Fatigue Cracking from Unexpected Conditions

This is a case study of the I-10 bridges over the Pascagoula River in Jackson County, Mississippi. The two-girder system has a unique framing plan where upper and lower lateral braces are used to provide a load path for wind on 16-foot deep box girders. The upper lateral bracing pattern is not uniform across the structure and some bays see force couples develop as live load moves across the bridge. As live load moves across the bridge, the upper lateral braces cycle between tension and compression, causing the bracing and connection plates to produce out-of-plane bending moments on the floorbeam.

Starting in 2013, cracks have been observed in the floorbeam webs at the tip of the upper lateral bracing tab plate and in the weld between the floorbeam web and the upper lateral bracing tab plate. The layout of the unique upper lateral bracing installed for wind load has created live load fatigue cracks in the main structural members. Placement of these braces was intended to carry wind load, but the unique framing system combined with the mis-fabricated detail has created fatigue cracks for loads that were not likely considered in design.

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