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The Long Truss Covered Bridge

Long Truss Bridge Truss Diagram

Colonel Stephen H. Long first patented a truss configuration in 1830. His focus was on a parallel chord truss made with heavy timbers and with crossed diagonals in each panel. A special feature of his bridge included the use of timber wedges at the intersections of the chords, posts, and diagonals. The wedges allowed builders and maintainers to adjust the shape of the panels, and provided the opportunity to adjust the initial camber.

In today's jargon, the wedges allowed builders to induce forced loads in the diagonals in a way that is described as pretensioning. It is extremely difficult to predict the amount of the induced prestressing force. Long's patent applications included images of wedges between the vertical and the chord and between the counter and the chords.

However, the wedges do increase the strength of the connection between the horizontal component of the load in the diagonal and the chord. The transfer of load without wedges flows from the end bearing on the diagonal to the cross grain bearing in the post, then from the cross grain bearing at the shoulder of the post back to the end grain bearing at the shoulder of the chord. Introducing the wedge distributes the bearing load from the chord over a much larger area of the post through the wedge in direct cross grain bearing.

The Long truss was adopted by many builders for use in highway and railway bridges, but the timing of its introduction meant that it was destined to be overtaken quickly in popularity by the Howe truss.

There are about 40 surviving bridges supported by the Long truss, with individual spans that range from 15.5 to .8 m (51 to 170 ft). The oldest extant Long truss was built in 1840, and the newest was built in 1987.