Bridge Description Crum Covered Bridge, Rinard Mills Ohio
Crum Bridge is a three-span multiple kingpost truss wooden covered bridge, with an auxiliary tied arch in the center span. It rests on cut stone abutments and piers. The total length of the bridge is 196'-0" along the lower chord, the spans measuring 48', 90', and 48'. The truss is 12'- 2" high from the top of the upper chord to the bottom of the lower chord with a roadway width of 12'-6".
The trusses of all three spans are framed as multiple kingpost trusses. The upper chord is two parallel 6x8" timbers with wooden shear blocks between them, fastened together with threaded rods that pass through the chords at each panel point and are fastened with a plate and nut on either side. The lower chord consists of two parallel 4x8" timbers with wooden shear blocks fastened together in a similar manner. The upper and lower chords are connected by vertical wooden posts spaced 7'-7" apart and diagonal members (5 1/2 x 6 1/2") between the posts. The posts of the center span (4x8") are smaller than those of the approach spans (5 x7 1/2"). The upper chord is notched around the posts on either side. The posts pass through the lower chord where they are set into notches and fastened with bolts. There are plank hub rails (1 1/2 X 9") along the length of the bridge trusses, centered about 1 1/2 feet above the deck.
The main (center) span has a pair of wooden arches sandwiching each truss. The arches span 96' between the piers and rise 10'-4" above the deck. Each arch is composed of paired, laminated arch ribs (four 2 1/2 x 5" planks bolted together) the ends of which are seated in sawtooth cuts in the lower chord.
Wooden floor beams placed transversely at each panel point make up the floor system. The beams rest on the lower chord and the lower lateral bracing (4x4" timbers) is fastened between them. The lower chords of the approaches rest on top of the lower chords of the main span. In order to keep the floor level, the floor beams on the approaches do not rest on the lower chord; rather they are suspended below the lower chords by means of looped iron hangers which wrap around wooden blocks resting on the lower chord and are secured underneath each floor beam with a plate and nuts. Five lines of wooden joists, or stringers, are laid on top of the floor beams and support the wood plank deck. The roadway surface is plank flooring laid diagonally. The center span is tied to the piers with a l"-diameter rod, one end of which is secured in a hole drilled in the stone masonry. The other passes through a wood block at the end of arch where it is fixed with a nut.
The upper lateral system consists of transverse tie beams notched into the upper chord and diagonal bracing between the tie beams. There are sway braces between the posts and tie beams. The rafters are notched into the tie beams and extend diagonally upward to meet an opposing rafter at the ridge. There are wooden purlins on the rafters to which the gable roofs metal sheathing is fastened.
Vertical wood siding covers the exterior of the bridge to about 1 1/2 feet below the upper chord. The sheathing is fastened to wooden nailers on the outer faces of the trusses. The portals are straight with hipped openings.
The abutments and piers are cut, squared stone with mortared joints. The lower chords of the bridge rest on bedding timbers on top of the abutment facewall. The backwalls above the abutments and behind the bedding timbers serve as retainers for the roadbed. Stone wingwalls extend from the backwall at the northerly end of the bridge along a steeply inclined approach.
The north approach has been reinforced with 13x24" rolled steel beams, which sit next to the trusses on the deck above the lower chord. They are attached to the floor beams by threaded rods that pass on either side of the floor beams and are fastened underneath with a metal plate and nuts.
Dating to the Middle Ages, the kingpost is the oldest and simplest bridge truss design. It is based on the inherently stable form of the triangle, which tends to resist deformation. The kingpost truss is essentially a triangle with a central post, known as the kingpost. The two diagonal timbers are braced on the ends of the lower chord and function in compression, transmitting loads from the center of the bridge back to the abutments. The multiple kingpost truss can span greater distances than the simple kingpost. According to the World Guide to Covered Bridges, there are approximately ninety-five examples of the kingpost truss design remaining in the United States.
Although Crum Bridge is usually described as a Burr truss, the center span actually incorporates a tied arch, which makes it somewhat unusual and interesting from an engineering perspective. In a tied arch, the ends of the arches are fitted into slots in the ends of the lower chord rather than extending below the bottom chord to the piers or abutments. The horizontal thrust of the arch is therefore resisted by the tension in the bottom chord, rather than being carried to the piers.
