Spans Across the Mississippi

March 14, 2005

In 2003, the Minnesota Department of Transportation awarded three of four major construction contracts for the $250 million upgrade of I-494, Highway 61 and the interchange connecting I-494 with Highway 61 in the cities of South St. Paul and Newport. The projects include 30 bridges, thousands of feet of concrete retaining wall and extensive use of high-performance concrete paving.

In 2003, the Minnesota Department of Transportation awarded three of four major construction contracts for the $250 million upgrade of I-494, Highway 61 and the interchange connecting I-494 with Highway 61 in the cities of South St. Paul and Newport. The projects include 30 bridges, thousands of feet of concrete retaining wall and extensive use of high-performance concrete paving. The existing highways, interchange and bridge over the Mississippi River, which were constructed in the 1960s, have severe geometric and traffic-capacity problems, causing lengthy traffic delays during the morning and evening rush hours. A key component of the upgrade is a new bridge carrying I-494 over the Mississippi River.

Because of the span lengths required to maintain the existing channel width for commercial navigation and clear existing bridge foundations, roadway width needed to fulfill the design vehicular capacity requirements, and to minimize interference to river traffic during construction a segmental design was chosen for the new river bridge.

The new Wakota Bridge will consist of twin long-span, cast-in-place, segmental concrete bridges. The new bridges will each have five lanes and two full-width shoulders and will replace the existing four-lane structure that includes a tied-arch main span.

The new bridges will be 97.8 and 85 ft in width, respectively, for westbound and eastbound traffic with a pedestrian walkway on the westbound structure. Pedestrian outlooks at each pier will provide a panoramic view looking north toward the city of St. Paul. Each bridge is continuous from abutment to abutment for a total length of 1,879 ft with spans of 266, 328, 466, 466 and 353 ft.

The superstructure is composed of a variable-depth trapezoidal box girder with a maximum depth of 24.1 ft at the piers and a minimum depth of 11.8 ft at midspan. The box girder is post-tensioned in the longitudinal direction with the top slab also post-tensioned in the transverse direction. A 2-in.-thick low-slump concrete wearing course will provide the riding surface on the top slab.

Foundations for the bridges include 42-in.-diam. steel pipe piles at piers 1, 2 and 3, 18-in.-diam. steel pipe piles at the west abutment and 14-in. H piles at the east abutment. The pier 2 and 3 foundations are located in or directly adjacent to the Mississippi River and require cofferdams with tremie seal to provide a dry working environment for substructure construction. All pilings are topped with a reinforced concrete footing. The pier 4 foundation on the east river bank is a spread footing bearing on rock.

Above the footing, the substructure consists of an icebreaker, a large concrete block with flared ends, which extends above water level to deflect and break ice floes that impact the piers. Twin stem walls, which flare in the transverse direction to match the width of the box girder soffit and web incline angle, top the icebreaker. The box girder is rigidly attached to the stem walls with the reinforcing steel from the stem wall extending into the box girder diaphragm walls.

Building a superstructure

The bridge superstructure will be constructed using a combination cast-in-place balanced cantilever with form travelers for the interior spans and cast-in-place on falsework for the end spans. The form travelers are supplied by Strukturas, a Norwegian company that specializes in the design and fabrication of bridge construction equipment. Each form traveler weighs 160 tons including formwork. Two pairs of form travelers will be used to cast the segments in 16.4-ft lengths, with a maximum segment weight of 420 tons consisting of 200 cu yd of concrete and 32,000 lb of reinforcing steel. Balanced cantilever construction will be used for the spans adjacent to piers 2, 3 and 4, allowing work to proceed above the river with no disruption to navigation.

The box-girder cross section widens to accommodate entry and exit lanes at each end of the bridge, and cast-in-place construction on falsework can most easily accommodate the required variable-width cross-section. At span 1 the falsework is supported on spread footings after preloading of the soil to minimize settlement. At span 5, H piles driven to rock support the falsework. The span 1 falsework must accommodate an active railroad and roadway, and approach embankments at each end of the bridge require falsework of up to 50 ft in height.

