Burr Arch Truss
The Burr Arch is basically a combination of a typical multiple Kingpost truss with a superimposed arch. The arch is added to allow heavier loads on the bridge and to starch their span capabilities to greater lengths.
Burr’s development was immediately popular with bridge builders and has proven durable. More existing North American covered bridges use the Burr arch then another type. Classic Burr arch supports the ends of the arch at the abutment, with no connection between the bottom chord and arch as they pass each other. A variation of the Burr arch – sometimes referred to as a modified Burr arch – terminates and ties the arch with a connection directly to the bottom chord (which is supported on the abutments).
J.J. Daniels, J.A. Britton, and the Kennedy Family used this truss most often. The majority of covered bridges in Indiana use this type of truss in their construction.
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 ties between any two vertical components.
The Queenpost truss is a simple modification of the kingpost truss. The Queenpost truss is simply a stretched-out version of the kingpost truss. This is accomplished by adding a central panel with extra horizontal top and bottom chords. Classic examples of Queenpost trusses do not have any diagonal web members in the central rectangular panel. Therefore, the simplest Queenpost trusses are not true trusses at all but rather frames.
The member forces and behavior in Queenpost trusses are very similar to those found in kingpost trusses: therefore, the design consideration for these two basic truss styles are equally similar: truss components are usually of single members, the key area of interest is the heel connection and some of the longer spans use subdivided panels, with sub diagonals, hanger rods, and extra floor beams.
This truss uses metal rods as the vertical members of what is otherwise a simple timber parallel-chord, cross-braced truss. This truss was the first one patent granted with a major structural component made with metal. One feature of this truss type was the easy to erect build and the readily prefabricated components that could be assembled on-site and adjusted via threaded connections at the rod ends.
Multiple Kingpost Truss
A 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. Most multiple Kingpost trusses have an even number of panels so that all the diagonals are in compression and are the verticals are in tensions under normal loads. There are very few multiple Kingpost trusses that have an odd number of panels, with opposing (or crossing) diagonals in the center panel.
Town Lattice Truss
The Town Lattice configuration relies on assembling relatively short and light planks in overlapping intersection of members with round timber dowels or pegs that are called trunnels. The plank intersections in the web may have one to three trunnels. Where the chord members intersect with web or lattice members, the overlapping zone may contain as many as four trunnels. The trunnels are often 1 ½ to 2 inches in diameter.
There are about 135 covered bridges left that employ the Town Lattice truss and none of them are in Indiana.
Patented by Colonel Stephen H. Long in 1830, the focus was on a parallel chord truss made with heavy timbers and with crossed diagonals in each panel. A special feature of this bridge included the use of timber wedges at the intersection of the chords, posts, and diagonals. The wedges allowed the builders and maintainers to adjust the shape of the panels, and provided the opportunity to adjust the initial camber (in today’s terms, this allowed the builders to induce forced loads in the diagonals in a way that is described as pretensioning).