But the report concluded that 'fatigue cracking of the deck truss is not likely' and said that the bridge 'should not have any problems with fatigue cracking in the foreseeable future'.
As a result, said the report, the Minnesota DOT did not need to prematurely replace the bridge because of fatigue cracking, 'avoiding the high costs associated with such a large project'.
However the report also highlights the lack of redundancy in the main truss system. The structure is a deck truss with steel multi-girder approaches and was built in 1967 to carry the I-35 over the Mississippi River just east of downtown Minneapolis. Only two planes of the main trusses supported the eight lanes of traffic. The truss was determinate and the joints were theoretically pinned. 'Therefore if one member were severed by a fatigue crack, that plane of the main truss would, theoretically, collapse,' the report says.
The report recommends that the members of the main truss should be inspected thoroughly, especially at the ends of the clips on the diaphragms in the tension members and at any intermittent fillet welds. This inspection should be carried out every two years.
But the lower chords and diagonals of all the floor trusses also have high stress ranges, the report says, and as they were accessible from the catwalk, they should be inspected every six months. Any manifestation of fatigue on the main structure was expected to be seen first in these members.
