Accelerated Construction of an Unbraced Network Tied Arch Bridge

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The 2nd Avenue Bridge in Detroit, Michigan, is unique not only due to its skewed, unbraced network arch configuration but also for the innovative accelerated bridge construction (ABC) process that utilized self-propelled modular transporters (SPMTs) to move the bridge into place during a brief closure of I-94.

The bridge skeleton—a hybrid system consisting of steel trapezoidal arch ribs, steel floor beams, post-tensioned concrete tie girders, end diaphragms, and knuckles—was initially assembled in a staging area near the bridge site. Then this structure, weighing just over 5 million pounds, was lifted and moved into place using SPMTs in July 2022. Following casting of the concrete deck, the bridge opened to traffic in December 2022.

As the primary freeway access point for urban Detroit, I-94 carries an estimated 175,000 vehicles per day and plays a crucial role in connecting the city. Originally constructed during the interstate highway expansion of the 1950s, the highway has served the area for decades. However, increased traffic volumes in this region have exceeded its original capacity, necessitating significant upgrades.

All of the original bridges constructed over the highway in the 1950s have reached or exceeded their intended 75-year lifespan. With these structures having outlived their useful service life, the Michigan Department of Transportation (MDOT) launched its Advanced Bridges program, replacing nearly two dozen of them—including the 2nd Avenue Bridge—in advance of the main corridor reconstruction project.

On 2nd Avenue, MDOT needed a bridge that could span a widened I-94 corridor without intermediate piers since future alignments for an adjacent freeway interchange are still unknown and MDOT wanted to avoid “throwaway” construction. The resulting bridge consists of a 245-foot-long, 96-foot wide, unbraced network arch span, which carries vehicular traffic, bicycles, and pedestrians in separate dedicated lanes alongside decorative planter boxes and lighting. In addition, the arch ribs are unbraced, creating a visually distinctive, “community connector” structure and a park-like environment for users.

MDOT selected the team of Tetra Tech and HDR (as Engineer of Record for the signature span) to design the 2nd Avenue bridge replacement with the following goals:

The planned widening of the I-94 corridor, resulting in a shift of the alignment, led to the bridge’s innovative network unbraced arch design. The bridge can accommodate a much wider roadway than the current configuration. A key design feature is the clear span arch, eliminating the need for future demolition and reconstruction of a median pier. The unbraced arches offer a cleaner, more open appearance, allowing the bridge to serve as a “community connector” rather than acting as a visual barrier within the surrounding environment.

In addition to the complexity of accommodating I-94 traffic, physical constraints made the site more challenging. The bridge could not be built in place; space to assemble the bridge was limited; and the depressed freeway was located nearly 20 feet below the elevation of the only feasible bridge construction and staging area.

A significant cultural landmark—United Sound Systems, a historic recording venue rooted in the origins of Motown Music— also stands just off the northeast corner of the bridge. To minimize adverse impacts, the design team skewed the ends of the bridge 18 degrees to avoid encroaching too closely on the fragile masonry building, a concern heightened by the significant abutment excavation and temporary earth retention systems required for the large abutments. The skew added significant geometrical complications and detailing such as unique knuckle dimensions and elevations at each corner, and it required further analysis to capture the structure’s behavior for temperature and live load effects.

At the project's outset, MDOT and the design team prioritized constructing the signature bridge without disrupting I-94 traffic for more than a few days. To avoid rush hour traffic, they aimed for as brief a closure as possible, such as from 10 p.m. Friday to 5 a.m. Monday. The project utilized the design-bid-build procurement method, and significant investigation into the constructability of the bridge was required during the design phase.

MDOT and the design team held confidential meetings with industry-leading specialty contractors to gather input on feasible methods for moving a comparable bridge into its final position. These one-on-one meetings proved invaluable, as contractors were more open to sharing ideas, knowing their competitors could not gain an unfair advantage.

After evaluating several alternatives, the design team defined a method using a performance specification where the bridge skeleton, essentially the entire arch span, excluding the concrete deck, would be built in a nearby parking lot and moved into place using SPMTs.

The 2nd Avenue Bridge design emphasizes aesthetics, as well as redundancy, resilience, and efficiency. The bridge accommodates both pedestrian and vehicular loads, supporting four lanes of traffic and a higher “festival” pedestrian load of 90 psf to handle peak pedestrian traffic during special events, with provisions for six vehicular lanes in the future. Substantial superimposed dead load is present with wide sidewalks and multiple planter boxes on the structure.

Each arch panel incorporated thirty 31/8-inch diameter ASTM A586 structural strand hangers. Arranged in an inclined, crisscrossing network, these hangers significantly enhance the bridge’s structural efficiency. The 63-degree hanger angle was carefully selected to more evenly distribute forces across the structure and minimize bending moments in both the arch rib and the tie girder. This orientation improves the bridge’s structural performance, making it lighter, stiffer, and more stable compared to traditional tied arch bridges with vertical hangers.

