Trefusis Bridge

This is the last remaining of several highway bridges built to a similar, distinctive design in the early 1830s over the line of the Grand Western Canal (Engineer – James Green). Arched cast-iron ribs, spanning 3.9m and spaced 1.25m apart, are seated on cast-iron cill beams along the top of the masonry abutments. Cast-iron road plates, bearing on the ribs, carry the actual road construction. The web of each edge rib is extended upwards to support the side of the road construction. Very flat masonry arches, one voussoir wide, support the parapets and disguise the constructional form of the deck. An assessment of the bridge�s load-carrying capacity revealed shortcomings in the cast-iron members, and instability in the parapet-support arches. As a result, W. S. Atkins South West (the County Council�s agents for highway works) designed a strengthening scheme comprising an overlying reinforced concrete slab, which carries traffic loads and into which the arches are tied. The slab is de-bonded from the cast iron, and the strengthening work is “reversible” with the exception of four holes drilled through each edge rib into the arch voussoirs. SIAS undertook a photographic and measured survey of the exposed bridge deck while strengthening work was being carried out during October 1997.²