I-35W Bridge Collapse - Week 23

Week 23 by Ivy Chang | September 28, 2010

On January 15, the National Transportation Safety Board announced that the collapse of the Interstate 35W bridge in Minneapolis may have originated with the gusset plates that were one-half inch too thin in the 1960s design. Gusset plates are flat pieces of steel used to connect adjacent truss members, such as box beams or H beams. They are either riveted or bolted into place.

Investigators found 16 gusset plates in the main span to be only one-half inch thick instead of one-inch thick, which would have been safe. In addition, MnDOT installed a center median barrier, built larger outside walls and increased the thickness of the bridge’s concrete deck by two inches in 1977 and 1998 without reinforcing support.

At the unused roadway of 1-35W, crews built casting beds for the main span of
the bridge and will pour concrete after all piers are completed.

States should review bridge designs

Chair Mark Rosenker said in the news conference that Minnesota’s upkeep of the bridge did not play a role in its collapse. He did say this finding is not complete and a more comprehensive report is still months away. The NTSB also warned bridge engineers across the country to review the designs and recalculate maximum loads on about 12,600 steel deck truss bridges that are similar to the I-35W bridge.

Gov. Tim Pawlenty ordered the Department of Transportation to recalculate the load capacities for 23 state bridges and review the load capacities for 36 local bridges all having similar design as the I-35W bridge.

Attacks follow party lines

Legislators, getting ready for their session to begin in February, continued their attacks on the opposing party with Pawlenty and Republican leaders saying that the investigation on the collapse shows that MnDOT did everything humanly possible to maintain state bridges and demanded that Democrats apologize to Transportation Commissioner Carol Molnau.

But Democrats warned that the commissioner, who also serves as lieutenant governor, will not be confirmed and that a decision has been made with the Democratic majority leader to hold confirmation hearings on Molnau’s ability to oversee the agency.

Last week, the Democratic Senate caucus had concerns about a showdown with Republicans during the 2008 Legislative session because it could impede passing a variety of bills, especially transportation bills. In recent sessions, Senate Democrats have twice passed comprehensive transportation bills that would have paid for part of road and bridge repair through a gas tax. Gov. Pawlenty vetoed the transportation bills.

Flatiron-Manson crews tie rebar to reinforce the stemwall at Abutment 1 on the south side of the river. Concrete has been poured to form the wall.

Major concrete work began

MnDOT and Flatiron-Manson Joint Venture completed 100 days of reconstruction work on the I-35W bridge. As part of the pre-cast work, crews have erected large heating sheds on the unused portion of I-35W that will provide a heated environment for the segments as the concrete cures. The sheds will move along the casting beds as concrete is poured into forms. Concrete for the first segment will be poured before the end of January.

The major concrete pour, for the footings at Pier 3, began during the January 12-13 weekend. Two concrete pumps allowed crews to pour approximately 200 cubic yards of concrete per hour for a total of 1,329 cubic yards. Crews also began work on forms for the eight piers that support the main bridge span over the river. Each pier, which will stand 70 feet tall when complete, will be poured in three stages.

On the north side of the Mississippi River, crews drilled shafts for the foundations of Abutment 5 and the Second Street bridge and drilled as many as five shafts each day to complete 69 total shafts. Three large air compressors powered the drills, creating noticeable noise at the higher elevation of this abutment.

Webcams record construction activities

During the work, area residents noticed some odors similar to asphalt because a coal gasification plant once occupied the area and emitted low-level tar. MnDOT doesn’t anticipate any air quality issues and monitored the situation closely. Crews are working day and night on both sides of the river, seven days a week, until all piers are complete.        

Open Houses will be held on January 22, 29 and 30 to inform the public on the progress of the construction, with project leaders providing information and answering questions.

A webcam that was installed on the roof of a University of Minnesota steam plant went live on January 4 and shows a panoramic view of the construction activities 24 hours a day. Log on to www.mndot.gov to see the view. Eventually, a time-lapse photography feature will be created to see the construction of the entire bridge from start to finish in rapid sequence.

A second camera will be installed on a light pole on the Tenth Avenue bridge and zooms in periodically on specific construction activities.