I-94 2nd Avenue Bridge Network Tied Arch

A core piece of Detroit’s $2 billion Interstate 94 corridor reconstruction and downtown revitalization made history with its daring engineering.

The new 245-ft bridge carrying Second Avenue over I-94 is the first skewed and unbraced network tied arch bridge in the United States. It was also erected offsite and moved into place.

A historic building adjacent to the bridge location required an 18° skew, which offset the bridge’s arches longitudinally by nearly 30 ft. Structural steel’s strength, low weight, and flexibility were essential to reduce the bridge skeleton’s dead load and facilitate the self-propelled modular transporter (SPMT) move and installation. During the design, one of the controlling load cases was the buckling capacity of the unbraced arch ribs, and the torsional stiffness of structural steel was up to the challenge.

All told, 800 tons of AASHTO M270 Grade 50 steel makes up the trapezoidal arch ribs, floor beams, and lateral bracing in the floor system. The unbraced arch ribs consist of trapezoidal box sections with PL 1¾ in. by 4¾ ft for the top flange and PL 1¾ in. by 2½ ft for the bottom flange. Web plates also consist of 1¾-in. material on a 1:3.74 inclination.

The elegant and sophisticated design uses four planes of steel hanger cables crossing from the top of the arch rib to the tie girder. These inclined cables dramatically increase structural stiffness, reduce dead and live load deflections to approximately 10% compared to a vertical hanger system, and increase structural redundancy.

The Michigan Department of Transportation (MDOT) wanted a durable, redundant structure to resist over-height vehicle strikes. The network hanger arrangement provides much greater redundancy than a conventional tied arch with vertical hangers. The structural system also reduces member forces in the rib and tie girders, which allows for more efficient use of materials and a slender, attractive appearance.

The design team also evaluated a conventional steel girder bridge as part of the preliminary design. However, MDOT’s plans to widen and shift I-94’s alignment in the future would require building a pier in the current median and then constructing a future median pier to accommodate the shifted alignment. Steel was also the preferred material for the later configuration due to the ability to design multiple support locations during the bridge’s life.

The tied arch span allowed the bridge to be constructed without the need for future demolition and reconstruction of a median pier that would have been required with a conventional steel girder bridge at that location. The tied arch spans completely over the current freeway alignment and was designed to be long enough so that the future shifted freeway will fit comfortably between the abutments, providing complete construction access for the current and future alignments.

Limited vertical clearance over I-94 demanded a strong, thin floor system for the network tied arch span. The design team chose welded steel I-girder floor beams for the 89½-ft span between tie girders. The floor beams required a bat wing profile to accommodate the roadway and SUP cross slopes. Each floor beam was slightly different than the others due to the skew and asymmetric vertical curve on the bridge, and these geometric differences were carefully documented in the shop drawing and fabrication process.

The steel design allowed for vertical warping of the floor system up to 3 in., and the contractor’s SPMT system used hydraulic jacks to make continual adjustments to ensure that the bridge remained safely within the tolerance.

The designers felt that a skewed arch with lateral bracing between the ribs would appear warped from the driver’s perspective and decided early on an unbraced arch solution would be the most appropriate. The ribs were erected with temporary lateral bracing in place that remained until after the bridge skeleton move finished and the deck casting was complete.

Lateral bracing in the floor system consists of WT9×48.5 members in an X-configuration. The design dictated a specific tightening sequence for the lateral bracing members, and the contractor completed that work without any significant problems.

The combination of skew and curvature meant no connections would be square, and each fabricated steel floor beam would be unique. Shop drawings were carefully prepared and reviewed to ensure the skewed connections fit properly in the field.

MDOT did not want I-94 closed for an elongated period, making offsite erection and fast installation an appealing solution. The bridge was erected on temporary supports at each corner that matched the elevations of the permanent bearings on the abutments. Normally, these geometric details are not difficult to accommodate on a conventional bridge. However, with a network tied arch transported using SPMTs skidded over the top of the abutment to another set of SPMTs, differing elevations must be considered at every step of the lift and move.

