along Boulder Creek—two primary concerns for CU. The two bridges were constructed sequentially, each taking about 10-months to complete; both were on time and on budget.
One of the unique aspects of the project was the progression of the design and engineering over the course of the two bridges. As they were constructed sequentially, rather than concurrently, the team was able to refine the design for the 19th Street Bridge and incorporate changes based on lessons learned from the 23rd Street Bridge. Since the design of the second bridge occurred during the construction of the first, the team was able to get real-time feedback and make modifications that would be better suited to the challenges of the second location.
For both bridges, the design team considered different alignments to find the optimum balance between cost, aesthetics, ease of construction and minimizing site impacts, especially along the steep and marginally stable hillside. As they were also charged with mandates to avoid significant tree removal, preserve historic rock walls, and create a path that was pleasing to the user, the resulting design essentially weaves across the creek and up the hillside through the abundant canopy of trees. Keeping the bridge at a height that is not overly tall and vertically imposing also steered the alignment.
To minimize fabrication efforts and construction costs, a series of identical straight 30-foot steel beam spans were designed at the maximum ADA compliant slope to climb grade as quickly as possible. Each pier provides an ADA landing that also serves as a turning point for the pathway to provide a sinusoidal effect, creating an overall curve shape out of a series of straight lines. The piers are supported by a single concrete column founded on either a caisson or a micropile foundation, depending on the terrain. Custom railings were developed using stainless steel handrails; weathering steel plates, pipes, tubes; and stainless-steel cables to minimize materials and limit obstruction to views of the natural beauty of the surrounding environment. The resulting effect has given the bridges a “light and airy” feeling and allows users to connect to the nature surrounding the bridge.
Bridge lighting is provided by LED lights mounted flush in the handrails that shine down onto the deck surface. This configuration directs light only towards the pathway where it is needed and minimizes glare into the riparian surroundings. Sensors at the ends of the bridges increase lighting intensity when pedestrians are present and dim lighting otherwise.
The 23rd Street Bridge connects the Buff Walk (the path between Folsom Field and the athletic center) to the Boulder Creek Path and Parking Lot 169, a popular tailgating area on game day. At the top of this bridge is a large landing that allows people to gather for game day activities and view the Flatiron formations west of the city and riparian corridor below. To provide an ample landing area atop the steep hillside, a retaining wall with high-density styrofoam blocks was used as backfill to reduce earth pressures and provide the necessary wall stability.
One key difference and a lesson applied from the 23rd Street Bridge to the 19th Street Bridge are the configurations of the pier landings. The 23rd Street Bridge incorporates thin concrete bearing seats for the bridge spans to sit upon, while the 19th Street Bridge provides a sleeker profile by eliminating this bearing seat and framing the bridge spans directly to the concrete landings via a steel landing connection frame. This configuration also allowed for easier fit-up of the bridge spans and railing.
Construction of the 19th Street Bridge also presented a unique set of challenges, different from the bridge at 23rd Street. The natural environment around both bridges is home to a plethora of wildlife, including foxes, deer, and numerous species of birds. In fact, the canopy surrounding the 19th Street Bridge has the highest number of bird species in Boulder County, so the design needed to also appease bird watchers. A large landing with a bench is included at the mid-point on the bridge to facilitate bird-watching or to just take a rest and enjoy the surroundings. The portion of the bridge spanning Boulder Creek also contains patterned markings in the concrete slab to celebrate the flow of the water below.
History played its part in the design process as well, presenting both preservation challenges and opportunities to celebrate the area’s past. A previous bridge crossing at the 23rd Street Bridge location was historic but had to be removed to achieve necessary hydraulic improvements. The historic masonry abutments were allowed to remain, and interpretive signage has been added at this bridge’s location so the community can see pictures of and read about the site as it once existed. Many historic elements are also visible from the 19th Street Bridge, including an old warming hut used when the area between Boulder Creek and the hillside would be intentionally flooded in the winter for ice skating. Historic stonework is also visible along the banks of Boulder Creek, placed there from 1938–1940 by workers associated with the Works Progress Administration (WPA), a New Deal program created by the Roosevelt Administration to give unemployed Americans the opportunity to help with public works. The stonework is easily observed from the new bridge, reintroducing pedestrians to these historic elements that were previously difficult to access and obscured from view.
Together, the bridges provide much-needed connectivity between CU’s main campus, the neighborhoods to the north, and the rest of Boulder via the creek path. But the bridges also provide the desired connection with nature and history. Pedestrians now have both a pathway to get to where they need to go and an opportunity to be immersed in the creek corridor’s wilderness along the way. In the end, Otak and CU were able to deliver two iconic bridges that will be a part of the University’s identity for generations.