The bridge central span has a rigid roadway (1) resting on end supports and on two piles. End spans are formed between a pile and an end support and the central span between two piles. At each end of the bridge is constructed a transverse beam (14,15) fixed to the roadway. The beam has a weight such that, in the modified bridge, the bending moment resulting from the sum of the weights of the beam and the adjacent lateral span balance the bending moment resulting from the weight of the central half span which it provides. The existing roadway is pre-stressed by means of longitudinal cables (18) anchored in the two transverse beams. The position of a pile (12,13) is changed so as to obtain the required increase in the central span. Under the central span (9) is provided a deviator (22) against which certain cables are supported so as to increase the resistance of the central span to loads carried by the bridge.
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