Segmental Precast Rises In Steel Town

From the Transportation Research Board’s January 2008 meeting in Washington, D.C. A Pennsylvania Turnpike project in planning used a design advisory team

From the Transportation Research Board’s January 2008 meeting in Washington, D.C. ÷

A Pennsylvania Turnpike project in planning used a design advisory team (DAT) consisting of laymen to select the height and design of a viaduct through the Borough of Turtle Creek in Allegheny County, just east of Pittsburgh. After careful consideration Û including review of a Harrisburg, Pa., precast segmental bridge Û a segmental girder design using either precast or steel was selected. The material is being worked out in the actual design stage, according to a paper, Empowered Public Participation in the Design of the Turtle Creek Viaduct, by Frederick Gottemoeller, Bridgescape LLC; Frank Kempf, P.E., chief bridge engineer for the Pennsylvania Turnpike Commission; and, James Long, Ph.D., Olszak Management Consulting.

Bridges are prominent features of many landscapes. They are justifiably called on to meet public objectives beyond their transportation function, such as the facilitation of economic development, enhancement of an urban environment, or even providing a symbol for the community they are in, the authors write. When such objectives are developed by means of a recognized community involvement process, meeting them becomes a legitimate use of public transportation funds. Identifying these objectives requires thoughtful interaction with the involved community.

The viaduct is a key part of the new MonFayette (MF) Expressway from Pennsylvania 51 to Interstate 376, a 24-mile urban toll road connecting the formerly industrial Monongahela River Valley communities to the Pennsylvania Turnpike at Monroeville and downtown Pittsburgh. PTC’s first goal was to set a new standard for urban expressway design with innovative aesthetic treatments, sensitive urban design features, and related enhancements, the authors report. Its principles include an extensive public outreach program, of which DATs constitute an important part.

The Turtle Creek DAT consists of 12 community stakeholders, along with five technical team members. The initial group of community participants was identified by asking known public entities and interest groups in Turtle Creek to name representatives. DATs thus formed were empowered by the PTC to make design decisions regarding the MonFayette Expressway in their community.

The title ÎDesign Advisory TeamÌ is a misnomer, as the team was essentially a design decision team, the authors affirm. [A]ll decisions made by the DAT were accepted as final by the PTC. Investing the DAT with decision-making power and making a commitment to implement each decision were key to the DAT’s success. This empowerment resulted in the establishment of trust early in the process. More important, having been empowered, the DAT acted in a responsible manner, considering all pros and cons, including costs. This experience contrasts with assumptions often made by transportation agencies that if they give decision-making power to the public, the public will not act responsibly.

Given the DAT’s keen interest in maximizing redevelopment potential of the areas around and under the viaduct, the appearance of the structure and the placement of piers became key issues, the authors note. The viaduct will function as the ceiling of a large outdoor area that the borough hopes will accommodate communal activities. Consequently, its first criterion was that the structure not provide roosting spots for birds.

The DAT did not want to see visitors to their town center dodging bird droppings or viewing the decay that bird droppings create, the professionals write. In part, this concern was the result of a DAT field trip to older viaducts in the Pittsburgh area, where they saw these problems for themselves. Also, the DAT asked that the entire viaduct be of a single structural type, so that it would have a consistent appearance throughout the downtown.

The design advisory team was emphatic as well that the piers not end up in front of a church door or in a church parking lot. With DAT participation, a plan was created indicating all locations where a pier could not be placed. When combined with clearances over streets and sidewalks, several spans of at least 195 ft. will be required.

All of these considerations, taken together, ruled out welded-plate and precast concrete girders, the authors observe. Further development was based on an assumption of either concrete box girders or steel tub girders. In order to become more familiar with these structural types, the DAT took a field trip to Harrisburg to view the PTC’s new mainline bridge over the Susquehanna River, which is a precast segmental concrete box girder.

Considering design options with spans ranging from 195 to 320 ft., the DAT viewed 3-D images of various span arrangements. After examining the renderings and simulated viaduct/pier locations in the field, the DAT decided that smaller spans placed too many piers in the town center, circumscribing views and reducing the sense of openness necessary for viable redevelopment. Accordingly, team members requested a plan that includes only 250-plus-ft. spans and places piers in optimal locations relative to each of the nearby churches. A revised plan satisfying these criteria incorporates two span lengths, 250 ft. and 305 ft. The revised plan, as well as the ultimate girder material, will be investigated further in a final design.

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