Replacing a Historic Bridge: Innovative Solutions Add Modern Charm to Grand Avenue Bridge in Des Moines
Contemplating the replacement of historic structures is never an easy decision for communities to make. Prominent structures have a way of growing into meaningful icons by striking a permanent pose against a city skyline. Unfortunately, the forces of time and weather are relentlessly at work, causing material deterioration to even the best-built structures. Having seasoned professionals involved and engaged, providing critical guidance, is a primary component in the decision-making process. A case in point is the recently completed Grand Avenue Bridge project in Des Moines, Iowa.
Opened in 1918, the Grand Avenue Bridge was one of a series of four bridges (Locust Street, Walnut Street, Grand Avenue, Court Avenue) built between 1907 and 1918 spanning the Des Moines River in the heart of Iowa’s capital city. Over the years, these ornately-decorated, reinforced-concrete arch structures became symbols of the city’s rich history and are even depicted on the City of Des Moines flag. While these bridges provided passage for horse-drawn carriages, pedestrians, and early automobiles, over decades, the Grand Avenue Bridge, in particular, became a critical link connecting the East Village with the downtown Des Moines business district. By a recent count, approximately 7,600 vehicles utilize it each day, making it one of the most heavily used bridges in the downtown region.
In the late 1960s, the Locust Street, Walnut Street, and Grand Avenue bridge decks were reconstructed along with other improvements. As a cost-saving measure, most of the original ornamentation was removed, leaving only the arched spans and piers. After another half-decade of use, the time came again in the 2010s to either renovate the existing structures or build entirely new bridges. Focusing on the Grand Avenue Bridge, the professionals with Shuck-Britson were brought on board to help provide guidance in this all-important, reconstruct or replace decision.
Deteriorated Structure Requires Complete Replacement
To kick start this process, the bridge experts with Shuck-Britson prepared a structural evaluation report and feasibility study to aid in the repair or replace decision of the century-old arch bridge. The engineering team found the bridge to be structurally deficient with scour-critical concrete piers and immediately set about creating design options for consideration.
During this phase, multiple public participation opportunities were held to inform the community of the findings, the design process, and receive feedback relating to the design alternatives, schedules, and costs. Shuck-Britson also enlisted the services of Substance Architects to help refine the design concepts. It was determined that renovating the existing bridge would cost between $7 and $10 million, but would only result in a short-term solution. Rebuilding a bridge in a similar arch-style could cost more than $30 million. However, a pretensioned, prestressed concrete (PPC) beam bridge would maximize the use of standard construction details and could be constructed for under $9 million.
Once the decision was made to replace the bridge with a modern concrete beam structure, an Aesthetic Steering Committee consisting of city personnel, architects, and other local stakeholders was established to evaluate the design options and make recommendations to the City Council. Keeping some of the same aesthetic details as the original Grand Avenue Bridge (namely, the iconic arches) was a critical requirement for many of the committee members. To meet this directive, the designers developed a concept that would use scalloped metal panels along the sides of the bridge reminiscent of the original arch span. This design, along with strategically-placed LED accent lighting, would create a new, signature Des Moines River bridge while also satisfying the requirements of efficiency and economy.
Unique Features, Bridge Details, & Design Challenges
The new four-span, 446-foot long, 54-foot wide, concrete-beam bridge was designed to support two lanes of traffic, on-street parking, bicycle lanes, and 14-foot wide sidewalks on both sides. A non-standard deck cross-section was incorporated to raise the pedestrian sidewalk above the roadway. A modified crash rail protects pedestrians, while the raised sidewalks with extended overhangs enhance the dramatic effect of the scalloped fascia panels.
The engineering team also added beamlines under the raised pedestrian sidewalk to support a snooper truck for future under-bridge inspections and maintenance. The additional sidewalk load resulted in nonuniform beam reactions requiring careful analysis to determine the proper reinforcing strand and shear stirrup patterns, as well as bearing pad sizes. Further, more than 30 separate utility conduits were incorporated into or below the new bridge deck.
Because of existing constraints at the bridge location, several design challenges needed to be addressed to accommodate the preferred bridge design. Foremost was the limited ability to raise the roadway from its current elevation and the preference to avoid changes to the existing floodwalls.
Since the levee system is critical for protecting the downtown region, there was a strong desire to leave the existing floodwalls and levees intact. Appropriately, the bridge abutments were designed to penetrate the levee on both river banks while still maintaining flood protection for the city, with the new bridge abutments sized to match the locations of the floodwalls. To minimize excavation and disturbance to the existing floodwalls, large portions of the original massive abutments were left in place. The new prototypical, semi-integral abutments were developed to account for the pile fixity caused by the rigid connection/incorporation into the existing abutments.
Due to the limited ability to raise the roadway, the Shuck-Britson engineers worked closely with the U.S. Army Corps of Engineers (USACE) and the Iowa Department of Natural Resources (DNR) to modify the existing hydraulic model, as well. With the pier footings for the new bridge supported on steel piles extending to bedrock, one less pier was required, increasing the flow area of the river by nearly twenty percent. Additionally, the bridge is designed to withstand the hydraulic effects of a 200-year frequency flood event and the intense water pressure acting against the bridge fascia.
Expert Coordination & Collaboration Lead to Aggressive Build Schedule
Following the demolition of the previous structure at the end of 2016, construction began on a new bridge. The shallow river depth allowed for the use of temporary causeways during construction with a minimal impact on the environment.
All plans were submitted on time, allowing the Iowa Department of Transportation (DOT) to let this comprehensive project on schedule. Shuck-Britson’s estimate of probable cost for the project was $8,612,304.00, with an estimated bridge cost of $7,749,615.00. The actual bid price for the project was $9,060,752.68, with an actual bridge bid cost of $8,379,396.23. The difference in the original cost estimate versus the bid cost is mostly attributed to site difficulties and aesthetic components.
In November of 2017, a mere 12 months after construction began, the bridge opened to one lane of vehicle traffic in each direction with partial pedestrian traffic. The project was substantially completed in spring 2018.
The Shuck-Britson team, along with parent company Snyder & Associates, made the project possible by collaborating to provide the development of project concept statement, preliminary bridge and roadway plans, hydraulic analysis and documentation, final bridge and roadway design plans, as well as the extensive coordination and development of aesthetic details. Also, the design team provided rapid response service for issues that arose during construction. These included addressing unanticipated site conditions, pile driving inquiries, and general construction observation.
Award-Winning Bridge Project
The success of the new Grand Avenue Bridge led to a similar design for the Locust Street Bridge location (currently under construction) with plans in the works to let the Walnut Street Bridge in 2022. The innovative aesthetics on the new bridges is viewed as a visual complement to the Iowa Women of Achievement Bridge located just upstream from Grand Avenue and helped to transform the downtown river area for a new generation. Shuck-Britson and their client, the City of Des Moines, were awarded the 2019 Honor Award for Engineering Excellence in the Transportation category by the American Council of Engineering Companies of Iowa (ACEC-IA) for the Grand Avenue Bridge project.