Interstate 74 is a major east-west link in the nation’s transportation system, a key corridor in the Midwest and an economic lifeline for the Quad Cities of Davenport and Bettendorf, Iowa, and Rock Island and Moline, Illinois.
The cities function as one large metropolitan area, thanks to a pair of Mississippi River suspension bridges that directly link Bettendorf and Moline. As part of a regional strategy to improve access across the major waterway, the Iowa and Illinois departments of transportation launched the I-74 Mississippi River Bridge Replacement Project more than 20 years ago. Construction began in 2017, and in December 2021, the bi-state project celebrated substantial completion.
Encompassing 6 miles of interstate through two cities in neighboring state, the project’s major components include:
- Twin signature east-west inclined basket-handle true arch bridges constructed along a new alignment east of the existing bridges
- New full-access interchanges, connector roadways and ramps
- Reconfigured local roads
- A multiuse path connecting trial systems in Bettendorf and Moline
The Iowa and Illinois DOTs and the Federal Highway Administration worked closely with transportation and community officials throughout the project’s development. In addition, the Bi-State Regional Commission, the metropolitan planning organization for the Quad Cities area, included it as a top-priority project in its long-range transportation plan. Other critical partners included municipalities, utilities, motorists, citizens, elected officials and local, state and federal agencies.
“Building strong relationships with our project partners and then maintaining those relationships through construction was our greatest achievement,” said Jim Schnoebelen, Iowa DOT District 6 engineer. “This collaborative effort ultimately improves safety and mobility for motorists, cyclists and pedestrians across the region.”
Engineering for service life
In addition to building a broad coalition of support, Iowa DOT built a lasting asset with a targeted 100-year service life.
“The investment in a longer life span will result in huge dividends as we look at future operation, maintenance and user costs,” Schnoebelen said.
“Serving as an extension of the Iowa DOT, we worked together to provide the Quad Cities with a durable, long-lasting structure,” said Travis Konda, HNTB construction engineering and inspection task lead.
Engineering for service life is an emerging trend, according to Jim Nelson, director of Iowa DOT’s Bridges and Structures Bureau.
“During the design phase, Modjeski and Masters was involved in developing AASHTO’s Guide Specification for Service Life Design of Highway Bridges. Consequently, some of the new guide’s concepts are incorporated in the bridge’s design,” Nelson said.
Service life concepts and other innovations include:
- Non-replaceable components. To avoid having the anchor bolts’ service life dictate the bridge’s service life, Iowa DOT chose bolts made of alloy 2507, a super duplex stainless steel designed for demanding applications. Iowa DOT selected the alloy after studying carbon steel, titanium and various stainless steels. The bridges have a total of 384 anchor bolts.
- Minimal maintenance. The bridge’s dual basket-handle arches are grade 70W high-performance steel coated with high-performance paint. The bridge decks are reinforced with stainless steel rebar to avoid chloride-induced corrosion damage.
- Winglets. Engineers designed winglets for the westbound bridge deck, a first for Iowa. The winglets are designed to stabilize the bridge deck by breaking up the way the wind attacks the bridge.
- A structural health monitoring system. The bridges are equipped with an array of sensors and gauges to detect stress, displacement, tilt, vibration, temperature and strain. The data will help DOT engineers analyze the bridge’s performance and facilitate preservation.
Erecting the arches
When citizens voiced their desire for an iconic gateway structure, the design team of Alfred Benesch & Associates and Modjeski and Masters responded with the true arch design, also part of the longevity strategy.
In a true arch design, the thrust from the arch is carried straight down from the arch rib into the piers through the foundation and into the drilled shafts, which are socketed into the underlying bedrock.
“Having bridge elements in compression rather than tension helps prevent fatigue and fractures,” Nelson said.
“We developed refined analytical models to ensure the stability of the slender arches and optimized the hanger spacing and arch bracing,” said Dr. Thomas Murphy, senior vice president for Modjeski and Masters and engineer of record for the arch superstructures. “Continuous coordination with the design team was equally essential, ensuring seamless integration of the design with the overall corridor plan.”
The bridges are each 3,387 ft long. The main spans support 795-ft-long by 72-ft-wide decks and feature 245-ft-tall true arch ribs.
“The combination of the structure, its location over the Mississippi River and the true arch design made the project unique,” Konda said.
The DOT, HNTB and Lunda Construction Company collaborated to identify issues and workable solutions to keep the multifaceted and exacting mission of arch erection moving forward.
“The arch bearing tolerances were tight, two-hundredths of an inch,” Konda said. “So, we worked with the contractor to develop a quality assurance survey plan to ensure the two connected arch segments joined perfectly.”
Erecting the arch to facilitate fit up and closure was a critical and ongoing part of the construction process.
