Middle Bar Bridge, Load Rating Analysis
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Originally built in 1912, the historic Middle Bar Bridge is a simply supported, steel-truss bridge that spans the Mokelumne River and links Calaveras and Amador counties. At 204’ long and 22’ wide, the Pratt Truss bridge links Gwin Mine Road in Calaveras County and Middle Bar Road in Amador County. The bridge’s steel was originally fabricated in 1901. First made of timber, the bridge’s deck was replaced in 1969 with corrugated metal planking filled with bituminous pavement.
Alta Vista Solutions (AVS) recently investigated the structural steel of this Northern California bridge. As part of a team led by Quincy Engineering, AVS extracted and tested the structural steel for use in determining how much weight the bridge can hold. Quincy analyzed and compiled AVS’s findings into a report for Calaveras County on the bridge’s load capacity.
The collapse of the Minneapolis Bridge in 2007 exposed the problem of aging infrastructure nationwide, and since then, public agencies have launched new efforts to ensure the structural soundness of their bridges. In the case of the Middle Bar Bridge, Caltrans determined that prior non-destructive testing was no longer sufficient and that new testing was crucial to determine MBB’s safety. AVS tested the bridge’s material properties to help Caltrans determine a more definitive load rating.
Challenges and Solutions
- AVS coordinated the bridge closure with both counties to avoid traffic disruptions during sample extraction.
- AVS made necessary field adjustments to complete work on time and with the utmost quality.
- To maintain material integrity, AVS’s plate removal was performed by drill and saw, as opposed to torch, which might be damaging. AVS maintained this approach even on horizontal surfaces, where it was quite difficult to operate drilling equipment.
- AVS assured complete compliance of specifications and performed verification of bolt installation, even in areas that are extremely difficult to access.
