Engineering Project Case Studies | Erdman Anthony

Bridge Design

Monroe County’s Rochester Pure Waters District (RPWD) constructed the Combined Sewer Overflow Abatement Program (CSOAP) tunnel system in the 1980s and 1990s to mitigate the occurrence of combined sewer overflows (CSOs) from the existing City of Rochester sewer collection system to the Genesee River and Irondequoit Bay.

As part of the comprehensive system of large-diameter lined and unlined deep-rock tunnels, RPWD also constructed a pedestrian arch bridge and access pathways across the Genesee River, between Maplewood and Seneca parks in the city, in 1985.

The bridge carries two 6-foot-diameter sewers underneath its deck that convey CSOAP flow from the west side of the city to the east side, to the St. Paul Boulevard Tunnel and on to the Frank E. VanLare Wastewater Treatment Facility. Crossing the gorge above rather than below the river surface eliminated the need for downstream pumping and also allowed the construction of the pedestrian walkway along the suspended conduit, providing access between parks located on either side of the gorge. A siphon was designed and constructed that took the flows from the east side of the river to the VanLare Wastewater Treatment Facility – some 3.5 miles – without any energy costs, by gravity.

Erdman Anthony was engineer of record for the original design of the St. Paul Boulevard Tunnel System, a subset of RPWD’s CSOAP. Our firm also provided construction support and construction observation for the project. Erdman Anthony designed the new 3-mile tunnel, the bridge, and two junction and control structures.

The form of the bridge was dictated in part by the high, steeply sloping banks of the gorge. Since each bank is in densely wooded parkland, the team had to develop a bridge that harmonized with its surroundings and carried the pipes at the required elevation or gravity flow.

As a bonus, the bridge carries a pedestrian path on top of the pipes to link the parkland on the two banks, giving the city access to a stretch of formerly inaccessible woodland.

The bridge’s twin arch ribs span 475 feet and rise 80 feet from their spring line. Steel bents, spaced 25 feet apart along the ribs, carry the pipes. The pipes carry wastewater and serve a structural function, acting as continuous beams with 25-foot spans that carry a precast-concrete deck.

An Environmental Protection Agency Guidance for Long-Term Control Plan document advocates the use of creative thinking in the development of CSO-reduction alternatives. It notes that the initial identification of alternatives should involve some degree of brainstorming and free thinking. It further notes that the CSO-control policy encourages and permits authorities to consider innovative and alternative approaches and technologies that achieve the objectives of the policy and the Clean Water Act. The document specifically calls out this Erdman Anthony project as an example of creative thinking in the resolution of CSO issues.

In 2015, NYSDEC performed a 20-year look-ahead for CSO abatement for the entire state. The agency noted that Buffalo, Syracuse, Albany, and New York City were in dire need of major funding. It also noted that the City of Rochester had already addressed the problem at a fraction of the cost.

Recently, Erdman Anthony’s structural staff reanalyzed the bridge, owing to the addition of a 24-inch pipe along the south fascia. The analyses were performed using the finite-element method. Computer run time in 1981 for the design with SAP IV was 18 hours. Today, with SAP2000 v21.02, run time for the same structure was 30 seconds.

Based on these analyses, it was discovered that this well-designed bridge could handle the additional weight from the new pipe – without issue.

Daniel Ziemianski, PE
Daniel Ziemianski, PE
 716-631-1241 x 1103