Minimum design compressive strength is 4,000 psi for the substructure concrete and 6,000 psi for the superstructure concrete, and all reinforcing steel above the footings is epoxy-coated. Typical post-tensioning consists of 23- x 0.6-in.-diam. strand tendons in the longitudinal direction and 4- x 0.6-in.-diam. strand tendons in the transverse direction.

Substructure elements with a thickness of greater than 3-ft are designated as mass concrete, and temperature controls are specified to minimize potential cracking as a result of excessive internal temperatures during the curing phase. A combination of Type II cement, ground granulated blast furnace slag, fly ash with a water-to-cementitious materials ratio of 0.45 is used to control the heat of hydration. The specified maximum concrete temperature is 150°F, with an additional requirement of a maximum differential temperature of 77°F for abutment and pier stems. Thermal modeling is used to predict maximum temperature and maximum differential temperature for different ambient temperatures and initial concrete mix temperatures. If specified maximum values are exceeded, modeling can predict the beneficial effects of different levels of initial concrete temperature reduction, surface insulation and thermal cooling pipes placed within the concrete.

Initially, the contractor proposed 0.62-in., 270-ksi, seven-wire super strand in place of the specified 0.6-in., 270-ksi, seven-wire strand post-tensioning tendons. The super strand has 7% more steel area and allows the elimination of one strand per tendon for tendons with more than 16 strands. As post-tensioning strand availability became an issue, the contractor returned to 0.6-in. strand for the transverse tendons and continuity tendons, and 0.5-in. special strand for the cantilever tendons.

So far, so good

Construction began in the spring of 2003 and is scheduled for completion in fall 2007. The westbound structure will be constructed first followed by demolition of the existing bridge after traffic has been moved to the new bridge in the fall of 2005. As demolition progresses work can begin on the substructure for the eastbound structure. Work is scheduled through the winter months to allow completion of the twin structures within the allotted contract time.

Currently construction progress for the westbound structure includes completion of all foundations and substructure. Superstructure construction has been completed at pier 4 and span 5 and is in progress at all other locations.

The pier table constructed on top of the twin stem walls of the piers is the starting point for balanced cantilever construction. After the pier table is complete, the form travelers can be placed on the pier table and construction of the 26 segments that make up the cantilevers can begin.

Segment construction alternates between up-station and down-station cantilevers. This sequence limits the cantilever out of balance to a maximum of one-half segment. As each segment is constructed a series of three post-tensioning tendons are installed and stressed. These cantilever tendons located in the top flange of the box girder are the primary tensile elements of the structure and precompress the concrete so that the concrete is always in compression with no tensile stresses occurring.

Balanced cantilever construction began at pier 4, and is now in progress at pier 2 and pier 3. The second pair of form travelers permitted construction to begin at pier 2 as construction at pier 4 was in progress. In parallel with the balanced cantilever construction, cast-in-place construction on falsework began at span 5 adjacent to the east abutment followed by span 1 adjacent to the west abutment.

Construction has been completed at span 5 and the pier 4 cantilevers, allowing the 2-m closure segment to be cast to connect the two sections of bridge and complete span 5. This is followed by a similar operation at span 2 after completion of span 1 and the pier 2 cantilevers. Finally, after completion of the pier 3 cantilevers closure segments are cast at span 4 followed by span 3.

During these final two closure operations, a horizontal jacking force of 790 kips is applied across the closure to offset some of the long-term shortening of the girder that will occur due to creep and shrinkage and reduce the residual bending moments in the substructure. A series of post-tensioning tendons are installed and stressed across each closure segment. These continuity tendons are located in the bottom flange of the box girder and resist tensile stresses in the midspan.

To ensure that these independently constructed sections of bridge will align properly during the closure operations, a computer model of the bridge is developed and analyzed in a step-by-step sequence that mimics the planned construction sequence and schedule. Each construction activity from launching the form travelers to casting the segments to stressing the post-tensioning tendons is modeled in the analysis. In addition the long-term effects from creep and shrinkage of the concrete are calculated.

From this analysis, the structural camber required to build the bridge is obtained. This analysis also verifies that stresses caused by the imposed construction loads, including form travelers, remain within specified allowable stresses. The roadway geometry plus structural camber and structural deformations obtained from the analysis defines the deflected shape of the bridge at each step of construction.

