The design principle behind the Burr arch truss is that the arch should be capable of bearing the entire load on the bridge while the truss keeps the bridge rigid. Even though the kingpost truss alone is capable of bearing a load, this was done because it is impossible to evenly balance a dynamic load crossing the bridge between the two parts. The opposite view is also held, based on computer models, that the truss performs the majority of the load bearing and the arch provides the stability. Either way, the combination of the arch and the truss provides a more stable bridge capable of supporting greater weight than either the arch or truss alone.
Sim Smith Covered Bridge, Parke County, Indiana. Notice the arch projects below the lower chords of the bridge.
Mansfield Covered Bridge, Parke County, Indiana. The roof was missing after a major storm and the interior design was easier to see.
Mansfield Covered Bridge. Spans could be added to extend the bridge further than bridges of similar design.