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Show United States Department of the Interior National Park Service / National Register of Historic Places Registration Form NPS Form 10-900 OMB No. 1024-0018 Weber River Railroad Bridge Weber County, Utah Name of Property County and State the company, in 1885, that he designed and patented his Pegram truss. Pegram worked at Edge Moor until 1886. The Edge Moor Bridge Works was operational until 1900 when it became one of the companies purchased by J.P. Morgan and his American Bridge Company.5 The Pegram Truss The Pegram truss is a hybrid between the Warren and Parker truss systems. A truss is a structural property that has an assemblage of relatively short components arranged in a series of triangles. Used primarily for roofs and framing in the 1700s, by the 1800s engineers explored truss systems for bridge use.6 A popular early truss, the Warren truss, developed in the 1840s, features diagonals that are heavier than the verticals and has parallel top and bottom chords, which lead to the use of more metal than was structurally necessary. Another early truss, the Parker, featured compression posts of several different lengths, as are the individual members of the top chord, which created a polygonal (and unparallel) shaped truss. At the manufacturing site, this required resetting the stamps for different lengths of steel several times during fabrication-it was this issue which George Pegram addressed in his original design.7 The Pegram design uses the same length for all of the top chord members, the same length for each of the compression posts, and the same length for the bottom chord members. Pegram's reasoning was that this simplification would save manufacturing costs by minimizing the stamp tool resetting time and, therefore, cost. Further, this would simplify erection of the final bridge in the field since the individual components of the three major segments (top chord, posts, and bottom chord) were essentially interchangeable. In practice, Pegram modified this design by using different post lengths to provide a more circular arc to the upper chord. The upper and lower components, however, stayed true to concept. When assembled, the Pegram truss produces a very distinguishable design. The geometry of the combination of elements produces a design which has the posts arranged at increasing angles from the vertical as one moves from the center of the truss toward the ends.8 The Weber River Bridge is an excellent example of this design. In addition to the declining number of intact railroad-related structures in Ogden, the once prolific Pegram truss bridges throughout the country have largely disappeared. Only twelve known bridges remain-Republican River (Concordia) Bridge, Kansas (converted for vehicles); Minneapolis Bridge, Kansas (relocated); Naches River (Yakima) Bridge, Washington (relocated); Conant Creek Bridge, Idaho (relocated); Grace Bridge Bridge, Idaho (relocated); St. Anthony Bridge, Idaho (relocated); Gimlet Bridge, Idaho (relocated); Cold Springs Bridge, Idaho (relocated); Ririe A Bridge, Idaho (relocated); Ririe B Bridge, Idaho (relocated); Santa Ana 5 Donald W. Watts, Pegram Truss Railroad Bridges of Idaho, National Register Multiple Property Documentation Form, 1997; and Delaware Department of Transportation, Online PDF, Truss Bridges, http://www.deldot.gov /archaeology/historic_pres/delaware_bridge_book/pdf/truss.pdf, Accessed February 7, 2014. 6 Richard L. Cleary, Bridges, (New York: W.W. Norton & Company, 2007), 127. 7 Watts, Pegram MPS. 8 Watts, Pegram MPS. 11 |