Geometry control procedures are used to monitor the deflected shape of the bridge and make corrections as necessary to achieve the design bridge geometry. Before each segment is cast, a table of elevations at 16 key points around the cross section is generated for setting the form traveler to the cambered elevation determined by the computer model. This setup survey is done at two different stages of construction—once just after the form traveler is advanced to the new casting position and again the day that the segment will be cast. The day after casting, an as-cast survey is performed. Based on this survey, adjustments are made to the setup elevations for the subsequent segment to correct variations between the actual and theoretical elevations. All surveys are done before dawn to minimize the effects of temperature.

Heightened tensions

In 2000, the durability of post-tensioned concrete bridges was brought into question when a bridge in Florida was found to have experienced corrosion of several post-tensioning tendons after a relatively short time in service. Poor-quality grout surrounding the tendons and anchorage protection details had allowed water to penetrate into the tendon. Two tendons had failed in less than eight years of service. After a complete inspection, an additional 11 tendons had to be replaced because of excessive corrosion.

This failure led to investigations of many post-tensioned bridges, both in service and under construction. The results of these investigations indicated that the design and construction practices pertaining to temporary and permanent corrosion protection of post-tensioning tendons were not sufficient to ensure long-term durability of the prestressing steel. All parties involved in the post-tensioned concrete bridge industry have worked to improve these practices, and the Wakota Bridge is a beneficiary of these improvements.

Specific improvements that have been implemented are:

Prepackaged grout: Manufactured products specifically formulated for the grouting of post-tensioning tendons have replaced cement and water grouts.

Improved workmanship, quality control and quality assurance: In 2001 the American Segmental Bridge Institute began a training program to certify grouting technicians.

Improved details: All post-tensioning duct connections use heat-shrink sleeves for more durable, leak-proof connections. Duct tape, the previous common practice, is prohibited. All anchorages have permanent plastic grout caps and are covered with an epoxy grout pourback for increased protection.

In addition, anchorage pourbacks at expansion joints are covered with an elastomeric coating.

Temporary corrosion inhibitor: During cold-weather months, when grouting is not possible, a water-soluble oil is applied to the strands during installation.

These improved practices and overall specification improvement will ensure that the post-tensioning steel is protected and that the structure is durable throughout a long service life.

Fighting a cold

The contract time available for construction requires that superstructure construction continue throughout the year. Year-round construction in a region known for severe winter weather requires that cold-weather concreting practices be carefully implemented and monitored.

The specifications require that the contractor maintain the concrete surface temperature above 52°F during the initial curing period, when the ambient air temperature is below 37°F. The initial curing period is defined as the period of time until the weight-supporting forms may be removed. By specification the forms may be removed after the concrete has attained a compressive strength of 4,000 psi and post-tensioning is complete.

With mean temperatures less than 37°F from November to March, a typical winter season will require five months of cold-weather concreting procedures. During January, the mean temperature falls to 15°F, highlighting the need for a well-thought-out cold-weather plan. To meet the specification, the contractor will implement a cold-weather-protection plan that uses enclosure of the form travelers combined with propane-fueled heaters and thermal curing blankets to maintain temperatures and allow the concrete to cure and gain strength. A thermal monitoring program using thermocouples embedded at critical locations in the segment will provide temperature data to confirm that minimum temperatures are maintained. Field-cured cylinders will be used to determine form-stripping strength. In addition, the collected temperature data will be used to correlate concrete strength to temperature using the maturity method.

The new Wakota Bridge and associated construction projects at the I-494 and Highway 61 interchange will greatly improve traffic flow along these busy corridors in the southeast metropolitan area of Minneapolis-St. Paul. The new bridge will incorporate the most up-to-date concrete segmental construction technology and will provide a visually pleasing structure across the Mississippi River.

For more information about the construction of the Wakota Bridge, visit the Minnesota Department of Transportation’s Wakota Bridge Project home website:

About The Author: Towell is a principal bridge engineer at Parsons Corp., Minneapolis.

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