The only feasible assembly area for the bridge was a parking lot approximately 500 ft from the final location, meaning an SPMT move would be required to install the bridge. The SPMT portion, though, had to navigate a 20 ft elevation difference between the staging area and I-94.

The design team considered the final condition for the bridge with unbraced ribs and a complete concrete deck and the load case where the bridge skeleton without deck would be transported using SPMTs. LARSA models were used to evaluate stress, deflection, and the potential for arch rib buckling at each load case.

Working directly with MDOT and the Federal Highway Administration, the design team solicited feedback from several heavy-lift contractors regarding the most feasible means of moving the bridge skeleton into place. Confidential oneon- one meetings helped gather ideas to incorporate into the assumed construction sequence without giving an unfair advantage to any contractor. MDOT brought on an independent peer review engineer about 80% of the way though the design stage.

The construction process used three separate accelerated bridge construction (ABC) operations in a single project:

• SPMTs moved the bridge skeleton from the staging area to the rear of the south abutment.

• A skidrail system launched the bridge skeleton over the south abutment.

• SPMTs and steel towers were again used to transport the bridge skeleton across the depressed freeway.

The bridge skeleton (consisting of arch ribs, tie girders, floor beams, and end diaphragms) weighs just over 2,500 tons. It was jacked, transferred to SPMTs, and driven over 500 ft to the I-94 crossing location as the first step in the installation process.

The team used four clusters of SPMTs--one at each bridge skeleton corner--supported by and driven by a single operator via a digital control panel. After assembling the structure, they jacked the skeleton nearly 8 ft, transferred the load to timber cribbage, and removed the temporary falsework used during assembly. They then drove the SPMTs beneath the bridge, removed the towers, and shifted the loads to the SPMTs.

SPMTs took several hours to move the bridge skeleton from the staging area to a location behind the south abutment, which included a 90° turn. When lined up with the permanent alignment, the skeleton’s leading end was transferred to a skidding system on the abutment, while the trailing end remained on the SPMTs.

At this point, MDOT closed the interstate and rerouted traffic to nearby freeways. The contractor delivered and compacted nearly 4,000 tons of crushed stone over the freeway to provide the SPMTs with a level driving surface. To manage a 20-ft elevation difference between the staging area and the freeway, the team relocated SPMTs onto the freeway and added a 34-ft-tall temporary tower to accept the bridge’s leading end from the transporters above.

The team transferred the structure via hydraulic jacks from skid tracks on the abutment to the SPMTs—the final step before driving the skeleton across I-94 and reversing the process to lower the bridge onto the permanent bearings.

Before reopening I-94, MDOT and the project team inspected the bridge skeleton to confirm the site was safe for the public. Moving the bridge skeleton from the staging area to its permanent location took less than a week, but years of planning, design, fabrication, and construction.

Once in place, MDOT saw the bridge as a community connector structure linking the Wayne State University campus to residential and commercial areas on the opposite side of I-94. The bridge has wide shared-use paths with continuous planter boxes as a separator between traffic and pedestrians and aesthetic lighting, creating a safe and inviting park-like environment for students and other users. The unbraced arch ribs provide an unobstructed view of the sky.

The tied arch span, now accented with energy-efficient LED lighting, is an aesthetic gem representing significant progress toward future freeway improvements.

Project Team

• Owner: Michigan Department of Transportation, Lansing, Mich.

• General contractor: Z Contractors, Shelby Township, Mich.

• Structural engineer: HDR, Ann Arbor, Mich.

• Erection engineer: Janssen & Spaans Engineering, Indianapolis

• Independent peer review: Parsons, Chicago

• Steel team:

  • Fabricator: Veritas Steel LLC, Eau Claire, Wis. *AISC full member; AISC-Certified fabricator*

  • Detailer: Tensor Engineering, Indian Harbour Beach, Fla. *AISC associate member*