A main girder of the Lafayette Street Bridge over the Mississippi River in St. Paul was discovered to be cracked on May 7, 1975. A detailed examination of the fracture is given in this paper. Fractographic examinations and analyses of crack growth showed that fatigue crack growth and brittle fracture both contributed to the members' failure. Fatigue cracks were found to originate at large lack of fusion areas in the lateral bracing gusset to transverse stiffener weld. After the crack had nearly penetrated the web thickness, it precipitated a brittle fracture. Subsequent intervals of fatigue crack growth and fracture were also observed. Recommendations are also provided on ways to retrofit the bridge details to prevent reoccurrence at other comparable details. In addition, a suggested detail configuration for this type of connection is recommended for future use.
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Fisher et al. (1977) studied this question.