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| Swinging the Bridge at Upper Nicholsons photo by: Ken W. Watson, 2004 |
| The prettiest bridges on the Rideau are arguably the unequal arm, center bearing timber swing bridges, such as the one at Upper Nicholsons lockstation. This centre bearing design can, as the picture shows, easily be swung by one person simply pushing the bridge. Of the 39 bridges along the canal, four are of this specific design (at Lower Brewers, Brass Point, Kilmarnock and here at Upper Nicholsons). They are sometimes erroneously called a Kingpost Truss Bridge, but they aren't (see explanation below). Their basic structure has been maintained on the Rideau Canal since 1864. Previous swing bridges (rim bearing and offset pivot swing) required two people to operate, whereas the centre bearing design can be operated by one person. This type of bridge was first installed at Upper Nicholsons in 1877 (it has been replaced several times since). Robert W. Passfield, a renowned expert on rideau bridge construction (he literally wrote the book on the topic) explains why the Rideau bridges are not Kingspost Truss Bridges: "The Rideau Canal swing bridges are strengthened by a triangular truss with a center post similar to a conventional kingpost truss, and have been commonly referred to as "kingpost truss swing bridges", but that is an error. In a kingpost truss the truss arms are in compression in carrying the weight of the bridge span, and its live load, to the extremities of the bridge on the abutments, and the kingpost is in tension in holding up the center of the span to keep it from sagging. Hence, the truss arms need to be of a rigid, heavy wood construction. In contrast, on the Rideau Canal swing bridges the main post is in compression over the pivot beam of the swing span in supporting the truss arms, which extend to both ends of the swing span. They are in tension in holding up the extremities of the swing span to keep them from sagging when the span is swung open. Hence, the truss arms are lighter wrought iron rods. Although the conventional kingpost truss and the Rideau Canal swing bridge truss are similar in appearance, they differ widely in their structural function." |
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