I-35W bridge fact sheet
While the cause of the I-35W bridge collapse is unknown at this time, facts and background information on the bridge are coming to light.
--Built in 1964 by Hurcon Inc. and Industrial Construction Company.
--Steel trusses and deck were constructed by Industrial Construction Company in the summer of 1965.
--Bridge opened to traffic in 1967.
--Scheduled for reconstruction in 2020-25.
--Bridge carries 144,000 vehicles per day; including 4,760 commercial vehicles.
--Similar bridges in Minnesota include the Hwy. 123 bridge in Sandstone and the Hwy. 23 bridge over the Mississippi River in St. Cloud.
--Deck steel truss is made up of three parts: deck, superstructure and substructure (the structure under water).
--Bridge has a split deck (longitudinally parallel to traffic) and is 113 feet, 4 inches wide.
--Size/length: 1,907 feet long, eight lanes.
--Had been inspected annually since 1993; before that, was inspected every two years
--Last fully inspected in 2006. Partial inspections were conducted in 2007; to be complete in fall 2007
--The 2006 Fracture Critical Bridge Inspection Report, prepared by a MnDOT bridge inspection team, describes specific problems that caused the superstructure (part of bridge above water) to receive a poor rating.
The poor rating can be attributed to corrosion at some areas where the paint system has deteriorated, poor weld details in the steel truss members and floor beams, bearings that are not moving as they were designed to move, and existing fatigue crack repairs to the truss cross beam and approach spans.
--Deficiencies were acknowledged in the 2005, 2006 and 2007 inspection reports.
--MnDOT had taken several steps to address these deficiencies. Some cracking in the approach spans was repaired or was being monitored.
The Bridge Office had contracted with the University of Minnesota in 1990 to evaluate the fatigue stresses within the truss. Field tests were conducted. Measured and calculated stress ranges were less than the fatigue threshold, therefore, it was concluded that fatigue cracking was not expected in the deck truss. The following actions were recommended:
--Structural components of the main truss with the highest stress ranges should be inspected thoroughly, every two years.
--Critical locations of the floor trusses had high stress ranges, and should be inspected every six months.
--Although the report concluded that fatigue cracking was not expected to be a problem for the weld details used on the truss, MnDOT contracted with URS (a private firm) in 2003 to do a more in-depth fatigue and fracture analysis, and to determine whether the fracture of any single truss member would result in collapse of the bridge or whether the traffic load would be safely carried by other members of the bridge.
URS made three recommendations in January 2007:
1) Add redundant plating over the most critical 52 truss members,
2) Conduct a visual examination of all suspected weld details and remove measurable defects at suspected weld details of all 52 fracture critical truss members, or,
3) Do a combination of both 1) and 2).
MnDOT had begun inspection of the weld details and no weld cracks were detected. Therefore, MnDOT did not proceed with option 1 at that time. MnDOT intended to complete the inspection of the weld details on all of the remaining members after the completion of the current construction project.
Structurally deficient bridges
--A bridge is rated as "structurally deficient" when part of the bridge is found to be in poor condition. Many bridges in poor condition are still safe for use.
As deterioration continues, engineering analysis is sometimes necessary to recompute the safe load capacity of the bridge. If the safe load capacity is less than today's legal truck load (80,000 pounds), the bridge is posted at the newly computed safe load capacity.
--The I-35W bridge was rated safe for legal truck loads and permitted overweight truck loads of up to 136,000 lbs. The bridge was not under any restrictions.
--The condition of different parts of a bridge is rated on a scale of 1 to 9 (7, 8, or 9 are good condition ratings, 6 is satisfactory, 5 is fair, 4 is poor, 3 is serious, 2 is critical and 1 is closed).
A structurally deficient bridge is one for which the deck, the superstructure or the substructures are rated in condition 4 or less. For this bridge, the superstructure was rated 4.
--In Minnesota, there are 1,097 bridges that are considered structurally deficient and that have a sufficiency rating less than or equal to 80. Of these bridges, 106 are on the state trunk highway system and 991 are on the local system.
Federal report on bridges (NBIS database)
--The National Bridge Inspection Standards require states to annually report condition ratings for all bridges in their states to the Federal Highway Administration (FHWA). Each MnDOT district has inspectors who are trained to inspect and rate bridge condition. That information is forwarded to MnDOT's Bridge Office where it is compiled and forwarded to the FHWA. The FHWA uses that data to determine which bridges are structurally deficient and functionally obsolete.
Recent work on the bridge
--Work involved concrete and joint repair, lighting and guardrail installation
--Work was scheduled to be complete Sept. 30.
--Cost for the work is $9 million.
(Source: Minnesota Department of Transportation)