Structural Modeling for Reconstruction of Arch Bridge

When a 70-year-old multiarch concrete bridge was to be reconstructed by removing the deck and vertical spandrel columns down to the arch, analysis was necessary for unloading and reloading. The arch bridge, located on TH-55 over the Minnesota River, was of world-record length for such a structure when it was originally constructed in 1926 through 1928. The amount of steel reinforcement in each of the two support arches was only 32 No. 9 bars of 28.6-mm (1.125-in.) diameter. The deck, which was 18.3 m (60 ft) wide and supported at 9.1 m (30 ft) by columns, was designed as a patented panel system. This involved the use of only 250 kg (550 lb) of reinforcement in each deck panel. The slab did not possess upper reinforcement in the center areas, so removal operations could not allow the slab to cantilever in these areas. As a result of the minimal steel reinforcement in the deck and arches, removal operations needed to be planned safely and effectively. At no time during the removal process could the arches be placed in flexural stress above their very limited carrying capacity. Prospective contractors were advised of this condition and were given 2 weeks beyond the original deadline to submit bids accordingly. The proposal and plans given to each prospective contractor for bidding showed a possible deck removal method using a crane. Structural modeling by the contractor's consultant and the Minnesota Department of Transportation using Staad III finite element analysis was necessary for the contractor's bridge deck removal and replacement method. The existing slab removal and replacement with a wider, 27.4-m (90-ft) deck was successful and the bridge was reopened to traffic in October 1994.