A Treatise On The Principles And Practice Of Harbour Engineering

PIERHEADS, QUAYS, AND LANDING-STAGES. 201 the behaviour of floating bodies have to be carefully studied and clearly understood. Landing-stages, whether adapted for the use of passengers, vehicles, goods, or cattle, fall naturally into two main types —fixed and floating. Fixed Landing’-stag’es.—Besides serving no apparent purpose, it would be difficult to make an exact differentiation between the functions of a fixed landing-stage and those of a wharf, pier, or quay. Generally speaking, the term stage is limited to framed structures of timber or iron, and fixed landing-stages are most commonly platforms of woodwork on a piled founda- tion. But even with this limitation, they possess no features which are not common to jetties and wharfs of similar construction (vide fig. 167). To pursue this branch of the subject further would be redundant, and therefore unnecessary. Floating 1 Landing-stages are decks or rafts of timber or iron work, either self-supporting or carried upon hollow pontoons which afford the necessary bearing power by reason of the buoyant properties of water. The pontoon is the more usual arrangement, and it is certainly the only system applicable to stages of size and importance. Pontoons. The stability of any statical structure depends, in the first instance, upon the equilibrium of the external forces which act upon it. In the case of a floating body, these external forces are essentially and practically two, and two only, viz., (1) gravity, due to the intrinsic weight of the body, acting vertically downwards, and (2) buoyancy, or the vertical component of hydro- static pressure, acting vertically upwards. It is true that, in addition, there are the horizontal components of hydrostatic pressure, but, as in still water, whatever the shape and size of the body, these components must always and exactly neutralise one another, there is no necessity to bring them into considération. We have therefore only to take into account two resultant forces, opposed to one another. In ordinary statics, equilibrium would be sufficiently assured by the fulfilment of the conditions, that (1) the lines of action must coincide, and (2) the forces must be equal in magnitude and opposite in direction. As regards floating bodies, these requirements are satisfied when the vertical line through the centre of gravity passes through the centre of buoyancy (or centre of gravity of the displaced fluid), and when the weight of the body is just equal to that of the fluid displaced. But the extreme mobility of water and the absence in a floating body of any perceptible inherent resistance to disturbance, involve another phase of equilibrium in its practical and working sense. It is obvious that there must be not only perfect balance at any instant, but also a disposition on the part of the body to right itself, or to recover its initial position, in the event of a slight or moderate displacement. Both these points have to be taken into considération in the design of floating vessels.