The Kingpost Truss
The Kingpost truss, in its simplest form, is found in frame buildings where there is a need to provide large spaces without columns or load bearing walls. The most common form found today supports the roofs of small commercial buildings and the ubiquitous post-World War II ranch- style homes those rambling houses with the low-pitched roofs and no useful attic space. These mass-produced roof trusses, found in any lumber yard, are descended from the structures used in the huge old barns and meeting houses that had cavernous interiors, constructed with post and beam and fastened with mortise and tenon.
The Kingpost truss was adapted to support the shorter bridge spans. It was probably first used with small open-work bridges, and it represented quite an improvement over stringer bridges, in which the trunks of trees were simply thrown over a stream.
In the Kingpost truss, a timber called the Kingpost, is in effect, suspended by its top end by two diagonal timbers whose ends are braced on the ends of the chord above the abutments on each stream bank. The center of the chord is suspended from the Kingpost. The weight of the bridge is transmitted to the ends of the chord through the Kingpost braces. The Kingpost braces are said to be in compression, squeezed between the chord-ends and the Kingpost by the weight carried by the Kingpost. The Kingpost, being stretched by the weight of the roadway and the outward thrust of the Kingpost braces, is said to be in tension. The chord is in tension as well, supported at the abutments and by the Kingpost and stretched by its own weight and the weight of the roadway. As long as the truss, its joints, and its timber members are strong enough to bear the stresses of compression and tension, the bridge will stand.
The carrying capability of the Kingpost truss requires that the angles between the Kingpost braces and the chords not get too small, meaning that a wider stream would require a really tall Kingpost. The maximum practical span for a Kingpost bridge is a little over forty feet. Longer spans require a more sophisticated truss. The Queenpost truss, for example.
From Spanning Time Vermont’s Covered Bridges, by Joseph C. Nelson