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The Multiple Kingpost Truss Covered Bridge

Multiple Kingpost Bridge Truss Diagram

A straightforward way to stretch the span capability of the queenpost truss is to add panels to the kingpost truss to create what is known as multiple kingpost trusses. Accordingly, the basic kingpost truss is sometimes referred to as a simple kingpost truss. Most of these trusses were built with an even number of panels so that all the diagonals are in compression and all the verticals are in tension under normal loading. Very few multiple kingpost trusses have an odd number of panels, with opposing (or crossing) diagonals in the center panel.

There is a lack of tensile capacity of the connection of the diagonals to posts. In this instance, the compressive force in the diagonals under the influence of the dead load of the bridge is usually much larger than the tensile force resulting from the passage of vehicles. Hence, under normal circumstances, the diagonals remain in compression under all combinations of loading, and the tensile connection is unnecessary.

The longer spans of the multiple kingpost truss, without increasing truss depth significantly, generate higher member forces, which require more capacity. Multiple kingpost truss chords are often comprised of twin members that sandwich a central plane of single web (vertical and diagonal) members. The longer chord members also usually require splices that typically are staggered along the truss length. This critical detail is meant to ensure that, at any particular cross section along the bridge, there is at least one unspliced bottom chord (tension) member in each longitudinal truss; more specifically, there should be 1 m (3.28 ft) separation between splices of adjacent members of the bottom chord.

The panels in multiple kingpost trusses are often quite short, which means that the transverse floor beams could be located abutting each vertical member. This minimal eccentricity between load application and truss joint location greatly reduces bending stresses in the bottom chord. In addition, these more closely spaced web members tend to have smaller member forces in the diagonals due to their geometry, so that the connection forces are somewhat smaller than those associated with kingpost or queenpost trusses.

The truss diagonals bear on shoulders cut into the sides of the vertical tension members. This means that the verticals must be made from substantially wide timbers. Unfortunately, this joint eccentricity means that the shoulders of the verticals are significantly overstressed in shear along the grain. Many truss verticals have failed in shear; it is common to find evidence of separation and slippage of the shoulder relative to the main portion of the vertical. This can happen at either the top or bottom of the post.

About 95 bridges using multiple kingpost trusses remain, or a little more than 10 percent of all covered bridges in the United States. Multiple kingpost trusses have spans that range from 11.0 to 41.1 m (36 to 124 ft), and they all seem to have been built between 1849 and 1983. Interestingly, comparing the span ranges and the construction dates between queenpost and multiple kingpost trusses, one may observe the similarity of these two features.