Abstract

Breyton bridge is a nine-span viaduct across the River Yare which will form part of the Great Yarmouth western bypass now under construction. The span over the navigation channel is a single leaf bascule bridge linked to an overhead counterweight. During the design process the overhead counterweight arms were investigated to ascertain their vulnerability to wind-induced excitation. Consultation with the National Maritime Institute revealed that within the range of depth contained in the tapered arms there was a risk of vortex excitation in the fundamental vertical mode, although because the arms were tapered it was difficult to predict this response on the basis of any previous tests. Because of this doubt a 1:20 scale model test was carried out in a wind tunnel at NMI. It revealed only weak excitation in the range of design wind speeds. It was concluded that no special precautions need to be taken on the full-scale structure. Calculations predicted stable conditions for the road deck structure when closed to shipping, but there was less certainty of its performance under wind excitation when raised to permit shipping to pass: this was tested in two different ways.

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