The Tohickon Aqueduct in .NET

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The Tohickon Aqueduct
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structure needed to be replaced. As if to place an exclamation mark on the statement that the aqueduct needed to be replaced, a September 16, 1999, storm in Point Pleasant, caused a portion of the sidewall of the aqueduct to collapse. Bill Collins was one of several community leaders who were very interested in replacing the aqueduct as a timber structure. The concept plans were presented at a meeting of community and elected of cials and of cials of the Pennsylvania Department of Conservation and Natural Resources, Bureau of Facilities Design and Construction. A unique agreement was reached, allowing the community to collaborate with the State Parks Department by providing engineering plans for the timber superstructure outside the normal procurement process. Apparently, it was an easy agreement to broker, as the state would not have to pay for the engineering. Fortunately, there was an organization interested in having the Tohickon Aqueduct constructed of wood. The Wood in Transportation Program of the U.S. Department of Agriculture Forest Service came to the rescue, providing ve small grants to successive phases of the project. The state would be responsible for administration of the project and design of the substructure and the interface of the trunkway with the canal. Initial funding for design of the superstructure was provided by U.S. Forest Service grants through the Wood in Transportation Program. Initial funding was minimal, so design proceeded slowly between 1992 and 1999. This was especially frustrating for the members of the Point Pleasant Community Association when I presented them with construction photographs of the New Covered Bridge in Old Salem, North Carolina, a similar project well underway, which had started at the same time. By contrast, the aqueduct project had gone nowhere. During the extended design phase, the proposed Town lattice truss had evolved into a Burr arch-truss. The geometry of the aqueduct was governed by the existing stone piers, the canal grades, and location of the towpath. It was soon apparent that a trapezoidal shaped trunkway would be the most logical cross-section allowing suf cient space for the ten foot wide standard canal boat or barge while minimizing the total amount of weight to be supported. Grades required the trunkway to be supported on transverse beams supported by the bottom chords of two parallel trusses. Interior diagonal braces provided lateral stability to the top of the trusses, completing the trapezoidal shape. The use of a Town lattice truss had strong historical precedence in Bucks County, with 11 covered bridges, so framed, remaining in 1989. Those covered bridges were built between 1832 and 1875, certainly within the active history of the canal. It is interesting to note that the counties in Pennsylvania that had been settled by the English usually built Town lattice framed covered bridges, while the counties settled by the Germans mostly built Burr arch framed bridges. We were about to upset the covered bridge continuum.
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The Tohickon Aqueduct
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There were a number of good reasons for selecting a Burr arch-truss system instead of a Town lattice truss design for the superstructure. First, the original aqueduct was a Town lattice, which was replaced after 50 or 60 years with an iron aqueduct. We were not able to locate information about the original design, its con guration, or its record of service. As a result, a Town lattice design would be open to almost as much speculation as any other system. In reality, the Town lattice would have been a dif cult system to construct, because it would have to be built on site with many trunnels driven into a large number of lapped joints of chords and lattice. The aqueduct, by its nature, needed to be built of pressure treated wood for durability. The problems of treating and drying pressure-treated timber would be virtually insurmountable using waterborne treatment such as Chromated Copper Arsenate (CCA). The acquisition of trunnel stock, which usually is of White Oak (impossible to pressure treat) and southern pine pressure-treated lattice and chord material, would require a very long lead time to dry, treat, and redry. Any excess moisture in the trunnels or lattice would result in splits forming in the lattice and the trunnels loosening as the structure reached its equilibrium moisture content. Although the goal was to design a bridge that timber framers could build, the assembly of a Town lattice structure requires speci c experience and skills unique to certain bridgewrights. Although the Town lattice has a reputation for toughness, it contains many built up and intersecting members with a potential for decay between members on hidden surfaces. Boring or cutting through CCA pressure-treated material is problematic because salt crystals associated with the preservative tend to dull tools. Driving trunnels through salt-treated lattice and chords is also an issue because of the brashness of the treated wood and the roughness of the salt-treated surfaces, which must engage during the driving process. The Town lattice truss is labor intensive and would have to be built continuous over three spans, extending beyond the abutments a distance equal to its depth. Although this would allow for the forces at the end supports to be distributed among a suf cient number of lattices, as opposed to an abrupt termination resulting in excessive stresses in the lattice, the total length of the trusses would increase by 24 feet. Repairing a Town lattice truss is extremely dif cult and labor intensive because of the closely spaced and tightly held lattice sticks. Again, this work would require the services of a bridgewright with speci c experience. The Burr arch system was designed as a redundant system with either the arch or the truss able to support all loads, independent of the other. The nineteenthcentury builders would proportion each and then simply yoke the two together. In this way, they could circumvent the question of whether the truss stiffens the arch, or the arch stiffens the truss.
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