Proceed with caution. This phrase summarizes the story of the Florence Bridge and the engineering teamwork needed to preserve the aging structure. On June 28th 2012, the Florence Bridge was shut down following a routine inspection. Inspectors observed buckling at one of the lift span’s trunnion support columns, along with lateral displacement of the sheave (a large pulley-like mechanism). Proceeding with caution, the Illinois Department of Transportation (IDOT) halted vehicular and barge traffic at the vertical lift bridge. IDOT then turned to trusted bridge design experts Modjeski and Masters (M&M) to assist with the emergency response.
On July 3rd, M&M dispatched movable bridge experts to the site, confirming that the bridge should remain closed to traffic to avoid further damage. Upon detailed evaluation, M&M determined that the sheave had “walked” itself along the trunnion shaft, laterally pushing the support column and bearing. To accommodate barge traffic and to protect the lift span during design and rehabilitation, the span was parked in the fully raised position. M&M then designed temporary full-height columns so that the counterweights could be jacked and unloaded while the span remained in the raised position – nearly 65 feet above the Illinois River.
Through close collaboration early on in the design phase, IDOT recommended evaluating the suitability of four spare sheaves in storage. M&M design engineers recommended a practical, holistic approach. The design specified that the spare sheaves be repurposed, existing shafts be recut, and that new counterweight ropes and bearings be installed. M&M also designed repair details for the buckled column, and the counterweight jacking plan to remove the existing wire ropes.
M&M’s movable bridge unit provided construction inspection during the counterweight jacking, column repairs, installation of the new lifting components, and strain gage balancing. Strain gage balancing, used to help align new machinery by measuring friction in the new bearings, was key in helping IDOT verify successful installation of the new parts.
The Florence Bridge reopened on April 22, 2013. Originally built in 1929, the bridge has been given a new lifecycle extension. Of four Strauss Vertical lift bridges built by the State of Illinois in the 1920s, only the Florence and the Joe Page Bridges remain. And M&M, proud to partner and assist a proactive client in IDOT, helped to preserve not only the Florence Bridge, but the Joe Page Bridge