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The Kingpost Truss Covered Bridge

Kingpost Truss Diagram

The most elementary heavy timber truss configuration is the kingpost. The inclined members of a kingpost truss serve both as the top chord and as the main diagonals, and resist compression forces. The horizontal member, along the bottom of the truss, is the bottom chord and acts in tension. A central vertical member (the kingpost), also acts in tension to support the floor loads and serves as the connecting element between the opposing main diagonals. The kingpost truss configuration has two panels. A panel is that portion of the truss that lies between any two vertical components.

In addition to resisting the tensile forces generated by the opposing diagonals, the bottom chord almost always supports the floor beams. In most kingpost truss bridges, the floor beams are located only at the ends of the bridge and next to the center kingpost. The floor beam point loading does not coincide with the intersections of the theoretical centerlines of the truss members. This connection eccentricity induces bending stresses in the bottom chord that may be large or negligible, depending on the distance of the floor beams from the joints and the depth of the bottom chord.

The dead and live loads are applied differently to kingpost trusses. Live traffic loads are carried to the truss through the central floor beam, while much of the bridge dead load is carried in the rafter plate, along the eaves of the roof. As a result, almost half of the bridge weight is carried to the end posts of the bridge, which transfer their loads directly to the foundation. The kingpost truss carries the centerline floor beam(s) and the inner ends of the four eave plates. Technically, the end posts and the eave struts are not structural members of the kingpost trusses, and their connections are not intended to transfer axial loads within the truss; they are simply members of the associated framework.

The inclination angle for the kingpost diagonals is restricted. Generally, steeper diagonals are more efficient at resisting shear forces in a truss. There are, however, compromises to consider when laying out the members in any truss. For instance, given a set span for a two-panel kingpost truss, steeper diagonals make taller trusses. Beyond the aesthetic issues of building unusually tall, but short-span structures, there are practical limits to the height of the bridge involving bracing and its connections. Hence, the span limit for this simplest truss is quite short, typically only about 7.6 to 9.1 m (25 to 30 ft).

Longer kingpost trusses have been built by including sub-diagonals. These members act as braces, from the bottom of the kingpost up to the midpoint of the main diagonals, thereby producing a minitruss within the larger kingpost truss. Short struts often extend above this junction to support the load from the roof eave plate. Vertical metal rod hangers may also be used from the intersection of these sub-diagonals downward to the bottom chord, allowing the installation of floor beams at this quarter point of the bridge. These modifications allowed builders to increase kingpost spans out to about 10.7 to 12.2 m (35 to 40 ft).

Most kingpost trusses were built with single-member components, usually large sawn or hand-hewn timbers. The most critical connection in kingpost trusses is the heel connection of the main diagonals to the bottom chord. These connections are prone to several weaknesses.

The kingpost truss is not very common in the extant United States covered bridge population. There are only about 30 kingpost covered bridges remaining in the United States, with spans ranging from 6.7 to 21.3 m (22 to 70 ft) (7 It is very unusual for a kingpost bridge to span 6.7 m (70 ft)-approximately 15.2 m (50 ft) would be the more common upper limit. The extant kingpost bridges were built between 1870 and 1976.