A Small Covered Bridge Glossary
Condensed from the book: Spanning Time by Joseph C. Nelson
For more on the book, please visit: www.vertmontbridges.com
Abutment. The abutments support the bridge at each shore of a stream. The abutment consists of a facewall, backwall, and wingwalls.
Bed timbers. Timber components typically located between the top of an abutment or pier and the underside of the truss bottom chord. Intended to serve as sacrificial components they are easily replaced when deteriorated from rot, thus protecting truss components from similar deterioration.
Bridge Deck. The roadway through the bridge.
Buttress. Timbers or iron rods placed along the outside of both sides of a bridge and connected to the ends of extended floor beams, the upper end attached to the top of the truss-work.
Camber. A curvature provided to compensate for dead load deflection, the bridge camber is an upward bowing of the bridge structure, highest at the center of the span. A sagging bridge is said to have negative camber, a sign of a failing bridge.
Chord. The upper and lower longitudinal members, extending the full length of the truss and carrying the forces of tension and compression away from the center of the span.
Compression member. An engineering term that describes a timber or other truss member that is subjected to squeezing or pushing. Also see tension member.
Counter brace. A diagonal timber in a truss which slants in the opposite direction from the brace.
Dead load. The static load imposed by the weight of materials that make up the bridge structure itself.
Kingpost. In a Kingpost truss, the vertical wooden post hung from the apex of the main diagonal braces. In a multiple-Kingpost truss, the vertical member paired with one or two diagonal braces.
King-rod. An iron rod substituted for, or augmenting, a Kingpost.
Portal. General term for the entrance of a covered bridge.
Post. The upright or vertical timber in a bridge truss.
Tension member. An engineering term. Any timber or rod of a truss which is subjected to pull, or stretch. See compression member.
Treenails. Pronounced "trunnels." The wooden pins driven into the holes drilled into the plank members of a lattice truss to fasten them together. Treenails are also used to pin a mortised joint together. Treenails are made of hardwood, usually oak.
Truss. A framework of beams usually connected in a series of triangles, used to support a roof or a bridge. The triangular element in the truss is desirable because the triangle is inherently stable and resists deformation.
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