This project in Rochester, NY, entailed replacing a bridge over the Genesee River. Originally, the Troup Howell Bridge carried Interstate 490 over the river (which bisects the city and flows northward to Lake Ontario) and several city streets and ramps. The highway is a major commuter route that connects the eastern and western suburbs to the center of the city.
In 1999, NYSDOT commissioned Erdman Anthony to perform a bridge replacement study. Six different bridge types were evaluated: short-span steel multigirder, long-span steel multigirder, prestressed concrete box girder, steel box girder, steel through arch, and cable-stayed.
After considering the bridge options at a public hearing, the community overwhelmingly chose the steel arch. There was consensus on a fundamental point: The site deserved a "gateway" or "signature" span that framed the river and the city skyline.
Given that several multispan masonry arch bridges are located downstream, the community decided a more modern arch would be the appropriate bridge type. Not only was there aesthetic benefit, the through arch structure allowed the I-490 profile to be lowered about 3.2 feet, which partially mitigated a nonstandard I-490 stopping sight distance.
The new eight-span structure is 364 meters (1,194 feet) long. The centerpiece is a 132 meter (433 foot) long through arch span crossing the Genesee River.
Most arches built in the last several decades have been of the tied arch style. One benefit of a tied arch is that it does not require a large foundation. In many cases, however, the tie becomes deeper than the arch rib to resist arch thrust.
When given the choice between a tied arch and a true arch design, the community chose the true arch. Feedback indicated that a thinner deck was more desirable. A true arch requires transmission of thrust to a solid competent foundation, preferably bedrock. Since extremely competent bedrock (i.e., allowable bearing pressure is 1.2 MPa or 25.0 tsf) was located within 4.0 meters (13.1 feet) of the ground surface, the choice of a true, two-hinged arch became structurally and economically feasible.
Erdman Anthony prepared preliminary and final plans, special specifications, and quantity estimates for the arch span. Our firm also performed three-dimensional structural analysis, including a multimode response-spectrum seismic analysis and wind-loading evaluation, and performed detailed design of the arches, braces, hangers, floor beams, stringers, and deck.
In the bridge section, it was necessary to increase median shoulder widths to meet standards, so it also became necessary to further separate the I-490 eastbound and westbound alignments. The amount of separation, however, had to be kept to a minimum because of the presence of structures located on the edges of both approaches. The historic Corn Hill neighborhood and the city’s public safety building are present on the west side, and bridge piers for an overhead ramp bridge are on the east side.
These constrictions precluded the use of four arch ribs, two supporting each travelway, which is the common arrangement for bridges of this type.
In the end, it was decided to support both eastbound and westbound travelways by only three ribs.