Bridge Description

Full-depth precast deck panels, precast cap beams, and precast ornamental railing panels were designed to replicate defining historic features.

over Mississippi River and West River Pkwy and intersecting East River Pkwy in the city of Minneapolis, Minnesota

Tier 6 (longer but reduced by months/years)

• ABC: 116 days (less than 4 months)
• Conventional: Cast-in-place construction would have ranged between 15-24 months, and at a minimum, two construction seasons

• reduced traffic impacts - the bridge serves as a critical link for local businesses and neighboring universities
• reduced onsite construction time
• improved work-zone safety
• improved site constructability
• improved material quality and product durability
• minimized environmental impacts
• reduced life-cycle cost

1049.94-ft-long and 66.3 to 76.3-ft-wide 5-span open spandrel concrete deck arch bridge (55.42 ft - 199.00 ft - 400.00 ft - 199.00 ft - 55.42 ft); 0º skew

9900

extended use of 4-mile detour

The existing 5-span open spandrel concrete deck arch bridge was 1049.94-ft long and 66.3-ft wide with six reinforced concrete supports. It had two 10-ft-wide traffic lanes, two 5-ft-wide bike paths, two 10-ft-wide shoulders (buffer) and two 7-ft-wide sidewalks. Built in 1923 and rehabilitated in 1970, the bridge is listed on the National Register of Historic Places and is recognized as a Minneapolis Landmark. Many of the original and rehabilitated elements were deteriorated and laden with chlorides. Degradation and changing usage warranted an extensive structural rehabilitation and required closure of the bridge for extensive concrete repair and full deck and spandrel cap beam replacement.

The newly rehabilitated bridge has two 11-ft-wide traffic lanes [each with 2-ft-wide striped shoulder (reaction area)] and two 17-ft-wide shared used paths from West abutment to Pier 4. The section tapers to four 11-ft-wide traffic lanes and two 12-ft-wide shared-use paths just east of Pier 4 and extends to the East abutment. The bridge has 4-ft-deep spandrel cap beams spaced from 13-ft to 28-ft, with 14-inch-thick 6,000 psi full-depth precast deck panels.

The bridge has two lines of arch ribs with no bracing between them. The column spacing was altered in the 1970 rehab from 14-ft spacing to 28-ft spacing, and the column size was also reduced by about ½. To replace the cap beams and keep traffic on the bridge would have required significant temporary works, would have put the public close to construction operations and would have brought several risks that could be avoided by closing the bridge. Therefore, the decision was made to close the bridge. Once the decision was made to close the bridge, the County and Public would not allow the bridge to be closed for 15-25 months. Thus, an ABC solution was moved forward to rehab the bridge during the summer months when the University of Minnesota was out of session. The Hennepin County/HNTB team engaged in intense planning with the general contractor, Kraemer North America (KNA), prior to the ABC period and established an erection sequence and construction contingencies.

Precast elements were designed to replace the existing bridge deck and cap beams while replicating the defining historic features. The bridge rehab required 350 full-depth precast deck panels, 43 precast spandrel cap beams, and 163 precast ornamental railing panels.

The contractor took advantage of a staging yard offered in the contract documents and developed a casting yard just upstream of the bridge at Bohemian Flats and self-performed the new mildly-reinforced bridge deck precast panels. The precast cap beams were fabricated by Forterra in Elk River, MN, and the ornamental railing panels were fabricated by American Artstone in New Ulm, MN. The yard was used to receive and store all precast elements.

The precast elements were fabricated prior to closure of the bridge. Numerous other pre-closure activities, including deck sawing and utility relocation, were also completed while traffic remained on the bridge.

The bridge was closed to traffic on May 8, 2016. The contractor worked 14 hours per day, 6 days a week, to replace the existing deck and cap beams with the precast elements. The precast elements were barged from the casting yard to the bridge as needed and hoisted to the deck by cranes. The cranes were also used to remove and lower sections of the old 1970s bridge deck and cap beams to barges below the bridge, to be ferried upriver and crushed to be hauled for recycling.

Expansion joints were reduced from 15 to six through the use of PTFE/stainless steel sliding bearings between cap beams and deck panels. Ultra-high performance concrete (UHPC) closure joints between deck panels created a continuous surface. Premixed Polymer Concrete (PPC) was used as a wearing surface and waterproofing layer to protect the structural deck. In addition to the spandrel cap beams and ornamental railing, the architectural details in the pier overlooks and lighting also provide accurate historical context.

The original contract allowed for three months of full closure and one month of partial closure. However, the County allowed four months of full closure for ABC with public support, as long as the bridge was open to traffic by September 1. Traffic was restored on September 1, 2016, 116 days after closure, completing the ABC period.

The contract had a disincentive of $10,000/day if the bridge was not open by September 1. The contract did not include incentives.

All stakeholders felt a good compromise was reached in all aspects of the project, especially the Minnesota Department of Transportation Cultural Resources Unit (MnDOT CRU) and the State Historic Preservation Office (SHPO) were pleased with how the historic aspects turned out. The public sent several positive emails –one stated they were happy their tax dollars went to restoring the bridge. Another noted the bridge was previously used to just cross the river, but now it is a place people want to go and enjoy the bridge and the river.

Lessons Learned:

The field team needs to be aware of how the concrete surface repairs will be inspected and handled in the field and that the contractor is familiar with historic structure restoration. Historic concrete does not have the same sound or feel as newer concrete when it is being removed with hammers.  Therefore, contractors that do not understand these subtleties would likely remove more historic concrete than necessary. This increases the repair cost and potentially introduces further damage to the structure.  Alternating frequently between historic concrete and newer concrete removals amplifies the likelihood of this potential and unwanted scenario.

Placing new precast elements on an existing substructure requires good understanding and survey of the existing bridge. Options include:

• Collect detailed survey during design and detail all precast elements to fit.
• Contractor collects survey during construction and new precast element plans are made by designer.
• Contractor collects survey and modifications made in shop drawings.
• Need to determine who does what. Telling the Contractor to collect survey and compare to the plans without providing direction on who will address the issue will create confusion.

Pre-planning cannot be stressed enough. Pre-planning meetings were held a full year before the bridge was closed. Bring all involved parties to the table and step through each aspect until all are comfortable. Require a Contingency Plan be developed that outlines what will happen if an issue arises.

Over pour UHPC. This can be ground by a walk-behind grinder even after reaching strength.

UHPC is very sensitive to temperature. Need to substitute ice cubes for water; do not let it get above 80 degrees. If have lower temperatures, will not start to cure. We had several days’ pours that sat over night when colder and were still plastic.

Require contractor to develop a UHPC pouring plan that shows how UHPC joints will be poured and protected such that they will not pour a joint and drive over it with equipment or buggies before it cures. Think “do not paint yourself in a corner”.

Understand and establish the tolerances required in the precast and the system the precast need to be a part of. Setting precast panels with SS plates on PTFE requires very precise placement of the cap beam, in all three directions, to achieve the bearing needed. Recommend on future project a pour back of some kind be implemented to allow contractor to “make up” differences.

Keep the PPC placement compact and do not let the screed get too far in front of the finishers.

In ABC, everything needs to go fast. Have a robust and well thought out QC/QA plan to check element fabrication and installation. Know what you will check, how you will check it and what you will do if the result is not per plan.

• Ultra-High Performance Concrete (UHPC) – deck closure joints
• Premixed Polymer Concrete (PPC) – overlay