Page 138 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 3 BRIDGE FAILURE STUDIES AND SAFETY ENGINEERING 113

2. Cracks in the I-95 bridge: A significant crack was discovered on an I-95 bridge over the
Brandywine River in Delaware. The steel girder bridge carriessix lanes of traffic just north of

downtown Wilmington. Thecrack was located on the fascia girder at midspan ofthe bridge’s

main span. The entire bottom flange was found to be fractured, with the crack extending
upwards to within0.3 meters of the upper flange. The circumstances leading up to the crack,

the cause of the crack, review of the repair strategy, and the results of load tests performed
prior to and during the repair are described in a paper by Michael Chajes, Dennis Mertz,
Spencer Quiel, Harry Roecker, and John Milius of the University of Delaware and DMJM
Harris.
3. Failure of I-35W Highway bridge over the Mississippi River in Minneapolis, Minnesota:
The following is the summary of recommendation of the National Transportation Safety
Board dated January 15, 2008: On August 1, 2007 there was a failure in the superstructure
of the 1000-foot-long deck truss portion of the 1900-foot-long bridge (Figure 3.14).
Approximately 110 vehicles were on the portion of the bridge that collapsed, and 17 vehicles
fell into the water. As a result of the bridge collapse, 13 people died, and 145 people were

injured. The bridge was originally opened to traffic in 1967. The steel deck truss portion of
the bridge consisted of two parallel main trusses connected through transverse fl oor trusses
supporting the reinforced concrete deck. It was considered to be fracture-critical because
the load paths in the structure were non-redundant, meaning that a failure of any one of a
number of structural elements in the bridge would cause a complete collapse of the entire
bridge. This type is also referred to as a non-load-path-redundant bridge.
4. The National Transportation Safety Board had a concern regarding certain elements of the
bridge (gusset plates), which has prompted issuance of the following safety recommenda-
tion:
The ends of the beams in the main trusses were connected by riveted gusset plates at 112
nodes (joints) along the deck truss portion.
Additional load: As part of renovations, the average thickness of the concrete deck was
increased from 6.5 inches to 8.5 inches, and the center median barrier and outside barrier

walls were increased in size. These changes added significantly to the overall weight of the
structure.

Figure 3.14 An aerial view of the failure of the I-35W bridge over the Mississippi River in
Minneapolis, Minnesota.