The 14-foot spacing of the steel floor beams greatly influenced the selection of the hanger angle, a decision that balanced structural requirements with long-term maintenance considerations. Per MDOT guidelines, floor beams spaced more than 14 feet apart require more rigorous arm’s-length, hands-on inspections. The design met structural needs by keeping the spacing at 14 feet while minimizing inspection demands. The floor system employed Grade 50 steel I-girder floor beams with variable web heights to efficiently respond to varying load conditions across the span. While the arch rib is the defining visual element of the structure, the core design decision lies in the use of post-tensioned concrete tie girders in place of the more typical steel alternative. This choice not only enhances durability but also avoids the fracture-critical classification commonly associated with steel tie girders, which are subject to non-redundancy considerations under AASHTO and FHWA guidelines. The 8,000-psi concrete tie girders are rectangular in shape, measuring 4 feet in depth and 3 feet 6 inches in width. Each girder contains twelve post-tensioning tendons, with each tendon consisting of nineteen 0.6-inch diameter, Grade 270 prestressing strands. HDR designed the concrete tie girders with a 1.25 overstrength factor, driven by their location directly above active traffic.

A strain compatibility analysis was used to evaluate the complex combination of tension, biaxial flexure, shear, and torsion within the tie section. To check global stability, HDR used an iterative P-Delta method to evaluate the critical buckling load of the trapezoidal steel arch ribs. Geometric non-linear analysis refined the arch rib design, accounting for out-of-plane deflections and providing more precise force predictions beyond conservative estimates. Both the global stability check and geometric non-linear analysis utilized LARSA 4D finite element software. Each arch rib features a constant-width trapezoidal cross-section fabricated Grade 50 steel, with 1¾ inch welded flanges and webs. The ribs maintain a constant height of 4 feet 2½ inches, with a top flange width of 4 feet 9 inches and a bottom flange width of 2 feet 6 inches. The trapezoidal cross-section offers increased torsional stiffness and resistance to lateral buckling, which is especially beneficial for an unbraced arch. In addition to its structural efficiency, the rib’s angular geometry contributes to the bridge’s aesthetics and reinforces its identity as a visually open, community-focused structure. All steel ribs, along with other steel components of the bridge, were galvanized and finished with an exposed epoxy paint system. This not only elevates the bridge’s appearance but also provides an added layer of protection against chloride exposure.

At each end of the bridge, the post-tensioned concrete knuckles serve as the critical anchorage points for the arch ribs, transferring forces into the supporting elements of the structure. HDR developed three-dimensional strut-and-tie models to validate the concrete knuckle design and ensure reliable load transfer between the arch rib, tie girder, and end diaphragms. The knuckles are anchored by twenty 1-3/8-inch diameter Grade 150 prestressing bars, which are protected by a four-level protection system comprised of grout, caps, applied coatings, and concrete enclosures, to ensure long-term durability. Between the knuckles, post-tensioned concrete end diaphragms provide lateral restraint and tie the arches together. The 6,500-psi concrete end diaphragms are rectangular in shape, measuring 6 feet 9 inches in depth and 6 feet in width. Each diaphragm contains eleven post-tensioning tendons, with each tendon consisting of nineteen 0.6-inch diameter, Grade 270 prestressing strands. The decision to use concrete for both the knuckles and diaphragms, rather than the more typical steel, was driven by durability, reduced maintenance, and the desire to avoid fracture-critical classifications associated with non-redundant steel components.

Redundancy is a critical design feature: the post-tensioned tie girders are internally redundant, with multiple tendons sharing the load. If one tendon fails, the others continue to carry the load, allowing the bridge to remain operational without a sudden loss of capacity. This multi-tendon system, combined with the ductile behavior of the concrete and controlled cracking under the compressive forces introduced by post-tensioning, enhances the tie girder’s resilience. The network hanger system works with the tie girders to ensure the bridge remains operational even if a hanger fails. A hanger loss analysis, modeled using LARSA 4D, was completed following the PTI Recommendations for Stay Cable Design, 6th Edition. HDR also incorporated visible hanger connections and exposed components for straightforward inspection and maintenance. Additionally, a replaceable sacrificial concrete nosing on the tie girders was placed to mitigate damage from high load hits.

Designers of cable supported bridges must carefully plan the construction method and sequence to ensure the permanent structure's integrity. They must anticipate reasonable installation sequences to withstand construction forces and estimate “locked-in” stresses. After selecting a construction sequence developed to meet project goals and informed by contractor outreach, HDR analyzed the bridge based on the locations of assumed temporary supports and the order in which components would be assembled.

