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Mr. D. J. Lee (F): I am going to raise a query on the shape of the river crossing and relate this to cantilever bridges in general. One can plot span against the span/
rise ratio for various bridges as shown in the diagram, Fig Dl. The upper band noted as typical prestressed concrete cantilever zone is placed arbitrarily to include several international and European examples of various spans, including the largest cantilever span, Bendorf, point 5. The Bettingen Bridge, point 3, has a span that is pretty well identical to the Kingston Bridge(point 1). Point 2 is the Medway Bridge. It will be seen that these two bridges are rather curiously located half-way between the cantilever zone and the concrete arch zone. Although one would not expect the two zones to have constant S/R values for any span, i.e. one would expect slightly lower S/R values the larger the span, nevertheless the trend as based on existing cases is interesting and I would welcome the authors’ comments.
Mr. A. Spruce (M): The main construction problems facing the contractor on this project were those associated with the ground conditions and with the interesting features of the construction.
Using the equivalent uniform moment approach that has previously been adopfed for unrestrained columns, an elastic design method is presented for columns laterally restrained at intervals along one flange and subjected to axial thrust in the presence of unequal terminal moments about the major axis. The criterion used is the attainment
of yield stress, allowance being made for imperfections in the columns. The analysis is then modified to allow for the formation of a plastic hinge (bending about the major
axis) at one end or, in the case of equal double curvature bending, of plastic hinges at both ends, such loading conditions being of importance in the plastic design of portal frames.
M.R. Horne and J.L. Ajmani