A Treatise On The Principles And Practice Of Harbour Engineering

2 i8 HARBOUR ENGINEERING. of depth) is proportional to the moment of the righting couple, tlie pontoon is in stable, neutral, or unstable equilibrium, according as We turn now to the question of load, which only concerns the pontoon in its empty condition. When in position a pontoon carries kelsons, or main llLUlLLLlLLlLLLLLLU ^_________________L nrnrrrnrrrrrrrrrn Fie. 192. Fig. 193. girders, which, in turn, receive the load of deck-beams and stringers. These imposed loads, beyond raising the centre of gravity, do not affect the external conditions of equilibrium; their immédiate interest is in regard to the conditions of internal equilibrium. The. problem of the internal stresses to which pontoons are subjected is not one which need cause any difficulty in the way of solution, the same methods of procedure being applicable as in dealing with ordinary beams. Ihere is Fia. 194. Fig. 195. but one difference, albeit a striking one, between the two cases ; but the effect of this is not nearly so embarrassing as might at first sight appear. In a beam the upward reaction is concentrated at isolated points of support ; in a pontoon the reaction is distributed over the whole of the immersed area. A very simple expedient serves, however, to put the two cases on an equal footing. Take fig. 192, representing a beam uniformly loaded and supported beneath at any two points. Now, invert the diagram, as in fig. 193, and we Fig. 196. Fig. 197. Fio. 198. have the case of a floating pontoon earrying two concentrated loads. 0b- viously, the same diagrams of shearing stress and bending moment will serve in both cases, and there will be no difficulty in proceeding by this method in most cases. Even when the pontoon does not float upon an even keel, a measure of the exact distribution of the upward force is given by the area of the buoyant section (fig. 194). Figs. 196-198 are consequently typical bending moment diagrams.