“Keeping track of the as-built geometry as the arch erection advanced was a combined effort between the contractor, the erection engineer, the designer, Iowa DOT and the field staff. Consistent coordination between all parties was key in understanding the geometry and making adjustments as the work progressed,” Konda said.
Each arch consists of 30 pieces. Barges and trucks delivered the heavy, fabricated sections to the job site, then cranes on barges lifted them into place.
The contractor faced some tough challenges during arch erection, including high water that prevented barge movement in the river. High winds, extremely cold winters and three floods were other factors that restricted work. Furthermore, the channel was never closed to recreational boats during construction. The contractor collaborated with the U.S. Coast Guard and the commercial shipping industry to establish short closures to set bridge pieces over the navigational channel.
Work on the steel portion of the first arch for the westbound bridge began in March 2019 and finished 14 months later in May 2020. Work on the second arch began in July 2020 and was completed 10 months later in May 2021.
Maintaining two-way traffic during construction
More than anything, Schnoebelen said the magnitude and duration of the project presented the biggest challenges, but the Quad Cities’ enthusiasm and support for the project never waned.
“Our community partners and residents understood firsthand the need for the project, and that kept them engaged,” Schnoebelen said.
During construction, the Quad Cities wanted to keep traffic flowing in both directions across the river, which posed numerous challenges and required extensive coordination.
George Ryan, corridor manager for the I-74 project, and his team monitored and adjusted on the fly the extensive maintenance of the traffic plan. They also orchestrated an exhaustive communications campaign to keep the public informed of the near-daily traffic pattern shifts that included executing an innovative zipper merge on the existing bridge and using part of the newly completed interstate as a local route to bypass a railroad crossing.
“Our team worked hard to adapt traffic configurations to new circumstances and ensure the public knew what to expect. We used both the new and old structures to keep traffic flowing between the states. Construction contracts were let in stages, and detours were timed with the completion of sections of the new structures to minimize user impacts,” Ryan said. “Overall, motorists responded very well to these changes.”
Designing a bridge as beautiful as it is functional
Based on stakeholder and community input, the bridge’s overall theme is “Reflections,” depicted with interlocked hands. Representing the Quad Cities’ spirit of partnership and interdependency, the pattern can be found throughout the corridor on bridge rails, retaining walls, shapes of lighting and the bridge’s distinctive Y-shaped piers.
“The curved, intertwined piers were a challenge for the engineer to model and detail for constructability, so we required the contractor to build custom formwork and demonstrate its use,” Nelson said. “There are 34 piers. The customized design was fairly efficient when amortized over the entire project.”
Other aesthetic elements include:
- A glass oculus embedded in the multiuse path as part of a scenic overlook
- Dramatic LED lighting
- A 14-foot-wide multiuse path that accommodates bicycles, pedestrians and wheelchairs and connects to paths in Bettendorf and Moline
- An elevator and other ADA accommodations
Realizing a community’s vision
When the first bridge opened in November 2020, many of the traffic issues the Quad Cities had experienced for years vanished. At the opening of the second bridge on Dec. 1, 2021, Iowa DOT Director Scott Marler officially declared the project a success.
“The new I-74 Mississippi River Bridge will join our states and communities, will strengthen our region, and will stand tall in welcoming new hope and opportunity to the Quad Cities and beyond for many years to come,” he said.
Preventing mussel loss
In September 2016, the Iowa and the Illinois departments of transportation undertook one of the largest mussel relocations in the country. Divers removed more than 150,000 of the bivalve mollusks (29 species) from the shorelines and around the piers of the I-74 project area before construction in 2017.
“It went well,” Schnoebelen said, “due to the great planning and the cooperative nature of all the regulatory agencies involved.”
The relocation, required by the Federal Endangered Species Act, is a culmination of a three-year process developed through a partnership of both state DOTs, the U.S. Fish and Wildlife Service, the Federal Highway Administration, and the departments of natural resources in both states.
As part of the mitigation, the Iowa DOT developed a virtual reality program with Iowa State University. The program helped the public understand the project by providing views of the old bridge and the mussels’ habitat. The exhibit toured various public buildings and was highly popular.
Phase 2 of the mussel relocation was completed in September 2021 prior to demolition of the existing I-74 bridge.
Dimensions of the new I-74 Mississippi River Bridges
- Total bridge length: 3,387 feet
- Arch height: 245 feet
- Main span length: 795 feet
- Number of approach spans: 14 over the river, 11 on the Illinois side, 3 on the Iowa side
- Number of interstate lanes: 3 in each direction
- Number of bridge lanes: 4 in each direction
- Navigational clearance: 60-foot minimum
- Arch bridge navigation envelop: 710 feet horizontal by 60 feet vertical (minimum from normal pool elevation)
