Before tackling construction of the 180-foot-span Presumpscot River Bridge in Falmouth, Maine, general contractor Cianbro Corp. realized it couldn't complete the job on schedule using conventional falsework to support forms for the structure's cast-in-place spandrel arch ribs. The state Environmental Protection Agency had set a stringent four-month time limit on construction activity in the river. To meet the tight schedule and minimize disruption to the river, Cianbro and construction engineer Finley McNary Engineers Inc. devised a plan for temporarily suspending the formwork for the bridge segments from cable stays.
Crews erected the parallel arch ribs from abutments on both sides of the river, beginning on the north bank to construct one half of the arch and moving to the south bank to build the other half. Each half-arch consisted of a pair of 13-foot-long base segments and three 25-foot-long segments. To form the segments, Cianbro used 5-foot-deep steel forms that were modified to accommodate installation of the cable stays and the curved bottom soffit of the ribs.