Presumpscot River Bridge

Construction was completed in November of 1995 for this $2.6M Presumpscot River Bridge near Falmouth, Maine. Cianbro Corporation, Pittsfield, Maine elected to build this conventional, 180’ span spandrel arch bridge with an unconventional method. Instead of using shoring in the river, (which was limited by the EPA to the period between June 1 and September 30) Cianbro elected to use cable stays to suspend the arch during erection, resulting in a substantial savings in schedule and cost.

The stay system consisted of 1” diameter high strength steel post-tensioning bars. The arch was divided into 25 ft. long sections with a stub of 13 ft. at the thrust block. At each 25 ft. segment, 2 bars were anchored to a “through” tube cast into the arch. The other end of the bars were anchored in the cap at the main bents. The backstays were anchored on the opposing face of the bent cap and anchored in the abutment footing. The thrust of approximately 500 kips was resisted by the weight of the abutment which was keyed into the rock ledge. Reinforcing dowels were drilled into the rock to provide a positive connection to the ledge and to provide an additional safety against slip and overturning.

Erection began by casting a 13 ft. stub section of the arch. The forms for this were supported on a temporary prop. The forms for the next 25 ft. segment were pre-assembled on the ground and flown into position using a crane parked on the old bridge (which was left in place for access). Once the formwork was stripped, the final 25 ft. closure at the top of the arch was cast re-using the forms, but supporting them on the free ends of the arch halves. The casting cycle for each pair of segments was approximately two weeks and the entire arch erection was completed in less than three months.