History of The Thomas Bridge
The Thomas Bridge is a highway bridge spanning Crooked Creek in Armstrong Township, Indiana County. The terraces adjacent to Crooked Creek have been farmed continually for more than a century and a half. Crooked Creek, a tributary of the Conemaugh River, flows north to south through the southern part of Armstrong Township. The Thomas Bridge was constructed, in 1879, at "Thomas Fording" and named for the local 19th century property owner, Amos M. Thomas. Mr. Thomas and stonemason, Ben McCreight, constructed the bridge while Robert Morris Fleming supervised the project. The structure cost $545 when it was built in 1879 (Quin 1993: 242). The Town Lattice Truss Bridge design was patented by Ithiel Town in 1820. Although this covered bridge type is "infrequently found" (Pennsylvania Historical and Museum Commission and Pennsylvania Department of Transportation 1986: 63), there are three Town type trusses in Indiana County (Zacher n.d.). The original construction of The Thomas Bridge had a clear span of 75.75 feet with an interior width of 13 feet 3 inches and an exterior width of 15 feet. The lattice of wood diagonals were connected by locust pins to the horizontal top and bottom chords. In fact, Thomas Bridge is the only known Town Lattice Truss Bridge with a single bottom chord in the United States. The lattice consisted of 2" x 10"s pinned with wood. These specifics are replicated in the reconstruction of Thomas Bridge. The structure originally rested on quarry dressed sandstone masonry abutments, which have been replaced by reinforced concrete faced to look like the original abutments. The roof on the reconstructed Thomas Bridge is a corrugated galvanized metal roof, which aesthetically matches the standing-seam roof the structure was assumed to have had.
It took eight (8) months to reconstruct the one hundred nineteen (119) year old structure. The existing floorbeam and lateral bracing system was replaced with steel beams and diaphragms; including PTFE coated stainless steel plates attached at the interface of the truss bottom chord and the diaphragms to allow vertical and horizontal translation while providing lateral/torsional stability.
The structural decking became glued-laminated panels with longitudinal deck planks to maintain the historic appearance.
The coordination between the Pennsylvania Department of Transportation, the Pennsylvania Department of Environmental Protection and the Pennsylvania Historical and Museum Commission was an outstanding achievement.