Did MnDOT Make a Fatal Choice?

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PHOTO: Harry Goldstein, IEEE Spectrum

A report in todayâ''s Minneapolis Star-Tribune states that the Minnesota Department of Transportation â''last winter considered bolting steel platesâ'' to the trusses on the 35W bridge â''to prevent cracking in fatigued metal,â'' and even went so far as to ask contractors for advice on the best way to accomplish the task.

The move was prompted by the URS Corporation June 2006 report, which recommended â''three equally viableâ'' retrofit approaches:

(1) Steel plating of all 52 fracture critical truss members. This approach will provide member redundancy to each of the identified fracture critical members via additional plates bolted to the existing webs. The critical issue of this approach is to ensure that no new defects are introduced to the existing web plates through drilled holes. This approach is generally most conservative but its relatively high cost may not be justified by the actual levels of stresses the structure experiences [although itâ''s worth noting that the simulation data used to calculate these stress levels might, by URSâ''s own admission, have been inadequate; see Reports on Bridge Condition Raise Questions].

(2) Non-destructive examination (NDE) and removal of all measurable defects at suspected weld details of all 52 fracture critical truss members. The critical issue of this approach is to ensure that no measurable defects are missed by the NDE efforts. The fracture mechanics analysis has indicated that the dimensions of preexisting surface cracks need to be at least one quarter of the web plate thickness in order to grow and subsequently cause member fracture under traffic load. This approach is most cost efficient.

(3) A combination of the above two approaches: steel plating of the 24 more fatigue sensitive membersâ'¿and NDE of the 28 more fracture sensitive members.

As MnDOTâ''s Dan Dorgan said yesterday at a press conference, â''We chose the inspection route.â'' And although Dorgan denies there was dissension among MnDOTâ''s engineers, the Star-Tribune quotes an anonymous source identified only as an â''industry officialâ'' as saying, â''There were people over there [in MnDOT] that were deathly afraid that this kind of tragedy was going to be visited on usâ'¿.There were people in the department that were screaming to have these replaced.â''

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