Located 75 MILES northwest of Minnesota’s capital, the Desoto Bridge in St. Cloud, Minn. for more than 50 years carried Highway 23 over the Mississippi River. The in-place truss bridge was built in 1957 and was named after Hernando Desoto, the first European to take credit for seeing the Mississippi River in 1541.

Following the I-35W bridge collapse in August 2007, one of the FHWA directives was to inspect all under-deck truss bridges. The Desoto Bridge was closed to traffic in March 2008 when minor distortion was found on a few of its gusset plates. Further analysis showed that the gusset plates had adequate strength but free edge stiffening would be required to remedy the minor distortions. The bridge was scheduled for replacement in 2010, but with these findings the district and the city decided to leave the bridge closed while accelerating design and construction so that a new bridge could be opened to traffic by November 2009. That required completing the preliminary and final design in a very short time—what typically would require 12 to 18 months had to be accomplished in less than two months.

Bridge Overview

Several options for spanning the river were considered, including prestressed concrete beams, post-tensioned concrete box girders, and continuous steel girders. In order to limit the number of piers in the river, which was a key issue in obtaining permitting, a continuous steel girder bridge was selected. An additional consideration for the selection was the Minnesota Department of Transportation (MnDOT) Bridge Office’s experience with steel plate girder bridges of this length. Using this in-house expertise was helpful in limiting the time required to create construction documents.

The new bridge  spans the  Mississippi River  with  three  continuous steel spans of approximately 222 ft, 345 ft, and 222 ft. For the river spans, the haunched plate girder web depth varies from 104 in. at mid-span to 168 in. over the supports. An additional 92-ft span crosses Riverside Drive on the north side of the river and uses 36-in. rolled beams.

Construction Contracts

The bridge was let in two contracts.  To ensure  timely fabrication and delivery of the girders, an early steel contract  was let on June  6, 2008, which  provided  additional  time  for  steel  procurement  and fabrication.  Upon award of the contract, the successful fabricator was to purchase the material, fabricate, and deliver the steel girders for the bridge to the project site by the date specified in the construction documents.

As completion  of the project  was tied to the on-time  delivery of steel, a monetary deduction  of $20,000 per calendar day was included  in the  contract  for any material  that  did not  meet  the delivery  schedule.  The steel contract  plans  were sent  to  NSBA for distribution to interested  fabricators.  The steel contract  was awarded to Chicago-based  Industrial Steel Construction, Inc.

The final construction contract was let on July 25, 2008. To ensure  opening  of the bridge  for traffic by November  1, 2009 a “Locked  In  Date”  incentive  of $1,000,000  was added. The final contract   was awarded  to  Lunda  Construction Company,  Black River Falls, Wis. The total  construction cost of the  project  was $19.5 million. The steel portion of the contract  was $7,278,000.

Design Considerations

Final bridge plans and the specifications were completed in 55 days. This required close coordination with the city’s visual quality task force, the geotechnical  engineers for the river foundation coordination, and the local contractors for input on bridge demolition  and  conflicts between  existing and  new river pier  foundations. The existing deck removal was done by saw cutting the deck and removing it in small sections. The pieces were dropped onto a barge to minimize damage  to the  riverbed. The in-place  truss had a “drop-in”  span about 145 ft long at the center of the bridge that was lowered intact onto the barge. Even though that was not the most efficient method for removing  the truss, it had the least impact on the river channel. The 220-ft-long approach spans were supported on temporary falsework for stability and removed in sections by barge.

More  than  20 engineers  and drafters  from MnDOT’s Bridge Office were  involved  in completing  the  bridge  design  and  construction  documents.  Daily meetings were held to coordinate  the work done  by team  members  in different  functional  groups. To meet the schedule, final design was started at the same time as the preliminary  plan group was finalizing the roadway alignment  and profile. Visual quality and environmental assessment are normally critical path items as they require input from outside agencies and citizens. The  Bridge  Office  already  was working  with  a citizen advisory committee  on the visual quality aspects of another  bridge of significance in the area. Due to the proximity of Desoto Bridge to that  bridge,  the  committee  decided  to use the  same aesthetic details. A categorical  Exclusion  Document was put  together   in place of an Environmental Assessment document  due to minimal environmental impact in the river.

Substructure Design

A geotechnical   consultant   contract   for  subsurface  investigation and completion  of the foundation report was let in April 2008. Design and drafting of the substructures was started ahead of foundation recommendation. The foundations were designed  based  on  existing  soil borings and pile records and had to be revised as new borings  were completed.  To minimize revisions and rework, the normal process of drafting  substructures foundations on up, a top down process was utilized.

Foundation    recommendations    were completed in June. The borings indicated dense granular soils in the  top  layers followed by dense loamy soil at all substructure locations except Pier 2. At Pier 2 granite rock was encountered at depths varying from 15 ft to 30 ft from the bottom of the footing. The west abutment, Pier 3 and east abutment are supported on spread footings, Pier 1 is on H-piling, and Pier 2 is on rotary drilled cast-in-place concrete piles.

Due to the proximity of the rock layer at Pier 2, the cofferdam design was challenging. Lowering the water level in the river by using the dam downstream was considered to meet  the  factors  of safety required  by MnDOT. But ice build-up in the river made this an unreliable option. The seal thickness varied from 10 ft to 16 ft and the seal design was revised using average thickness.

Superstructure Design

Special deck design considerations  were required  due to a deck span of 15 ft, 6 in. and the addition of overlook areas. The pier overlooks were supported using steel plate brackets fanning from the girder stiffeners.The delivery schedule for steel plates and the availability of high-strength steel were discussed with NSBA, which suggested notusing high-performance steel due to uncertainty in steel delivery schedule.

The  plate  girder   used  two  different web  thicknesses—¾  in.  for  outer  spans and over the river piers and 13∕16 in. for the middle span between the river piers. Longitudinal stiffeners were required  over the river  piers. The  thickness  of the  flanges varied from 1¼ in. to 3¾ in. Lateral  bracing was required  to resist wind loads during construction.

The bridge  also carries  a 24-in. water main   hung   underneath  the   deck   that required special diaphragm details. Haunch girders and the presence of the water main resulted   in seven different  types  of  diaphragms. Weathering steel was used for the girders, which is MnDOT’s standard practice. The end 5 ft of the beams are painted with a zinc-rich epoxy paint system to pro- vide protection against corrosion. Also, the exposed face of fascia girders was painted to meet visual quality requirements.

Conclusions

A project this size typically requires 12 to 18 months for completion and the final design is normally handled by a group consisting of a principal  engineer,  two senior engineers   and  four  drafters.  Thanks  to very good  coordination between  various agencies  and  district  offices, using  more than  20 engineers  and drafters  made  the delivery  of  this  project  possible  in  less than   two  months.   Also, by  letting   the project  in  two  contracts,  the  early  steel contract provided additional time for steel procurement and fabrication.

Steel proved to be cost competitive  for this project, and the cost of replacing the Desoto  Bridge was well below MnDOT estimates. The bridge opened to traffic on October 29, 2009, renamed the Granite City Crossing  Bridge to pay tribute  to the many granite quarries found in the vicinity.

Owner: Minnesota Department of Transportation

Structural Engineer: Minnesota Department of Transportation– Bridge office

Steel Fabricator: Industrial Steel Construction, Inc., Chicago (AISC Member)

General Contractor: Lunda Construction Company, Black river Falls, Wis. (Impact Member)