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 Av- enue 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, approxi- mately 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, re- construct 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 de- sign 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 Archi- tects to help refine the design concepts. It was determined that reno- vating the existing bridge would cost between $7 and $10 million but Grand Avenue Bridge in Des Moines Replacing a Historic Bridge: Innovative Solutions Add Modern Charm to Grand Avenue Bridge in Des Moines
would only result in a short-term solution. Rebuilding a bridge in a similar arch-style could cost more than $30 million. However, a pre- tensioned, 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 con- crete 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 GrandAvenue Bridge (namely, the iconic arches) was a critical require- ment 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 rein- forcing 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.
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october 2020
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