NTM Engineering provided bridge design services and hydrologic and hydraulic (H&H) analysis for the Salford Station Road (SR 1024) project in Upper Salford Township, Montgomery County, Pennsylvania. The project involved two proposed bridge replacements. The first bridge (SR 1024 Section NRV) is a 40-foot-long, single-span, rolled-steel, I-beam bridge over the Mill Race. This bridge, built in 1933, was structurally deficient and posted for 16 tons due to its condition. The second bridge, over Perkiomen Creek, was a 266-foot-long, three-span, throughgirder floorbeam stringer bridge, which was closed in 2008 due to substructure deterioration.
NTM was responsible for preliminary and final design of the Mill Race Bridge and associated 200-foot retaining walls. The firm conducted a complicated H&H analysis, including a multiple opening HEC-RAS model, for the Mill Race Bridge and the adjacent three-span structure over Perkiomen Creek.
NTM investigated six bridge replacement alternatives and three alternatives for the retaining wall. NTM’s final recommendation for the Mill Race Bridge was a single-span, pre-stressed concrete spread box beam superstructure with the same opening as the existing structure. The Mill Race Bridge was situated on a tight 134-foot radius curve with a 43 degree skew. To satisfy design specifications and to reduce the deck overhangs, the beams were flared from 35 degrees to 47 degrees. NTM proposed rehabilitating the existing retaining walls, which consisted of replacing the concrete cap and adding a new concrete facing. Rehabilitation was the most cost-effective alternative and did not require specialized equipment or extensive excavation.
NTM modeled both bridges in HEC-RAS using the multiple opening analysis approach. Both structures share the same floodplain and encroach on the floodway of a detailed FEMA study area. The 100-year FEMA flow was evaluated to ensure that no water surface increase would occur as a result of the proposed replacement. The temporary conditions hydraulic model included staged, partial-width causeways with pipes and cofferdams on the Perkiomen Creek. A HEC-18 scour analysis was performed for both proposed structures.
NTM received an “Exceeds Expectations” score from PennDOT for work on this project.