The contract plans outlined this conceptual construction sequence, enabling contractors to bid on the project using available, though specialized, SPMT equipment. These plans provided assumed lifting points, pick weights, and suggested temporary bracing locations but did not include member or connection details. Contractors had to submit a detailed structural analysis and temporary member designs stamped by a licensed professional engineer in Michigan consistent with their proposed means and methods.

During the design of the 2nd Avenue Bridge project, the project team, including MDOT, HDR, and other members, conducted a thorough review of the construction method, sequence, and approach. As a result, the project team incorporated several proactive measures:

Given the complex bridge design and erection sequence, the involvement of a third-party reviewer added an additional level of quality control. Before accepting the erection procedure, three structural analysis models—completed by the project’s Engineer of Record, Independent Peer Reviewer, and Erection Engineer, each using different software—were compared for convergence.

Assembly of the bridge skeleton was performed on temporary supports in a parking lot approximately 500 feet from the final location of the bridge. The assembly work faced many challenges, not the least of which was the COVID pandemic and related work delays and supply chain shortages. The bridge skeleton, weighing more than 5,000 kips and nearly the size of a football field, was ready to be moved into place by the start of July 2022.

The transfer of the arch span to SPMTs required the use of stacked timber dunnage in what the project team referred to as the “Jenga system.” This allowed the span to be lifted and lowered in increments of 4 inches with climbing jacks. Each layer of timber was offset 90 degrees from the previous layer. The timbers were made from Ekki/Azobe hardwood from Africa or an epoxy-injected composite with a very high compressive strength capable of resisting the imposed loads.

Three 12-axle SPMTs were linked to lift each corner of the bridge skeleton with a total reaction to each group of approximately 1,300 kips. All SPMTs were controlled by a single operator through the move operations. After the load of the bridge skeleton was transferred to the SPMTs, the temporary abutments were removed from the staging area, and the span was made ready for transportation.

The first stage of the move transported the span from the staging area to a location immediately behind the south abutment. This operation was performed on July 19, 2022. Due to the limited space available in the staging area, the bridge was carefully manipulated to avoid the adjacent Wayne State University parking garage and law school.

The next day, the leading end of the skeleton was then transferred over the top of the south abutment and onto SPMTs on I-94 using a skid track system. This maneuver, called the “handoff,” was performed in two steps, the first of which was to transfer the load to the skid tracks prior to stopping traffic on I-94. SPMTs were removed from beneath the leading end of the skeleton, while the SPMTs at the rear of the skeleton remained in place.

In the second step, on July 22, 2022, traffic was stopped on I-94, and a layer of compacted aggregate created a smooth-running surface for the SPMTs. The handoff then transferred the skeleton to the SPMTs positioned on I-94.

The final stage of the bridge move, transporting the bridge across I-94 and onto the north abutment, occurred on July 24, 2022. During the overnight hours prior to the move, a heavy rainfall soaked the compressed aggregate infill on I-94. To avoid creating ruts, the contractor installed a layer of 1-inch thick steel plates to create a durable runway surface.

The bridge move proceeded without significant incident save for a few minor support elevation adjustments during the operation. These adjustments were necessary to keep the four corners of the bridge in the same plane at which they were cast to avoid damage to concrete and maintain stress levels within AASHTO limits. The bridge was designed to withstand out of plane movement of up to 3 inches at a corner.

In addition, the contractor was required to actively monitor the bridge during the move. Monitoring was done with a total station survey instrument, and readings were taken each time the move operation was halted. A pair of electrically charged piano wires stretched diagonally from corner-to-corner of the span were also used to monitor warping within the 3 inch value determined during the design phase.

Following the bridge move across I-94, the load was transferred to the abutments with a skid track system and the span was lowered onto the abutment bearings, once again using a Jenga tower system. A thorough inspection by HDR and MDOT staff ensured that it was safe to resume I-94 traffic. I-94 was reopened on July 29, one week after it was closed to begin move preparations. The concrete bridge deck was placed, a second stage of tie girder post-tensioning was performed and final adjustments made to the hangers prior to opening 2nd Avenue to traffic on December 22, 2022.

The bridge's opening capped a process of remarkable design and construction. By thinking creatively and collaboratively, the team behind the 2nd Avenue Bridge reduced construction work, limited traffic disruption and met demanding site constraints to deliver a signature structure that will serve its community for decades to come. ■

About the Authors

Matt Longfield, PE, SE, is the Bridge Section Manager for HDR in Michigan and provided design services during construction, including review of contractor erection analysis and extensive on-site technical assistance. (Matt.Longfield@hdrinc.com)

Ryan Bersano, PE, is a Bridge Engineer at HDR in Illinois and served as the lead designer for various components of the bridge. (Ryan.Bersano@hdrinc.com)